A Comparison of Cavitation Inception Index Measurements to the Spatial Pressure Distribution Within a 2D Cavity Shear Flow

Abstract

Direct cavitation inception index measurements and observation on occurrence of cavitation are compared to results of novel spatial pressure distribution measurements in a 2D cavity shear flow. This non-intrusive technique utilizes four-exposure PIV to measure the distribution of material acceleration, and integrating it by means of omni-directional virtual boundary integration algorithm to obtain the pressure distribution (Liu and Katz, 2006). Consequently, it provides the instantaneous spatial distributions of velocity, material acceleration and pressure over a sample area along with their statistics. The present Reynolds numbers based on the cavity length vary from 1.7×105 to 3.4×105. High-speed imaging of cavitation inception, recorded at 30,000 fps, indicates that for this 2D cavity flow, the onset of cavitation always occurs on the top of the cavity trailing edge, regardless of the free stream speed. With decreasing pressure cavitation intermittently expand to the region located just in front of the cavity. The time-averaged spatial pressure distribution has a minimum just above the trailing edge due to the interaction of the impinging shear layer with the trailing wall. Around the cavity trailing edge, the mean flow first decelerates due to the impingement, but then accelerates right above the trailing edge, creating a local pressure minimum there. RMS values and PDFs of pressure fluctuations show that the highest fluctuations occur around the cavity trailing edge, and that the pressure peaks are consistent with the measured cavitation inception indices. There is also agreement between pressure statistics and conditions of appearance of cavitation in front of the trailing edge. The paper also provides the first directly measured experimental data on pressure-velocity correlation and pressure diffusion terms that appear in the evolution equation for turbulent kinetic energy. Results compared to other terms that act as sources and sinks in the turbulent kinetic energy balance. It is evident that near the trailing edge of the cavity, the contribution of pressure diffusion is comparable to that of turbulent kinetic energy production rate, and is much larger than the turbulent diffusion rate. Trends and spatial distribution of pressure diffusion also differs from those of turbulence diffusion.