Experimental and numerical study of cavitating flow around a surface mounted semi-circular cylinder

Abstract

In this paper, the cavitating flow around a bluff body is studied both experimentally and numerically. The bluff body has a finite length with semi-circular cross section and is mounted on a surface in the throat of a converging-diverging channel. This set-up creates various 3D flow structures around the body, from cavitation inception to super cavities, at high Reynolds numbers ( Re = 5 . 6 × 10 4 − 2 . 2 × 10 5 ) and low cavitation numbers ( σ = 0 . 56 − 1 . 69 ). Earlier studies have shown this flow to be erosive and the erosion pattern varies by changing the flow rate and w/o the cylinder; hence, this study is an attempt to understand different features of the cavitating flow due to the cylinder effect. In the experiments, high-speed imaging is used. Two of the test cases are investigated in more detail through numerical simulations using a homogeneous mixture model. Non-cavitating simulations have also been performed to study the effect of cavitation on the flow field. Based on the observed results, vortex shedding can have different patterns in cavitating flows. While at higher cavitation numbers the vortices are shed in a cyclic pattern, at very low cavitation numbers large fixed cavities are formed in the wake area. For mid-range cavitation numbers a transitional regime is seen in the shedding process. In addition, the vapour structures have a small effect on the flow behaviour for high cavitation numbers, while at lower cavitation numbers they have significant influence on the exerted forces on the bluff body as well as vortical structures and shedding mechanisms. Besides, at very low cavitation numbers, a reverse flow is observed that moves upstream and causes the detachment of the whole cavity from the cylinder. Such a disturbance is not seen in non-cavitating flows.

Numerical simulation and analysis of multi-scale cavitating flows

“…These values correspond to test case 3 in the experimental work with a cavitation number of (Ghahramani et al. 2020). Furthermore, a no-slip condition is applied on all solid boundaries including the bluff body.…”

“…All of the experimental results that are shown below are from an earlier study (Ghahramani et al. 2020) and in all figures, the flow is from left to right. Additionally, in all of the time series of the flow that are illustrated in this paper, s. The cavitation pattern of the described test case is shown in figure 9.…”

“…The experimental data have been captured using high-speed imaging in an earlier study (Ghahramani et al. 2020) for a vast range of cavity patterns. Further details about the set-up as well as the experimental procedure can be found in the same paper.…”

“…To study the influence of the grid resolution on the predicted results, in a previous study (Ghahramani et al. 2020), the cavitating flow (by the Eulerian model) was calculated using a finer grid with 10.8 million cells as well. From the predicted values for the pressure drop in the channel, as well as the drag coefficient on the bluff body, it was found that using a finer grid does not alter the averaged flow parameters significantly and therefore the 4.8 million cell grid is used in this study.…”

“…In a recent study (Ghahramani et al. 2020), the Eulerian model simulation could not sufficiently capture the cavity development in a multi-scale flow around a bluff body. The investigated bluff body is a surface-mounted semi-circular cylinder, which generates various cavity structures of different length scales.…”