Within a wind farm, multiple turbine wakes can interact and have a substantial effect on the overall power production. This makes an understanding of the wake recovery process critically important to optimizing wind farm efficiency. Vertical-axis wind turbines (VAWTs) exhibit features that are amenable to dramatically improving this efficiency. However, the physics of the flow around VAWTs is not well understood, especially as it pertains to wake interactions, and it is the goal of this thesis to partially fill this void. This objective is approached from two broadly different perspectives: a low-order view of wind farm aerodynamics, and a detailed experimental analysis of the VAWT wake.
One of the contributions of this thesis is the development of a semi-empirical model of wind farm aerodynamics, known as the LRB model, that is able to predict turbine array configurations to leading order accuracy. Another contribution is the characterization of the VAWT wake as a function of turbine solidity. It was found that three distinct regions of flow exist in the VAWT wake: (1) the near wake, where periodic blade shedding of vorticity dominates; (2) a transition region, where growth of a shear-layer instability occurs; (3) the far wake, where bluff-body oscillations dominate. The wake transition can be predicted using a new parameter, the dynamic solidity, which establishes a quantitative connection between the wake of a VAWT and that of a circular cylinder. The results provide insight into the mechanism of the VAWT wake recovery and the potential means to control it.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Dabiri, John O.}, } @phdthesis{10.7907/Z9057CX7, author = {Martinez-Ortiz, Monica Paola}, title = {Fluid Transport by Aggregations of Small Swimming Organisms}, school = {California Institute of Technology}, year = {2016}, doi = {10.7907/Z9057CX7}, url = {https://resolver.caltech.edu/CaltechTHESIS:12232015-091951061}, abstract = {Diel vertical migration of zooplankton has been proposed to affect global ocean circulation to a degree comparable to physical phenomena. Almost a decade after shipboard measurements showed high kinetic energy dissipation rates in the vicinity of migrating krill swarms, the hypothesis that biogenic mixing is relevant to ocean dynamics and local fluid transport has remained controversial due to the inability to directly measure the efficiency of this biological process. In situ field measurements of individual swimming jellyfish have demonstrated large-scale fluid transport via Darwinian drift, but it has remained an open question how this transport mechanism is manifested in smaller species of vertically-migrating zooplankton that are sufficient in number to be accountable in the dynamics. The goals of the present study are, first, to devise and implement experimental instruments and develop methodologies to investigate this biological process in a laboratory setting and, second, to determine whether efficient fluid transport mechanisms become available during vertical collective motion and, if so, analyze how energy is distributed within the flow. By leveraging the phototactic abilities of zooplankton, a multi-laser guidance system was developed to achieve controllable vertical migrations of A. salina concurrently with laser velocimetry of the surrounding flow. Measurements show that the hydrodynamic interactions between neighboring swimmers during vertical migration result in the development of a pronounced jet opposite to animal motion. In non-stratified fluid, this hydrodynamic feature is shown to trigger a Kelvin-Helmholtz instability that results in the generation of eddy-like structures with characteristic length scales much larger than the individual size of the organisms. Experiments in a thermally stratified water column also display the presence of a downward jet despite the strong stable stratification. Furthermore, overturning regions larger than the size of an individual organism are observed adjacent to the migrating aggregation, suggesting an alternate energy transfer route from the small scale of individual swimmers to significantly larger scales, at which mixing can be efficient via a Rayleigh-Taylor instability. The computed velocity spectrum is consistent with these findings and displays energy input at scales larger than the body length of a single swimmer. The mixing efficiency, inferred from the spectral energy distribution with and without stratification, matches experimentally achieved mixing efficiencies via a Rayleigh-Taylor instability within a stable stratification. According to our findings, biogenic mixing does have the potential to redistribute temperature, salinity and nutrients effectively. We propose the employment of laser control to examine additional species as well as alternative oceanic environments and interrogate its effect on the efficiency of biogenic mixing.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Dabiri, John O.}, } @phdthesis{10.7907/AFAA-KF43, author = {O’Farrell, Clara}, title = {A Dynamical Systems Analysis of Vortex Pinch-Off}, school = {California Institute of Technology}, year = {2013}, doi = {10.7907/AFAA-KF43}, url = {https://resolver.caltech.edu/CaltechTHESIS:05032013-161632237}, abstract = {Vortex rings constitute the main structure in the wakes of a wide class of swimming and flying animals, as well as in cardiac flows and in the jets generated by some moss and fungi. However, there is a physical limit, determined by an energy maximization principle called the Kelvin-Benjamin principle, to the size that axisymmetric vortex rings can achieve. The existence of this limit is known to lead to the separation of a growing vortex ring from the shear layer feeding it, a process known as `vortex pinch-off’, and characterized by the dimensionless vortex formation number. The goal of this thesis is to improve our understanding of vortex pinch-off as it relates to biological propulsion, and to provide future researchers with tools to assist in identifying and predicting pinch-off in biological flows.
To this end, we introduce a method for identifying pinch-off in starting jets using the Lagrangian coherent structures in the flow, and apply this criterion to an experimentally generated starting jet. Since most naturally occurring vortex rings are not circular, we extend the definition of the vortex formation number to include non-axisymmetric vortex rings, and find that the formation number for moderately non-axisymmetric vortices is similar to that of circular vortex rings. This suggests that naturally occurring vortex rings may be modeled as axisymmetric vortex rings. Therefore, we consider the perturbation response of the Norbury family of axisymmetric vortex rings. This family is chosen to model vortex rings of increasing thickness and circulation, and their response to prolate shape perturbations is simulated using contour dynamics. Finally, the response of more realistic models for vortex rings, constructed from experimental data using nested contours, to perturbations which resemble those encountered by forming vortices more closely, is simulated using contour dynamics. In both families of models, a change in response analogous to pinch-off is found as members of the family with progressively thicker cores are considered. We posit that this analogy may be exploited to understand and predict pinch-off in complex biological flows, where current methods are not applicable in practice, and criteria based on the properties of vortex rings alone are necessary.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Dabiri, John O.}, } @phdthesis{10.7907/5ZTQ-2J09, author = {Nawroth, Janna C.}, title = {Conceptual Framework and Physical Implementation of a Systematic Design Strategy for Tissue-Engineered Devices}, school = {California Institute of Technology}, year = {2013}, doi = {10.7907/5ZTQ-2J09}, url = {https://resolver.caltech.edu/CaltechTHESIS:12172012-143708325}, abstract = {Tissue-engineered and biologically inspired devices promise to advance medical implants, robotic devices and diagnostic tools. Ideally, biohybrid constructs combine the versatility and fine control of traditional building substrates with dynamic properties of living tissues including sensory modalities and mechanisms of repair, plasticity and self-organization. These dynamic properties also complicate the design process as they arise from, and act upon, structure-function relationships across multiple spatiotemporal scales that need to be recapitulated in the engineered tissue. Biomimetic designs merely copying the structure of native organs and organisms, however, are likely to reflect evolutionary constraints, phenotypic variability and environmental factors rather than rendering optimal engineering solutions.
This thesis describes an alternative to biomimetic design, i.e., a systematic approach to tissue engineering based on mechanistic analysis and a focus on functional, not structural, approximation of native and engineered system. As proof of concept, the design, fabrication and evaluation of a tissue-engineered jellyfish medusa with biomimetic propulsion and feeding currents is presented with an emphasis on reasoning and strategy of the iterative design process. A range of experimental and modeling approaches accomplishes mechanistic analysis at multiple scales, control of individual and emergent cell behavior, and quantitative testing of functional performance. The main achievement of this thesis lies in presenting both conceptual framework and physical implementation of a systematic design strategy for muscular pumps and other bioinspired and tissue-engineered applications.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Dabiri, John O.}, } @phdthesis{10.7907/SC4M-8896, author = {Whittlesey, Robert Wells}, title = {Dynamics and Scaling of Self-Excited Passive Vortex Generators for Underwater Propulsion}, school = {California Institute of Technology}, year = {2013}, doi = {10.7907/SC4M-8896}, url = {https://resolver.caltech.edu/CaltechTHESIS:05282013-114822808}, abstract = {A series of experiments was conducted on the use of a device to passively generate vortex rings, henceforth a passive vortex generator (PVG). The device is intended as a means of propulsion for underwater vehicles, as the use of vortex rings has been shown to decrease the fuel consumption of a vehicle by up to 40% Ruiz (2010).
The PVG was constructed out of a collapsible tube encased in a rigid, airtight box. By adjusting the pressure within the airtight box while fluid was flowing through the tube, it was possible to create a pulsed jet with vortex rings via self-excited oscillations of the collapsible tube.
A study of PVG integration into an existing autonomous underwater vehicle (AUV) system was conducted. A small AUV was used to retrofit a PVG with limited alterations to the original vehicle. The PVG-integrated AUV was used for self-propelled testing to measure the hydrodynamic (Froude) efficiency of the system. The results show that the PVG-integrated AUV had a 22% increase in the Froude efficiency using a pulsed jet over a steady jet. The maximum increase in the Froude efficiency was realized when the formation time of the pulsed jet, a nondimensional time to characterize vortex ring formation, was coincident with vortex ring pinch-off. This is consistent with previous studies that indicate that the maximization of efficiency for a pulsed jet vehicle is realized when the formation of vortex rings maximizes the vortex ring energy and size.
The other study was a parameter study of the physical dimensions of a PVG. This study was conducted to determine the effect of the tube diameter and length on the oscillation characteristics such as the frequency. By changing the tube diameter and length by factors of 3, the frequency of self-excited oscillations was found to scale as f~D_0^{-1/2} L_0^0, where D_0 is the tube diameter and L_0 the tube length. The mechanism of operation is suggested to rely on traveling waves between the tube throat and the end of the tube. A model based on this mechanism yields oscillation frequencies that are within the range observed by the experiment.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Dabiri, John O.}, } @phdthesis{10.7907/3XEF-X568, author = {Peng, Jifeng}, title = {A Lagrangian Approach to Transport of Momentum and Biomass in Aquatic Biological Systems}, school = {California Institute of Technology}, year = {2010}, doi = {10.7907/3XEF-X568}, url = {https://resolver.caltech.edu/CaltechETD:etd-07212009-132708}, abstract = {In recent years, a Lagrangian Coherent Structures (LCS) method was developed to identify boundaries between distinct kinematic regions in unsteady flows. Many fluid transport processes can be described in terms of these kinematic boundaries in the flow. The method has since been applied to many engineering, biological, and geological fluid systems, but primarily on transport of homogenous fluid mass.
In this thesis, with emphases on aquatic biological transport systems, the LCS analysis is further developed to study momentum transport in animal locomotion and biomass transport in animal predation. Three independent studies are included in this thesis.
In the first study, LCS analysis is used to identify the boundary of the vortex attached to the fin in sunfish pectoral fin locomotion. A potential flow, deformable body theory is used to describe the dynamics of the vortex. The hydrodynamic forces acting on the fin are evaluated from the linear momentum of the vortex itself and its added-mass. The quantification of instantaneous locomotive forces provides necessary information for studying complicated locomotive behaviors such as motion control and maneuvers.
In the second study, the LCS analysis is applied to a numerically simulated undulatory swimming and shows existence of ‘upstream fluid structures’ that are invisible in Eulerian velocity/vorticity fields. These structures indicate the exact portion of fluid that interacts with the swimmer. A mass flow rate and a momentum flux are then defined. A metric for propulsive efficiency is established using the momentum flux, which can be used to measure and compare the efficiency of other engineering and natural propulsion systems.
In the third study, a framework is developed to study transport of zooplankton prey in the feeding currents generated by a predator jellyfish. An equation of motion is proposed to describe the dynamics of prey in the flow. Then the concept of particle Lagrangian Coherent Structures (pLCS) is introduced to separate prey encounter regions from prey escape regions. The framework provides a mechanical basis for evaluating the predatory role of medusae in marine planktonic ecosystems. It can also be used to study transport and mixing in multiphase and granular flows in general.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Dabiri, John O.}, } @phdthesis{10.7907/QJAM-9228, author = {Young, Kakani Katija}, title = {Effect of In Situ Animal-Fluid Interactions on Transport and Mixing}, school = {California Institute of Technology}, year = {2010}, doi = {10.7907/QJAM-9228}, url = {https://resolver.caltech.edu/CaltechTHESIS:05252010-172302796}, abstract = {
Traditional studies of animal-fluid interactions have led to the understanding of factors that affect the distribution, ecology and energetics of swimming organisms. These interactions are commonly investigated by using quantitative flow measurement techniques, which include digital particle image velocimetry. Due to limitations in quantitative flow measurements in the natural environment, animal measurements are conducted in laboratories. Laboratory measurement techniques have been shown to have an altering impact on animal behavior and resulting flow fields. Hence, it is reasonable to question conclusions made about the impact of background flows in the natural environment from measurements conducted in the laboratory. Therefore, an apparatus that will enable the quantitative measurement of flows surrounding a swimming animal in the field is needed to accurately address the effect of background flows on animal swimming and fluid transport.
We describe the development of a self-contained underwater velocimetry apparatus that achieves the goal of real-time, quantitative field measurements of aquatic animal-fluid interactions. Using this apparatus, we obtain measurements of flow fields surrounding animals in the field and analyze the effect of background flows on swimming animals. Using a dynamical systems technique called Lagrangian coherent structures to quantitatively compare laboratory and field-generated flows, we find that background flow structures alter fluid transport by swimming jellyfish. From these studies, we define a biologically-relevant metric for animal feeding that is based entirely on the volume of fluid that interacts with the swimming animal. The ability to quantify background flows and their influence on animal-fluid interactions will allow us to broaden our concept of animal-fluid interactions to include the effects swimming animals have on their surrounding environment. This represents a paradigm shift in the analysis of animal-fluid interactions.
Recent studies have provided heated debate about whether biologically-generated (or biogenic) mixing can have an impact in the ocean. Arguments for biogenic mixing lacked an efficient mechanism for fluid transport in viscous and stratified flow environments. We present an effective mechanism for biogenic mixing called drift, which is active during swimming, and results in permanent displacement of fluid in the direction of the animal’s motion (in unstratified flow). We show that unlike mechanisms that rely on turbulent mixing generated by wake structures, drift is enhanced as viscous effects are increased. While drift has been observed in jellyfish and copepods, to understand its relevance in the global ocean, the effects of stratification need to be considered. By conducting simulations of moving bodies in stratified flow, we show that at buoyancy frequencies on the order of the mean ocean, fluid transport due to drift remains a powerful mechanism through which swimming animals may provide a significant contribution to mixing in the oceans.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Dabiri, John O.}, } @phdthesis{10.7907/7JWD-TB88, author = {Ruiz, Lydia Ann}, title = {The Role of Unsteady Hydrodynamics in the Propulsive Performance of a Self-Propelled Bioinspired Vehicle}, school = {California Institute of Technology}, year = {2010}, doi = {10.7907/7JWD-TB88}, url = {https://resolver.caltech.edu/CaltechTHESIS:05272010-122004587}, abstract = {Aquatic animals differ from typical engineering systems in their method of locomotion. In general, aquatic animals propel using unsteady dynamics producing vortex rings. Researchers have long shown interest in designing devices that resemble their shape and propulsive behavior. Traditional definitions of propulsive efficiency used to model these behaviors have not taken unsteady effects into account and are typically based on steady flow through propellers or rocket motors. Measurements of aquatic animals based on these quasi-steady metrics have suggested propulsive efficiencies over 80% when utilizing certain swimming kinematics. However, the mechanical efficiency of muscle-actuated biological propulsion has been found to be much lower, typically less than 20%. It is important to take into account the total efficiency of the system, the product of the mechanical and propulsive efficiency, when designing and implementing a biologically inspired propulsive device.
The purpose of my research is to make a direct, experimental comparison between biological and engineering propulsion systems. For this study, I designed an underwater vehicle that has the capability of producing either a steady or unsteady jet for propulsion, akin to a squid and jellyfish, while utilizing the same mechanical efficiency. I show that it is unnecessary to take an approach that mimics animal shape and kinematics to achieve the associated propulsive performance. A bioinspired, propeller-based platform that mimics animal wake dynamics can be similarly effective.
A study on how vortex dynamics plays a key role in improving the propulsive efficiency of pulsed jet propulsion was conducted. Measurements of propulsive performance resulted in superior performance for the pulsed-jet configuration in comparison to the steady jet configuration particularly at higher motor speeds. The analysis demonstrated that vortex ring formation led to the acceleration of two classes of ambient fluid, entrained and added mass, and this consequently led to an increased total fluid impulse of the jet and propulsive performance. The first source of ambient fluid acceleration investigated was entrained mass. The magnitude of the entrainment ratio was measured and found to be smaller for the steady jet mode of propulsion in comparison to the pulsed jet mode of propulsion given comparable motor speeds. The role of the added mass effect was also investigated in increasing propulsive performance. A model developed by Krueger is used to determine the fraction of the total impulse imparted to the flow that was contributed by the added mass effect. Results demonstrated that the added mass effect associated with the acceleration of ambient fluid at the initiation of a starting jet provides an increase in the total impulse and is thus a source for increased propulsive performance. Last, a model was developed to investigate how an increase in the total fluid impulse due to vortex ring formation is related to the propulsive efficiency. Results obtained using the model are in agreement, within uncertainty, with previous experimental results for the measurement of propulsive efficiency. The results support that the additional force generated from the acceleration of two classes of ambient fluid are the source of increased propulsive efficiency for the pulsed jet configuration in comparison to the steady jet configuration. This model serves as an additional metric for determining the propulsive efficiency of a system utilizing pulsed jet propulsion.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Dabiri, John O.}, }