Characterisation of Flow Around a Sphere

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SPHERE TEST: REVISIT WIND TUNNEL TURBULENCE

ABSTRACT
One of the most widely used methodologies in characterising the quality of a wind tunnel is to study the flow over a sphere.The flow around the bluff body was characterised using smoke test, and the region of flow separation was analysed. The drag characteristics of 3 spheres of different diameters were studied for a wide range of Reynolds number. The Reynolds number at which the transition occurs is strongly dependent on the degree of turbulence in the wind tunnel. Based on the following tests, the quality of the wind tunnel was determined. The turbulence level in the wind tunnel was experimentally studied. The sphere test results were in good agreement with the literature and the quality of the wind tunnel was found to be fairly good.

CHAPTER 1 INTRODUCTION

FIGURE 1.1. Flow around a sphere

FIGURE 1.1 shows that whenever a flow encounters a body, the flow tends to curve around that body. As air flows around the sphere, the flow gets deflected due to the shape and there is a difference in pressure at various points on the sphere. Pressure decreases as we move from front to the top point and increases as we move from top to the rear. For the latter part there is a chance for flow separation due to adverse pressure gradient. Spheres are known to have a distinct critical Reynolds number above which the flow on the upstream face of the sphere is fully turbulent causing the drag coefficient to drop dramatically. This is because the turbulent boundary layer results in separation further aft than a laminar boundary layer, thus producing a smaller wake. The critical Reynolds number for the three spheres was determined by examining the measured drag coefficient CDp as a function of Reynolds number. To understand the quality of flow in the test section, turbulence level is the flow quality parameter. In this experiment, the level of turbulence and resultant turbulence factor for the wind tunnel was



References: 1. J.D. Anderson, Jr.”Fundamentals of Aerodynamics”, McGraw-Hill, 2001. 2. E.L .Houghton and P.W. Carpenter “Aerodynamics for Engineering Students” Fifth Edition, Butterworth-Heinemann publications, 2003. 3. B.R Munson, P.F Young, T.H Okiishi, ”Fundamentals of Fluid Dynamics” John Wiley & Sons, 2002. 4. Barlow, J.B., Rae Jr., W.H., Pope, A., Low-Speed Wind Tunnel Testing Wiley & Sons, Inc., New York, 1999. pp 147-150. 5. Robert C. Platt,”Turbulence Factors of NACA Wind Tunnels As Determined Sphere Tests”1937. 6. Dryden, H.L., Keuthe, A.M., “Effect of Turbulence in Wind Tunnel Measurements” NACA Report 342, 1929. 21

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