Pressure Fields and Cavitation in Turbulent Shear Flows

Abstract

Cavitation in turbulent shear flows is the result of a complex interaction between an unsteady pressure field and a distribution of free stream nuclei. Experimental evidence indicates that cavitation is incited by negative peaks in pressure that are as high as ten times the rms level. This paper reviews the current state of knowledge of turbulent pressure fields and presents new theory on spectra in Lagrangian frame of reference. Cavitation data are analyzed in terms of the available theory on the unsteady pressure field. It is postulated that one heretofore unconsidered factor in cavitation scaling is the highly intermittent pressure fluctuations which contribute to the high frequency end of the pressure spectrum. Because of limitations on the response time of cavitation nuclei, these pressure fluctuations play no role in the inception process in laboratory experiments. However, in large scale prototype flows, cavitation nuclei are relatively mroe responsive to a wider range of the pressure spectrum and this can lead to substantially higher values of the critical cavitatino index. Unfortunately, this issue is coulded by the fact that higher cavitation indices can be found in prototype flows because of gas content effects. Some cavitation noise data are also examined within the ocntext of available theory. The spectrum of cavitation noise in free shear flows has some similarity to the noise data found by Blake et al. (1977) with the exception that there appears to be a greater uncertainty in the scaling of the rate of cavitation events which leads to a substantial spread in available data.