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Effect of wall confinement on the aerodynamics of bluff bodies El-Sherbiny, Saad el-Sayed

Abstract

The effect of wall confinement on the aerodynamics of a set of stationary circular cylinders and flat plates, representing the blockage ratio range of 3 - 35.5%, is investigated experimentally to obtain data on mean and unsteady pressure distributions, Strouhal number, and wake geometry. In general, the influence of the Reynolds number in the range of 10⁴ -12 x 10⁴ was found to be confined to the mean pressure distribution at the higher blockage (circular cylinder only) and the unsteady surface loading. The results showed the base pressure to decrease and consequently the drag coefficient to increase with bluffness; however, the pressure distribution in the potential flow region remained relatively unaffected by the confinement. The wake geometry does not change appreciably under constraint and thus leads to a similarity of the several flow parameters. Variation in the vortex shedding frequency was found to be essentially proportional to the increase of separation velocity in accordance with Roshko's universal Strouhal number. The shape of the fluctuating surface pressure distribution curves also remains unaffected by the constraint. However, the pressure intensity increases with an increase in bluffness and shows considerable dependence on the Reynolds number and the three-dimensionality of the flow. Validity of the correction methods due to Glauert and Maskell for the mean drag coefficient is checked in the light of the experimental results. They were found to be inadequate particularly at higher blockage ratios. However, modification of these methods through inclusion of the higher order terms improved their applicability. Least square fit of the results showed linear variation [e.g., C[sub D] = C[sub D[sub c]] + Const (C[sub D] S/C)] of the parameters. A potential flow model is developed for two-dimensional symmetrical bluff bodies under wall confinement. It provides a procedure for predicting the mean surface loading on a bluff body over a range of blockage ratios. Experimental results with normal flat plates and circular cylinders for blockage ratios up to 35.5% substantiate the validity of the approach.

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