Flow Characteristics Within and Downstream of a Double-Row Array of Shallow Spherical and Cylindrical Dimples

Abstract

The experimental program was performed in the U.S. Air Force Academy water tunnel to visualize details of the unsteady flow structure in front of, within and downstream of a double-row array of shallow (h/D = 0.1) spherical and cylindrical dimples placed on a flat plate. The dimple projected diameter was 50.8 mm. The center of the first array was located at 88,0 mm downstream of the elliptically-shaped leading edge of the plate, the spanwise dimple pitch is 76.2 mm (Sz/D = 1.50). The second array was arranged in a staggered mode with the same spanwise pitch and the axial pitch between rows of 88.0 mm (Sx/D = 1.73). The diameter-based Reynolds number ReD ranged from 3,260 to 23,450, while the length-based Reynolds number Rex in front of the first array varied from 4,010 to 28,840. The laminar flow existed upstream of the first array for all flow conditions studied. In front of the second array the flow structure appeared to have either laminar or turbulent depending on the Reynolds number and streamline location. Flow visualizations were performed at 13 different flow speeds using the dye visualization technique. All recordings were made with a SONY-DCR VX2000 video camera. Adobe Premiere 6.5 software was used to analyze the flow characteristics using the slow motion feature. The results presented include details of the unsteady flow structure, bulk flow fluctuations and laminar-turbulent flow transition. The upstream vortex structures reduce bulk flow fluctuations beyond the second array making them smaller than those after the first array. At ReD > 13,000 the spherical dimples in the second array produce more significant bulk flow fluctuations than cylindrical dimples.