Flow structure due to dimple depressions on a channel surface

Instantaneous, dynamic and time-averaged characteristics of the vortex stru ctures which are shed from the dimples placed on one wall of a channel are described. The dimpled test surface contains 13 staggered rows of dimples i n the streamwise direction, where each dimple has a print diameter of 5.08 cm, and a ratio of depth to print diameter of 0.2. Considered are Reynolds numbers (based on channel height) Re-H from 600 to 11 000, and ratios of ch annel height to dimple print diameter H/D of 0.25, 0.50, and 1.00. For all three H/D, a primary vortex pair is periodically shed from the central port ion of each dimple, including a large upwash region. This shedding occurs p eriodically and continuously, and is followed by inflow advection into the dimple cavity. The frequency of these events appears to scale on time-avera ged bulk velocity and dimple print diameter, which gives nondimensional fre quencies of 2.2-3.0 for all three H/D values considered. As H/D decreases, (i) the strength of the primary vortex pair increases, and (ii) two additio nal secondary vortex pairs (which form near the spanwise edges of each dimp le) become significantly stronger, larger in cross section, and more appare nt in flow visualization images and in surveys of time-averaged, streamwise vorticity. The locations of these primary and secondary vortex pairs near the dimpled surface coincide closely with locations where normalized Reynol ds normal stress is augmented. This evidences an important connection betwe en the vortices, Reynolds normal stress, and mixing. The large-scale unstea diness associated with this mixing is then more pronounced, and encompasses larger portions of the vortex structure (and thus extends over larger volu mes) as H/D increases from 0.25 to 1.0. (C) 2001 American Institute of Phys ics.