Bubble-induced sheet cavitation inception on an isolated roughness element

Abstract

The nucleation process on an isolated roughness element, located at the point of minimum pressure of a NACA 0015 hydrofoil was studied experimentally and computationally. The objective of this study was to investigate the working mechanism of bubble-induced sheet cavitation inception. High-speed micro-scale observations show the generation of a streak of cavitation-attached to the roughness element-in the wake of the bubble. Below its critical diameter, the bubble can detach from the streak cavity and travel on while the streak cavity remains. The solutions of a Rayleigh-Plesset equation along a streamline extracted from a RANS calculation show strong similarities with the experimental observations, but a factor 5 to 10 higher frame rate is needed to validate the calculations. 1. Introduction Sheet cavitation inception does not necessarily occur on foils and propellers when the local pressure decreases below the vapour pressure. The condition of inception is also dependent on the characteristics of the fluid, the solid surface and the local flow. Although seeding the flow with micro bubbles and applying leading edge roughness with a Reynolds-dependent grain size is adequate in most cases [1], the detailed working mechanism of the sheet cavitation inception process is not fully understood. A hypothesis formulated in [2] states that a roughness element causes a very local but significant additional pressure decrease in which a micro bubble can expand and induce a streak of cavitation. This local pressure decrease is dependent on the thickness of the boundary layer compared to the height of the roughness element and therefore also dependent on the Reynolds number [2]. High-speed micro-scale observations were made on a hydrofoil to investigate the nuclei-induced sheet cavitation inception process in more detail. The first analysis was focussed on the diameter, form and trajectories of the nuclei [3]. In the present study, a further analysis is made of the nucleation process on an isolated roughness element caused by free stream bubbles. To obtain the pressure and velocity distribution around the roughness element, the foil with the roughness element were numerically modelled and the steady wetted flow was solved using a RANS code. To study the bubble dynamics, a variant of the Rayleigh-Plesset equation has been solved along calculated streamlines passing close to the roughness element.

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