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Boundary Integral Methods for Cavitation Bubbles Near Boundaries
No full textThe paper will review the art, craft and science of modelling cavitation bubbles near boundaries through the use of the boundary integral method. The presentation will come in four parts, namely, a discussion of recent experimental studies that have motivated our current research, understanding the physics associated with the phenomena, a detailed presentation on the boundary integral method and, finally, a detailed comparison between experiment and theory, indicating the improved understanding and interpretation of behaviour that the computed results provide
Stokes Wave Over Cavitating Vortex
No full textFollowing Stokes, several authors have studied waves with 120-degree angles at their crests caused on a free surface of fluid by the gravity force. Mathematical analysis, however, shown that similar Stokes waves can be caused by centrifugal force on surfaces where the pressure is constant, and cavitating tip vortex can be examined as a surface of an eventual appearance of axissymmetrical Stokes waves. Determination of shape of such vortices is a novel non-linear problem in potential theory. Thus, a special attention is paid to successive validation of intermediate computations. The main characteristics of the cavity-wave couples past a semi-ellipsoid are computed. An attempt to establish an association between the considered utmost steady flow and experimental data related to breakdown of cavitating vortices is done
The Effect of Water Quality Characteristics on Cavitation Noise
No full textThis study investigated the effects of seawater on cavitation noise using a water jet test, a two-dimensional wing test, and a three-dimensional wing test. A model propeller cavitation test was conducted in a cavitation tunnel using seawater, in order to determine the effects of propeller cavitation more precisely. In the cavitation tests, measured propeller performance for both cavitation noise inception and desinence, as well as the cavitation noise spectrum (see, for example, figure 7). On the basis of the results of the present propeller cavitation test and tests that were carried out previously, we discuss how to control the water quality in the cavitation tunnel to reproduce the cavitation phenomena, especially cavitation noise, in seawater (see, for example, figure 9)
Joint Time Frequency Analysis Techniques: A study of Transitional Dyamics in Sheet/Cloud Cavitation
No full textAn introduction to, and the use of, Joint Time Frequency Analysis techniques is given. Special emphasis is made on Time Frequency Distribution series. This method is demonstrated on selected experimental data. Attention is given the transition region in the dynamic nature of sheet/cloud cavitation. Wavelets and a method that is a variation of phase-portraits are also presented and discussed with regard to cavitation dynamics. An example on
how experimental techniques have been improved by the above mentioned methods is given
Experimental Study of Cavitation in a Kaplan Model Turbine
No full textThe cavitation processes present in a Kaplan model turbine was studied with the aim to identify mechanisms that promote erosive cavitation. The studies were carried out with high-speed filming, video filming and visual observations with stroboscopic light. A periodic pattern of the cavitating tip vortex was observed. The main
feature of this pattern is that the cavitating vortex is bent towards the blade surface and transformed into cloud formations. These clouds were collapsing in a manner that suggests this to be an erosive process. The blade periphery, where the cloud collapses was observed, is known as an area likely to sustain damage. It was also found that
these cloud formations appears in bands, with a periodicity which corresponds approximately to the spacing of the guide vanes. Bubble clouds and cavitating vortices was found to be shed from the sheet cavities which showed signs of re-entrant jets. The cavities at the blade root seem to be mainly of sheet or travelling bubble type, depending on running condition, some facts indicate the contribution
of vortex motion as well
Cavitation Erosion Prediction From Inferred Forces Using Material Resistance Data
No full textTo improve cavitation erosion prediction based on induced
vibrations, some experimental work has been carried out on two
different materials, a stainless steel and a protective coating.
First, the erosive characteristics of the materials have been
analyzed in a cavitating vortex generator. Pit counting results
have been correlated to inferred force intensities occurring on
the eroded area from measured vibrations considering the
transmissibility from the excitation location to the measuring
position. The relationship between the pit area and the force
intensity has been calculated. To validate these results, the same
materials have been tested in a hydrodynamic tunnel where the
force intensities have been inferred in the same way. Then, the
eroded area has been calculated using the erosion characteristics
previously determined. The results obtained compare well with with
the actual erosion measured on the tunnel specimens. Next step is
to apply this method to hydraulic machines
Numerical Analysis of Temperature Fields Inside Nonspherical Bubbles in the Final Stage of Collapse
No full textThe temperature fields inside a collapsing bubble near a plane rigid wall are investigated. We develop a numerical method based on the boundary element method combined with the finite volume method. In this method, the adaptive unstructured grids which are generated by Delaunay triangulation are used inside the largely deformable bubble to consider the heat transfer of the internal gas. We compare the present numerical results with those using structured grids. The result shows that the present method has sufficient accuracy to investigate the bubble motion. It is also shown that the present method can be applied to the analysis for the temperature fields inside tiny bubbles in which the thermal boundary layer is thick. Since the unstructured grids are more flexible for the regeneration and resizing of grids, the present method is applicable to the dynamics of toroidal bubbles by taking the thermal diffusivity of the internal gas into account
Numerical Simulation of Cavitating Flows in Diesel Injectors by a Homogeneous Equilibrium Modeling Approach
No full textDue to excessive stress in the orifice, cavitation occurs in high-pressure Diesel injectors. As experiments are very hard to manage for injection conditions (small-scaled, high-speed flow), a numerical model seems to be the right tool to get a better understanding of the flow features inside and at the exit of the injector nozzle.
The purpose of this paper is to present a simulation code based on a Homogeneous Equilibrium Model. The validation of the code for typical cavitating flow configuration is presented. Then numerical results of cavitating flows in Diesel injectors are shown. Then we report concluding remarks and objectives
Tip Leakage Cavitation, Associated Bubble Dynamics, Noise, Flow Structure and Effect of Tip Gap Size
No full textThis paper focuses on the onset of tip-leakage cavitation on a fixed hydrofoil. The objectives are to investigate the effect of gap size on the flow structure, conditions of cavitation inception, the associated bubble dynamics and cavitation noise. The same hydrofoil with three tip gap sizes of 12%, 28% and 52% of the maximum tip thickness have been studied. Controlled cavitation tests are performed after de-aerating the water in the tunnel and using electrolysis to generate cavitation nuclei. The experiments consist of simultaneously detecting cavitation inception using a 2000fps digital camera (visual) and two accelerometers ("acoustic") mounted on the test-section windows. Good agreement between these methods is achieved when the visual observations are performed carefully. To obtain the time dependent noise spectra, portions of the signal containing cavitation noise are analyzed using Hilbert and Wavelet transforms. Rates of cavitation events as a function of the cavitation index for the 3 gap sizes are also measured. The observations demonstrate that high amplitude noise spikes are generated when the bubbles are distorted and "shredded" – broken to several bubbles following their growth in the vortex core. Mere changes to bubble size and shape caused significantly lower noise. High resolution Particle Image Velocimetry with a vector spacing of 180um is used to measure the flow, especially to capture the slender tip vortices where cavitation inception is observed. The instantaneous realizations are analyzed to obtain probability density functions of the circulation of the leakage vortex. The circulation decreases with increasing gap sizes and minimum pressure coefficients in the cores of these vortices are estimated using a Rankine model. The diameter of the vortex core varied between 540 – 720um. These coefficients show a very good agreement with the measured cavitation inception indices
Overview of Industrial and Rocket Turbopump Inducer Design
No full textHigh performance inducers for rocket turbopumps and industrial low suction pressure pumps have been designed frequently during the past century. Some design lore has evolved to guide this design process; additionally, some detailed flow observations have been made which shed light on the basic flow process. Design methods have been recommended during the past decades to guide inducer design. This paper reviews some of the design methods, some of the flow observations, and some of the design practice which is used for inducer design. The purpose of this review is to bring together a good portion of prior art and focus it from a modern designer's perspective. Some suggestions for future improvements are provided. A useful overview of current design practice is given which should provide guidance to current inducer pump design