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Numerical Simulation of Bubbly Cavitating Flow in Shock Wave Lithotripsy
No full textThe bubbly cavitating .ow generated by a lithotriptor is computed using an ensemble averaged two-phase .ow
model. The time-dependent, compressible .ow computation is divided into two separate calculations: the
refocusing of a spherical pulse by an ellipsoidal re.ector, and the evolution of the steepening wave including
the cavitating bubble cloud it generates. The .rst computation is single phase and is done in prolate
spheroidal coordinates in order to have the surface of the ellipsoidal re.ector aligned with the computation
grid. The output of this simulation is then fed to the two-phase cylindrical coordinates domain. Preliminary
results and qualitative comparison to experimental observation are presented
Acoustic Emission from Micro Bubbles in Ultrasound Field
No full textUltrasound is widely applied in the clinical field. It is essential to take a real understanding of dynamics of micro bubbles in these applications. But it has not been clarified yet. In this paper we numerically simulate a single micro bubble and bubble cloud, which consists of micro bubbles, in water. In the case of a single bubble, we conduct FFT analysis of acoustic pressure emitted from the bubble in ultrasound field, and we investigate the amplitude of second-harmonics and sub-harmonics. In the case of bubble cloud, we observe that pressure at the center of cloud drastically changes depending on the frequency of ultrasound
On the Use of Ultrasonics in Hydrodynamic Cavitation Control
No full textIn a recent study, we have shown the potential for the use
of ultrasonics in travelling bubble cavitation control near inception. Here, we present results which demonstrates control under developed cavitation conditions. In addition we also discuss effects of different methods of excitation of a piezoelectric crystal on control
A Cavitation Erosion Model for Ductile Materials
No full textAn analytical model is proposed for the prediction of cavitation erosion of ductile materials. It is based upon a physical analysis of the work-hardening process due to the successive bubble collapses. The material is characterized by its classical stress-strain relationship and its metallurgical behaviour is analysed from microhardness measurements on cross sections of eroded samples. The flow aggressiveness is determined from pitting tests, using the material properties to go back to the impact loads. The histogram of impact loads is applied numerically a large number of times on the material surface and the evolution of the mass loss with the exposure time is computed. The approach is supported by experimental tests
Prediction of the 2-d Unsteady Supercavity Shapes
No full textA method to calculate a length and a shape of two--dimensional unsteady
supercavities past
slender wedges and around slender hydrofoils is developed. In case of
arbitrary time--dependence, the finite difference time discretization
is used. In each time step, the solution is constructed by the method of
integral equations. The variable supercavity length is found
by numerical solving the equation of the mass of gas in the cavity balance.
Examples of calculation of evolution of both the natural supercavity and
the ventilated cavity in cases of aperiodic time--dependence and sinusoidal
oscillation of hydrofoil are presented
Cavitation Bubbles in a Starting Submerged Water Jet
No full textThe behavior of cavitation bubbles in a starting submerged water jet discharging from a circular nozzle is studied by a simple photography technique in a moderately low range of jet exit velocity. A number of small spherical bubbles are initially generated in a starting vortex formed at the jet tip and often connected circumferentially with each other in the form similar to a vortex ring. Nearly axisymmetric lumps of disconnected bubbles are also observed frequently. By analyzing photographic data acquired from the side and end view pictures of the ring-like bubbles, their average properties, such as trajectory, geometry and size, are evaluated
Cavitating Flow Simulations Based on the Bubble Dynamics
No full textA new numerical method for simulating cavitating flows is developed. The cavitation is modeled with the dynamics of bubbles which radii change with Rayleigh-Plesset equation. The pressure inside of a bubble is modeled from the results of very precise direct simulation of single bubble motion. The bubbles are also allowed to have slip velocities so that the bubble accumulation could be simulated. Pseudocompressibility concept for incompressible flow simulation is extended for the present systems of equations for efficient and robust computations. Computations of two- and three-dimensional cavitating flows verify the present method and show its capability to the practical applications of cavitating flows
Modeling of the Thermal Behavior Inside a Bubble
No full textCavitation occurs by the sudden expansion and the volumetric oscillation of bubble nuclei in the water due to the ambient pressure change. The size of bubble nuclei is O(10mm) and the thermal damping effects on the bubble motion is dominant in comparison with the acoustic and viscous damping effects. Because the thermal damping effect strongly depends on the thermal phenomena inside the bubble, it is important to simulate the detailed thermal behavior inside the bubble. The full DNS (Matsumoto and Takemura (1994) and Takemura and Mastumoto (1994)) is the most useful method to obtain the detailed structure. However, since it requires a long computational time to conduct the full DNS, a simple modeling for the internal thermal behavior is required. As we have known that there are many simple models for calculating the bubble oscillation such as the Rayleigh-Plesset equation etc. Nevertheless, most of previous works have assumed the thermal process of content inside the bubble to be adiabatic or isothermal and have neglected the thermal damping effect. In the present study, a simple model of the thermal behavior inside a spherical bubble is developed coupling with the Rayleigh-Plesset equation. The behavior of a spherical bubble in an acoustic field is numerically obtained by the full DNS, the present model and the conventional ones under adiabatic and isothermal assumptions
Simple Rules for Cavitation Instabilities in Turbomachinery
No full textThe present paper focuses on the correlation of cavitation instabilities in turbomachinery with steady cavity length, or a parameter sigma/(2*alpha). A linear stability analysis shows that various types of cavitation instabilities start to occur when the steady cavity length becomes larger than 65% of the blade spacing. Experimental evidence for this will be reviewed. If we apply this rule in a cross flow plane, the effects of leading edge sweep to suppress cavitation instabilities can be reasonably explained by using a corrected parameter in cross flow. This will also be addressed
Collapse and Rebound of a Laser-Induced Cavitation Bubble
No full textSingle cavitation bubble luminescence (SCBL) induced by
laser in contrast to single bubble sonoluminescence (SBSL) has no need in a sound field for a strong collapse and for light emission. The cavitation bubbles are produced by focused laser light and make the single strong collapse. As shown experimentally, the number of emitted photons from cavitation luminescence is much greater than it was observed in sonoluminescence due to the large bubble size during the final stage of collapse.
To describe the process of laser--induced bubble collapse a mathematical
model is proposed, which is based upon the spherically--symmetric motion.
The basic results of the numerical solution are presented for the bubbles
with initial radii of about 1~mm. According to the observed results the
minimum bubble radius in collapse is about 15 microns, and the mass decreases
up to 5 per cent of the initial value. The calculations show a good agreement
with experimental measurements