Pressure fluctuations and their effect on cavitation inception within water jets

Abstract

Instantaneous and phase averaged pressure distributions in the near field of a jet, and their effects on the conditions for the onset of cavitation are studied in detail. The measurements are performed by using microscopic bubbles as pressure sensors, and holography as a means of detecting them. Experiments are performed at Red exceeding 4 × 105, with and without acoustic excitation. The results show that the highest negative pressure peaks (–0.97) and the resulting cavitation inception occur because of vortex pairing. Prior to pairing the negative peaks are between –0.8 and –0.9. Weak acoustic excitation changes the entire flow structure and the spatial distributions of bubbles, but has little effect on the onset of cavitation. Downstream of the potential core the highest pressure peaks (∼ –0.6) are considerably smaller, in agreement with the occurrence of cavitation there. It is also shown that although the r.m.s. values of pressure fluctuations do not vary with the jet speed, the probability distribution changes significantly, causing a reduction in the inception index with increasing velocity. The probability of cavitation inception is estimated from the distributions of bubbles and pressure peaks. It is shown that the actual, non-uniform bubble distribution increases the probability of inception owning to migration of the bubbles to the low pressure regions.

Cavitation inception during the interaction of a pair of counter-rotating vortices

“…The resulting cavitation inception location can occur at random sites throughout the shear layer. [2][3][4] Similar observations have been made in jets 5,6 and in the wake of cavitating wedges. 7 In propellers for marine applications, tip vortex cavitation is generally the first form of cavitation to be observed.…”

Lagrangian statistics of pressure fluctuation events in homogeneous isotropic turbulence

“…It puts forward the main ingredients for turbulence-induced inception: That the cavitation nuclei are subject to pressure fluctuations as measured in a Lagrangian frame, and that (a) the pressure must dip below a critical level (p cav , the cavitation pressure, also called Blake threshold 11 ), and (b) the low pressure must persist below the critical level for a time that exceeds the time scale for bubble growth. The picture has been confirmed and enriched over the years by several authors [12][13][14][15] , including the role of coherent structures and the possibility of using incip-ient cavitation bubbles as pressure sensors.…”

“…The pressure PDF in HIT is known to be negatively skewed and exhibit an exponential tail at very low pressures [2][3][4] .On the basis of physical intuition, several authors have argued that the frequency ζ(δp) of pressure fluctuations below δp should also exhibit an exponential tail for large and negative δp 10,12,14 . Mathematically, this is far from obvious.…”

Cavitating flow in annular jet pumps

“…Kudirka and Decoster (1979) found that the vapor bubble firstly occurred inside the jet boundary in CJP. Then, Ran and Katz (1994) validated that the cavitation inception appeared at the vortex center of the jet, which was related to the fluctuation of the shearing layer, and it will also form in the vortex center of the co-flow with the cavitation nuclei growing. Hence the inception of cavitation in CJP can be due to high shear and vortex in jet boundary (namely shearing layer).…”