Experimental investigation of boundary layer transition in flow past a bluff body

Abstract

We explore the phenomenon of drag crisis observed for the flow over bluff bodies at high Reynolds numbers. The drag coefficient reduces significantly beyond a certain Re due to the transition of the boundary layer from laminar to turbulent state. Flow past a smooth sphere and a circular cylinder is experimentally investigated for 1.0 × 105 ≤ Re ≤ 5.0 × 105 via unsteady force, surface-pressure and 2-D Particle Image Velocimetry(PIV) measurements. In case of a smooth sphere, the drag crisis is observed for Re > 3.3 × 105. The unsteady force measurements reveal that the fluctuations in the force coefficients initially increase with Re in the high subcritical regime and then experience a steep fall in the critical regime. It is found from the PIV measurements that the normal Reynolds stresses in the separated shear layer from the sphere are one order lower in magnitude for the supercritical regime in comparison to the subcritical regime. In the case of flow past a smooth circular cylinder, a two-stage drag crisis is captured using surface-pressure measurements where the boundary layer over one side of the cylinder undergoes transition around Re = 3.9 × 105 and that over the second side transitions around Re = 4.8 × 105. The transition is accompanied with increased fluctuations in the surface-pressure coefficients near the shoulders of the cylinder.