Flow Characteristics Within and Downstream of a Single Shallow Cylindrical and Spherical Dimple: Effect of Pre-Dimple Boundary Layer Thickness

The objective of this study is to investigate the details of the average and unsteady flow structures in front, inside and after shallow (h/D = 0.1) spherical and cylindrical dimples placed on a flat plate at the different distances with different pre-dimple boundary layer thicknesses. The dimple projected (surface) diameter was 50.8 mm with the dimple centers located at 88 mm and 264 mm downstream of the elliptical leading edge of the flat plate. Experimental program was established in the U.S. Air Force Academy water tunnel, both dimple configurations were tested across the range of freestream water velocities from 0.07 to 0.52 m/s corresponding with diameter based Reynolds numbers ReD ranging from 3,200 to 23,500. The length based Reynolds number Rex ranged from 3,940 to 110,450 while the non-dimensional boundary layer thickness δ0/h ranged from 0.28 to 1.18. The inlet flow turbulence was below 1% at all flow speeds. Laminar flow existed upstream of the dimple for all of the flow conditions studied. Flow visualizations were performed inside and downstream of each dimple at 10 to 13 different flow speeds. All recordings were made with a SONY-DCR VX2000 video camera. Five different colors of dye were injected through five cylindrical ports, 1.0 mm in diameter, positioned at locations upstream and inside the dimples. Adobe Premiere 6.5 software was used to analyze the flow characteristics using the slow motion feature. LDV measurements were made both in front of and downstream of the dimple. The results presented include the vortex patterns, in-dimple separation zone extent, unsteady flow phenomena (bulk flow oscillations), velocity profiles after the dimple, and some features of the laminar-turbulent flow transition downstream of a single cylindrical dimple. The data obtained revealed three-dimensional and unsteady flow structures inside and downstream of the dimples, the important role of the pre-dimple boundary layer thickness. Increasing the δ0/h ratio reduces the downstream bulk flow oscillations at very low Reynolds numbers. However, at ReD>16,500 for the cylindrical dimple and at ReD>24,000 for the spherical dimple the boundary layer thickness had little effect on the bulk flow oscillations. A comparison of both spherical and cylindrical dimple geometric configurations was made to assess their relative benefits.