Flow structure due to dimple depressions on a channel surface

Abstract

Instantaneous, dynamic and time-averaged characteristics of the vortex structures 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 in 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) ReH from 600 to 11 000, and ratios of channel 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 portion of each dimple, including a large upwash region. This shedding occurs periodically and continuously, and is followed by inflow advection into the dimple cavity. The frequency of these events appears to scale on time-averaged bulk velocity and dimple print diameter, which gives nondimensional frequencies 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 additional secondary vortex pairs (which form near the spanwise edges of each dimple) become significantly stronger, larger in cross section, and more apparent 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 Reynolds normal stress is augmented. This evidences an important connection between the vortices, Reynolds normal stress, and mixing. The large-scale unsteadiness associated with this mixing is then more pronounced, and encompasses larger portions of the vortex structure (and thus extends over larger volumes) as H/D increases from 0.25 to 1.0.

An Experimental Study on Heat Transfer Enhancement of Flat Plates Using Dimples

“…It is obvious that 'h' increases as compared to flat plates because the dimple arrangement causes delay in the development of the thermal boundary layer & hence increases the local heat transfer in the reattachment region and increases the heat transfer coefficient [1].It can also be seen that 'Q' increases with decrease in orientation angle of dimples. Because the near-wall turbulent mixing intensity downstream the dimple increases due to the vortex flow shedding from the dimples [6,15]. …”

Static pressure correction in high Reynolds number fully developed turbulent pipe flow

“…Alternatively, visualizations by Ligrani et al (2001) of the flow over a dimpled surface show that a pair of counter-rotating vortices is formed above two recirculation zones within each dimple. This mechanism may also be applicable to the tapping problem, hinting at some three-dimensional aspects of the flow.…”

Effect of rounded edged dimple arrays on the boundary layer development

“…Early investigations of the effects of dimple cavities have focused mainly on the changes in the flow structures or heat transfer either inside or downstream of a single or multiple dimples on a channel wall Isaev et al, 2000;Ligrani et al, 2001 andLigrani, 2000;Mahmood and Ligrani, 2002). These favorable heat transfer effects have led to the proposed use of dimpled surfaces ranging from micro-scale passages for electronics cooling devices to cooling passages in large-scale turbine airfoils used for electrical power generation .…”