Cavitation Scaling Experiments With Headforms: Bubble Acoustics

Recently Ceccio and Brennen [1][2][3] have examined the interaction between individual traveling cavitation bubbles and the structure of the boundary layer and flow field in which the bubble is growing and collapsing. They were able to show that individual bubbles are often fissioned by the fluid shear and that this process can significantly effect the acoustic signal produced by the collapse. Furthermore they were able to demonstrate a relationship between the number of cavitation events and the nuclei number distribution measured by holographic methods in the upstream flow. Kumar and Brennen [4][5] have further examined the statistical properties of the acoustical signals from individual cavitation bubbles on two different headforms in order to learn more about the bubble/flow interactions. All of these experiments were, however, conducted in the same facility with the same size of headform (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 will describe further results from experiments conducted in order to try to answer some of these important questions regarding the scaling of the cavitation phenomena. These experiments (see also Kuhn de Chizelle et al. [6][7]) were conducted in the Large Cavitation Channel of the David Taylor Research Center in Memphis Tennessee, on 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

Cavitation Event Rates and Nuclei Distributions

This paper examines the relationship between the cavitation event rates on axisymmetric headforms and the nuclei distributions in the incident flow. An analytical model is developed to relate these quantities and the results are compared with experimental cavitation event rates measured in the Large Cavitation Channel (LCC) at David Taylor Research Center (DTRC) on three different sizes of Schiebe body. The experiments were carried out at various cavitation numbers, tunnel velocities and air contents. Boundary layer, bubble screening and observable cavitation bubble size effects on the event rates are examined. The trends in the event rates with changing cavitation number and body size are consistent with those observed experimentally. However the magnitudes of the event rates are about an order of magnitude larger than the experimental data. Nevertheless it is shown that the cavitation inception values predicted using a certain critical event rate are consistent with those observed experimentally

Observations and scaling of travelling bubble cavitation

Recent observations of growing and collapsing bubbles in flows over axisymmetric headforms have revealed the complexity of the ‘micro-fluid-mechanics’ associated with these bubbles (van der Meulen & van Renesse 1989; Briancon-Marjollet et al. 1990; Ceccio & Brennen 1991). Among the complex features observed were the bubble-to-bubble and bubble-to-boundary-layer interactions which leads to the shearing of the underside of the bubble and alters the collapsing process. All of these previous tests, though, were performed on small headform sizes. The focus of this research is to analyse the scaling effects of these phenomena due to variations in model size, Reynolds number and cavitation number. For this purpose, cavitating flows over Schiebe headforms of different sizes (5.08, 25.4 and 50.8 cm in diameter) were studied in the David Taylor Large Cavitation Channel (LCC). The bubble dynamics captured using high-speed film and electrode sensors are presented along with the noise signals generated during the collapse of the cavities. In the light of the complexity of the dynamics of the travelling bubbles and the important bubble/bubble interactions, it is clear that the spherical Rayleigh-Plesset analysis cannot reproduce many of the phenomena observed. For this purpose an unsteady numerical code was developed which uses travelling sources to model the interactions between the bubble (or bubbles) and the pressure gradients in the irrotational flow outside the boundary layer on the headform. The paper compares the results of this numerical code with the present experimental results and demonstrates good qualitative agreement between the two

Scaling Experiments on the Dynamics and Acoustics of Travelling Bubble Cavitation

Ceccio and Brennen (1991 and 1989) recently examined the interaction between individual cavitation bubbles and the structure of the boundary layer and flow field in which the bubble is growing and collapsing. They were able to show that individual bubbles are often fissioned by the fluid shear and that this process can significantly effect the acoustic signal produced by the collapse. More recently Kumar and Brennen (1991-1992) have closely examined further statistical properties of the acoustical signals from individual cavitation bubbles on two different headforms in order to learn more about the bubble/flow interactions. All of these experiments were, however, conducted in the same facility with the same size of headform (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 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

Rapport I.19 Simulation d’écoulement partiellement cavitant sur un profil tronqué monte en veine, confrontation avec des mesures de pression

Direct methods of potential flow calculation were succesfully applied to the calculation of the flow around a partial cavitating hydrofoil : i.e. by intoducing the direct calculation in an iterative scheme to approach the state corresponding to a constant pressure on the cavity. An application of an inverse design method, using small perturbations of a direct calculation, to the resolution of the problem of the cavity is presented here. For a given cavity length, the geometry of the profile is considered as constant except on the location of the cavity. Only one or two iterations are necessary to obtain the form of the cavity and its corresponding cavitation number for a given length. Results are then compared to measurements in the high speed cavitation tunnel of the IMHEF.Des méthodes de calcul potentiel direct ont été appliquées avec succès dans le calcul de l'écoulement autour d'un profil en cavitation partielle. Ceci a été réalisé moyennant l'introduction du calcul dans un processus itératif afin de parvenir à un état de pression constante à la surface de la poche. On présente ici une application, dans cette même situation, d’un programme inverse, utilisant des petites perturbations d'un calcul direct, moyennant un blocage de la géométrie du profil en dehors de la poche, sa longueur étant fixée. Une à deux itérations suffisent à approcher l’épaisseur de la cavité et le chiffre de cavitation correspondant à cette longueur de poche. Les calculs sont ensuite confrontés à des mesures réalisées dans le tunnel de cavitation à grande vitesse de l’IMHEF.Favre J.-N., Avellan François, Kuhn de Chizelle Y. Rapport I.19 Simulation d’écoulement partiellement cavitant sur un profil tronqué monte en veine, confrontation avec des mesures de pression. In: Machines hydrauliques. Conception et exploitation. Développements récents et Applications aux différents secteurs industriels. Vingtièmes journées de l'hydraulique. Lyon, 4-6 avril 1989. Tome 1, 1989