The topology and unsteady behaviour of ventilated and natural cavity flows over a two-dimensional (2-D) wall-mounted fence are investigated for fixed length cavities with varying free-stream velocity using high-speed and still imaging, X-ray densitometry and dynamic surface pressure measurement in two experimental facilities. Cavities in both ventilated and natural flows were found to have a re-entrant jet closure, but not to exhibit large-scale oscillations, yet the irregular small-scale shedding at the cavity closure. Small-scale cavity break-up was associated with a high-frequency broadband peak in the wall pressure spectra, found to be governed by the overlying turbulent boundary layer characteristics, similar to observations from single-phase flow over a forward-facing step. A low-frequency peak reflecting the oscillations in size of the re-entrant jet region, analogous to ‘flapping’ motion in single-phase flow, was found to be modulated by gravity effects (i.e. a Froude number dependence). Likewise, a significant change in cavity behaviour was observed as the flow underwent transition analogous to the transition from sub- to super-critical regime in open-channel flow. Differences in wake topology were examined using shadowgraphy and proper orthogonal decomposition, from which it was found that the size and number of shed structures increased with an increase in free-stream velocity for the ventilated case, while remaining nominally constant in naturally cavitating flow due to condensation of vaporous structures.
The influence of nucleation on cavitation inception in a high Reynolds number shear layer in the wake of a backward facing step was investigated experimentally in a water tunnel. The flow was investigated for two nuclei populations, the one naturally occurring in the water and for the water artificially seeded with monodisperse nuclei. Incipient events were observed to form in stretched quasi-streamwise vortices. The collapse of an incipient cavity resulted in a microbubble cloud dispersed into the shear layer and the step re-circulation zone. These microbubbles, generally larger than those naturally occurring in the water, act as preferential sites for re-nucleation, triggering the formation of developed cavitation. This phenomenon rendered statistical characterization of cavitation inception impractical for the natural nuclei population. The re-nucleation issue was addressed by seeding the flow with a population of large monodisperse nuclei, with a critical pressure higher than that of cavitation products. Spatial distribution of the nuclei within the seeded plume was characterized using a volumetric measurement based on Mie-Scattering Imaging. Ability to discern individual incipient events enabled examination of the effect of cavitation number and the nuclei injection rate on the inception event rate. The event rate was found to follow a power law with cavitation number and vary linearly with the injection rate. Mapping of spatial distribution of cavitation susceptibility was obtained by combining the spatial distributions of incipient events and nuclei concentration. The current work provides a valuable dataset for development of computational tools for modelling of cavitation inception in nucleated flows.
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