THE COUNTER-JET FORMATION IN AN AIR BUBBLE INDUCED BY THE IMPACT OF SHOCK WAVES

The interaction of an air bubble (isolated in water or attached to a boundary) with shock waves induced by electric sparks is investigated by high-speed photography.The interaction is closely related to the counter-jet induced by the impact of shock waves.The formation of a counter-jet in an air bubble is related to the liquid jet formed in the same air bubble,but the mechanism is different with that of the counter-jet formation in a collapsing cavitation bubble.The formation of a counter-jet in an air bubb...     

INTERACTION OF A CAVITATION BUBBLE AND AN AIR BUBBLE WITH A RIGID BOUNDARY

The motion of a spark-induced cavitation bubble and an air bubble near a rigid boundary is experimentally studied by using high-speed photography. Several dimensionless parameters are used to describe the geometrical configuration of the bubble-bubble-boundary interaction. The bubble-bubble interaction can be considered in two different conditions. The cavitation bubble will collapse towards the air bubble if the air bubble is relatively small, and away from the air bubble if the air bubble is relatively large. The two zones are identified in the bubble-boundary interaction, and they are the danger zone and the safety zone. The relative position, the bubble-boundary distance and the bubble-bubble distance play important roles in the bubble-bubble-boundary interaction, which can be considered in several conditions according to the responses of the bubbles. Air jets are found to penetrate into the cavitation bubbles. The cavitation bubble and the air bubble (air jet) move in their own way without mixing. The motion of a cavitation bubble may be influenced by an air bubble and/or a rigid boundary. The influence of the air bubble and the influence of the boundary may be combined, like some thing of a vector.     

THE BUBMAC METHOD-A NUMERICAL METHOD BUBBLE DYNAMICS IN VORTEX CORE

This paper presents a numerical method called BUBMAC method,whichis based on the Marker And Cell(MAC) technique,to numerically simulate the problemof bubble dynamics in vortex core. With the incorporation of the azimuthal velocity intothe momentum equation, the model takes into account the complete interaction betweenthe bubble dynamics and the vortex flow field.The three momentum equations (N-Seqs.) are solved numerically by the finite-difference methed, and the motion of bubblesurface is described by tracing massless marker particles distributed only on the surface ofthe bubble. With some important medifications to the original MAC method, the numeri-cal accuracy of the method is conisderably increased. The validation of the proposed BUB-MAC method is checked by comparing the numerical results with some available analyticalsolutions in the cases of spherical bubble evolution and with numerical results in the casesof nonspherical collapse of bubble and bubble evolution in tip-vortex core. The compar-iso     

VIBRATION OF A SINGLE PROTEIN BUBBLE IN BINGHAM LIQUID

The vibration of a single protein bubble may take place under the action of high pressure difference. In this process, the bubble wall may experience a finite deformation. The equation describing the dynamics of the protein bubble with viscoelastic film in Bingham liquid is derived. A numerical solution to this equation is carried out to study the effect of liquid pressure, the characteristic parameters of Bingham liquid and the viscosity of the protein film on the finite deformation of the bubble. The results show that the vibration of the protein bubble wall is caused by the action of pressure difference, the elastic stress in finite deformation and the dissipation of viscosity of the protein film and Bingham liquid. The vibration is nonlinear. Decreasing the pressure difference between gas and Bingham liquid on both sides of the protein bubble will lead to a change of vibration performance. The frequency and amplitude are reduced, tegether with the speed of vibration damping. In addition, the deformation rate of the bubble is smaller when the amplitude of vibration is reduced, which means shorter time to reach a balance state. On the other side, the increase of the magnitude of viscosity of the protein film or the plastic viscosity of Bingham liquid can restrain the vibration of the protein bubble wall in the course of finite deformation, as a result, the load bearing capacity of bubble is enhanced.     

VISCOSITY EFFECTS ON THE BEHAVIOR OF A RISING BUBBLE

In the incompressible fluid flow regime,without taking consideration of surface tension effects,the viscosity effects on the behavior of an initially spherical buoyancy-driven bubble rising in an infinite and initially stationary liquid are investigated numerically by the Volume Of Fluid(VOF) method.The ratio of the gas density to the liquid density is taken as 0.001,and the gas viscosity to the liquid viscosity is 0.01,which is close to the case of an air bubble rising in water.It is found by numerical exp...     

EFFECT OF A PROPELLER AND GAS DIFFUSION ON BUBBLE NUCLEI DISTRIBUTION IN A LIQUID

A multi-bubble dynamics code accounting for gas diffusion in the liquid and through the bubble wall was developed and used to study the modification of a bubble nuclei population dynamics by a propeller.The propeller flow field was obtained using a Reynolds-Averaged Navier-Stokes(RANS) solver and bubble nuclei populations were propagated in this field.The numerical pro-cedure enabled establishment of the possibility of production behind the propeller of relatively large visible bubbles starting from typical ocean nuclei size distributions.The resulting larger bubbles are seen to cluster in the blade wakes and tip vortices.Parametric investigations of the initial nuclei size distribution,the dissolved gas concentration,and the cavitation number were conducted to ide-ntify their effects on bubble entrainment and the resultant void fractions and bubble distribution modifications downstream from the propeller.Imposed synthetic turbulence-like fluctuations unto the average RANS flow field were also used to study the effect avera-ging in the RANS procedure has on the results.     

INFLUENCE OF INCLINATION ANGLE ON THE MOTION OF TAYLOR BUBBLES IN UPWARD SLUG FLOW

1 .　ＩＮＴＲＯＤＵＣＴＩＯＮＳｌｕｇｆｌｏｗｉｓｏｎｅｏｆｔｈｅｍｏｓｔｃｏｍｐｌｉｃａｔｅｄｆｌｏｗｐａｔｔｅｒｎｓａｎｄｏｃｃｕｒｓｏｖｅｒａｗｉｄｅｒａｎｇｅｏｆｇａｓａｎｄｌｉｑｕｉｄｆｌｏｗｒａｔｅｓｉｎｓｍａｌｌａｎｄｍｅｄｉｕｍｓｉｚｅｔｕｂｅｓ .Ｉｔｉｓｃｈａｒａｃｔｅｒｉｚｅｄｂｙｌａｒｇｅｂｕｌｌｅｔ ｓｈａｐｅｂｕｂｂｌｅｓ (ｒｅ ｆｅｒｒｅｄｔｏａｓｔｈｅＴａｙｌｏｒｂｕｂｂｌｅｓ) ,ａｌｍｏｓｔｆｉｌｌｉｎｇｔｈｅｔｕｂｅ ,ｗｈｉｃｈａｒｅｓｅｐａｒａｔｅｄｂｙｌｉｑｕｉ…     

DIGITAL IMAGE MEASUREMENT OF BUBBLE MOTION IN AERATED WATER FLOWS

Digital image measurement method, as an ex-tension of Particle Image Velocimetry of single-phase flowmeasurement, was investigated for application to air-watertwo-phase flows. The method has strong potential ability inmeasuring bubble geometrical features and moving velocitiesfor complex bubble motion in aerated water flow. Both dilutedand dense bubble rising flows are measured using the digitalimage method. Measured bubble shapes and sizes, and bubblevelocities are affected by threshold selection for binary image.Several algorithms for selecting threshold are compared andmethods for calculating the time-averaged void fraction arediscussed.     

NUMERICAL SIMULATION OF BUBBLE FLOW INTERACTIONS

Bubble flow interaction can be important in many practical engineering applications. For instance, cavitation is a problem of interaction between nuclei and local pressure field variations including turbulent oscillations and large scale pressure variations. Various types of behaviours fundamentally depend on the relative sizes of the nuclei and the length scales of the pressure variations as well as the relative importance of bubble natural periods of oscillation and the characteristic time of the field pressure variations. Similarly, bubbles can significantly affect the performance of lifting devices or propulsors. We present here some fundamental numerical studies of bubble dynamics and deformation, then a practical method using a multi-bubble Surface Averaged Pressure (DF-Multi-SAP©) to simulate cavitation inception and scaling, and connect this with more precise 3-D simulations. This same method is then extended to the study of two-way coupling between a viscous compressible flow and a bubble population in the flow field.     

A LEVEL SET METHOD FOR SIMULATION OF RISING BUBBLE

A level set method, the TVD scheme of second-order upwind procedure coupled with flux-limiter, and SIMPLE algorithm were incorporated to simulate the flow and interracial motion of immiscible two-fluids with large density ratio and viscosity ratios, large topology distortion and surface tension. As a numerical example axi-symmetric rising bubbles were investigated. It is found that the method is numerically stable and has good convergence property and the results are in good agreement with other works.     

SUPERCAVITY MOTION WITH INERTIAL FORCE IN THE VERTICAL PLANE

The curvilinear motion in a vertical plane is one of the most important features of the supercavitating vehicle. It is of great significance to study the controllability and the maneuverability of the supercavitating vehicle. Models are built for the effects of the angle of attack, the gravity and the inertial force in the curvilinear motion in the vertical plane. Numerical simulations are carried out for the supercavity motion based on these models combined with the Logvinovich model. It is shown that the maximum deviation displacement in the outward normal direction of the trajectory with a constant curvature, which occurs in the tail of the supercavity, increases as the cavitation number or the curvature radius of the supercavity trajectory decreases under the condition that other model and flow parameters are kept constant. For a varied curvature, the supercavity shape changes evidently because of the change of the ambient pressure, but with the same trend as in constant curvature. The deviation displacement increases along the supercavity length gradually.     

PROPULSIVE PERFORMANCE AND VORTEX SHEDDING OF A FOIL IN FLAPPING MOTION

1 .　INTRODUCTIONInsectsandfishhaveexperiencedabillions yearprocessofevolutionwithnaturalselectionfortheirsurvivalandhavedevelopedtheirsuperiorandcomplete performanceofflightandswim ming .Usually ,aflappingmotionisabasicmodeoflocomotionforinsects ,birds ,andfish .Thrustandliftaregeneratedwhentheflappingwingsortailsinteractwiththesurroundingfluids .Becauseofthehighlyunsteadynatureofviscousflowaroundaflappingwing ,itisfarfromsatisfactorytounderstandthephysicalmechanismandvortexsheddinginuns…     

NUMERICAL COMPUTATION OF PRESSURE GRADIENT FOR THE FLOW OF POWER-LAW FLUID IN ANNULUS WITH INNER CYLINDER EXECUTING A PLANETARY MOTION

With the governing equations for the flow of the power-law fluid in annulus with the inner cylinder executing a planetary motion being transformed by using the undetermined coefficient method, the formula of pressure gradient for this flow under the given flow rate and the relevant numerical calculation method are given in this article. Through the experiments on the flow of Hydrolyzed Polyacrylamide (HPAM) aqueous solution which can be regarded as a power-law fluid, the calculated results of the pressure gradient values are compared with measured data, the mean relative error between them is smaller than 5%, which verifies the results presented in this article.     

A COUPLING MODEL OF WATER FLOWS AND GAS FLOWS IN EXHAUSTED GAS BUBBLE ON MISSILE LAUNCHED UNDERWATER

The gas and water flows during an underwater missile launch are numerically studied. For the gas flow, the explicit difference scheme of Non-oscillation and Non-free-parameter Dissipation (NND) is utilized to solve the Euler equations for compressible fluids in the body-fitted coordinates. For the water flow, the Hess-Smith method is employed to solve the Laplace equation for the velocity potential of irrotational water flows based on the potential theory and the boundary element method. The hybrid Eulerian-Lagrangian formulation for the free boundary conditions is used to compute the changes of the free surface of the exhausted gas bubble in time stepping. On the free surface of the exhausted gas bubble, the matched conditions of both the normal velocities and pressures are satisfied. From the numerical simulation, it is found that the exhausted gas bubble grows more rapidly in the axial direction than in the radial direction and the bubble will shrink at its "neck" finally. Numerical results of the movement of the shock wave and the distribution of the Mach number and the gas pressure within the bubble were presented, which reveals that at some time, the gas flow in the Laval nozzle is subsonic and the gas pressure in the nozzle is very high. Influences of various initial missile velocities and chamber total pressures and water depths on both the time interval when the gas flow in the nozzle is subsonic and the peak of the gas pressure at the nozzle end were discussed. It was suggested that a reasonable adjustment of the chamber total pressure can improve the performance of the engine during the underwater launch of missiles.     

DIRECT NUMERICAL SIMULATIONS OF TURBULENT CHANNEL FLOWS WITH CONSIDERATION OF THE BUOYANCY EFFECT OF THE BUBBLE PHASE

Turbulent channel flows with consideration of the buoyancy effect of the bubble phase is investigated by means of the Direct Numerical Simulation(DNS).This two-phase system is solved by a two-way coupling Lagrangian-Eulerian approach.The Reynolds number based on the friction velocity and the half-width of the channel is 194,and the gravitational acceleration varies from-0.5 to 0.5,ranging from the upflow to the downflow cases.This study aims to reveal the influence of buoyancy on the turbulence behavior and the bubble motion.Some typical statistical quantities,including the averaged velocities and velocity fluctuations for the fluid and bubble phases,as well as the flow structures of the turbulence fluctuations,are analyzed.     

NUMERICAL STUDY ON THE VORTEX STRUCTURES IN THE WAKE OF AN OSCILLATING CIRCULAR CYLINDER IN A UNIFORM FLOW

ＮＵＭＥＲＩＣＡＬＳＴＵＤＹＯＮＴＨＥＶＯＲＴＥＸＳＴＲＵＣＴＵＲＥＳＩＮＴＨＥＷＡＫＥＯＦＡＮＯＳＣＩＬＬＡＴＩＮＧＣＩＲＣＵＬＡＲＣＹＬＩＮＤＥＲＩＮＡＵＮＩＦＯＲＭＦＬＯＷＬｕＸｉ－ｙｕｎ；ＴｏｎｇＢｉｎｇ－ｇａｎｇ；ＺｈｕａｎｇＬｉ－ｘｉａｎ...     

MOTION OF TRACER PARTICLES IN A CENTRIFUGAL PUMP AND ITS TRACKING CHARACTERISTICS

The Basset-Boussinesq-Oseen (BBO) equation can be used for most flows to trace the motion of a particle, but in a centrifugal pump, among the forces that act on the particles, one should also include those due to the impeller rotation, as additional effects. This paper firstly reviews various approximations of the BBO equation for the motion of dispersion particles in a viscous fluid. Then based on the motion equation for particles in low Reynolds number centrifugal pumps, a formula for calculating the tracking characteristics of tracer particles is deduced through the Fourier integral transformation. After that the deviations of the particle motion from the fluid motion, as predicted by the various approximations, are discussed and compared. At last, with an emphasis on the Particle Image Velocimetry (PIV) results, the tracking characteristics of particles are estimated. Also, advantages and disadvantages of different tracer particles are discussed and suitable tracer particles for application in PIV studies for flow fields in centrifugal pumps are suggested.     

气泡在波浪中运动的实验研究

本文对气泡在波浪中的运动进行了实验研究，得到了气泡在波浪中运动的平均上升速度及上升运动轨迹，并同理论进行了比较。发现当气泡直径小于１ｍｍ时，气泡在波浪中的平均上升速度与静水中的稳定上升速度基本一致，波浪的影响很小，且是随机的。当气泡直径大于１ｍｍ时，气泡在波浪中的平均上升速度不再随直径增大而增大，而是趋向于某一定值。这与气泡在静水中稳定上升速度的变化趋势是一致的。当气泡直径小于１ｍｍ且气泡到液面的     

ANALYSIS OF BASSET FORCE OF PARTICLE MOTION IN TURBULENT FLOWS WITH SPECTRAL METHOD

1 .　ＩＮＴＲＯＤＵＣＴＩＯＮＷｉｔｈｔｈｅｗｉｄｅａｖａｉｌａｂｉｌｉｔｙｏｆｃｏｍｐｕｔａｔｉｏｎａｌｔｅ ｃｈｏｌｏｇｙ ,ｔｈｅｎｕｍｅｒｉｃａｌＬａｇｒａｎｇｉａｎｃｏｍｐｕｔａｔｉｏｎｆｏｒｆｌｏｗｓｉｎｖｏｌｖｉｎｇｄｉｓｐｅｒｓｅｄｓｏｌｉｄｐａｒｔｉｃｌｅｓｏｒａｉｒｂｕｂｂｌｅｈａｓｉｎｃｒｅａｓｅｄｄｒａｍａｔｉｃａｌｌｙｉｎｒｅｃｅｎｔｙｅａｒｓ .ＡｌｌｏｆｔｈｅｓｅｃｏｍｐｕｔａｔｉｏｎｓｕｓｕａｌｌｙｓｔａｒｔｅｄｗｉｔｈＬａｇｒａｎｇｉａｎｅｑｕａｔｉｏｎｏｆ ｐａｒｔ…