Abstract: This paper presents a review of some of the recent developments in our understanding of the dynamics and instabilities caused by cavitation in pumps. Focus is placed on presently available data for the transfer functions for cavitating pumps and inducers, particularly on the compliance and mass flow gain factor that are critical for pump and system stability. The resonant frequency for cavitating pumps is introduced and contexted. Finally emphasis is placed on the paucity of our understanding of pump dynamics when the device or system is subjected to global oscillation.

ID: CaltechAUTHORS:20170630-101600501

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Abstract: In this opening lecture I will summarize some of the fundamentals of cavitation in the hope that this will allow attendees greater insight into the more advanced lectures which follow. Whether your primary interest is in the turbomachinery field or in the biological and bioengineering contexts in which cavitation is important these fundamentals are important in understanding the observed phenomena.

ID: CaltechAUTHORS:20111208-111956559

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Abstract: Shock propagation through a bubbly liquid filled in a deformable cylindrical tube is considered. Quasi-one-dimensional bubbly flow equations that include fluid-structure interaction are formulated, and the steady shock relations are derived. Experiments are conducted in which a free-falling steel projectile impacts the top of an air/water mixture in a polycarbonate tube, and stress waves in the tube material are measured. The experimental data indicate that the linear theory cannot properly predict the propagation speeds of shock waves in mixture-filled tubes; the shock theory is found to more accurately estimate the measured wave speeds.

ID: CaltechAUTHORS:20111221-123516479

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Abstract: The effect of distributed bubble size on shock propagation in homogeneous bubbly liquids is computed using a continuum two-phase model. An ensemble-averaging technique is employed to derive the statistically averaged equations and a finite-volume method is used to solve the model equations. The bubble dynamics are incorporated using a Rayleigh-Plesset-type equation which includes the effects of heat transfer, liquid viscosity and compressibility. For the case of monodisperse bubbles, it is known that relaxation oscillations occur behind the shock due to the bubble dynamics. The present computations for the case of polydisperse bubbles show that bubble size distributions lead to additional damping of the shock dynamics. If the distribution is sufficiently broad, the statistical effect dominates over the physical damping associated with the single bubble dynamics. This smooths out the oscillatory shock structure.

ID: CaltechAUTHORS:20111213-124427981

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Abstract: In many cavitating liquid flows, when the number and concentration of the bubbles exceeds some critical level, the flow becomes unsteady and large clouds of cavitating bubbles are periodically formed and then collapse when convected into regions of higher pressure. This phenomenon is known as cloud cavitation and when it occurs it is almost always associated with a substantial increase in the cavitation noise and damage. These increases represent serious problems in devices as disparate as marine propellers, cavitating pumps and artificial heart valves. This lecture will present a brief review of the analyses of cloud cavitation in simplified geometries that allow us to anticipate the behavior of clouds of cavitation bubbles and the parameters that influence that behaviour. These simpler geometries allow some anticipation of the role of cloud cavitation in more complicated flows such as those in cavitating pumps.

ID: CaltechAUTHORS:20111213-112942130

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Abstract: In many cavitating liquid flows, when the number and concentration of the bubbles exceeds some critical level, the flow becomes unsteady and large clouds of cavitating bubbles are periodically formed and then collapse when convected into regions of higher pressure. This phenomenon is known as cloud cavitation and when it occurs it is almost always associated with a substantial increase in the cavitation noise and damage. These increases represent serious problems in devices as disparate as marine propellers, cavitating pumps and artificial heart valves. This lecture will present a brief review of the analyses of cloud cavitation in simplified geometries that allow us to anticipate the behavior of clouds of cavitation bubbles and the parameters that influence that behaviour. These simpler geometries allow some anticipation of the role of cloud cavitation in more complicated flows such as those in cavitating pumps.

ID: CaltechAUTHORS:20111213-112144056

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Abstract: In many cavitating liquid flows, when the number and concentration of the bubbles exceeds some critical level, the flow becomes unsteady and large clouds of cavitating bubbles are periodically formed and then collapse when convected into regions of higher pressure. This phenomenon is known as cloud cavitation and when it occurs it is almost always associated with a substantial increase in the cavitation noise and the potential for material damage associated with the cavitation. These increases represent serious problems in devices as disparate as marine propellers, cavitating pumps and artificial heart valves. This lecture will present examples of the phenomenon and review recent advances in our understanding of the dynamics and acoustics of clouds of bubbles and cloud cavitation. Both analyses of these complex multiphase flows and experimental observations will be used to identify the key features of the phenomenon and the parameters that influence it.

ID: CaltechAUTHORS:20111208-104034508

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Abstract: n/a

ID: CaltechAUTHORS:20130930-101528489

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Abstract: There are an increasing number of biological and bioengineering contexts in which cavitation is either utilized to create some desired effect or occurs as a byproduct of some other process. In this review an attempt will be made to describe a cross-section of these cavitation phenomena. In the byproduct category we describe some of the cavitation generated by head injuries and in artifical heart valves. In the utilization category we review the cavitation produced during lithotripsy and phacoemulsification. As an additional example we describe the nucleation suppression phenomena encountered in supersaturated oxygen solution injection. Virtually all of these cavitation and nucleation phenomena are critically dependent on the existence of nucleation sites. In most conventional engineering contexts, the prediction and control of nucleation sites is very uncertain even when dealing with a simple liquid like water. In complex biological fluids, there is a much greater dearth of information. Moreover, all these biological contexts seem to involve transient, unsteady cavitation. Consequently they involve the difficult issue of the statistical coincidence of nucleation sites and transient low pressures. The unsteady, transient nature of the phenomena means that one must be aware of the role of system dynamics in vivo and in vitro. For example, the artificial heart valve problem clearly demonstrates the importance of structural flexibility in determining cavitation occurrence and cavitation damage. Other system issues are very important in the design of in vitro systems for the study of cavitation consequences. Another common feature of these phenomena is that often the cavitation occurs in the form of a cloud of bubbles and thus involves bubble interactions and bubble cloud phenomena. In this review we summarize these issues and some of the other characteristics of biological cavitation phenomena.

ID: CaltechAUTHORS:BREwimrc06

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Abstract: Recent testing of high speed cavitating turbopump inducers has revealed the existance of more complex instabilities than the previously-recognized cavitating surge and rotating cavitation. This paper explores one such instability which is uncovered by considering the effect of a downstream asymmetry such as a volute on a rotating disturbance similar to (but not identical to) that which occurs in rotating cavitation. The analysis uncovers a new instability which may be of particular concern because it occurs at cavitation numbers well above those at which conventional surge and rotating cavitation occur. This means that it will not necessarily be avoided by the conventional strategy of maintaining a cavitation number well above the performance degradation level. The analysis considers a general surge component at an arbitrary frequency, ω, present in a pump rotating at frequency, Ω, and shows that the existence of a discharge asymmetry gives rise not only to beat components at frequencies, Ω − ω and Ω + ω (as well as higher harmonics) but also to circumferentially-varying components at all these frequencies. In addition, these interactions between the frequencies and the basic and complementary modes lead to “coupling impedances” that effect the dynamics of each of the basic frequencies. We evaluate these coupling impedances and show not only that they can be negative (and thus promote instability) but also are most negative for surge frequencies just a little below Ω. This implies potential for an instability involving the coupling of a basic mode with a frequency around 0.9Ω and a low frequency complementary mode about 0.1Ω. We also examine how such an instability would be manifest in unsteady pressure measurements at the inlet to and discharge from a cavitating pump and establish a “footprint” for the recognition of such an instability.

ID: CaltechAUTHORS:BREcav06b

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Abstract: The behavior of liquid-solid flows varies greatly depending on fluid viscosity, particle and liquid inertia, and collisions between particles. While particle collisions in inviscid fluids can be understood statistically, liquid-solid flows are complicated by the fluid viscosity and forces acting on the particles (e.g. lift, drag, added mass). These flows were first studied by Bagnold, whose investigation found two different flow regimes: a macro-viscous regime where the shear and pressure forces are proportional to the shear rate, and a grain-inertia regime defined by a dependance on the square of the shear rate [1, 2]. The scaling relations he developed have been used to model and understand natural phenomena since.

ID: CaltechAUTHORS:20111215-143708669

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Abstract: Set of DVD videos of seven special lectures given at the NASA George Marshall Space Flight Center, Huntsville, Alabama, on Dec. 13 and 14, 2005.

ID: CaltechAUTHORS:20120424-105426839

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Abstract: There are an increasing number of biological and bioengineering contexts in which cavitation is either utilized to create some desired effect or occurs as a byproduct of some other process. In this review an attempt will be made to describe a cross-section of these cavitation phenomena. In the byproduct category we describe some of the cavitation generated by head injuries and in artificial heart valves. In the utilization category we review the cavitation produced during lithotripsy and phacoemulsification. As an additional example we describe the nucleation suppression phenomena encountered in supersaturated oxygen solution injection. Virtually all of these cavitation and nucleation phenomena are critically dependent on the existence of nucleation sites. In most conventional engineering contexts, the prediction and control of nucleation sites is very uncertain even when dealing with a simple liquid like water. In complex biological fluids, there is a much greater dearth of information. Moreover, all these biological contexts seem to involve transient, unsteady cavitation. Consequently they involve the difficult issue of the statistical coincidence of nucleation sites and transient low pressures. The unsteady, transient nature of the phenomena means that one must be aware of the role of system dynamics in vivo and in vitro. For example, the artificial heart valve problem clearly demonstrates the importance of structural flexibility in determining cavitation occurrence and cavitation damage. Other system issues are very important in the design of in vitro systems for the study of cavitation consequences. Another common feature of these phenomena is that often the cavitation occurs in the form of a cloud of bubbles and thus involves bubble interactions and bubble cloud phenomena. In this review we summarize these issues and some of the other characteristics of biological cavitation phenomena.

ID: CaltechAUTHORS:BREcav03

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Abstract: The Rayleigh-Plesset equation and its extensions have been used extensively to model spherical bubble dynamics, yet radial diffusion equations must be solved to correctly capture damping effects due to mass and thermal diffusion. The latter are too computationally intensive to implement into a continuum model for bubbly cavitating flows, since the diffusion equations must be solved at each position in the flow. The goal of the present research is to derive a reduced-order model that accounts for thermal and mass diffusion. Motivated by results of applying the Proper Orthogonal Decomposition to data from full radial computations, we derive a model based upon estimates of the average heat transfer coefficients. The model captures the damping effects of the diffusion processes in two ordinary differential equations, and gives better results than previous models.

ID: CaltechAUTHORS:PREcav03

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Abstract: We present here an example of an important biomedical technique in which it is critical to understand and avoid bubble nucleation in supersaturated aqueous solutions of oxygen. By doing so it is possible to inject highly supersaturated oxygen solutions through a small capillary without the formation of significant gas bubbles. The potential medical benefits of a successful technique of this kind are substantial and multi-faceted. Deprivation of oxygen even for brief periods of time such as occur during heart attacks or strokes results in cell damage or death - and is a primary cause of permanent physiological damage. Consequently rapid therapeutic oxygen delivery systems could substantially enhance the treatment, for example, of acute myocardial infarction or acute cerebral stroke.

ID: CaltechAUTHORS:20111213-103456946

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Abstract: The Rayleigh-Plesset equation has been used extensively to model spherical bubble dynamics, yet it has been shown that it cannot correctly capture damping effects due to mass and thermal diffusion. Radial diffusion equations may be solved for a single bubble, but these are too computationally expensive to implement into a continuum model for bubbly cavitating flows since the diffusion equations must be solved at each position in the flow. The goal of the present research is to derive reduced-order models that account for thermal and mass diffusion. We present a model that can capture the damping effects of the diffusion processes in two ODE's, and gives better results than previous models.

ID: CaltechAUTHORS:PREfedsm02

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Abstract: Suppression of cavitation is a relatively common goal of fluid engineers and therefore examples of bubble nucleation suppression in other technological contexts are useful in suggesting ways in which such suppression might be achieved. In this paper we describe a remarkable example of bubble nucleation suppression achieved by a combination of the elimination of nucleation sites and the reduction of bubble growth time. The context is the invention of a device that allows the injection of aqueous solutions highly supersaturated with oxygen into the bloodstream without the formation of significant gaseous oxygen bubbles.

ID: CaltechAUTHORS:CREjsmemec02

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Abstract: High-speed observations (for example, Lauterborn and Bolle 1975, Tomita and Shima 1990, Frost and Sturtevant 1986) clearly show that though a collapsing cavitation bubble approaches its minimum size as a coherent single volume, it usually reappears in the first rebounding frame as a cloud of much smaller bubbles or as a highly distorted single volume (see, for example, figure 2). This paper explores two mechanisms that may be responsible for that bubble fission process, one invoking a Rayleigh-Taylor stability analysis and the other utilizing the so-called microjet mechanism. Both approaches are shown to lead to qualitatively similar values for the number of fission fragments and the paper investigates the flow parameters that effect that number. Finally, we explore the effective damping of the Rayleigh-Plesset single bubble calculation which that fission process implies and show that it is consistent with the number of collapses and rebounds which are observed to occur in experiments.

ID: CaltechAUTHORS:CEBcav01

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Abstract: The Rayleigh-Plesset equation is used extensively to model spherical bubble dynamics, yet is has been shown that it cannot correctly capture damping effects due to mass and thermal diffusion. Full single bubble models have been successfully used to study these diffusion effects, but these are to computationally expensive to implement into the continuum model for bubbly cavitating flow since the diffusion equations must be solved in the radial direction at each position in the flow. The focus of the present research is the development of simpler and more efficient bubbly dynamic models that capture the important aspects of the diffusion processes. We present some preliminary results from a full bubbly model that has been developed to provide insight into possible simplifications. This in turn can be used to develop and validate simpler models. The full model is contrasted to the Rayleigh-Plesset equations, and a suggestion for possible improvement to the Rayleigh-Plesset equation is made.

ID: CaltechAUTHORS:PREcav01

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Abstract: This study details experiments investigating a previously unrecognized surge instability on a cavitating propeller in a water tunnel. The surge instability is furst explored through visual observation of the cavitation on the propeller blades and in the tip vortices. Similarities between the instability and previously documented cavitation phenomena are noted. Measurements of the radiated pressure are then obtained, and the acoustic signature of the instability is identified. The magnitudes of the fluctuating pressures are very large, presumably capable of producing sever hull vibration on a ship. The origins of the instability are explored through separate investigation of the cavitation dynamics and the response of the water tunnel to volumetric displacement in the working section. Experiments are conducted to quantify the dynamics of the propeller vacitation. Finally, a model is developed for the complete system, incorporating both the cavitation and facility dynamics. The model predicts active system dynamics (linked to the mass flow gain factor familiar in the context of pump dynamics) and therefore potentially unstable behavior for two distinct frequency ranges, one of which appears to be responsible for the instability.

ID: CaltechAUTHORS:DUTcav01

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Abstract: This paper presents the results of an investigation assessing the role experimental facility dynamics might play in determining the nature of a recently observed instability on a cavitating propeller (Duttweiler and Brennen 1999). To address this question, a theoretical model of the facility dynamics is developed. Experiments were conducted to measure the response of the water tunnel facility to volumetric excitations of varying amplitude and frequency, and the measurements are compared with the response predicted by the model. The dynamics of the propeller cavitation are characterized by estimating two parameters (cavitation compliance and mass flow gain factor) previously employed in developing a system transfer function for cavitating pumps (Brennen 1994). Finally, the characteristics of a model for the complete system, incorporating both the cavitating propeller and the experimental facility dynamics, are discussed.

ID: CaltechAUTHORS:DUTasmefedsm00

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Abstract: The effects of unsteady bubble dynamics on cavitating flow through a converging-diverging nozzle are investigated numerically. A continuum model that couples the Rayleigh-Plesset equation with the continuity and momentum equations is used to formulate unsteady, quasi-one-dimensional partial differential equations. These equations are solved numerically using a Lagrangian finite volume method. Special formulations are used at the boundary cells to allow Eulerian boundary conditions to be specified. Flow regimes studied include those where steady state solutions exist, and those where steady state solutions diverge at the so-called flashing instability. These latter flows consist of unsteady bubbly shock waves travelling downstream in the diverging section of the nozzle. The computations show reasonable agreement with an experiment that measures the spatial variation of pressure, velocity and void fraction for steady shockfree flows, and good agreement with an experiment that measures the shock position and throat pressure for flows with bubbly shocks.

ID: CaltechAUTHORS:PREfedsm00

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Abstract: This paper presents results from experiments investigating an instability observed on a cavitating propeller. Preliminary visual observations were made of the attached cavities on the blades of the propeller, and particular note was made of similarities between the behavior of the re-entrant jets and that found recently by Laberteaux and Ceccio (1998). It was also noted that the nature of the instability is closely related to the partial cavity instability observed on single, two-dimensional foils. (Knapp, 1955; Wade and Acosta, 1966; Brennen, 1994, 95). The flow conditions (cavitation number and advance ratio) under which the instability occurs were mapped and it is shown that the onset corresponds to a specific configuration of attached cavity lengths on the propeller. Pressure measurements were obtained from two different locations within the experimental facility, and the acoustic signature of the instability is identified. A simple model based on cavity volume estimates obtained from high speed video footage is developed, and the predictions of the model are compared with the experimentally obtained pressures.

ID: CaltechAUTHORS:DUTfedsm99

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Abstract: This paper focuses on the different forms that individual cavitating events may take when the cavitation number is below the inception value (but not so low as to produce only attached cavities) and individual nuclei trigger individual cavitation events. It is a sequel to those of Kuhn de Chizelle et al. (1992a, 1992b, 1995) which described a set of cavitation scaling observations on simple Schiebe headforms conducted in the US Navy Large Cavitation Channel (LCC). The most common events observed in those experiments were traveling, hemi-spherical shaped bubbles which grew and collapsed as they were convected through the low pressure region on the headform. Several interesting variations were also observed, including the development of bubble tails and the triggering of patches, or local regions of attached cavitation. In the present paper, the frequency of occurrence of the various types of events is analyzed as well as how those probabilities changed with cavitation number, velocity and headform size. In general, the probabilities of tails and patches increased with decreasing cavitation number, but they also increased with increasing headform size and increasing velocity. A specific parametric dependence on these variables is suggested.

ID: CaltechAUTHORS:WANfedsm99

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Abstract: This paper reviews some of the literature on partial cavity instabilities on single hydrofoils and then summarizes the striking differences in the appearance and behavior of partial cavities on swept foils (as opposed to two-dimensional, unswept foils) as rcently highlighted by de Lange et al. (1994) and Laberteaux and Ceccio (1998). These demonstrate the importance of the spanwise evolution of the re-entrant jet, and the consequences for the characteristics of the cavity closure flow. It is suggested in this paper that several variants of this evolution can be seen in the photographs of cavitation on single hydrofoils foils and on propellers. What is common to many of these variants is that, the spanwise evolution of the cavity and the re-entrant jet can give rise to conditions at some particular spanwise location(s) which initiate partial cavity instability. In this paper we present information on an instability that was observed to occur on a cavitating propeller of modern US Navy design. Detailed photographic examinations show that the instability oscillations involve spanwise development of a re-entrant jet and behavior similar to that of the partial cavity oscillations previously observed on two-dimensional foils.

ID: CaltechAUTHORS:DUT18b

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Abstract: Impedance based techniques have been used to quantify air entrainment by a stationary breaking wave at the bow of a ship. The present paper describes an impedance based void fraction meter which was developed to make measurements in this high speed, unsteady, multiphase flow, and details of its calibration are provided. In addition, air entrainment data from an experimental simulation of a bow wave are presented. The local, time averaged void fraction was mapped for flow cross sections beneath the plunging wave jet, revealing the location of the clouds of bubbles formed by that jet impacting the incoming water surface. Size distribution functions for the bubbles within the bubble clouds are also presented. The results are correlated with the wave structure described in Waniewski et al. (1997).

ID: CaltechAUTHORS:WANfedsm98

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Abstract: A recent significant advance in our understanding of cavitating flows is the importance of the interactions between bubbles in determining the coherent motions, dynamic and acoustic, of the bubbles in a cavitating flow. This lecture will review recent experimental and computational findings which confirm that, under certain conditions, the collapse of clouds of cavitating bubbles involves the formation of bubbly shock waves and that the focussing of these shock waves is responsible for enhanced noise and potential damage in cloud cavitation. The recent experiments of Reisman et al. (1998) complements the work begun by Mørch and Kedrinskii and their co-workers and demonstrates that the very large impulsive pressures generated in bubbly cloud cavitation are caused by shock waves generated by the collapse mechanics of the bubbly cavitatting mixture. Two particular types of shocks were observed: large ubiquitous global pressure pulses caused by the separation and collapse of indiviual clouds from the downstream end of the cavitation and much more localized local pressure pulses which occur much more randomly within the bubbly cloud. One of the first efforts to model cloud cavitation was due to vanWijngaarden (1964) who linked basic continuity and momentum equations for the mixture with a Rayleigh-Plesset equation for the bubble size in order to study the behavior of a bubbly fluid layer next to a solid wall. In the 1980s there followed a series of papers on the linearized dynamics of clouds of bubbles (for example, d’Agostino et al. 1983, 1988, 1989). But highly non-linear processes such as the formation of shock waves require computational efforts which are capable of resolving these phenomena in both time and space. A valuable first effort to do this was put forward by Kubota et al. (1992) but by limiting the collapse of individual bubbles they prevented the formation of the large pressure pulses associated with bubble collapse. Wang et al. (1994, 1995) and Reisman et al. (1998) present accurate calculations of a simple spherical cloud subject to a low pressure episode and show that, for a large enough initial void fraction, the collapse occurs as a result of the formation of a shock wave on the surface of the cloud and the strengthening of this shock by geometric focussing as the shock propagates inward. This review will discuss other efforts to investigate these phenomena computationally. Wang and Brennen (1997, 1998) have extended the one-dimensional methodology used for the spherical cloud to investigate the steady flow of a bubbly, cavitating mixture through a onvergent/divergent nozzle. Under certain parametric conditions, the results are seen to model the dynamics of flashing within the nozzle. Moreover, it is clear from these steady flow studies that there are certain conditions in which no steady state solution exists and it is speculated that the flow under those conditions may be inherently unstable. Of course, it has frequently been experimentally observed that cavitating nozzle flows can become unstable and oscillate violently. Finally, we will also describe recent efforts (Colonius et al. 1998) to extend the code to two and three space dimensions. A simple example of such a calculation is the collision of a plane pressure pulse with a cylindrical or spherical cloud of bubbles. When the pressure pulse is negative, the growth and subsequent collapse of the cloud is particularly interesting and is seen to involve the formation and propagation of a shock waves within the cloud. Moreover, the non-linear scatterring of the pressure waves into the far field provides valuable information. The long term objective is to develop computational techniques and experience which would allow practical calculation of much more complex bubbly flows such as occur on hydrofoils, on propellers and in pumps where there is a real need for CFD methodologies which allow calculation of the noise and damage potential of these flows.

ID: CaltechAUTHORS:CEBicmf98

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Abstract: This paper presents a numerical investigation of some of the phenomena involved in the nonlinear dynamics of a homogeneous bubbly mixture bounded by an oscillatory wall. This problem represents an idealization of the flow in a typical vibratory cavitation damage device. Results are presented showing that wave steepening and ultimately shock wave formation occur as the magnitude of the excitation increases. The propagation characteristics of the waves through the bubbly medium have also been studied. Strong pressure peaks of short duration, corresponding to the coherent collapse of the bubble clusters, are computed and accurately resolved, both in space and time. As the amplitude of the excitation is increased a series of period doubling bifurcations occurs. The nonlinear dynamics of the oscillating bubble cluster are observed to follow a subharmonic route to chaos.

ID: CaltechAUTHORS:CEBcav98

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Abstract: Fluid-induced rotordynamic forces produced by the fluid in an annular seal or in the leakage passage surrounding the shroud of a pump or turbine, are known to contribute substantially to the potential excitation forces acting on the rotor. In this paper we explore some of the important features of the equations governing bulk-flow models of these flows. This in turn suggests methods which might be used to solve these bulk-flow equations in circumstances where the linearized solutions (such as those of Childs 1987, 1989) will no longer be accurate. An example of a numerical solution is then presented.

ID: CaltechAUTHORS:CEBisromac98

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Abstract: The rotordynamic forces generated by the fluid flow through the impeller leakage path of a centrifugal pump are now well established. The present paper examines the effects of modifying the leakage path geometry by changing the front shroud, from a conical shape to a more typical curved design, and the effects of low pressure seal design on these forces. It is found that only the cross-coupled stiffness is affected by the change of path geometry. Changing the low pressure seal from an axial to a radial clearance does, however, significantly affect the rotordynamic forces. A bulk flow numerical model is found to predict the same general result for the low pressure seal tests. The model agrees with the general trends with increasing leakage flow coefficient exhibited by the data, but appears to underpredict the magnitude of the normal force.

ID: CaltechAUTHORS:UYRicfe97

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Abstract: One of the most destructive (and noisy) forms of cavitation is that referred to as "cloud cavitation" because it involves a large collection of bubbles which behave as a coherent whole. The present paper presents the results of an experimental study of the processes of collapse of a cavitation bubble cloud, specifically that generated by an oscillating hydrofoil in a water tunnel. Measurements of the far-field noise show that this is comprised of substantial pulses radiated from the cloud at the moment of collapse. Also, transducers within the cavitation zone encounter very large pressure pulses (or shock waves) with amplitudes of the order of tens of atmospheres and typical durations of the order of tenths of a millisecond. These shock waves appear to be responsible for the enhanced noise and damage potential which results from that phenomenon.

ID: CaltechAUTHORS:REIissw97

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Abstract: Unsteady forces generated by the fluid flow through the impeller leakage path of a centrifugal pump were investigated. The effect of leakage path inlet (pump discharge) swirl on the rotordynamic forces was examined for various ratios of fluid tangential velocity to impeller tip speed. It was observed that changing the inlet swirl velocity does not appear to significantly affect the measured forces for a given leakage flow coefficient. A bulk flow numerical model was found to predict the same general result. The model agreed with experimental data for small values of the leakage flow coefficient.

ID: CaltechAUTHORS:UYRfedsm97

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Abstract: In flows around ships, the bow wave can entrain a significant amount of air as it breaks continuously on the free surface. The resulting air bubbles persist in the ship wake affecting its radar cross section as well as acting as cavitation nuclei in the flow entering the ship's propeller. In the present investigation, the formation of a bow wave on a ship was simulated in the laboratory using a deflecting plate in a supercritical free surface flow. The experiments were conducted at two scales. The present paper focuses on how the bow wave changes with the angles and flow parameters, information which is a necessary prerequisite for understanding the air entrainment process. Flow visualization studies were performed and an electronic point gage was used to study the three-dimensional shape of the bow waves and the manner in which they break.

ID: CaltechAUTHORS:WANfedsm97

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Abstract: This paper describes experiments investigating the effects of air injection on the cloud cavitation of an oscillating hydrofoil. The effects of continuous air injection were investigated using two different hydrofoils. Measurements of the acoustic pressure were made on the downstream test section floor and on the surface of one of hydrofoils, and the extent of noise reduction provided by air injection at various volume flow rates was determined. The acoustic surface pressure measurements were also correlated with visual observations made using high speed motion pictures of the cloud cavitation. Thus the effects of continuous air injection on specific cavitation structures could be identified. In addition, the effectiveness of pulsed air injection in achieving greater reductions in cavitation noise at volume flow rates equal to those used in continuous air injection experiments was investigated.

ID: CaltechAUTHORS:REIfedsm97

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Abstract: A non-barotropic continuum bubbly mixture model is used to study the one-dimensional cavitating flow through a converging-diverging nozzle. The nonlinear dynamics of the cavitation bubbles are modeled by the Rayleigh-Plesset equation. Analytical results show that the bubble/bubble interaction through the hydrodynamics of the surrounding liquid has important effects on this confined flow field. One clear interaction effect is the Bernoulli effect caused by the growing and collapsing bubbles in the nozzle. It is found that the characterisitics of the flow change dramatically even when the upstream void fraction is very small. Two different flow regimes are found from the steady state solutions and are termed: quasi-steady and quasi-unsteady. The former is characterized by large spatial fluctuations downstream of the throat which are induced by the pulsations of the cavitation bubbles. The quasi-unsteady solutions correspond to flashing flow. Bifurcation occurs as the flow transitions from one regime to the other. An analytical expression for the critical bubble size at the bifurcation is obtained. Physical reasons for this quasi-static instability are also discussed.

ID: CaltechAUTHORS:WNGfedsm97

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Abstract: Several recent experimental and analytical investigations of cavitating flows have revealed new phenomena which clearly affect how we should view cavitation growth and collapse and the strategies used to ameliorate its adverse effects. On the scale of individual bubbles it is now clear that the dynamics and acoustics of single bubbles are severely affected by the distortion of the bubble by the flow. This distortion depends on the typical dimension and velocity of the flow (as well as the Reynolds number) and therefore the distortion effects are very important in the process of scaling results up from the model to the prototype. The first part of the lecture will discuss the implications of these new observations for the classic problem of scale-up. Another recent revelation is the importance of the interactions between bubbles in determining the coherent motions, dynamic and acoustic, of a cloud of cavitation bubbles. The second part of the lecture focusses on these cloud cavitation effects. It is shown that the collapse of a cloud of cavitating bubbles involves the formation of a bubbly shock wave and it is suggested that the focussing of these shock waves is responsible for the enhanced noise and damage in cloud cavitation. The paper describes experiments and calculations conducted to investigate these phenomena in greater detail as part of an attempt to find ways of ameliorating the most destructive effects associated with cloud cavitation.

ID: CaltechAUTHORS:CEBksme97b

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Abstract: When a deep bed of granular material is subjected to vertical, sinusoidal oscillations, a number of interesting phenomena appear including heaps, convection cells, surface waves, and arches. This paper examines the convection cell phenomena associated with vertical side walls using two-dimensional discrete element simulations. Measurements from the simulations indicate that when the container aspect ratio, defined as the depth of the granular bed, H, divided by the width of the container, W, is large, convection cells interact and the boundary layer width of the downward flow of particles against the walls varies linearly with the container width. However, when the container aspect ratio [is] small and the convection cells do not interact, the boundary layer width remains at a nearly constant value of ten particle diameters. Other simulation measurements show that the vertical location of the convection cell center remains close to the free surface regardless of container aspect ratio. Additional measurements show that the particle flow rate per oscillation cycle in the boundary layer increases with increasing vibration amplitude and velocity. Lastly, the asymmetric drag mechanism proposed as the cause of the side wall convection cells is briefly examined.

ID: CaltechAUTHORS:WASwcce96

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Abstract: This paper investigates the effects of bubble dynamics on the stability of bubbly and cavitating jets of low void fraction. The equations of motion for the bubbly mixture are linearized for small perturbations and the parallel flow assumption is used to obtain a modified Rayleigh equation governing the inviscid stability of a two-dimensional jet. Inertial effects associated with the bubble response and energy dissipation due to the viscosity of the liquid, the heat transfer between the two phases, and the liquid compressibility are included. Numerical solutions of the eigenvalue problem for the modified Rayleigh equation of a Bickley jet are obtained by a multiple shooting method. Depending on the jet velocity, the void fraction, and the ambient pressure, the presence of air bubbles can induce significant departures from the classical results for a single phase fluid.

ID: CaltechAUTHORS:20130725-165119685

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Abstract: This paper reports on experiments conducted in the Rotor Force Test Facility at the California Institute of Technology to examine the effects of a tip leakage restriction and swirl brakes on the rotordynamic forces due to leakage flows on an impeller undergoing a prescribed circular whirl. The experiments simulate the leakage flow conditions and geometry of the Alternate Turbopump Design (ATD) of the Space Shuttle High Pressure Oxygen Turbopump and are critical to evaluating the pump's instability problems. Results indicate the detrimental effects of a discharge orifice and the beneficial effects of adding swirl brakes. Plots of the tangential and normal forces versus whirl frequency ratio show a substantial increase in these forces along with destabilizing resonances when a discharge orifice is added. When swirl brakes are added, some of the detrimental effects of the orifice are reduced. For the tangential force, a significant reduction occurs and a destabilizing resonance appears to be eliminated. For the normal force, although the overall force is not reduced, once again a destabilizing resonance appears to be eliminated.

ID: CaltechAUTHORS:SIVtpdrm94

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Abstract: This paper presents a summary of some recent observations of the interaction between individual traveling cavitation bubbles and the nearly solid surface. These reveal a complex micro-fluid-mechanics associated with the interaction of the bubble with the boundary layer, the surface and the large pressure gradients exterior to the boundary layer which normally occur in the vicinity of a minimum pressure point. The details are important because they affect how the bubble collapses and therefore influence the noise produced and the damage potential of the cavitating flow. We present data showing how the noise from an individual event can be affected by these interaction effects. Since the scaling of cavitation phenomena is always an important issue we also describe the results of some experiments carried out to investigate the scaling of these interaction effects.

ID: CaltechAUTHORS:CEBiwmf93

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Abstract: Nonlinear interactive effects in a bubbly cloud have been studied by investigating the frequency response of a bubble layer bounded by a wall oscillating normal to itself. First, a Fourier analysis of the Rayleigh-Plesset equation is used to obtain an approximate solution for the nonlinear response of a single bubble in an infinite fluid. This is used to solve for nonlinear effects in a semi-infinite layer containing bubbles with a distribution size. A phenomena termed harmonic cascading is seen to take place due to presence of distribution of bubble sizes. This phenomena consists of a large response at twice the excitation frequency when the mixture contains bubbles with a natural frequency equal to twice the excitation frequency. The ratio of the amplitude of the second harmonic response to the amplitude of the first harmonic response is observed to increase when the number of small bubbles is increased relative to the number of large bubbles. The response is also seen to be weakened by an increase in the total number of bubbles per unit liquid volume at constant void fraction.

ID: CaltechAUTHORS:KUMpc92

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Abstract: Several experiments by Ceccio and Brennen (1991, 1989) and Kumar and Brennen (1992, 1991) have closely examined the interaction between individual cavitation bubbles and the boundary layer, as well as statistical properties of the acoustical signals produced by the bubble collapse. All of these experiments were, however, conducted in the same facility with the same headform size (5.08cm in diameter) and over a fairly narrow range of flow velocities (around 9m/s). Clearly this raises the issue of how the phenomena identified change with speed, scale and facility. The present paper describes experiments conducted in order to try to answer some of these important questions regarding the scaling of the cavitation phenomena. The experiments were conducted in the Large Cavitation Channel of the David Taylor Research Center in Memphis Tennessee, on geometrically similar Schiebe headforms which are 5.08, 25.4 and 50.8cm in diameter for speeds ranging up to 15m/s and for a range of cavitation numbers.

ID: CaltechAUTHORS:KUHattc92

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Abstract: This paper reviews the current state of knowledge of rotordynamic forces caused by the discharge-to-suction leakage flows in centrifugal pumps. The indications that these flows could contribute significantly to the rotordynamics motivated the fabrication of an experiment in which measurements of rotordynamic forces would be made on simulated leakage flows in which the flow rate, clearance, eccentricity and other parameters would be exercised in order to understand the phenomena. Sample data is presented and demonstrates substantial rotordynamic effects which could be potentially destabilizing. The rotordynamic forces appear to be inversely propertional to the clearance and change significant with the flow rate.

ID: CaltechAUTHORS:GUIaetopt90

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Abstract: The speed and attenuation of small amplitude kinematic waves were measured in vertical bubbly and particulate flows in a continuous medium of water. This was done by evaluating the time delay and phase lag of coherent random fluctuations in the volume fraction signal at two measuring locations. The volume fraction was monitored using two closely spaced Impedance Volume Fraction Meters (Kytomaa (1986)). Using the broad-band volume fraction perturbations yields the dependence of the kinematic speed and attenuation of wave number from a single experiment for one set of conditions. The kinematic waves were found to be non-dispersive. Bubbly flows are observed to undergo a change in flow regime at an approximate volume fraction of 45%. Prior to onset of churn-turbulence, a sharp drop in kenematic wave attenuation is observed above volume fractions of 40%. When further increase in volume fraction is attempted, the homogeneous dispersion suddenly becomes unstable. The particulate flows remain uniformly dispersed for all volume fractions, but above a value of ~55%, the mixture flows like a solid plug. The volume fraction fluctuations become incresingly persistent as the volume fraction approaches the solidification value, but no instability is observed. It is argued that the inability of air-water flows to withstand bubble-bubble forces without break-up may account for the differences between the bubbly and particulate flow results.

ID: CaltechAUTHORS:KYTnhtc87

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Abstract: Forces are exerted on a centrifugal pump impeller, due to the asymmetry of the flow caused by the volute of diffuser, and to the motion of the center of the impeller whenever the shaft whirls. Recent work in the measurement of these forces as a function of the whirl speed to shaft speed ratio, and the influence of the volute, is reviewed. These forces may be decomposed into a steady force, a static stiffness matrix, a damping matrix and an inertia matrix. It is shown that for centrifugal pumps of the moderate specific speed typical of boiler feed stages, there is a region of potential shaft vibration excitation from the hydrodynamic forces if the operating speed is well above the first flexural critical speed.

ID: CaltechAUTHORS:JERsips85

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Abstract: Increasing use is being made of transmission matrices to characterize unsteady flows in hydraulic system components and to analyze the stability of such systems. This paper presents some general characteristics which should be examined in any experimentally measured transmission matrices and a methodology for the analysis of the stability of transmission matrices in hydraulic systems of order 2. These characteristics are then examined for cavitating pumps and the predicted instabilities (known as auto-oscillation) compared with experimental observations in a particular experimental system.

ID: CaltechAUTHORS:BREiahrs80

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