Cavitation Inception in Separated Flows.

Abstract

The phenomenon of cavitation was studied on four axisymmetric bodies whose boundary layers underwent a laminar separation and subsequent turbulent reattachment. The non-cavitating flow was studied by holography and the Schlieren flow visualization technique. Surface distributions of the mean and the fluctuating pressures were also measured. The conditions for cavitation inception and desinence were determined and several holograms were recorded just prior to and at the onset of cavitation. The population of microbubbles and the nature of the subsequent development of visible cavitation was determined from the reconstructed image. High rms and peak values of the fluctuating pressure were measured (up to 90 percent of the dynamic head), the negative peaks being larger than the positive ones except for the reattachment zone where large positive peaks existed. The power spectra contained peaks thought to originate within the large eddies of the mixing layer and in one case there were also peaks due to the laminar boundary layer instability waves. Cavitation inception occurred in the turbulent shear layer downstream of the transition region. When the separation zone was large the inception region was located within the most developed section of the mixing layer but upstream of the reattachment zone. When the separation region was small inception occurred close to the reattachment zone but still detached from the body surface. A comparison between the surface minimum pressure and the cavitation inception indices also indicated that inception could not occur near the surface of the bodies having a large separation region. The appearance of visible cavities was preceded by the appearance of a cluster of microbubbles only in the cavitation inception region. The nuclei population in the other sections of the flow field remained fairly uniform. This observation supports the assumption that cavitation is initiated from microscopic free stream nuclei. The rate of cavitation events was estimated from the nuclei population and from the dimensions of the separation region. It was shown for one of the bodies that at least one bubble larger than 10 micrometers radius was exposed every second to a pressure peak which was sufficiently large to cause a cavitation event.