Comparisons of flow structure above dimpled surfaces with different dimple depths in a channel

Abstract

Flow structural characteristics over dimple surfaces located on one wall of a rectangular channel with three different dimple depths ͑␦ / D = 0.1, 0.2, and 0.3͒ are studied experimentally. Reynolds number based on channel height Re H ranges from 2100 to 20 000, and the ratio of channel height to dimple print diameter H / D is 1.0. Presented are instantaneous flow visualization images, spectra of longitudinal velocity fluctuations, vortex pair frequency information, and time-averaged surveys and profiles of different quantities. Regardless of dimple depth, primary vortex pairs are periodically ejected from the central parts of each dimple and exist in conjunction with edge vortex pairs present near the spanwise edges of staggered dimples. As dimple depth increases, larger deficits of total pressure and streamwise velocity are present, along with higher magnitudes of time-averaged streamwise vorticity, vortex circulation, and longitudinal Reynolds normal stress. Bigger and stronger vortices with increased turbulence transport capabilities are thus produced by deeper dimples. Ensemble-averaged power spectral density profiles show that primary vortex pair ejection frequencies range from 7 to 9 Hz, and edge vortex pair oscillation frequencies range from 5 to 7 Hz, with similar distributions as the Reynolds number varies, regardless of dimple depth.

Effect of rounded edged dimple arrays on the boundary layer development

“…Comparing the mean velocity contours and surface visualization obtained in the present study with those of Won et al (2005) and the flow patterns observed by Ligrani et al (2001) reveals why this is so. Despite flow visualization by Won et al (2005) showing the periodic fluid shedding from the dimple center, this phenomenon is similarly not detected in their mean velocity contours.…”

“…Despite flow visualization by Won et al (2005) showing the periodic fluid shedding from the dimple center, this phenomenon is similarly not detected in their mean velocity contours. The contours they obtained on the other hand are similar to those obtained in the present study, showing depressions along the dimple center.…”

“…It is interesting to note that while the periodic vertical ejection of fluid is observed from the center of the dimple by many researchers (eg. Won et al, 2005;Ligrani et al, 2001 and, the depression in the velocity contours shown in Fig. 3 is more commonly associated with vertical downflow at the depression position (Bakchinov et al, 1995 andMitsudharmadi et al, 2006), in this case along the dimple center.…”

“…Recent work done by Won et al (2005) shows that as the dimple depth increases, larger losses in total pressure and streamwise velocity results, accompanied by higher magnitudes of time averaged streamwise vorticity, vortex circulation, and longitudinal Reynolds normal stress. Deeper dimples produce larger and stronger vortices that increase turbulent transport in the flow.…”

Numerical Study on the Effect of Tube Rows on the Heat Transfer Characteristic of Dimpled Fin

“…Their results showed that the dimples could produce vortex, secondary flow structure, and flow mixing, and the vortex pairs were periodically shed from the dimples and become stronger as the ratio of channel height to dimple diameter decreased. Won et al [5] studied the flow structural characteristics over dimpled surfaces located on one wall of a rectangular channel with three different dimple depths. Their results indicated that the deeper dimples ejected stronger primary vortex pairs, which could result in greater mixing, higher local levels of turbulence transport, and turbulence production.…”