Modern aerodynamic technologies such as unmanned aerial systems and horizontal axis wind turbines must regularly contend with forces from highly stochastic and turbulent atmospheric gusts. Conventional methods for modeling and controlling fluid flows are limited in their ability to mitigate these aerodynamic forces in real-time. By applying modern machine learning techniques in an experimental setting, this thesis demonstrates the utility of machine learning in addressing these important problems. We follow two complementary approaches towards this goal.
First, we find an end-to-end solution for control in a gusty environment with model-free reinforcement learning. We deploy state-of-the-art reinforcement learning algorithms on a generalized aerodynamic test-bed consisting of an airfoil with motorized trailing edge flaps. The system features embedded flow sensors, enabling the inclusion of flow measurements in state observations. We place this system in a highly irregular wake behind a bluff-body, dynamically mounted on elastic bands and therefore free to oscillate, and train reinforcement learning agents to minimize the net lifting force on the system by controlling the position of the trailing edge flaps. We find that model-free reinforcement learning agents can outperform basic linear controllers in this gusty, turbulent environment. We also show that augmenting state observations with flow measurements can lead to more consistent learning of the system dynamics.
Next, we explore Fourier neural operators (FNOs) as a method for forecasting the time evolution of turbulent fluid flows. FNOs are capable of learning underlying operator solutions to families of partial differential equations and can be evaluated in just milliseconds. We specifically focus on training FNOs with experimentally measured velocity fields of bluff body wakes in the subcritical regime. To the best of our knowledge, this is the first application of operator learning for fluid mechanics that features experimental measurements. We find that FNOs can accurately predict the evolution of these turbulent wakes even when trained with imperfect measurements. We then show that FNOs can quickly adapt to unseen conditions with minimal data and training through transfer learning. Finally, we consider the performance of FNOs over longer prediction horizons. This approach could enable real-time gust prediction capabilities and monitoring for applied aerodynamic systems.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Gharib, Morteza}, } @phdthesis{10.7907/fpcj-w268, author = {Oshima, Emile Kazuo}, title = {Experimental Studies of Flow Control Techniques for Future Aircraft}, school = {California Institute of Technology}, year = {2023}, doi = {10.7907/fpcj-w268}, url = {https://resolver.caltech.edu/CaltechTHESIS:02042023-015312785}, abstract = {From the signing of the Paris Agreement to the COVID-19 outbreak, the past decade has truly challenged the aviation industry to adapt. New technologies need to be developed constantly to meet the increasing commercial and defense demands for more efficient, quiet, safe, and agile aircraft. To keep up with these rapidly changing times, an approach that marries a fundamental understanding of aerodynamics with systems design and optimization is necessary. This thesis explores two promising concepts for controlling flow over next-generation aircraft: active control on a swept wing for airplane applications, and passive control on a rotating blade for drone applications. In each, force measurements are combined with advanced flow visualization techniques to create a research framework that is both data-driven and physics-informed.
In Part I, a comprehensive wind tunnel campaign is carried out on a swept wing model of modular geometry equipped with an array of sweeping jet actuators, which have demonstrated tremendous promise for flow control authority in both laboratory settings and full-scale flight tests. The flow physics and performance of the wing is investigated first without actuation, revealing separation behaviors at both the leading and trailing edges that are crucial to consider when flow control is applied. This paves the way for an optimization study in a newly proposed framework that relies on fluid power coefficients rather than the momentum coefficient that has been the accepted parameter of choice for characterizing blowing systems over the past seven decades of active flow control research.
Part II explores the feasibility of a “prop-shroud” concept for small-scale aerial vehicles, in which the shroud is directly attached to the blade tips and thus co-rotates with the propeller. Such a configuration has the potential to provide the various aerodynamic and engineering benefits of a shrouded propeller without the associated costs and complexities of its installation. The hover efficiency of a prop-shroud is shown to be comparable to commercially available drone propellers, even without a rigorous optimization of its geometry. The effect of the co-rotating shroud is then analyzed in detail on the time-averaged, phase-averaged, and unsteady features of the flow field. A model based on vortex formation time is developed, laying out a foundation for future research and understanding.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Gharib, Morteza}, } @phdthesis{10.7907/exsa-nm30, author = {Rosakis, Alexandros Yiannis}, title = {The Hemodynamics of Native and Surgical Aortic Valves with Regards to Wall Shear Stress and Residence Time}, school = {California Institute of Technology}, year = {2023}, doi = {10.7907/exsa-nm30}, url = {https://resolver.caltech.edu/CaltechTHESIS:10032022-224522966}, abstract = {Cardiovascular diseases are the leading causes of illness and death all around the world. The third most common cardiovascular disease is aortic stenosis (AS). AS is most commonly characterized as a stiffening of the native trileaflet aortic valve, which impedes blood flow into the aorta and puts extra stress on the heart. The aorta is the main artery that supplies oxygenated blood to the body. AS has been widely studied in the past. However, there has been little work in understanding the complex effects that non uniform stiffening of the aortic valve can have on the hemodynamics inside the aorta. The most effective treatment for AS is to replace the stiffened valve with a prosthetic valve. Care must be taken to ensure that the replacement actually performs better hemodynamically. A major metric for prosthetic valve performance is the transvalvular pressure drop which is a measure of how much pressure, and energy, is lost as the heart pumps blood through the valve. Generally speaking, larger valves exhibit a smaller pressure drop because they restrict the flow to a lesser degree. This phenomenon has led to a trend for surgeons to implant the largest prosthetic valve possible, and in some cases, to expanding the aorta to fit even larger valves. However, there has been relatively little work done on determining the effects of valve oversizing on the blood flow inside the Aorta. The aims of this study were two-fold. First, a model of AS was tested inside an in vitro aortic simulator in order to identify how different individual leaflet stiffnesses would affect blood flow. Digital particle image velocimetry (DPIV) was used to measure velocity profiles inside a model aorta. The DPIV results were used to estimate the wall shear stress and blood residence time. Our analysis suggests that leaflet asymmetry greatly affects the amount of WSS by vectoring the systolic jet and that stiffened leaflets have an increased residence time. This study indicates that valve leaflets with different stiffness conditions can have a more significant impact on wall shear stress than stenosis caused by the uniform increase in all three leaflets (and the subsequent increased systolic velocity) alone. Second, the experimental apparatus was used to test different prosthetic valve sizes and valve mounting methods in order to identify how they affected residence time inside the sinus bulge. Dye residence experiments and DPIV were used to measure fluid stasis in several different combinations of prosthetic valve sizes, sinus sizes, and valve mounting methods. Our results indicate that valve to sinus sizing and mounting method is very important and can lead to greatly increased residence time and thrombosis risk. We have also identified a metric that can predict the threshold at which valves become oversized.}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Gharib, Morteza}, } @phdthesis{10.7907/w49w-qy54, author = {Veismann, Marcel}, title = {Axial Descent of Multirotor Configurations – Experimental Studies for Terrestrial and Extraterrestrial Applications}, school = {California Institute of Technology}, year = {2022}, doi = {10.7907/w49w-qy54}, url = {https://resolver.caltech.edu/CaltechTHESIS:01252022-055518852}, abstract = {Axial descent, specifically the vortex ring state (VRS), poses great challenges for rotorcraft operation as this flight stage is typically accompanied by severe aerodynamic losses and excessive vibrational loads due to the re-ingestion of rotor downwash. Given the hazardous nature of this flight stage, its fluid dynamic properties in regards to single, large-scale rotors have been extensively investigated since the early stages of manned helicopter flight. In light of the rapidly expanding use of small-scale multirotor systems, the field of VRS research has recently received increased interest, with a shifted focus towards small-scale rotors, as the thrust generation and stability of these aerial systems have also been shown to be adversely affected by complex descent aerodynamics. While experimental studies have started examining low Reynolds number rotor aerodynamics in steep or vertical descent, the influence of small-scale rotor geometry and aerodynamic coupling between neighboring rotors have not yet been sufficiently explored.
The objective of this work is, therefore, to extend the current understanding of rotorcraft vortex ring state aerodynamics to low Reynolds number multirotor systems. A series of experimental studies employing various wind tunnel setups and flow visualization techniques is presented with the aim of identifying the underlying fluid-structure interactions, and quantifying rotor performance losses during multirotor axial descent. The work is divided into two fundamental experimental approaches, one utilizing statically mounted rotor systems and one utilizing free-flight testing.
The first part of this work (Chapters 4 and 5) presents the results of wind-tunnel tested statically-mounted rotors for precise aerodynamic identification of rotor performance under simulated descent conditions. Chapter 4 covers a parametric analysis to comprehensively assess the extent to which relevant geometric parameters of a small-scale rotor influence its descent characteristic. Chapter 5 then explores the influence of separation between rotors and identifies potential rotor-rotor interactions in the VRS. The studies in this part of the thesis also make use of PIV setups for visualizing the flow field around small-scale rotors in the axial descent regime, subject to changing geometric parameters and rotor separation.
In the second part (Chapters 6 and 7), a series of free-flight investigations is described for realistically simulated axial descent scenarios. Chapter 6 introduces the methodology for quantifying thrust generation of a multirotor in free-flight without rigid attachment to a load cell, and presents the results of exploratory axial flight studies. Chapter 7 discusses a study on axial descent of variable-pitch multirotor configurations, which was carried out to evaluate the feasibility of deploying a future Mars helicopter in mid air. Findings from this study helped to inform the entry descent and landing (EDL) strategy for JPL’s future Martian rotorcraft missions.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Gharib, Morteza}, } @phdthesis{10.7907/tzpw-pt75, author = {Hanania, Haeri Park}, title = {Nanophotonic Application to Biomedical Devices}, school = {California Institute of Technology}, year = {2022}, doi = {10.7907/tzpw-pt75}, url = {https://resolver.caltech.edu/CaltechTHESIS:02182022-230421298}, abstract = {Nanophotonics is the study of interactions between nanoscale structures and light. It has greatly expanded the fields of application over the past decades, taking advantage of the advancement in MEMS technology. The most common nanophotonic structures consist of either dielectrics, metals, or both. When a nanophotonic structure contains metals, it is considered as a plasmonic structure. Plasmonics is a field of light-metal interactions. Due to the negative permittivity of metals, the electromagnetic energy of light is focused at the metal-dielectric interface and creates plasmons-a collective motion of electrons in the conduction band of metals. By shaping metals into different structures to achieve a desired performance, plasmonics have been successfully applied to many fields including photovoltaics, spectroscopy, and biomedical devices.
This thesis provides 3 different applications of biomedical devices in which nanophotonics-articularly plasmonics-was applied. Chapter 1 discusses the application of nanophotonics to molecular sensing. In this chapter, an open-top, tapered waveguide that serves as a 3-dimensional plasmon cavity is demonstrated and achieves a near or single molecular detection. Chapter 2 discusses the application of nanophotonics to an implantable intraocular pressure sensor. In this chapter, an array of gold nanodots are introduced on a flexible membrane to optimize the performance of the sensor. Chapter 3 discusses the application of nanophotonics to angle-and-polarization independent pressure or strain sensing, which reduces the need for precise alignment or a trained technician, and therefore can be easily applied to moving subjects in diverse environments. Inspired by the geometry and optical principles of butterfly corneas, an array of gold paraboloids is designed to support a surface plasmon resonance that is angle-and-polarization independent. This array is integrated onto a hermetically sealed cavity with a flexible membrane and enables angle-and-polarization independent pressure/strain sensing.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Gharib, Morteza}, } @phdthesis{10.7907/293w-ev66, author = {Dougherty, Christopher John}, title = {On the Experimental Simulation of Atmospheric-Like Disturbances Near the Surface}, school = {California Institute of Technology}, year = {2022}, doi = {10.7907/293w-ev66}, url = {https://resolver.caltech.edu/CaltechTHESIS:05272022-085410375}, abstract = {Any and every ‘decision-maker’’ gravity-bound to the planetary surface (or very nearly so) must contend with the frictional complexities confined to its relatively small surface layer. From the perspective of the near-surface bound small autonomous flyer, it is the microclimatic local set of atmospheric conditions (i.e. the weather), characterized by moisture, temperature, and the parameters describing wind, that determines the baseline flowfields within which these flyers must navigate and negotiate. Unlike their human-on-board counterparts, mission parameters relegate small (nearly) massless autonomous flyers to the lower regions of the atmospheric boundary layer, where they may not be fortuned enough to soar above the effects of friction or wait for clearer skies. Relatively little focus has been placed on the experimental strategies of how these machines might learn to function in challenging scenarios well-before encountering them in the real-world. To address such shortcomings, this work focuses on the experimental simulation of flight-relevant environments through the development of multi-source wind generating apparatuses (i.e. fan arrays) that can initialize velocity distributions discretely-individually or in-concert to produce appropriate mean and fluctuating velocities through an ample open-air test envelope that enables full-scale conventional statically-mounted aerodynamic-characterizations up through free-flight autonomous vehicle testing. Though outside the scope of current experimental work, as full of an environmental description (i.e. moisture, temperature, and wind) is given as possible, prior to ultimately reducing the scope to a neutrally stable atmosphere devoid of any major weather events other than a reasonably strong prevailing wind. Nearly always set amongst the backdrop of a high Reynolds number turbulent flowfield, two primary prototypical flowfields (continuous-gust and discrete-gust) are identified as meriting consideration for mainstay experimental simulation. The core features within the spectral overlap of these windy disturbance environments with the response characteristics of flyers of interest ensure that the turbulence of consideration is nearly always of the mechanical-type. Unlike air motions far above local effects in the inertial sublayer (ISL), the dominant flow mechanism within regions of interest near canopied surfaces is augmented by the presence of coherent structures due to the prevalence of locally initiated mixing layers and wakes such that the task becomes one of simulation of suitable forcing spectra in the physical domain for the regions of interest during anticipated times-of-flight.
Likely to prove challenging to the small autonomous flyer are encounters of a change in wind state that occur upon piercing the dividing streamline of air masses of two different velocities. From the view of the flyer navigating the built-up environment, intermittent free shear layers due to wind-interactions with surface roughness elements are unavoidable and are experienced discretely when the flyer and shear layer dynamics are decoupled. Fan array techniques for the generation of mixing layers, the basic building block of any such free shear layer, is explored as a candidate flowfield for the experimental simulation of a discrete gust forcing input for the flyer near the surface. Both initialized dual-stream and triple-stream mixing layers at flight-relevant freestream velocity differences are explored and found to principally behave like the mixing layers developed in a more conventional splitterplate experiment. The Reynolds number Reδω based on the velocity difference ΔU and vorticity thickness δω (both outer scale parameters) is shown to linearly increase with downstream development as the vorticity thickness increases commensurately. The spectral analysis along the centerline confirms local isotropy for every tested case.
The continuous-gust flowfield (simply referred to as ’turbulence) is prevalent throughout the atmospheric boundary layer as are quasi-coherent flowfields of superimposed wakes within canopied environments. Because velocity fluctuations manifest as (predominantly) random deviations at any given instant, these flowfields are good candidates for statistical analysis. Generation techniques to produce such turbulent flowfields are introduced and compared against the uniform flow modality (i.e. all fan units set to produce nominally the same initial velocity condition to develop a well-mixed turbulent flowfield beyond x/L ∼ 0.5 with ReλT = 135). The random-phase (R-P) perturbation technique proves useful in increasing ReλT upwards of nearly sevenfold with only a slight further-loss-of-uniformity (to within 3.7% of the mean). The uniform flow modality with the (R-P) perturbation activated is shown, through the presence of a -5/3 slope power law region, to be locally isotropic at relevant freestream velocities. Significant increases in ReλT are made through a static-reconfiguring of the discrete source fan units into a so called quasi-grid (Q-G) configuration. The highest recorded Taylor microscale Reynolds number was found to be ReλT = 2700, likely accompanied by a non-negligible loss of uniformity at the fixed measurement location, though traverses were not undertaken during this campaign so no direct statement of homogeneity is put forth.
For all the flow modalities presented (i.e uniform, pseudo-random, quasi-coherent, and mixing layer), the high-Re number criteria (Reδω ≈ 104 , ReλT ≈ 102) has been met. This serves, then, as a necessary minimum benchmark in the development of multi-source wind tunnels with intended use as environmental simulators for flyers near the surface and also provides the basis for a spectral framework of comparison to enable systematic development of flowfields in future work. Characteristics of the evolving flowfields can further be tuned through the introduction of perturbation techniques applied as initial conditions to both increase the standard deviation of the fluctuating velocities about a desired mean as well as to initiate, evolve, and combine flowfields in representative ways. A preliminary example of one such combination of flow modalities (pseudo-random and mixing layer) indicates significant alteration of flow development compared to a nominal mixing layer case.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Gharib, Morteza}, } @phdthesis{10.7907/atnt-8p46, author = {Narasimhan, Vinayak}, title = {Bioinspired Nanostructures for Biomedical Applications}, school = {California Institute of Technology}, year = {2021}, doi = {10.7907/atnt-8p46}, url = {https://resolver.caltech.edu/CaltechTHESIS:07242020-111050846}, abstract = {Nature boasts a myriad examples of coloration achieved purely through the physical interaction of light with nano-scale features also known as biophotonic nanostructures. From reptiles to insects, birds to flora, structural coloration has been achieved through a variety of fascinating nano-architectures that leverage different physics. Beyond structural coloration, these nanostructures are often truly multifunctional. For instance, biophotonic nanostructures can also serve as self-cleaning and bactericidal surfaces, gas and thermal sensors, waveguides and beam splitters. With the growing need for robust and compact biomedical devices, the requirement to embed multiple functionalities towards sensing, monitoring, diagnostics and therapeutics within a diminutive device footprint becomes crucial. In this regard, inspiration from the multifunctionality of biophotonic nanostructures can prove to be greatly beneficial for medical applications. Consequently, this work attempts to showcase various examples of the utilization of nanostructures inspired from biophotonic nanostructures for biomedical applications under various overlapping themes such as ophthalmic sensors, bioinspired optics and plasmonic biosensing.
This thesis is summarized in two parts. The first part (Chapters 2–4) introduces a proof-of-concept optical intraocular pressure (IOP) sensor implant and various challenges faced during its in vivo implementation. In Chapter 3, nanostructures inspired by light-trapping epidermal micro-/nanostructures on flower petals are proposed and embedded onto the sensor platform to improve its in vivo optical signal-to-noise ratio and biocompatibility. Chapter 4 covers nanostructures inspired by biophotonic nanostructures on longtail glasswing butterfly wings that improve the in vivo angle of acceptance and biocompatibility of the sensor.
The second part (Chapters 5 and 6) presents the use of bioinspired nanostructures in plasmonic biosensors. Chapter 5 discusses an on-chip platform consisting of bioinspired plasmonic nanostructures to detect various nucleic acid sequences of relevance in the pathogenesis of HIV-1 via plasmon-enhanced fluorescence. Chapter 6 describes the employment of bioinspired quasi-ordered nanostructuring on flexible substrates for broadband surface-enhanced Raman spectroscopy (SERS). Here, SERS-based biosensing enabled by quasi-ordering is used to detect uric acid – a biomarker of various pathologies in human tears.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Choo, Hyuck}, } @phdthesis{10.7907/m3a3-4610, author = {Bedrossian, Manuel M.}, title = {A Novel Digital Holographic Microscope (DHM) to Investigate and Characterize Microbial Motility in Extreme Aquatic Environments}, school = {California Institute of Technology}, year = {2020}, doi = {10.7907/m3a3-4610}, url = {https://resolver.caltech.edu/CaltechTHESIS:04272020-152058259}, abstract = {Recent shifts in the astrobiological community have prompted the development of methods for the direct search for extant life within our solar system. In order to look for life elsewhere in our solar system, it is important to also investigate the broad spectrum of extant life on Earth. Over millions of years of evolution, life has continually adapted such that an ‘extreme’ environment has become a relative term. What is considered extreme for one type of organism is home to another and vice versa. Furthermore, very little is known about the organisms that inhabit these extreme environments, and even less in known about their in situ behavior. Investigating various extreme environments around Earth in order to understand the in situ behavior of organisms that inhabit it will better inform the astrobiological community when planning future space missions for the direct search for extant life within our solar system. However, no suitable instrument exists to conduct these in situ field campaigns, while also being physically robust enough to withstand the rugged terrains that can be expected from extreme environments.
This thesis describes the development of a novel off-axis digital holographic microscope (DHM) for the direct in situ observation of microscale organisms in extreme aquatic environments. The hardware developments of this instrument are introduced and validated experimentally as well as software developments including autonomous particle detection and tracking algorithms. This instrument is then used in novel laboratory experiments involving the development of optical phase contrast agents, as well as deployed to multiple field campaigns where off-axis DHM is used to observe the in situ behavior of microorgansisms in various extreme aquatic environments around North America.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Gharib, Morteza}, } @phdthesis{10.7907/741T-MN38, author = {Huang, Jinglin Alice}, title = {Investigations of Different Methods to Promote Drug Mixing in the Eye}, school = {California Institute of Technology}, year = {2020}, doi = {10.7907/741T-MN38}, url = {https://resolver.caltech.edu/CaltechTHESIS:12122019-162434400}, abstract = {Age-related macular degeneration (AMD) is the leading cause of central vision loss in the developed world. In the case of wet AMD, it can be managed through serial intravitreal injections of anti-vascular endothelial growth factor (anti-VEGF) agents. However, sometimes the treatment is ineffective. Given that half-life time of the drug is limited, one possible cause of the ineffective treatment is inefficient drug mixing in the eye. Here, we focus on the understanding of drug mixing in vitreous chamber and parameters that could potentially influence mixing profiles. Both movement-driven method and thermal-driven method are explored. The in-vitro study outcomes will not only be useful for achieving fundamental understandings of fluid dynamics in the eye, but also helpful in developing a better strategy for intravitreal injection and improving the quality of care for patients.}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Gharib, Morteza}, } @phdthesis{10.7907/71ak-w328, author = {Mittelstein, David Reza}, title = {Modifying Ultrasound Waveform Parameters to Control, Influence, or Disrupt Cells}, school = {California Institute of Technology}, year = {2020}, doi = {10.7907/71ak-w328}, url = {https://resolver.caltech.edu/CaltechTHESIS:05242020-045332969}, abstract = {Ultrasound can be focused into deep tissues with millimeter precision to perform non-invasive ablative therapy for diseases such as cancer. In most cases, this ablation uses high intensity ultrasound to deposit non-selective thermal or mechanical energy at the ultrasound focus, damaging both healthy bystander tissue and cancer cells. Here we describe an alternative low intensity pulsed ultrasound approach known as “oncotripsy” that leverages the distinct mechanical properties of neoplastic cells to achieve inherent cancer selectivity. We show that when applied at a specific frequency and pulse duration, focused ultrasound selectively disrupts a panel of breast, colon, and leukemia cancer cell models in suspension without significantly damaging healthy immune or red blood cells. Mechanistic experiments reveal that the formation of acoustic standing waves and the emergence of cell-seeded cavitation lead to cytoskeletal disruption, expression of apoptotic markers, and cell death. The inherent selectivity of this low intensity pulsed ultrasound approach offers a potentially safer and thus more broadly applicable alternative to non-selective high intensity ultrasound ablation.
In this dissertation, I describe the oncotripsy theory in its initial formulation, the experimental validation and investigation of testable predictions from that theory, and the refinement of said theory with new experimental evidence. Throughout, I describe how careful modifications to the ultrasound waveform directly can significantly impact how the ultrasound bio-effects control, influence, or disrupt cells in a selective and controlled manner.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, } @phdthesis{10.7907/ZN7F-ZF71, author = {Cho, Hyunjun}, title = {Real-Time Biosensing and Energy Harvesting on Human Body}, school = {California Institute of Technology}, year = {2020}, doi = {10.7907/ZN7F-ZF71}, url = {https://resolver.caltech.edu/CaltechTHESIS:07252019-145728798}, abstract = {
This thesis covers two technologies that can be applied to the human body for real-time applicable usages: biosensors and energy harvesters. The first part of the thesis describes optical biosensing techniques based on surface-enhanced Raman spectroscopy (SERS). Our large-scale spatially uniform Raman enhancing substrates allow low-level bio molecule detection due to their strong plasmonic enhancement of the 3D Au-NP clusters. This method also enables low-level insulin sensing as well as insulin concentration analysis in islet secretion. These results can lead to developing simple and easy biosensing methods allowing real-time biosensing applications including convenient monitoring of health, early disease detection, and diabetes-related clinical measurements.
The second part of the thesis suggests an energy harvesting method using vocal vibrations. The vocal folds produce mechanical vibrations that can serve as an energy source with consistent amplitude and frequency. The vibration hotspots exist at various locations on the human upper body. The energy harvesting system consisting of piezoelectric devices and energy harvesting circuits generates 3.99 mW of electrical power. The amount of energy generated from vocal vibrations is sufficient to charge a Li-Po battery which can drive an LCD display or charge Bluetooth headphones. This method demonstrating a relatively high power generation and convenience of practical use can provide a real-time complementary charging technique for wearable electronics like wireless headphones and smart glasses as well as medical implantable devices such as deep brain stimulators, cochlear implants and pacemakers.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Choo, Hyuck and Gharib, Morteza}, } @phdthesis{10.7907/326X-M576, author = {Huertas-Cerdeira, Cecilia}, title = {On the Dynamics of Flat Plates in a Fluid Environment: A Study of Inverted Flag Flapping and Caudal Fin Maneuvering}, school = {California Institute of Technology}, year = {2019}, doi = {10.7907/326X-M576}, url = {https://resolver.caltech.edu/CaltechTHESIS:06072019-103225366}, abstract = {Despite serving analogous functions, the mechanical designs conceived by human engineering and those that result from natural evolution often possess fundamentally differing properties. This thesis explores the use of principles that stem from natural evolution to improve the performance of engineered mechanisms, focusing on systems whose role is to interact with a fluid environment. Two different principles are considered: the use of compliance, abundant in nature’s structures, and the use of flapping propulsion, prevalent among nature’s swimmers.
The first part of this thesis is dedicated to investigating the physics that govern the behavior of an inverted-flag energy harvester; an unactuated flexible cantilever plate that is clamped at its trailing edge and submerged in a flow. The resonance between solid motion and fluid forcing generates large-amplitude unsteady deformations of the structure that may be used for energy harvesting purposes. The effect of the flag’s aspect ratio on its stability is first evaluated. Flags of very small aspect ratio are demonstrated to undergo a saddle-node bifurcation instead of a divergence instability. The angle of attack of the flag is then modified to reveal the existence of dynamical regimes additional to those present at zero angle of attack. A side-by-side flag configuration is finally explored, highlighting the presence of an energetically favorable symmetric flapping mode among other coupled dynamics.
The second part of this thesis delves into the analysis of underwater flapping propellers and the optimization of their three-dimensional motion to generate desired maneuvering forces, with the objective of obtaining an appendage for use in autonomous underwater vehicles that can perform both fast maneuvering and efficient propulsion. An experimental optimization procedure is employed to obtain the most efficient trajectory that generates a specified side force. The effect of increasing the fin’s aspect ratio is examined, and a highly efficient trajectory, that makes use of high three-dimensionality and rotation angles, is obtained for a fin of AR=4. The use of a flexible fin is then analyzed and shown to be detrimental to the maneuvering efficiency of the system.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Gharib, Morteza}, } @phdthesis{10.7907/FT8M-PM75, author = {Wang, Cong}, title = {On the Manipulation of a Turbulent Boundary Layer by Unsteady Boundary Conditions}, school = {California Institute of Technology}, year = {2019}, doi = {10.7907/FT8M-PM75}, url = {https://resolver.caltech.edu/CaltechTHESIS:06072019-114300433}, abstract = {Reducing the frictional drag generated by a turbulent boundary layer (TBL) is critical for many engineering applications. Motivated by existing turbulent drag reduction methods, this study explores the possibility of sustaining wall-attached air-films and manipulating the near-wall turbulence in hydrodynamic TBL. An innovative air-retaining system is designed to sustain and dynamically modulate the wall-attached air-films in TBL. In still water, the oscillating air-films induce vortical motions in the near-region of air-films. In TBL, phenomena such as Stokes-type oscillatory motion, zero- shear-stress layer, ‘inactive’ turbulence and reduced viscous shear stress are observed in the vicinity region of air-films. The analysis shows that TBL momentum transfer toward the wall is suppressed and a turbulence re-laminarization mechanism is induced in the near-wall region. One potential physical mechanism points to the process of vorticity generation in the near-region of oscillating air-films, which ‘pushes’ the TBL near-wall vortical structures away from the wall. With this viewpoint, the phenomena mentioned above can be explained. The modified momentum transfer mechanism and turbulence re-laminarization process are shown to be the potential cause of suppressed viscous shear stress in the near-wall region. Estimated using the Clauser chart method, the turbulent wall-skin friction shows a noticeable decrease in the presence of air-films.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Gharib, Morteza}, } @phdthesis{10.7907/Q6CG-QY57, author = {Martin, Nathan Koon-Hung}, title = {Analysis of Flapping Propulsion: Comparison, Characterization, and Optimization}, school = {California Institute of Technology}, year = {2018}, doi = {10.7907/Q6CG-QY57}, url = {https://resolver.caltech.edu/CaltechTHESIS:06072018-133402239}, abstract = {In recent decades, the development of autonomous underwater vehicles (AUVs) has rapidly increased and inspiration for novel designs has recently come from nature, primarily based on the fast, efficient, and maneuverable flapping motion of fish. Due to its potential, flapping propulsion is investigated through three studies.
The first study involves the comparison between swimming by flapping and by periodic contractions. A direct comparison is made between the two propulsion mechanisms by simplifying the motions, utilizing a machine that can operate in either mode of propulsion, and evaluating the average thrust generated and the average input power required per cycle between the two mechanisms when the overall kinematics are identical. The two propulsion mechanisms are tested using a variety of overall kinematics, flexible plates, and modified duty cycles, all of which suggest that flapping propulsion is the more efficient; however, periodic contractions with a modified duty cycle are shown to generate more thrust per cycle.
The second study involves the characterization of the impact of chord-wise curvature on the hydrodynamic forces and torques, motivated by the dorso-ventral bending of a fish’s caudal fin during locomotion. The impact of curvature is shown to depend on the planform area of the flapping plate. Plates with a smaller or an identical planform area compared with a baseline rigid flat rectangular plate either decrease or increase the generated thrust, respectively. These phenomena are utilized to develop an actuated plate for velocity modulation and a snap-buckling plate to provide a greater thrust and efficiency compared with a rigid plate propulsor.
The third study involves the development and demonstration of a method to experimentally optimize an arbitrary three-dimensional trajectory for a flapping propulsor. The trajectory is parameterized by variables inspired by birds and fish, executed by a mechanism that can actuate an arbitrary motion in a hemisphere, and optimized using an adaptive evolutionary strategy. The trajectories are scored based upon their difference from a desired force set-point and their efficiency. All trajectory searches demonstrate good convergence properties and match the desired force set-point almost immediately. Additional generations primarily improve the efficiency. This novel approach finds optimal trajectories for generating side-forces, similar to how a fish’s pectoral fin or a bird’s wing functions, and for generating thrust, similar to how a fish’s caudal fin operates.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Gharib, Morteza}, } @phdthesis{10.7907/Z9736P2V, author = {Grivel, Morgane Anne Marie}, title = {On the Effect of Large-Scale Patterned Wettability on Contact Line Hydrodynamics}, school = {California Institute of Technology}, year = {2018}, doi = {10.7907/Z9736P2V}, url = {https://resolver.caltech.edu/CaltechTHESIS:08182017-103752052}, abstract = {Numerous studies have investigated how liquid water behaves on solid surfaces with uniformly hydrophilic or uniformly hydrophobic wetting properties. In particular, uniformly hydrophobic surfaces have been widely studied for modifying flow behavior of rivulets and drops at smaller scales, as well as for drag reduction on ships or other free-surface-piercing bodies at larger scales. Despite the extensive body of work on surfaces with uniform wetting properties, minimal work has been done to investigate how combining hydrophilic and hydrophobic regions onto a single surface to create macroscopic non-uniform wetting properties affects flows. Research in this vein has predominantly focused on low Reynolds number flows, such as in microfluidic channels or droplet impacts.
This thesis expands on the current literature by investigating contact line dynamics and global flow behavior on surfaces with larger-scale non-uniform wetting properties. Experiments were first carried out to study thin sheet flow down an inclined plate at Re ~ 50 - 1200. The plate’s wetting condition was changed by introducing alternating hydrophilic and hydrophobic bands 2-25 mm wide oriented at different angles with respect to the flow direction. Results show that the contact line of such flows is heavily modified compared to the uniform cases. At low Reynolds numbers, large-scale wettability heterogeneities are observed to tune the fingering instability wavelength if the bands are parallel to the flow direction and to dampen finger oscillations if the bands are perpendicular to the flow direction. At higher Reynolds numbers, roller structures are introduced at every hydrophilic-to-hydrophobic junction, modifying the global flow morphology. Entrained air bubbles are also captured and observed to coalesce if the bands are perpendicular to the flow direction.
These experiments were then extended to a surface-piercing hydrofoil coated with alternating hydrophilic and hydrophobic bands. Experiments were run in Caltech’s Free Surface Laboratory water tunnel for Re on the order of 104 to 105. The experiments demonstrate that the contact line is modulated in this context, alternating from concave to convex over the different wettability regions. The modulation of the contact line propagates to the rest of the water free-surface via the generation of standing waves and further modifies the free-surface separation point’s location and steadiness. In addition, changes in wettability are observed to generate side force, which is of interest for vessel maneuvers in naval applications.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Gharib, Morteza}, } @phdthesis{10.7907/Z998856J, author = {Lin, Ben Albert}, title = {Ultrasound Speckle Image Velocimetry: Studies on System Performance and Application to Cardiovascular Fluid Dynamics}, school = {California Institute of Technology}, year = {2018}, doi = {10.7907/Z998856J}, url = {https://resolver.caltech.edu/CaltechTHESIS:08152017-194113650}, abstract = {Knowledge of detailed blood flow characteristics can be extremely valuable in a variety of settings. Examples range from studying disease processes such as atherosclerosis to aiding in the design of medical devices such as prosthetic cardiac valves. For in vivo and optically inaccessible in vitro flows, accurate measurements of velocity fields and shear stresses can be difficult to obtain. Doppler ultrasound and magnetic resonance imaging are the most commonly used techniques, but have important limitations. Recently, there has been increased interest in the application of particle image velocimetry principles towards tracking of ultrasound speckle patterns to determine multidimensional flow velocities with increased temporal resolution. We refer to our implementation as ultrasound speckle image velocimetry (USIV). In this research project, our first objective was to obtain a detailed characterization of the factors unique to ultrasound imaging that can influence the accuracy of velocity measurements. By conducting in vitro experiments with uniform speckle phantom translation as well as steady tube flow, we have shown that characteristics such as transducer focal depth and beam sweep speed as well as particle motion direction and velocity can all influence USIV results. Our second objective was to demonstrate the utility of USIV for analyzing in vivo blood flows. After administering ultrasound contrast agent to anesthetized pigs, we were able to obtain detailed images of both left ventricular flow and abdominal aortic flow. Velocity profiles were measured during both left ventricular filling and ejection. Our most interesting finding was the presence in certain cases of highly asymmetric retrograde flow in the infrarenal aorta. The factors that lead to such flows may have relevance to the development of atherosclerosis and abdominal aneurysms. USIV is likely to be very useful for further studies both in vivo and with in vitro elastic aorta models.}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Gharib, Morteza}, } @phdthesis{10.7907/Z9N014KR, author = {Hirsch, Damian George}, title = {An Experimental and Theoretical Study of Active Flow Control}, school = {California Institute of Technology}, year = {2017}, doi = {10.7907/Z9N014KR}, url = {https://resolver.caltech.edu/CaltechTHESIS:06092017-112408552}, abstract = {The accelerating growth of environmental awareness has not stopped at the aerospace industry. The need for greener and more efficient airplanes threatens to outpace the flow of new technology. This has ignited development in several fields, one of which is active flow control (AFC). Active flow control has quickly proven its tremendous potential for real applications. Even though the roots of this technology date back a century, we still lack fundamental understanding. This thesis combines both modern and traditional approaches to lay out a new foundation for future research.
The thesis first focuses on the rising stars of active flow control: the so-called fluidic oscillators or sweeping jet actuators. These devices consist of simple, rigid internal geometries that create a sweeping output jet motion. The fluid dynamic interactions with the internal geometry are studied in detail using high-speed Schlieren imaging. Additionally, the influence of adjacent sweeping jets is investigated. It is revealed that the internal driving mechanism is far stronger than the fluid dynamic interactions at the outlet, resulting in a completely independent jet behavior.
Next, a high-lift airfoil design is combined with active flow control, and an extensive wind tunnel study is carried out. It is shown that for the given wing design active flow control leads to much higher lift benefits when applied to the trailing edge. Applied to the leading edge active flow control disrupts the vortex lift of the high-lift airfoil, resulting in a deleterious lift effect; however, it shows potential for pitch moment control. This project also underlines the advantages of jet-like active flow control over steady blowing actuation at limited available mass flow rates.
The momentum input coefficient as an important parameter in active flow control is discussed in detail, identifying common misconceptions and difficulties that hinder its proper calculation. An innovative, much simpler approach is introduced. This allows a detailed study of the underlying physics, unveiling unknown limitations of active flow control. The approach is then used as a model to derive the novel concept of thermal active flow control. Experimental studies, including a wind tunnel test campaign, are performed to confirm the viability of the concept for practical applications.
The new calculation method of the input momentum coefficient emphasizes its weakness as a similarity parameter in active flow control studies. The extended mass flow coefficient is introduced as a new parameter. It is shown that it can overcome the deficiencies of the input momentum coefficient without suffering other disadvantages. Its further investigation leads to a deeper understanding of active flow control, which is supported by PIV experiments. The main findings of this investigation divide active flow control into three different “states”: boundary layer thickening, separation control, and supercirculation.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Gharib, Morteza}, } @phdthesis{10.7907/Z97P8WFW, author = {Roh, Chris}, title = {Hydrodynamics of Insects. Part 1. Jetting of the Dragonfly Larvae. Part 2. Honeybee at the Air-water Interface: Surfing with the Capillary Wave}, school = {California Institute of Technology}, year = {2017}, doi = {10.7907/Z97P8WFW}, url = {https://resolver.caltech.edu/CaltechTHESIS:06082017-183218154}, abstract = {This thesis presents the study on the hydrodynamics of two insects commonly known for their aerial adaptation: the dragonfly and the honeybee.
Part 1: Anisopteran dragonflies live underwater in their larval stages. The key factor for their aquatic adaptation is the modified hindgut chamber that is used as a pump. The two main functions of this biological pump are jet propulsion and respiration. Both functions involve jetting and refilling of the chamber through an orifice guard by a tri-leaflet anal valve. Despite it being a unique machinery among insects, associated hydrodynamic studies are limited thus far. In the first part of this thesis, various aspects of the hydrodynamics of the dragonfly larvae’s ventilatory flow are studied. The flow visualization showed that the respiratory flow is laminar but the propulsion flow is turbulent. The hydrodynamic force analysis showed that jetting and refilling phase forces are dominated by quasi-steady momentum flux and unsteady acceleration, respectively. Finally, simultaneous measurement of the anal valve kinematics and jet flow showed that the larvae could influence the direction and magnitude of the jet by controlling the anal valve leaflets.
Part 2: Water-collecting honeybees often fall onto water surfaces. However, bees trapped by the “stickiness” of the water can propel by vibrating their wings, often making it to shore. In the second part of this thesis, the honeybee’s propulsion mechanisms at the air–water interface is studied. The result shows that the bees can achieve three body-lengths per second propulsion speed. High-speed video of their wing motion shows that honeybee’s propulsion involves pulling blobs of water with the underside of the wing, while pushing on a surface wave with its trailing edge. This propulsion mechanism resembles surfing on a self-generated capillary wave. Moreover, their wing vibration generates complicated surface waves and flows, below which the deeper water flow shows a single jet stream. From the wave and flow field measurements, the average force imparted to the surrounding fluid is estimated and compared to the average force calculated from the bee’s body motion. The resulting average forces are of the same order of magnitude, which means that generating wave and flow are both important for the bee’s propulsion.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Gharib, Morteza}, } @phdthesis{10.7907/BJGT-TB74, author = {Lyon, Bradley Joseph}, title = {A Multi-Scale Approach to Shaping Carbon Nanotube Structures for Hollow Microneedles}, school = {California Institute of Technology}, year = {2014}, doi = {10.7907/BJGT-TB74}, url = {https://resolver.caltech.edu/CaltechTHESIS:05302014-012121120}, abstract = {The concept of a carbon nanotube microneedle array is explored in this thesis from multiple perspectives including microneedle fabrication, physical aspects of transdermal delivery, and in vivo transdermal drug delivery experiments. Starting with standard techniques in carbon nanotube (CNT) fabrication, including catalyst patterning and chemical vapor deposition, vertically-aligned carbon nanotubes are utilized as a scaffold to define the shape of the hollow microneedle. Passive, scalable techniques based on capillary action and unique photolithographic methods are utilized to produce a CNT-polymer composite microneedle. Specific examples of CNT-polyimide and CNT-epoxy microneedles are investigated. Further analysis of the transport properties of polymer resins reveals general requirements for applying arbitrary polymers to the fabrication process.
The bottom-up fabrication approach embodied by vertically-aligned carbon nanotubes allows for more direct construction of complex high-aspect ratio features than standard top-down fabrication approaches, making microneedles an ideal application for CNTs. However, current vertically-aligned CNT fabrication techniques only allow for the production of extruded geometries with a constant cross-sectional area, such as cylinders. To rectify this limitation, isotropic oxygen etching is introduced as a novel fabrication technique to create true 3D CNT geometry. Oxygen etching is utilized to create a conical geometry from a cylindrical CNT structure as well as create complex shape transformations in other CNT geometries.
CNT-polymer composite microneedles are anchored onto a common polymer base less than 50 µm thick, which allows for the microneedles to be incorporated into multiple drug delivery platforms, including modified hypodermic syringes and silicone skin patches. Cylindrical microneedles are fabricated with 100 µm outer diameter and height of 200-250 µm with a central cavity, or lumen, diameter of 30 µm to facilitate liquid drug flow. In vitro delivery experiments in swine skin demonstrate the ability of the microneedles to successfully penetrate the skin and deliver aqueous solutions.
An in vivo study was performed to assess the ability of the CNT-polymer microneedles to deliver drugs transdermally. CNT-polymer microneedles are attached to a hand actuated silicone skin patch that holds a liquid reservoir of drugs. Fentanyl, a potent analgesic, was administered to New Zealand White Rabbits through 3 routes of delivery: topical patch, CNT-polymer microneedles, and subcutaneous hypodermic injection. Results demonstrate that the CNT-polymer microneedles have a similar onset of action as the topical patch. CNT-polymer microneedles were also vetted as a painless delivery approach compared to hypodermic injection. Comparative analysis with contemporary microneedle designs demonstrates that the delivery achieved through CNT-polymer microneedles is akin to current hollow microneedle architectures. The inherent advantage of applying a bottom-up fabrication approach alongside similar delivery performance to contemporary microneedle designs demonstrates that the CNT-polymer composite microneedle is a viable architecture in the emerging field of painless transdermal delivery.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Gharib, Morteza}, } @phdthesis{10.7907/X7S3-CS74, author = {Cossé, Julia Theresa}, title = {On the Behavior of Pliable Plate Dynamics in Wind: Application to Vertical Axis Wind Turbines}, school = {California Institute of Technology}, year = {2014}, doi = {10.7907/X7S3-CS74}, url = {https://resolver.caltech.edu/CaltechTHESIS:05272014-160129404}, abstract = {
Numerous studies have shown that flexible materials improve resilience and durability of a structure. Several studies have investigated the behavior of elastic plates under the influence of a free stream, such as studies of the fluttering flag and others of shape reconfiguration, due to a free stream.
The principle engineering contribution of this thesis is the design and development of a vertical axis wind turbine that features pliable blades which undergo various modes of behavior, ultimately leading to rotational propulsion of the turbine. The wind turbine design was tested in a wind tunnel and at the Caltech Laboratory for Optimized Wind Energy. Ultimately, the flexible blade vertical axis wind turbine proved to be an effective way of harnessing the power of the wind.
In addition, this body of work builds on the current knowledge of elastic cantilever plates in a free stream flow by investigating the inverted flag. While previous studies have focused on the fluid structure interaction of a free stream on elastic cantilever plates, none had studied the plate configuration where the trailing edge was clamped, leaving the leading edge free to move. Furthermore, the studies presented in this thesis establish the geometric boundaries of where the large-amplitude flapping occurs.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Gharib, Morteza}, } @phdthesis{10.7907/Z9DR2SFM, author = {Pahlevan, Niema Mohammed}, title = {A Systems Approach to Cardiovascular Health and Disease with a Focus on Aortic Wave Dynamics}, school = {California Institute of Technology}, year = {2013}, doi = {10.7907/Z9DR2SFM}, url = {https://resolver.caltech.edu/CaltechTHESIS:05082013-152249157}, abstract = {Cardiovascular diseases (CVDs) have reached an epidemic proportion in the US and worldwide with serious consequences in terms of human suffering and economic impact. More than one third of American adults are suffering from CVDs. The total direct and indirect costs of CVDs are more than $500 billion per year. Therefore, there is an urgent need to develop noninvasive diagnostics methods, to design minimally invasive assist devices, and to develop economical and easy-to-use monitoring systems for cardiovascular diseases. In order to achieve these goals, it is necessary to gain a better understanding of the subsystems that constitute the cardiovascular system. The aorta is one of these subsystems whose role in cardiovascular functioning has been underestimated. Traditionally, the aorta and its branches have been viewed as resistive conduits connected to an active pump (left ventricle of the heart). However, this perception fails to explain many observed physiological results. My goal in this thesis is to demonstrate the subtle but important role of the aorta as a system, with focus on the wave dynamics in the aorta.
The operation of a healthy heart is based on an optimized balance between its pumping characteristics and the hemodynamics of the aorta and vascular branches. The delicate balance between the aorta and heart can be impaired due to aging, smoking, or disease. The heart generates pulsatile flow that produces pressure and flow waves as it enters into the compliant aorta. These aortic waves propagate and reflect from reflection sites (bifurcations and tapering). They can act constructively and assist the blood circulation. However, they may act destructively, promoting diseases or initiating sudden cardiac death. These waves also carry information about the diseases of the heart, vascular disease, and coupling of heart and aorta. In order to elucidate the role of the aorta as a dynamic system, the interplay between the dominant wave dynamic parameters is investigated in this study. These parameters are heart rate, aortic compliance (wave speed), and locations of reflection sites. Both computational and experimental approaches have been used in this research. In some cases, the results are further explained using theoretical models.
The main findings of this study are as follows: (i) developing a physiologically realistic outflow boundary condition for blood flow modeling in a compliant vasculature; (ii) demonstrating that pulse pressure as a single index cannot predict the true level of pulsatile workload on the left ventricle; (iii) proving that there is an optimum heart rate in which the pulsatile workload of the heart is minimized and that the optimum heart rate shifts to a higher value as aortic rigidity increases; (iv) introducing a simple bio-inspired device for correction and optimization of aortic wave reflection that reduces the workload on the heart; (v) deriving a non-dimensional number that can predict the optimum wave dynamic state in a mammalian cardiovascular system; (vi) demonstrating that waves can create a pumping effect in the aorta; (vii) introducing a system parameter and a new medical index, Intrinsic Frequency, that can be used for noninvasive diagnosis of heart and vascular diseases; and (viii) proposing a new medical hypothesis for sudden cardiac death in young athletes.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Gharib, Morteza}, } @phdthesis{10.7907/B6MF-FX89, author = {Aria, Adrianus Indrat}, title = {Control of Wettability of Carbon Nanotube Array by Reversible Dry Oxidation for Superhydrophobic Coating and Supercapacitor Applications}, school = {California Institute of Technology}, year = {2013}, doi = {10.7907/B6MF-FX89}, url = {https://resolver.caltech.edu/CaltechTHESIS:06012013-192515668}, abstract = {
In this thesis, dry chemical modification methods involving UV/ozone, oxygen plasma, and vacuum annealing treatments are explored to precisely control the wettability of CNT arrays. By varying the exposure time of these treatments the surface concentration of oxygenated groups adsorbed on the CNT arrays can be controlled. CNT arrays with very low amount of oxygenated groups exhibit a superhydrophobic behavior. In addition to their extremely high static contact angle, they cannot be dispersed in DI water and their impedance in aqueous electrolytes is extremely high. These arrays have an extreme water repellency capability such that a water droplet will bounce off of their surface upon impact and a thin film of air is formed on their surface as they are immersed in a deep pool of water. In contrast, CNT arrays with very high surface concentration of oxygenated functional groups exhibit an extreme hydrophilic behavior. In addition to their extremely low static contact angle, they can be dispersed easily in DI water and their impedance in aqueous electrolytes is tremendously low. Since the bulk structure of the CNT arrays are preserved during the UV/ozone, oxygen plasma, and vacuum annealing treatments, all CNT arrays can be repeatedly switched between superhydrophilic and superhydrophobic, as long as their O/C ratio is kept below 18%.
The effect of oxidation using UV/ozone and oxygen plasma treatments is highly reversible as long as the O/C ratio of the CNT arrays is kept below 18%. At O/C ratios higher than 18%, the effect of oxidation is no longer reversible. This irreversible oxidation is caused by irreversible changes to the CNT atomic structure during the oxidation process. During the oxidation process, CNT arrays undergo three different processes. For CNT arrays with O/C ratios lower than 40%, the oxidation process results in the functionalization of CNT outer walls by oxygenated groups. Although this functionalization process introduces defects, vacancies and micropores opening, the graphitic structure of the CNT is still largely intact. For CNT arrays with O/C ratios between 40% and 45%, the oxidation process results in the etching of CNT outer walls. This etching process introduces large scale defects and holes that can be obviously seen under TEM at high magnification. Most of these holes are found to be several layers deep and, in some cases, a large portion of the CNT side walls are cut open. For CNT arrays with O/C ratios higher than 45%, the oxidation process results in the exfoliation of the CNT walls and amorphization of the remaining CNT structure. This amorphization process can be implied from the disappearance of C-C sp2 peak in the XPS spectra associated with the pi-bond network.
The impact behavior of water droplet impinging on superhydrophobic CNT arrays in a low viscosity regime is investigated for the first time. Here, the experimental data are presented in the form of several important impact behavior characteristics including critical Weber number, volume ratio, restitution coefficient, and maximum spreading diameter. As observed experimentally, three different impact regimes are identified while another impact regime is proposed. These regimes are partitioned by three critical Weber numbers, two of which are experimentally observed. The volume ratio between the primary and the secondary droplets is found to decrease with the increase of Weber number in all impact regimes other than the first one. In the first impact regime, this is found to be independent of Weber number since the droplet remains intact during and subsequent to the impingement. Experimental data show that the coefficient of restitution decreases with the increase of Weber number in all impact regimes. The rate of decrease of the coefficient of restitution in the high Weber number regime is found to be higher than that in the low and moderate Weber number. Experimental data also show that the maximum spreading factor increases with the increase of Weber number in all impact regimes. The rate of increase of the maximum spreading factor in the high Weber number regime is found to be higher than that in the low and moderate Weber number. Phenomenological approximations and interpretations of the experimental data, as well as brief comparisons to the previously proposed scaling laws, are shown here.
Dry oxidation methods are used for the first time to characterize the influence of oxidation on the capacitive behavior of CNT array EDLCs. The capacitive behavior of CNT array EDLCs can be tailored by varying their oxygen content, represented by their O/C ratio. The specific capacitance of these CNT arrays increases with the increase of their oxygen content in both KOH and Et4NBF4/PC electrolytes. As a result, their gravimetric energy density increases with the increase of their oxygen content. However, their gravimetric power density decreases with the increase of their oxygen content. The optimally oxidized CNT arrays are able to withstand more than 35,000 charge/discharge cycles in Et4NBF4/PC at a current density of 5 A/g while only losing 10% of their original capacitance.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Gharib, Morteza}, } @phdthesis{10.7907/2C8T-TB84, author = {Azizgolshani, Hesham}, title = {Tissue Engineering Active Biological Machines: Bio-Inspired Design, Directed Self-Assembly, and Characterization of Muscular Pumps Simulating the Embryonic Heart}, school = {California Institute of Technology}, year = {2013}, doi = {10.7907/2C8T-TB84}, url = {https://resolver.caltech.edu/CaltechTHESIS:05232013-150116734}, abstract = {Biological machines are active devices that are comprised of cells and other biological components. These functional devices are best suited for physiological environments that support cellular function and survival. Biological machines have the potential to revolutionize the engineering of biomedical devices intended for implantation, where the human body can provide the required physiological environment. For engineering such cell-based machines, bio-inspired design can serve as a guiding platform as it provides functionally proven designs that are attainable by living cells. In the present work, a systematic approach was used to tissue engineer one such machine by exclusively using biological building blocks and by employing a bio-inspired design. Valveless impedance pumps were constructed based on the working principles of the embryonic vertebrate heart and by using cells and tissue derived from rats. The function of these tissue-engineered muscular pumps was characterized by exploring their spatiotemporal and flow behavior in order to better understand the capabilities and limitations of cells when used as the engines of biological machines.}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Gharib, Morteza}, } @phdthesis{10.7907/DBKG-EJ21, author = {Meier, John Allen}, title = {A Novel Experimental Study of a Valveless Impedance Pump for Applications at Lab-On-Chip, Microfluidic, and Biomedical Device Size Scales}, school = {California Institute of Technology}, year = {2011}, doi = {10.7907/DBKG-EJ21}, url = {https://resolver.caltech.edu/CaltechTHESIS:05262011-111659863}, abstract = {In 1954, Gerhart Liebau demonstrated a simple valveless pumping phenomenon utilizing the periodic compression of a compliant tube and some systematic asymmetry to pump water out of a bucket. Liebau’s goal was to explain peculiarities seen in the human circulatory system. In the years that have followed, the Liebau phenomenon has been studied in a variety of open and closed loop configurations, through experimental, computational, and analytical studies.
Recent advances in microfluidic and microelectromechanical systems (MEMS) technology have enabled a wide range of small scale engineering systems. The further development of many important systems is limited by the absence of an appropriate means of fluid transport. Valveless pumps based on the Liebau phenomenon show great promise, particularly in lab-on-chip (LOC), biological, and medical applications in which biocompatibility and the ability to move sensitive molecules without damage are key design requirements.
The purpose of this thesis is to synthesize previous studies of the Liebau phenomenon and produce the first extensive experimental study of a novel valveless pump at size scales and geometries that are relevant to lab-on-chip, microfluidic, and biomedical device applications. For the first time, detailed, dynamic pressure and flow data have been recorded during the operation of these valveless pumps for a large range of operating parameters. This dynamic data allowed us to identify new flow regimes and observe previously undocumented pump behaviors and performance. Parameters investigated include pump material properties and geometry, working fluid density and viscosity, pump excitation properties (amplitude, offset, location, and frequency), and flow loop/system properties. A critical relationship between the relative volumetric compliance of the valveless pump to the system it acts upon is identified, and the implications for practical implementation of valveless pumps at small size scales are discussed.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Gharib, Morteza}, } @phdthesis{10.7907/8VZJ-7Z78, author = {Kim, Daegyoum}, title = {Characteristics of Three-dimensional Vortex Formation and Propulsive Performance in Flapping Locomotion}, school = {California Institute of Technology}, year = {2010}, doi = {10.7907/8VZJ-7Z78}, url = {https://resolver.caltech.edu/CaltechTHESIS:06072010-114858790}, abstract = {Three-dimensional vortex formation and propulsive performance were studied experimentally to identify some of the main characteristic mechanisms of flapping locomotion. Mechanical models with thin plates were used to simulate flapping and translating motions of animal propulsors. Three-dimensional flow fields were mapped quantitatively using defocusing digital particle image velocimetry.
First, vortex structures made by impulsively translating low aspect-ratio plates were studied. The investigation of translating plates with a 90 degree angle of attack is important since it is a fundamental model for a better understanding of drag-based propulsion systems. Rectangular flat-rigid, flexible, and curved-rigid thin plastic plates with the same aspect ratio were used to compare their vortex structures and hydrodynamic forces. The interaction of the tip flow and the nearby vortex is a critical flow phenomenon to distinguish vortex patterns among these three cases. In the flexible plate case, slow development of the vortex structure causes a small initial peak in hydrodynamic force during the acceleration phase. However, after the initial peak, the flexible plate generates large force magnitude comparable to that of the flat-rigid plate case.
Drag-based paddling propulsion was also studied to explain some of the fundamental differences in vortex formation of lift-based and drag-based propulsions. While the temporal change of the inner area enclosed by the vortex loop is an important factor in thrust generation of lift-based propulsion, the temporal change of the vortex strength becomes more important in drag-based propulsion. Spanwise flow behind the paddling plate plays an important role in tip vortex motion and thrust generation. The distribution of spanwise flow depends on the propulsor shape and the Reynolds number. A delta-shaped propulsor generates strong spanwise flow compared to a rectangular propulsor. For the low Reynolds number case, the spanwise flow is not as strong as that of the high Reynolds number case. The flexible propulsor can smooth out force peaks during impulsive motions without sacrificing total impulse, which is advantageous in avoiding structural failures and stabilizing body motion. The role of the stopping vortex was addressed in optimizing a stroke angle of paddling animals.
In addition, vortex formation of clapping propulsion was investigated by varying aspect ratio and stroke angle. A low aspect-ratio propulsor produces larger total impulse than a high aspect-ratio propulsor. As the aspect ratio increases, circulation of the vortex is strengthened, and the inner area enclosed by the vortex structure tends to enlarge. Moreover, in terms of thrust, the advantage of a single plate over double clapping plates is larger for the lower aspect-ratio case. These results offer information to better understand the benefit of low aspect-ratio wings in force generation under specific locomotion modes. When a pair of plates claps, a vortex loop forms from two counter-rotating tip vortices by a reconnection process. The dynamics of wake structures are dependent on the aspect ratio and the stroke angle.
Vortex formation and vorticity transport processes of translating and rotating plates with a 45 degree angle of attack were investigated as well. In both translating and rotating cases, the spanwise flow over the plate and the vorticity tilting process inside the leading-edge vortex were observed. The distribution of spanwise flow is a prominent distinction between the vortex structures of these two cases. While spanwise flow is confined inside the leading-edge vortex for the translating case, it is widely present over the plate and the wake region of the rotating case. As the Reynolds number decreases, due to the increase in viscosity, leading-edge and tip vortices tend to spread inside the area swept by the rotating plate, which results in lower lift force generation.
Lastly, for translating motion, the dynamics of the vortex in corner regions was compared among three different corner shapes. For a large corner angle, the forward movement of the vortex tends to be uniform along the plate edges. However, for a small corner angle, the vortex close to the corner moves forward following the plate while the vortex away from the corner retards its forward movement.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Gharib, Morteza}, } @phdthesis{10.7907/THMJ-C355, author = {Grosberg, Anna}, title = {A Bioinspired Computational Model of Cardiac Mechanics: Pathology and Development}, school = {California Institute of Technology}, year = {2008}, doi = {10.7907/THMJ-C355}, url = {https://resolver.caltech.edu/CaltechETD:etd-05292008-152117}, abstract = {In this work we study the function and development of the myocardium by creating models that have been stripped down to essentials. The model for the adult myocardium is based on the double helical band formation of the heart muscle fibers, observed in both histological studies and advanced DTMRI images. The muscle fibers in the embryonic myocardium are modeled as a helical band wound around a tubular chamber. We model the myocardium as an elastic body, utilizing the finite element method for the computations. We show that when the spiral band architecture is combined with spatial wave excitations the structure is twisted, thus driving the development of the embryonic heart into an adult heart. The double helical band model of the adult heart allows us to gain insight into the long standing paradox between the modest, by only 15 %, ability of muscle fibers to contract, and the large left ventricular volume ejection fraction of 60 %. We show that the double helical band structure is the essential factor behind such efficiency. Additionally, when the double helical band model is excited following the path of the Purkinje nerve network, physiological twist behavior is reproduced. As an additional validation, we show that when the stripped down double helical band is placed inside a sack of soft collagen-like tissue it is capable of producing physiologically high pressures.
We further develop the model to understand the different factors behind the loss of efficiency in heart with a common pathology such as dilated cardiomyopathy. Using the stripped down model we are able to show that the change to fiber angle is the much more important factor to heart function than the change in gross geometry. This finding has the potential to greatly impact the strategy used in certain surgical procedures.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Gharib, Morteza}, } @phdthesis{10.7907/0K4J-0548, author = {Rinderknecht, Derek Gresham}, title = {Development of a Microimpedance Pump for Pulsatile Flow Transport - Part 1: Flow Characteristics of the Microimpedance Pump. Part 2: A Systematic Study of Steady and Pulsatile Transport in Microscale Cavities}, school = {California Institute of Technology}, year = {2008}, doi = {10.7907/0K4J-0548}, url = {https://resolver.caltech.edu/CaltechETD:etd-05292008-115810}, abstract = {Microfluidics offers an effective means to carry out a wide range of transport processes within a controlled microenvironment by drawing on the benefits imparted by increasing surface area to volume ratio at the microscale. Critical to the impact of microfluidics on integrated devices in the fields of bioengineering and biomedicine is the ability to transport fluids and biomolecules effectively particularly at the size scales involved. In this context a bio-inspired pumping mechanism, the valveless impedance pump, was explored for applications in microfluidics ranging from micro total analysis systems to microchannel cooling. Adhering to the basic principles of the impedance pump mechanism, pumps have been constructed at a variety of size scales from a few centimeters to a few hundred microns. The micro impedance pump is valveless, bidirectional, and can be constructed simply from a wide range of materials. Depending on the size of the pump flow rates range from nL/min to mL/min and pressures can be generated that exceed 20 kPa. Another benefit of the impedance pump is the pulsatile flow output which can be used in the context of microfluidic applications to enhance transport at low Reynolds numbers as well as metering in drug delivery.
Pulsatile flow was therefore investigated as a method of augmenting transport in microfluidic systems. Micro PIV was used to study the affect of both steady and pulsatile flows on transport at low Reynolds number was examined in microscale rectangular cavities. Ventilation of the cavity contents was examined in terms of the residence time or average time a particle remains in the cavity region. Lagrangian coherent structures (LCS) were applied to empirical velocity fields to determine the impact of unsteadiness on time dependent boundaries to fluid transport present in the flow. Experimental results show that there are both frequencies which are beneficial and detrimental to cavity ventilation as well as certain frequencies which more evenly distribute particles originating in the cavity throughout the freestream.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Gharib, Morteza}, } @phdthesis{10.7907/4JFZ-AG10, author = {Lu, Jian}, title = {Quantitative Three-dimensional Imaging of Droplet Convection and Cardiac Cell Motions Based on Micro DDPIV}, school = {California Institute of Technology}, year = {2008}, doi = {10.7907/4JFZ-AG10}, url = {https://resolver.caltech.edu/CaltechETD:etd-02012008-150234}, abstract = {
Biomechanical forces such as blood flow induced shear stress as well as genetic programming are widely acknowledged as critical factors regulating vertebrate heart development. While mechanisms of genetic regulation have been well studied, effects of biomechanics are poorly understood due to the lack of proper imaging tools with sufficient spatial and temporal resolutions for quantitative analysis of the mechanical stimuli in complex three-dimensional (3D) living systems. 3D quantitative flow visualization by tracking microscale particles has become an invaluable tool in microfluid mechanics. Defocusing digital particle image velocimetry (DDPIV) can recover depth coordinates by calculating the separation between defocused images generated by an aperture mask with a plurality of pinholes forming an equilateral triangle. In this thesis, a novel high-speed 3D micro-PTV system was developed based on this technique with laser-induced fluorescence to achieve microscale velocity field measurements. Application of this technique to microscale imaging was validated by calibration of targets spread over the image field. A micro volume of 400x300 µm2 with 100 µm depth has been mapped using an inverted microscope equipped with a 20X objective lens. The proposed technique was successfully applied to 3D tracking of 2-µm fluorescent particles inside an evaporating water droplet, exhibiting convective flow induced by Marangoni effects.
The microscopic imaging system was then utilized to acquire 3D time series data of highly dynamic cell motions in living embryonic zebrafish hearts. 1-µm and 500-nm fluorescent tracer particles were injected into the blood stream of developing zebrafish embryos at 32 hours post fertilization (hpf) to 59 hpf to help describe cardiac cell motions. Microinjection was delicately performed at the fish tail to minimize the influence to normal cardiovascular functions. The measurable depth in an embryonic heart is about 40 µm. 3D velocities of cardiovascular blood flow and trajectories of heart-wall motions were obtained, showing dynamic changes of the flow field and phase differences of wall movements between the atrium and the ventricle during heart beating. Endocardial ventricular strains were calculated based on the reconstructed coordinates of two particles adhered to the endocardium.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Gharib, Morteza}, } @phdthesis{10.7907/S27Z-J533, author = {Klamo, Joseph Thomas}, title = {Effects of Damping and Reynolds Number on Vortex-Induced Vibrations}, school = {California Institute of Technology}, year = {2007}, doi = {10.7907/S27Z-J533}, url = {https://resolver.caltech.edu/CaltechETD:etd-11292006-120631}, abstract = {
Vortex-induced vibrations have been studied experimentally with emphasis on damping and Reynolds number effects. Our system was an elastically-mounted rigid circular cylinder, free to oscillate only transverse to the flow direction, with very low inherent damping. We were able to prescribe the mass, damping, and elasticity of the system over a wide range of values, with the damping controlled by a custom-made variable magnetic eddy-current damping system.
Special emphasis is put on a nontraditional parameter formulation. The advantages of this formulation are explained, and an important new parameter, effective stiffness, is introduced. Using this new formulation, the amplitude and frequency responses are only a function of damping, Reynolds number, and effective stiffness. We show the effects that damping and Reynolds number each have on the amplitude and frequency response profiles and make the interesting observation that changes in damping or Reynolds number have similar effects.
The maximum amplitudes of our systems are studied in detail. We theoretically show that they should be functions of both damping and Reynolds number. This allows us to create constant-Reynolds-number curves of maximum amplitude over a large range of damping values, which we call a “generalized” Griffin plot. We also define maximum amplitudes in the case of zero damping as limiting amplitudes, and show that they are only a function of Reynolds number. We experimentally determine our limiting amplitude dependence on Reynolds number over the range 200 < Reynolds number < 5050.
Discontinuities in the amplitude response profile are also investigated. The discontinuity between the initial branch and the large-amplitude, upper branch is studied in two ways. First, the time-averaged behavior is examined to understand what controls the discontinuity and look for damping and Reynolds number effects. Second, we track the cycle-by-cycle transient response through this discontinuous amplitude change, induced either by changes in the tunnel velocity or system damping. Finally, we also find a new discontinuity hysteresis region between the lower branch and the desynchronized region, which appears to be a low Reynolds number effect and is only seen in systems with Reynolds number < 1000.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Leonard, Anthony}, } @phdthesis{10.7907/TJ7E-PK42, author = {Sansom, Elijah Bodhi}, title = {Experimental Investigation on Patterning of Anchored and Unanchored Aligned Carbon Nanotube Mats by Fluid Immersion and Evaporation}, school = {California Institute of Technology}, year = {2007}, doi = {10.7907/TJ7E-PK42}, url = {https://resolver.caltech.edu/CaltechETD:etd-05072007-114349}, abstract = {Pattern formation by capillary forces in a nanoscale system was studied experimentally. Densely packed, vertically aligned mats of order 100 microns in height comprised of 20 nm diameter multi-walled carbon nanotubes were fabricated and treated with various liquids. The carbon nanotubes deflected and rearranged under the action of surface tension as the liquids evaporated, and remained fixed once dried. The size analysis of the resulting patterns in these experiments and in the literature showed they are distributed within one standard deviation from the mean, and there are, in general, many more small sizes than large ones within a pattern.
Preexisting defects in the mats were found to play a significant role in the pattern formation process, both in this work and in the literature, whereas the properties of the specific liquid used and the height of the mats did not.
A novel method for anchoring the aligned mats within another material using spin-coating was developed. An anchored mat made in this way was successfully held in place even under the application of a 5.5 m/s water jet.
The anchoring method allowed the first known investigation of the role of boundary conditions in this pattern formation process. Under identical experimental conditions to cases where patterns are formed in the unanchored mats, it was found that no pattern formation occurs in the anchored mats.
A population balance model based on conservation of area was applied to the pattern formation process, but sufficient details are lacking to make predictions.
The anchoring method and its prevention of pattern formation is a very important finding, and is relevant to applications of the aligned mats, such as field emission displays, supercapacitors, tissue culture scaffolds, and friction drag reducing surfaces.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Gharib, Morteza}, } @phdthesis{10.7907/RQE4-MA23, author = {Nasiraei Moghaddam, Abbas}, title = {Measurement and Analysis of Structure and Function of Myocardium in Embryonic and Adult Heart}, school = {California Institute of Technology}, year = {2007}, doi = {10.7907/RQE4-MA23}, url = {https://resolver.caltech.edu/CaltechETD:etd-06012007-143736}, abstract = {Congestive heart failure is the most common and costly medical problem in the modern world. Current disease management procedures are mostly limited to treating the symptoms of this disease. The effective treatment, however, needs a deep understanding of the normal structure-function relationships of the myocardium.
The research of this study is concerned with the relationship between the structure and function of the myocardium in both embryonic and adult hearts. This relationship was investigated through an in-depth analysis of the spatial distribution of the local contractile function in the myocardium. The analysis is based on the heart kinematics captured through the tissue tracking of the myocardium.
Advanced imaging techniques, such as DENSE MRI and confocal microscopy, were used for tissue tracking in adult and embryonic myocardium, respectively. The acquired data, together with continuum mechanics concepts and computational methods, were exploited in a Lagrangian framework to measure appropriate characteristic parameters that describe local contribution of the myocardium in its global functionality.
This method resulted in novel understandings of the local and global functions in each of these hearts. In particular, it was observed in the adult heart that the left ventricle functionality is not uniformly distributed. Instead, the regions with higher effect on the pumping process form a helical band which wraps around the heart. This is the first time that such a myocardium macro-structure, which is supported by the established histological evidence, is revealed from its function in a beating heart. It can be considered as a landmark in connecting the structure and function of the heart through imaging. Furthermore, the compatibility of this model with microscopic observations about the fiber direction is investigated.
A similar approach was applied to embryonic zebrafish heart with GFP labeled myocytes. It identified distribution of regions that play an active role in functionality of the heart tube. This new understanding has provided better insights into the pumping mechanism of the embryonic heart.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Gharib, Morteza}, } @phdthesis{10.7907/MT2J-AR81, author = {Loumes, Laurence}, title = {Multilayer Impedance Pump:A Bio-Inspired Valveless Pump with Medical Applications}, school = {California Institute of Technology}, year = {2007}, doi = {10.7907/MT2J-AR81}, url = {https://resolver.caltech.edu/CaltechETD:etd-01082007-103832}, abstract = {This thesis introduces the concept of multilayer impedance pump, a novel pumping mechanism inspired from the embryonic heart structure.
The multilayer impedance pump is a composite two-layer fluid-filled elastic tube featuring a thick, gelatin-like internal layer similar in nature to the embryonic cardiac jelly, and that is used to amplify longitudinal elastic waves. Pumping is based on the impedance pumping mechanism. Elastic waves are generated upon small external periodic compressions of the elastic tube. They propagate along the tube’s walls, reflect at the tube’s extremities and drive the flow in a preferential direction. This fully coupled fluid-structure interaction problem is solved for the flow and the structure using the finite element method over a relevant range of frequencies of excitation. Results show that the two-layer configuration can be an efficient wave propagation combination, and that it allows the pump to produce significant flow for small excitations. The multilayer impedance pump is a complex system in which flow and structure exhibit a resonant behavior. At resonance, a constructive elastic wave interaction coupled with a most efficient energy transmission between the elastic walls and the fluid is responsible for the maximum exit flow. The pump efficiency reaches its highest at resonance, highlighting furthermore the concept of resonance pumping.
Using the proposed multilayer impedance pump model, we are able to bring an additional proof on the impedance nature of the embryonic heart by comparing a peristaltic and an impedance multilayer pump both excited in similar fashion to the one observed in the embryonic heart.
The gelatin layer that models the embryonic cardiac jelly occupies most of the tube walls and is essential to the propagation of elastic waves. A comparison between the exact same impedance pump with and without the additional gelatin layer sheds light on the dynamic role of the cardiac jelly in the embryonic heart and on nature’s optimized design.
Finally, several biomedical applications of multilayer impedance pumping are presented. A physiologically correct model of aorta is proposed to test the pump as an implantable cardiovascular assist device.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Gharib, Morteza}, } @phdthesis{10.7907/Z9HM56F8, author = {Graff, Emilio Castaño}, title = {On the Development of Defocusing Digital Particle Image Velocimetry with Full Characterization}, school = {California Institute of Technology}, year = {2007}, doi = {10.7907/Z9HM56F8}, url = {https://resolver.caltech.edu/CaltechETD:etd-05252007-140239}, abstract = {Defocusing Digital Particle Image Velocimetry is the first volumetric, three-dimensional PIV method ever put into practice. This manuscript contains the details of its development, a detailed analysis of its performance (both through simulation and real measurements), and a series of experimental demonstrations of the capability of the technique. The system is capable of resolving upwards of 7,000 vectors per pair with an absolute error on the order of 0.03% of the volume size.}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Gharib, Morteza}, } @phdthesis{10.7907/M1QB-4Y59, author = {Kheradvar, Arash}, title = {The Role of Vortex Ring Formation and Pressure Drop on Dynamics of the Left Ventricle during Diastole}, school = {California Institute of Technology}, year = {2007}, doi = {10.7907/M1QB-4Y59}, url = {https://resolver.caltech.edu/CaltechETD:etd-12132006-103720}, abstract = {
In the field of cardiology, the current ability to accurately detect diastolic dysfunction is unsatisfactory due to the lack of an effective diagnostic index. Currently, assessments of diastolic dysfunction are based on echocardiographic measurements that are assumed to be correlated with progression from mild dysfunction to more severe disease. However, relying on existing ultrasonic indices for diagnosis of diastolic failure leads us to underestimate the progress of dysfunction. The presence of vortical flow that develops along with a strong propulsive trans-mitral jet during diastole in a normal left ventricle has been demonstrated by different imaging modalities. Thus, physical characteristics of these vortical structures may provide more effective indices of diastolic function than existing ones. In the first few chapters of this thesis, I fully describe the relationship between physical characteristics of these vortices and the dynamics of mitral valve during diastole. We found that regardless of the valve size and the pressure drop time-constant, the mitral annuls recoil computed would be maximized when the trans-mitral vortex ring pinches off in a range of formation time between three and five.
In chapter five, I introduce a novel technique that can estimate the viscoelastic properties of the left ventricle based on harmonic behavior of the ventricular chamber. Elastic deformations resulting from the changes in the ventricular mechanical properties of myocardium are represented as a time-varying spring, while the viscous components of the model include a time-varying viscous damper, representing relaxation and the frictional energy loss.
In the final chapter, I discussed about effect of isovolumic relaxation phase on diastolic rapid filling in the process of post-infarction cardiac remodeling in sheep. The results of this study confirmed that the post-infarction changes in isovolumic relaxation phase have direct influence on diastolic rapid filling phase, which leads to complex variations in end-diastolic lengthening and end-systolic shortening of the LV contractile elements.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Gharib, Morteza}, } @phdthesis{10.7907/Z0X2-HG40, author = {Forouhar, Arian Soroush}, title = {Dynamic Views of Structure and Function during Heart Morphogenesis}, school = {California Institute of Technology}, year = {2006}, doi = {10.7907/Z0X2-HG40}, url = {https://resolver.caltech.edu/CaltechETD:etd-06282006-130509}, abstract = {Congenital heart defects remain the most common birth defect in humans, occurring in over 1% of live births. The high prevalence of cardiac malformations can be partially attributed to limited knowledge regarding the embryonic roots of the disease. A variety of congenital heart defects are thought to arise from combinations of genetic and epigenetic factors. In an effort to better understand this dynamic relationship, our study explores the structure and function of the developing heart and valves and examines hemodynamic factors influencing valvulogenesis. In order to study cardiac mechanics, we employed novel high-speed confocal microscopy and four-dimensional visualization techniques. A dynamic four-dimensional dataset describing heart and valve development along with blood flow patterns throughout cardiac morphogenesis is presented. Utilizing newly developed tools, we propose a novel pumping mechanism in the valveless embryonic heart tube via elastic wave propagation and reflection. We show that this form of pumping leads to oscillatory shear stresses in the developing atrio-ventricular canal, a phenomenon that had not previously been documented. An in vivo method to modulate trans-valvular oscillatory flows is described and used to test our hypothesis that oscillatory shear stress across the primitive valve cushions stimulates heart valve leaflet formation. Our results suggest hemodynamic forces contribute to valvulogenesis and enhance our understanding of normal and abnormal heart valve development.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Gharib, Morteza}, } @phdthesis{10.7907/SQQK-FD11, author = {Hickerson, Anna Iwaniec}, title = {An Experimental Analysis of the Characteristic Behaviors of an Impedance Pump}, school = {California Institute of Technology}, year = {2005}, doi = {10.7907/SQQK-FD11}, url = {https://resolver.caltech.edu/CaltechETD:etd-05232005-141405}, abstract = {When a fluid-filled pliant tube is connected to tubing of a different impedance, a net flow in either direction can be induced by periodically compressing the pliant section asymmetrically from the ends. An experimental analysis of the characteristic behaviors of such a pump has been done demonstrating interesting results not predicted by prior analytical and computational results. Measurements show a complex non-linear behavior in response to the compression frequency, including distinct resonance peaks and reversals in flow direction. Ultrasound imaging provided a unique view of the tube wall and flow within, allowing us to visualize the wave propagation and reflection. Measurements include transient responses, resonant responses, and bulk flow behaviors for a variety of configurations. Net flow rates can exceed the volumetric displacement done by active compression demonstrating that, as a first approximation, this pump can have a higher efficiency than peristaltic pumping. Elasticity has been shown not to be a necessary factor in stimulating net forward flow.
Results from this study have helped show that a zebrafish (a model for human cardiac development) may utilize impedance pumping to drive circulation in early embryonic stages prior to valve formation as opposed to peristaltic pumping as was once thought. Additional research is being conducted to develop a micro-scaled version with applications in medicine, heat transfer, lab-on-chip technology, and micro-mixing.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Gharib, Morteza}, } @phdthesis{10.7907/Q9A2-1M37, author = {Acevedo Bolton, Gabriel Alejandro}, title = {Blood Flow Effects on Heart Development and a Minimally Invasive Technique for in vivo Flow Alterations}, school = {California Institute of Technology}, year = {2005}, doi = {10.7907/Q9A2-1M37}, url = {https://resolver.caltech.edu/CaltechETD:etd-05242005-120722}, abstract = {A series of experiments were conducted on zebrafish (Danio rerio) in order to gain a better understanding of how blood flow and blood flow related forces, such as shear stress, affect vertebrate heart development. Zebrafish were used as a model due to their external fertilization and optical accessibility to the heart and vasculature. The flow field inside the 4.5 day post fertilization (dpf) embryo was analyzed using a combination of manual particle tracking and digital particle image velocimetry (DPIV) software. Our results present the first case of intracardiac microscale DPIV. Additionally, a minimally invasive and reversible technique of delivering and localizing magnetic microspheres inside the vasculature of the embryo was developed. The results of blocked flow induced with this method were compared with previous experiments and controls.
The results of the flow field analysis showed the existence of an extremely dynamic flow environment containing jets with a velocity of 5 mm/s and regions of vorticity in a low Reynolds number environment. Calculations of the flow at the 4.5 dpf A-V resulted in wall shear stress levels of 70 dynes/cm2, levels much higher than needed for endothelial cell response.
We also showed that injected magnetic microspheres can be delivered and localized within the embryonic vasculature to reversibly block blood flow in the dorsal artery and at the inflow to the heart. Blocked blood flow of 12 hours and longer resulted in lower blood velocity and a less developed heart, exhibiting edema, regurgitance, decreased contractile function, and delayed development. These findings are consistent with previous studies showing that blood flow is a necessary factor for heart development. Furthermore an unexpected result was observed. Exposure to a localized magnetic field eventually caused the absorption of magnetic microspheres into the surrounding tissue. It is theorized that this could be utilized in future studies modeling the effects of reduced cardiac contractility.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Gharib, Morteza}, } @phdthesis{10.7907/XMW0-DJ25, author = {Zhou, Jijie}, title = {Nanowicking: Multi-scale Flow Interaction with Nanofabric Structures}, school = {California Institute of Technology}, year = {2005}, doi = {10.7907/XMW0-DJ25}, url = {https://resolver.caltech.edu/CaltechETD:etd-04202005-172426}, abstract = {Dense arrays of aligned carbon nanotubes are designed into strips — nanowicks — as a miniature wicking element for liquid delivery and potential microfluidic chemical analysis devices. The delivery function of nanowicks enables novel fluid transport devices to run without any power input, moving parts or external pump. The intrinsically nanofibrous structure of nanowicks provides a sieving matrix for molecular separations, and a high surface-to-volume ratio porous bed to carry catalysts or reactive agents.
This work also experimentally studies the spontaneous fluid transport along nanowicks. Liquid is conveyed through corner flow, surface flow, and interstitial flow through capillary force and the Marangoni effect. The main course for corner flow and surface flow follows Washburn behavior, and can deliver liquid centimeters away from the input blob with a speed on the order of millimeters per second depending on the nanowick configuration and the amount of input liquid. Corner flow can be minimized and even eliminated through proper nanowick and input design. Otherwise, corner flow interacts with surface flow in the first 2mm of the pathway closest to the input point. Interstitial flow dominates the late stage. It is driven by both capillary force and concentration-gradient-induced Marangoni force. The concentration gradient is determined by two competing rates: surfactant diffusion in solution and adsorption onto nanotube surfaces. The flow inside nanowicks may wick hundreds of microns in seconds or tens of seconds. A non-conventional advancing front may develop in the flow around nanowicks. They are seen as (i) Rayleigh instability-induced fingering in surface flow on millimeter-wide nanowicks, (ii) viscous instability-induced branching near almost-stagnant surface film at low surfactant concentration, and (iii) disjointed wetting domains at very low concentration.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Gharib, Morteza}, } @phdthesis{10.7907/EH41-N436, author = {Dooley, Bradley Scott}, title = {Stereo Digital Particle Image Velocimetry Investigation of a Free Surface Mixing Layer}, school = {California Institute of Technology}, year = {2005}, doi = {10.7907/EH41-N436}, url = {https://resolver.caltech.edu/CaltechETD:etd-06022005-180557}, abstract = {Shear flows in the vicinity of a free surface are a problem with numerous applications, perhaps the most obvious being the wakes of seagoing surface vessels. The flow behind a full-scale ship is extremely complex – so much so that it is frequently more instructive to consider simpler cases highlighting particular elements of the larger problem. To that end, an experimental investigation has been conducted to study the behavior of a turbulent plane mixing layer intersecting a free surface at low Froude number. The local Reynolds number, based on the velocity differential across the layer and the momentum thickness, was approximately 10,000.
The technique of Stereoscopic Digital Particle Image Velocimetry (SDPIV) was implemented to obtain instantaneous three-component velocity measurements within planar slices of the steady-state, spatially developing mixing layer flow. Guided by previous studies of the same flow conditions, specific depths were chosen at a single downstream station for investigation – specifically those in and around counter-rotating streamwise vortices known to exist in the mean flow very near the free surface. 3,000 consecutive SDPIV image pairs were recorded at a rate of 15 per second at each location, giving ample data for Reynolds decomposition and spectral analysis of the velocity fields.
The present study has found that the anisotropy known to exist in some other free surface flows, such as surface-parallel submerged jets, is also present in the case of the mixing layer. Power spectra of all three velocity components are shown to capture part of the inertial subrange; the isotropic energy cascade seen to be present away from the free surface is also seen to disappear near the surface, as surface-normal velocity fluctuations are severely attenuated.
Additionally, a low-frequency spanwise oscillation is deduced from the velocity power spectra and cospectra in the immediate vicinity of the mean streamwise vortices. Not present at all at significant depth, the motions at this frequency are also observed to markedly decrease – in all components – at locations closer to the surface. These observations appear to have both parallels and key differences compared to previously observed meandering of model boat wakes, and the possibility that the oscillation stems from the vortex-pair instability is discussed.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Gharib, Morteza}, } @phdthesis{10.7907/QV8Y-YZ12, author = {Dabiri, John Oluseun}, title = {Unsteady Fluid Mechanics of Starting-Flow Vortex Generators with Time-Dependent Boundary Conditions}, school = {California Institute of Technology}, year = {2005}, doi = {10.7907/QV8Y-YZ12}, url = {https://resolver.caltech.edu/CaltechETD:etd-04112005-151435}, abstract = {Nature has repeatedly converged on the use of starting flows for mass, momentum, and energy transport. The vortex loops that form during flow initiation have been reproduced in the laboratory and have been shown to make a proportionally larger contribution to fluid transport than an equivalent steady jet. However, physical processes limit growth of the vortex loops, suggesting that these flows may be amenable to optimization. Although it has been speculated that optimal vortex formation might occur naturally in biological systems, previous efforts to quantify the biological mechanisms of vortex formation have been inconclusive. In addition, the unsteady fluid dynamical effects associated with starting flow vortex generators are poorly understood.
This thesis describes a combination of new experimental techniques and in vivo animal measurements that determine the effects of fluid-structure interactions on vortex formation by starting flow propulsors. Results indicate that vortex formation across various biological systems is manipulated by these kinematics in order to maximize thrust and/or propulsive efficiency. An emphasis on observed vortex dynamics and transient boundary conditions facilitates quantitative comparisons across fluid transport schemes, irrespective of their individual biological functions and physical scales.
The primary contributions of this thesis are the achievement of quantitative measures of unsteady vortex dynamics via fluid entrainment and added-mass effects, and the development of a robust framework to facilitate the discovery of general design principles for effective fluid transport in engineering technologies and biological therapies. The utility of this new research paradigm is demonstrated through a variety of examples.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Gharib, Morteza}, } @phdthesis{10.7907/0J1D-1B18, author = {Bobba, Kumar Manoj}, title = {Robust Flow Stability: Theory, Computations and Experiments in Near Wall Turbulence}, school = {California Institute of Technology}, year = {2004}, doi = {10.7907/0J1D-1B18}, url = {https://resolver.caltech.edu/CaltechETD:etd-05282004-143324}, abstract = {Helmholtz established the field of hydrodynamic stability with his pioneering work in 1868. From then on, hydrodynamic stability became an important tool in understanding various fundamental fluid flow phenomena in engineering (mechanical, aeronautics, chemical, materials, civil, etc.) and science (astrophysics, geophysics, biophysics, etc.), and turbulence in particular. However, there are many discrepancies between classical hydrodynamic stability theory and experiments. In this thesis, the limitations of traditional hydrodynamic stability theory are shown and a framework for robust flow stability theory is formulated. A host of new techniques like gramians, singular values, operator norms, etc. are introduced to understand the role of various kinds of uncertainty. An interesting feature of this framework is the close interplay between theory and computations. It is shown that a subset of Navier-Stokes equations are globally, non-nonlinearly stable for all Reynolds number. Yet, invoking this new theory, it is shown that these equations produce structures (vortices and streaks) as seen in the experiments. The experiments are done in zero pressure gradient transiting boundary layer on a flat plate in free surface tunnel. Digital particle image velocimetry, and MEMS based laser Doppler velocimeter and shear stress sensors have been used to make quantitative measurements of the flow. Various theoretical and computational predictions are in excellent agreement with the experimental data. A closely related topic of modeling, simulation and complexity reduction of large mechanics problems with multiple spatial and temporal scales is also studied. A nice method that rigorously quantifies the important scales and automatically gives models of the problem to various levels of accuracy is introduced. Computations done using spectral methods are presented.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Doyle, John Comstock and Gharib, Morteza}, } @phdthesis{10.7907/907K-2F28, author = {Ringuette, Matthew James}, title = {Vortex Formation and Drag on Low Aspect Ratio, Normal Flat Plates}, school = {California Institute of Technology}, year = {2004}, doi = {10.7907/907K-2F28}, url = {https://resolver.caltech.edu/CaltechETD:etd-05292004-183807}, abstract = {Experiments were done to investigate the role of vortex formation in the drag force generation of low aspect ratio, normal flat plates starting from rest. This very simplified case is a first, fundamental step toward understanding the more complicated flow of hovering flight, which relies primarily on drag for propulsion. The relative importance of the plate’s free end, or tip, with varying aspect ratio was also studied.
Identifying the relationship among aspect ratio, vortex formation, and drag force can provide insight into the wing aspect ratios and kinematics found nature, with the eventual goal of designing man-made flapping wing micro air vehicles.
The experiments were carried out using flat plate models in a towing tank at a moderate Reynolds number of 3000. Two aspect ratios, 6 and 2, were considered, the latter in order to have a highly tip-dominated case. A force balance measured the time-varying drag, and multiple, perpendicular sections of the flow velocity were measured quantitatively using digital particle image velocimetry. Vorticity fields were calculated from the velocity data, and features in the drag force for different aspect ratios were related to the vortex dynamics. Finally, since the flow is highly three-dimensional, dye flow visualization was done to characterize its structure and to augment the two-dimensional digital particle image velocimetry data.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Gharib, Morteza}, } @phdthesis{10.7907/C9NQ-N832, author = {Pottebaum, Tait Sherman}, title = {The Relationship Between Near-Wake Structure and Heat Transfer for an Oscillating Circular Cylinder in Cross-Flow}, school = {California Institute of Technology}, year = {2003}, doi = {10.7907/C9NQ-N832}, url = {https://resolver.caltech.edu/CaltechETD:etd-05202003-145011}, abstract = {A series of experiments were carried out in order to understand the relationship between wake structure and heat transfer for a transversely oscillating circular cylinder in cross-flow and to explore the dynamics of the vortex formation process in the wake. The cylinder’s heat transfer coefficient was determined over a range of oscillation amplitudes up to 1.5 cylinder diameters and oscillation frequencies up to 5 times the stationary cylinder natural shedding frequency. The results were compared to established relationships between oscillation conditions and wake structure. Digital particle image thermometry/velocimetry (DPIT/V) was used to measure the temperature and velocity fields in the near-wake for a set of cases chosen to be representative of the variety of wake structures that exist for this type of flow. The experiments were carried out in a water tunnel at a Reynolds number of 690.
It was found that wake structure and heat transfer both significantly affect one another. The wake mode, a label indicating the number and type of vortices shed in each oscillation period, is directly related to the observed heat transfer enhancement. The dynamics of the vortex formation process, including the trajectories of the vortices during roll-up, explain this relationship. The streamwise spacing between shed vortices was also shown to affect heat transfer coefficient for the 2S mode, which consists of two single vortices shed per cycle. The streamwise spacing is believed to influence entrainment of freestream temperature fluid by the forming vortices, thereby affecting the temperature gradient at the cylinder base. This effect may exist for other wake modes, as well.
The cylinder’s transverse velocity was shown to influence the heat transfer by affecting the circulation of the wake vortices. For a fixed wake structure, the effectiveness of the wake vortices at enhancing heat transfer depends on their circulation. Also, the cylinder’s transverse velocity continually changes the orientation of the wake with respect to the freestream flow, thereby spreading the main source of heat transfer enhancement–the vortices near the cylinder base–over a larger portion of the cylinder surface.
Previously observed heat transfer enhancement associated with oscillations at frequencies near the natural shedding frequency and its harmonics were shown to be limited to amplitudes of less than about 0.5 cylinder diameters.
A new phenomenon was discovered in which the wake structure switches back and forth between distinct wake modes. Temperature induced variations in the fluid viscosity are believed to be the cause of this mode-switching. It is hypothesized that the viscosity variations change the vorticity and kinetic energy fluxes into the wake, thereby changing the wake mode and the heat transfer coefficient. This discovery underscores the role of viscosity and shear layer fluxes in determining wake mode, potentially leading to improved understanding of wake vortex formation and pinch-off processes in general.
Aspect ratio appears to play a role in determining the heat transfer coefficient mainly for non-oscillating cylinders. The heat transfer is also affected by aspect ratio for oscillation conditions characterized by weak synchronization of the wake to the oscillation frequency.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Gharib, Morteza}, } @phdthesis{10.7907/VNJW-6592, author = {Zuhal, Lavi Rizki}, title = {Formation and Near-Field Dynamics of a Wing Tip Vortex}, school = {California Institute of Technology}, year = {2001}, doi = {10.7907/VNJW-6592}, url = {https://resolver.caltech.edu/CaltechTHESIS:02072013-122723580}, abstract = {The search for a more efficient method to destroy aircraft trailing vortices requires a good understanding of the early development of the vortices. For that purpose, an experimental investigation has been conducted to study the formation and near-field dynamics of a wing tip vortex.
Two versions of the Digital Particle Image Velocimetry (DPIV) technique were used in the studies. Planar DPIV was used to obtain velocity fields adjacent to the wing surface. Stereoscopic DPIV, which allows instantaneous measurements of all three components of velocity within a planar slice, was used to measure velocity fields behind the wing. The trailing vortex was produced by a rectangular half-wing model with an NACA 0012 profile. All measurements were made at Reynolds number, based on chord length, of 9040.
The present study has found that the wing sheds multiple vortices. A structure that closely resembles a wing tip vortex is first observed on the suction side of the wing near the tip at the mid-chord section of the wing. At the trailing edge of the wing, a smaller vortex with an opposite sense of rotation is observed next to the tip vortex. In addition to the two vortices, two vortex layers with opposite sense of rotation, one on the pressure side and one on the suction side, are apparent at the trailing edge. Farther downstream, most of the vorticity in the vortex layer, with the same sense of rotation as the tip vortex, rolls up into the wing tip vortex. The vortices, with opposite sense of rotation, break up into smaller vortices which orbit around the tip vortex. At least one relatively strong satellite vortex appears in some of the instantaneous fields. The studies found that the interaction of the tip vortex and satellite vortices give rise to the unsteady motion of the wing tip vortex. In addition, the studies also examined the effects of the boundary layer and the tip geometry to the strength and motion of the trailing vortex.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Gharib, Morteza}, } @phdthesis{10.7907/016w-2332, author = {Ol, Michael Volf}, title = {The Passage Toward Stall of Nonslender Delta Wings at Low Reynolds Number}, school = {California Institute of Technology}, year = {2001}, doi = {10.7907/016w-2332}, url = {https://resolver.caltech.edu/CaltechTHESIS:11192010-085724406}, abstract = {Separated flow over the leeside of relatively nonslender delta wings was studied experimentally. Such flowfields are more complex than those of the slender delta wing of very low aspect ratio. A version of Stereo Digital Particle Image Velocimetry was applied to measurements in a low speed water tunnel, at Reynolds numbers below 20,000, for delta wing models of 50° and 65° leading edge sweep angles and 30° windward-side leading edge bevels. Since the objective was to draw comparisons to the stall of classical high aspect ratio wings, low angles of attack were emphasized, with most data points taken in the 5°-20° angle of attack range. Measurements were taken over the starboard portion of the wing planform in crossflow planar slices near the apex region, yielding all three components of the velocity field, albeit restricted to planar cuts. Vorticity and circulation were calculated from these measurements. All three components of vorticity were obtained in select cases, by central-differencing velocity data across triplets of adjacent interrogation planes. In addition, flow visualization by dye injection into the windward apex stagnation region was used to confirm the presence of primary and secondary leading edge vortices, to qualitatively verify the locations of vortex breakdown, and to verify the stereo digital particle image velocimetry results. Both delta wings exhibit stable, coherent leading edge vortices at very low angles of attack, down to 2.5°. Results for the 65° wing were in accordance with the literature. The 50° wing, however, exhibited flow characteristics akin to both slender delta wings, and wings of high aspect ratio, and generally exhibited stronger and more robust leading edge vortices than usually observed. For the 50° wing, the primary leading edge vortices were stable below 10° angle of attack, with gradual and steady upstream progression of the vortex breakdown region with increasing angles of attack, from aft of the trailing edge to approximately the midchord. Secondary leading edge vortices were found to decay more abruptly, and at lower angle of attack than the primaries, all but disappearing by 10° angle of attack. This fact has the potential of serving as the basis for a predictive criterion for breakdown of the primary vortices. The entire vortex system undergoes large-scale instabilities in the 12°-20° angle of attack range. While the flow visualization was inconclusive, particle image velocimetry confirmed that breakdown sweeps over the entire forward third of the wing planform in going from 12.5° to 15° angle of attack. This change is characterized by a sharp drop in axial velocity in the primary leading edge vortex core region, along with a loss of coherent vortical structure normally associated with this region. The leading edge shear layer, however, remains in an organized rolled-up state. By 20°, the flow over the leeward side of the wing is at the threshold of complete separation, with flow along the wing centerline stalling as the left and right separated regions grow and merge. Both wings exhibited a largely stagnant region outboard of the primary LEV and inboard of the leading edge shear layer, especially at angles of attack beyond 10°. This phenomenon is consistent with some prior observations at Reynolds numbers on the order of 20,000 and below, and differs sharply from that at higher Reynolds numbers. Further experiments are necessary to elucidate the cause and extent of Reynolds number influence on separation near the leeward surface. Also, the 50° wing is probably of too high sweep to be a true limiting case for the existence of coherent leading edge vortices, for the conditions of the present experiment. But the abruptness of its stall and the close relationship between the leading edge vortex flow and the leeward surface boundary layer are qualitatively indicative of such a transitional case from slender delta wing separation to classical airfoil stall.}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Gharib, Morteza}, } @phdthesis{10.7907/3QWF-8G05, author = {Krueger, Paul Samuel}, title = {The Significance of Vortex Ring Formation and Nozzle Exit Over-Pressure to Pulsatile Jet Propulsion}, school = {California Institute of Technology}, year = {2001}, doi = {10.7907/3QWF-8G05}, url = {https://resolver.caltech.edu/CaltechETD:etd-09142005-111030}, abstract = {NOTE: Text or symbols not renderable in plain ASCII are indicated by […]. Abstract is included in .pdf document. Pulsatile jet propulsion can be accomplished using a fully-pulsed jet (i.e., a periodic series of starting jets or pulses), the unsteady nature of which engenders vortex ring formation. The significance of vortex ring formation for this type of propulsion is studied experimentally using a piston-cylinder mechanism to generate starting and fully-pulsed, round jets of water into water at a maximum jet Reynolds number of 13,000. Starting jets are considered separately since they are the limiting case of a fully-pulsed jet at zero pulsing frequency. Direct measurements of the total impulse per pulse (starting jets) and time-averaged thrust (fully-pulsed jets) are made using a force balance. Hotfilm anemometry is used to measure the jet velocity and Digital Particle Image Velocimetry (DPIV) is used to measure vortex ring position, vorticity, energy, circulation, and impulse. The pulses for both types of jets are generated using piston stroke to diameter ratios (L/D) in the range 2 to 8 for piston velocity programs in a generally positive-sloping (PS) or negative-sloping (NS) family. The range of L/D considered brackets the transition between the case where an individual vortex ring is produced with each pulse (small L/D) and the case where the vortex ring stops growing and pinches off from its generating jet, producing a trailing jet (large L/D). This transition occurs at a higher L/D for the PS ramps, allowing the effects of vortex ring formation and pinch off to be illuminated by comparison of the results for the NS and PS ramps. The significance of vortex ring formation is first analyzed for starting jets. Measurements of the total impulse per pulse as a function of L/D show that a leading vortex ring adds more impulse per unit L/D than a trailing jet. This leads to a maximum in the average thrust during a pulse at the L/Ds just before vortex ring pinch off is observed for both the PS and NS ramps. The propulsive benefit provided by a leading vortex ring over a trailing jet is connected to over-pressure at the nozzle exit plane during vortex ring formation. DPIV measurements demonstrate that nozzle exit over-pressure also makes an important contribution to energy and circulation. It is shown that this over-pressure can be related to the momentum that must be supplied by the forming vortex ring to ambient fluid in the form of added and entrained mass. A model is proposed for nozzle exit over-pressure near the initiation of an impulsive velocity program where entrainment can be ignored. The model readily accounts for the pressure contribution to circulation in the NS ramps, but modeling of entrainment is required to properly determine impulse and energy. For the fully-pulsed jet experiments, a normalized thrust, […], is introduced to characterize the pressure effects associated with vortex ring formation. The pulsing frequency is expressed in dimensionless form as […], which is between 0 and 1 for all fully-pulsed jets. A propulsive benefit from pressure ([…]) is observed for all L/D and […] considered. At low […], the results are similar to those for the starting jets. At higher […], […] decreases with L/D as with the starting jets, which is related to the existence of vortex ring pinch off for all observed […]. At a fixed L/D, two dominant decreasing trends in […] with […] appear and seem to be related to the effects of previously ejected pulses on forming vortex rings. No dramatic increase in […] with […] (associated with the increased convective velocity of multiple coaxial vortex rings over that of individual vortex rings) is observed since (a) the ring separation is never reduced low enough to see an increase in the ring velocity (even for […]), and (b) the vortex rings don’t remain coaxial or coherent as […].}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Gharib, Morteza}, } @phdthesis{10.7907/MB7E-7950, author = {Mohseni, Kamran}, title = {A: Universality in Vortex Formation. B: Evaluation of Mach Wave Radiation Mechanisms in a Supersonic Jet}, school = {California Institute of Technology}, year = {2000}, doi = {10.7907/MB7E-7950}, url = {https://resolver.caltech.edu/CaltechTHESIS:10082010-114955709}, abstract = {In this thesis two distinct features of coherent structures are investigated. In Part I a model for the pinch-off process in vortex ring formation is developed. The predicted nondimensional stroke length L/D (referred to as “formation number”) satisfactorily matches experimental observations. The model introduces two nondimensional parameters that govern the limiting value of the formation number: a nondimensional energy and circulation, E_(nd) and Γ_(nd), respectively. The predicted value of E_(nd) also matches well with the experimental data. The limiting value for the new nondimensional circulation is predicted to be in the range 1.77 ≲ Γ_(nd) ≲ 2.07. We perform detailed computations of vortex ring formation by nonconservative forcing. The validity of the assumptions in our model is verified in these computations. Some techniques for generating fat rings are successfully investigated, resulting in generation of vortex rings with Hill’s like properties. We consider thermodynamics of the vorticity density field (w/r), and we develop a statistical equilibrium theory for axisymmetric flows. It is shown that the statistical equilibrium of an axisymmetric flow is the state that maximizes an entropy functional constrained to the invariants of motion. Furthermore, it is shown that the final equilibrium state satisfies a variational principle similar to Kelvin’s variational principle. In Part II Mach wave radiation mechanisms in a fully expanded supersonic jet is studied. We compare a direct numerical simulation (DNS) of a 1.92 Mach number jet with a linearized Navier-Stokes (LNS) simulation. The numerical integration technique, inflow boundary conditions, and grid distributions are the same in both simulations. We found that the generated noise in the DNS calculation is dominated by the first two azimuthal modes, and contributions from all other azimuthal modes were limited to less than 1.5 dB in the acoustic field. The total directivity of the sound field in the LNS matches reasonably well with the sound field of the DNS data. At the peak Strouhal frequency, particularly for the azimuthal mode n = 1, the amplification of flow variables in the LNS closely matches that of the DNS data. However, for frequencies away from the peak Strouhal number the DNS data shows amplification rates comparable to those of the peak Strouhal number, while in the LNS data any disturbances away from the peak Strouhal number are highly damped. These extra noise sources in the DNS data have the characteristics of a nonlinear interaction among various modes. The noise generated by the first two modes in the linearized computation is substantially weaker than in the DNS. For example, in the near acoustic field, at a distance of 6 jet diameters from the jet centerline, the sound pressure level in the linearized computation is as much as 8 db smaller than the DNS results. We observed that the maximum amplification rate for the DNS data occurs at an axial location further downstream than for the LNS data, which corresponds to regions around and beyond the end of the potential core. Our results indicate that the missing sound generation mechanisms in the LNS computation at the frequencies considered in this study can be attributed to the non-linear sound generation mechanisms, that are not captured in linear theories.}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Colonius, Timothy E. and Gharib, Morteza}, } @phdthesis{10.7907/ky58-en52, author = {Jeon, David S.}, title = {On Cylinders Undergoing One- and Two-Degree of Freedom Forced Vibrations in a Steady Flow}, school = {California Institute of Technology}, year = {2000}, doi = {10.7907/ky58-en52}, url = {https://resolver.caltech.edu/CaltechTHESIS:09302010-151500998}, abstract = {Formation of vortices in the near wake of circular cylinders is discussed. Two different cases are compared: starting flow around an initially stationary cylinder and flow around an oscillating cylinder in a steady freestream. The effects of formation time on the morphology of the vortices are shown, as well as some consequences thereof. For starting flows, the critical formation time defines the point where the wake transitions from the initially symmetrical state to the intermediate asymmetrical state. The asymmetrical state breaks down into the periodic shedding state normally associated with cylinder flows. It appears that the wake reaches a critical level of vorticity annihilation at the critical time. This triggers an exponential growth of asymmetry in the near wake. Evidence of this process can also be seen in the early time force data. For oscillating flows, the critical time defines the transition from vortex to vortex-and-tail morphology. First, phase averaged vorticity fields are presented showing the changes in the wake with forcing frequency and streamwise motion. These changes are related to the formation time, and related to similar effects seen in other flows. In addition, prolonged formation is related to the observed switch in the phase of the vortex shedding. The effects of streamwise motion are also shown, including the increased phase coherence of the shedding via coordination of the shedding process and the ability to adjust formation time via streamwise acceleration. The latter was used to demonstrate a plausible explanation for the vortex pair formation process observed by some researchers by showing how the formation process affects the number of vortices generated per cycle.}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Gharib, Morteza}, } @phdthesis{10.7907/ERAB-MJ26, author = {Mahmoudi Zarandi, Mehrdad}, title = {Steady and Pulsatile Flow in Curved Vessels}, school = {California Institute of Technology}, year = {2000}, doi = {10.7907/ERAB-MJ26}, url = {https://resolver.caltech.edu/CaltechETD:etd-09232005-081558}, abstract = {NOTE: Text or symbols not renderable in plain ASCII are indicated by […]. Abstract is included in .pdf document. An experimental investigation is carried out on the nature of secondary flow patterns in a curved vessel. This study concentrates on the role of the upstream spatial boundary conditions and the time-periodicity of the formation of the secondary flow patterns in curved vessels. Four sets of studies are made. In the first set, steady flow is compared to a pulsatile flow simulating the physiological conditions in the human aorta. To study the effect of spatial upstream boundary conditions, we investigated the effect of an upstream constriction, which represents the actual physiological conditions in the human aorta. All in vitro studies are carried out in models with dimensions close to those of the human aorta and with the Reynolds number (1400 < Re < 7000), Dean number (1470 < De < 7300) and Womersley number (8.6 < […] < 12.6) all in a range close to the physiological values. The technique of Digital Particle Image Velocimetry is used to measure instantaneous and average flow fields. In the first stage of this research, an orifice with a stenosis ratio of ~3 = 80% is used to simulate the upstream constriction and study its effect on the secondary flow patterns. It is found that the existence of the upstream constriction profoundly changes the nature of the secondary flow pattern. In the presence of the upstream constriction, the double circulation pattern of Dean flow is substituted with a single circulation pattern. Next, to investigate where the transition from one pattern to the other occurs, similar sets of experiments are carried out with constrictions of different sizes corresponding to stenosis ratios of, […] = 65% and […] = 88%. In addition, to investigate the sense of rotation for the single circulation pattern, an orifice with an asymmetric opening mounted at different angular positions is used. The comparison of the secondary flow patterns for steady versus pulsatile flow revealed that the flow pattern does not change its main structure due to the time-periodicity of the flow. However, the pulsatile flow in general shows secondary flow rates greater than the steady flow. On the other hand, the spatial boundary conditions are found to be central in determining the secondary flow patterns. First, the presence of an upstream constriction or stenosis ratio of […] = 65% will result in a single circulation pattern. Therefore, the transition between the double circulation pattern of the non-constrained flow and the single circulation pattern occur at constrictions less than […] = 65%. Secondary flow velocities and shear rate along the vessel wall are much higher for the flow with an upstream constriction than those for the flow without an upstream constriction. In the presence of an upstream constriction, the axial velocity and secondary flow maximum velocity are of the same order of magnitude. The results of the experiments with the asymmetric orifice show that the sense of rotation depends on the position of the upstream opening with respect to the central axis of the vessel. The clinical data show that the aortic valve opening, even at the full open stage, is less than the maximum diameter of the sinus of valsalva at the base of the human aorta by at least 30% which is equivalent to a stenosis ratio of […] = 51%. Therefore, our results are important in the understanding of the nature of shear stress development along the vessel walls and in the study of the radial distribution of blood cells in a secondary flow field. Similarly, for industrial application these results are important for the assessment of the transport properties in coiled heat exchangers or fluid-fluid absorption systems. In order to investigate the spatial condition which determines the sense of rotation in single-circulation patterns, a series of experiments were carried out changing the position of orifice opening in […] increments with respect to anterior-posterior wall axis. These results strongly support the conjecture that the incoming jet impingement on the vessel wall is redirected by the local curvature of the vessel wall into a helical pattern which overcomes the Dean’s flow and causes the single-circulation patterns. To summarize, three distinct secondary flow patterns are observed in our experimental study. The first one is the known Dean flow, which is a double-circulation pattern. The second and third patterns, which are discovered for the first time, are the clockwise and counterclockwise single-circulation patterns. The transition from one pattern to another is dependent on the spatial boundary conditions and is independent of the temporal boundary conditions. The secondary flow velocity gradient and hence its shear stress is comparable to the axial flow velocity gradient and shear stress in the single-circulation pattern. For the Dean flow or double-circulation secondary flow pattern, shear stress values are much less than those for the axial flow. This finding can greatly contribute to our understanding of blood flow-related pathology. In addition, the position of the upstream orifice opening with respect to the anterior-posterior vessel wall is an important factor in the design of artificial aortic valves. The dependence of the sense of the rotation of single-circulation patterns on the relative position of orifice opening with respect to the anterior-posterior vessel wall axis suggests that the out of plane curvature of aorta may play an important role in the re-direction of the incoming jet in the arch.}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Gharib, Morteza}, } @phdthesis{10.7907/TCTV-BM26, author = {Maheo, Patrice Michel}, title = {Free-Surface Turbulent Shear Flows}, school = {California Institute of Technology}, year = {1999}, doi = {10.7907/TCTV-BM26}, url = {https://resolver.caltech.edu/CaltechETD:etd-02212008-104541}, abstract = {The structure and dynamics of turbulent wakes and shear layers in the presence of a clean free surface have been investigated experimentally using digital particle image velocimetry (DPIV). The purpose of this study was to determine the extent and characteristics of the influence, if any, of the free surface on these underlying turbulent shear flows.
The free surface was found to affect the dynamics of turbulence within a surface layer on the order of one half-width of the submerged wake and one half of the local vorticity thickness of the submerged shear layer. Within this layer, the vertical velocity fluctuations are inhibited and the turbulence kinetic energy is redistributed to the horizontal components. The self-induced motion of surface-parallel vortical structures under the influence of their images was shown to lead to large-scale mean streamwise secondary flows and associated outward surface currents-symmetric for the wake and asymmetric for the shear layer. This motion was the origin of the significantly higher lateral spreading rates of these surface shear flows compared to the spreading rates of their fully-submerged counterparts — 20% and 25% for the wake and shear layer respectively. In addition, the evolution of the streamwise and surface-normal enstrophy components within the surface layer was consistent with the normal connection of vortical structures required at a free surface.
The influence of the secondary flows was tracked back to the splitter plate’s turbulent boundary layers where they were hence deduced to originate. A simple analysis of the mixed-boundary corner flows of the splitter plate made using the mean streamwise vorticity equation coupled with the evolution of the values of the transverse velocity confirmed the latter. In this picture of the mean flow, the secondary flows present in the near-surface edges of these shear flows were related to the pair of outer secondary vortices generated thereby. Furthermore, using a simplified equation for the surface-normal Reynolds stress, it was shown that the mutual interaction of the surface-parallel vortical structures with their images yielded a decrease in vertical velocity fluctuations as the free surface was approached. This equation shed further light on the redistribution of the vertical kinetic energy of turbulence into the other two Reynolds normal stresses. The resulting free-surface Reynolds-stress anisotropy in turn gave birth to the two streamwise secondary flows.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Gharib, Morteza}, } @phdthesis{10.7907/C9KN-RQ12, author = {Park, Han G.}, title = {A Study of Heat Transport Processes in the Wake of a Stationary and Oscillating Circular Cylinder Using Digital Particle Image Velocimetry/Thermometry}, school = {California Institute of Technology}, year = {1998}, doi = {10.7907/C9KN-RQ12}, url = {https://resolver.caltech.edu/CaltechETD:etd-04132004-150955}, abstract = {NOTE: Text or symbols not renderable in plain ASCII are indicated by […]. Abstract is included in .pdf document. An experimental investigation is carried out on the processes of heat transfer associated with a heated circular cylinder in crossflow. Two studies are made. First, a study of the transport of heat in the near wake (x/D<5) of a stationary and transversely oscillated cylinder is made at Reynolds number of 610. Second, a study is made of the surface heat transfer from a cylinder which is undergoing forced oscillations in the transverse direction. The studies are made using the technique of Digital Particle Image Velocimetry/Thermometry (DPIV/T) which allows simultaneous measurements of both the velocity and temperature fields of the flow. The temperature is measured by seeding the flow with thermochromic liquid crystal (TLC) particles which change their reflected wavelength as function of temperature. By calibrating reflected wavelength versus temperature using a color multi-CCD camera, the local temperature of the flow may be deduced. The velocity is measured by using the same particles as Lagrangian flow tracers, and local velocity or displacement of the flow may be measured by cross-correlating two sequential images. A limitation of DPIV/T, which is the low level of precision (5% - 20% of the temperature span of TLC particles), may be overcome by a process in which the temperature at a given location is computed by averaging the temperatures of the particles within a specified sampling window. This process increases the precision to 2% - 10%. In the study of the heat transport in the near wake, the velocity and temperature measurements obtained from DPIV/T are decomposed into their mean, coherent, and incoherent components using the triple decomposition. It is found that the heat from the cylinder is transported down the wake mostly by the mean heat flux and is laterally transported out of the wake by the coherent and the incoherent heat fluxes. In examining the direction of the turbulent heat flux vectors, the vectors are found not to be co-linear with the gradient of mean temperature. This misalignment implies that the gradient transport models are inappropriate for modeling the turbulent heat transport in the near wake of a circular cylinder. In examining the production of turbulence, it is found that that kinetic energy fluctuations are produced in the saddle regions (regions where the fluid is being stretched in one direction and compressed in another) while the temperature fluctuations are produced at the edges of center regions (regions where the fluid is rotating), i.e., the edges of the vortex cores. From the study of the heat convection from a cylinder as function of forced oscillation frequency […] and amplitudes (A/D=0.1, 0.2), it is found that besides the previously known increase near the natural vortex shedding frequency, there also exists a large increase in the heat transfer at approximately three times this frequency for A/D=0.1. For A/D=0.2, there exist large increases at roughly two and three times the natural vortex shedding frequency. From a DPIV/T study, it is found that the wake pattern becomes synchronized with the mechanical oscillation of the cylinder at these frequencies where the heat transfer increases significantly. At the frequencies corresponding to roughly two and three times the unforced vortex shedding frequency, the wake pattern may become synchronized by processes of period doubling and tripling with respect to the cylinder oscillation period, respectively. The increase in the heat transfer rate is found to correlate with the distance at which vortices roll-up behind the cylinder. The distance is observed to decrease sharply at the frequencies corresponding to a sharp increase in the heat transfer. Therefore, the near wake is found to play a critical role in the heat transfer from the surface of a circular cylinder, and the cause of the increase in heat transfer is believed to the removal of the stagnant and low heat convecting fluid at the base of the cylinder during the roll-up of the vortices.}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Gharib, Morteza}, } @phdthesis{10.7907/1xg2-yy62, author = {Warncke, Amy E.}, title = {The Effects of Surfactants on Free-Surface Flows}, school = {California Institute of Technology}, year = {1997}, doi = {10.7907/1xg2-yy62}, url = {https://resolver.caltech.edu/CaltechETD:etd-01172008-092233}, abstract = {This experimental investigation into the nature of free surface flows is to study the effects of surfactants on the boundary condition at the free surface and the resulting flow field. In particular, the flow field associated with a stationary Reynolds ridge was investigated as well as the wake behind a surface-piercing cylinder, where experimental techniques such as Digital Particle Image Velocimetry and a new surface slope measurement technique were utilized. Results show a large change in the flow field in the free-surface vicinity depending on the presence of surface tension gradients and thus shear stresses at the free surface. In particular, the boundary layer beneath a Reynolds ridge was measured and it is shown that the primary source of vorticity at the free surface can be attributed to the free surface deceleration at the ridge. Also, in the wake of the cylinder, depending on the surface condition, the connection of the shedding vortex filaments was found to be greatly altered with the propensity of surface tension gradients to redirect the vorticity near the free surface to that of the surface-parallel component. Thus it is shown that surfactants can dramatically alter the flow field due to the change in the free-surface boundary condition and resulting vorticity generation and conversion in the vicinity of the free surface.}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Gharib, Morteza}, } @phdthesis{10.7907/2wbk-9k81, author = {Valluri, Siddhartha}, title = {Bluff Body Flows in the Presence of a Free Surface}, school = {California Institute of Technology}, year = {1996}, doi = {10.7907/2wbk-9k81}, url = {https://resolver.caltech.edu/CaltechETD:etd-06082007-075443}, abstract = {An experimental study is performed in a water tunnel (Re = 40,000 to Re = 60,000) to study the interaction between the wake of a circular disk and the free surface. The deformation of the free surface is correlated with the behavior of the wake by utilizing surface pictures, wake flow visualization, drag measurement and Digital Particle Image Velocimetry techniques. It is observed that the wake can exist in two modes with different stabilities. The flow can switch between these two modes and the switching process exhibits hysteresis. The topological differences between these modes and their relation to the observed surface patterns are discussed. The changes in the wake are reflected by an increase in Cd which reaches a maximum value when the upper edge of the disk is 0.125 diameters from the surface. Comparison is also made with a disk approaching a solid boundary.}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Roshko, Anatol and Gharib, Morteza}, }