Effect of rounded edged dimple arrays on the boundary layer development

Abstract

The effect of a turbulent boundary layer subjected to a series of rounded edged shallow dimple arrays with dimple depth ratios, d/D of 4%, 8% and 12% were experimentally studied. Measurements show the existence of a higher flow speed region at the center of each dimple. The spanwise distribution of the mean wall shear stress immediately downstream of the centers of the last row of dimples does not vary with dimple depth, and is about 45% over that without the dimple array. Turbulence measurements and surface flow visualization shows that the flow over the shallowest dimple differs from the deeper dimples. Flow separation observed with the deeper rounded edged dimples produce similar flow structures as those from sharp edged dimples reported in the literature. However flow separation is not observed when d/D = 4% but instead two other higher speed regions either side accompany the central flow. The effects of the dimples are rapidly suppressed by the flat surfaces between of the dimples, and the flow rapidly reverts back to an unmanipulated flat boundary layer flow in these areas.

Mechanics of drag reduction by shallow dimples in channel flow

“…It has been shown that dimples are able to generate streamwise vortices which in turn introduce spanwise flow components near the wall. 24 We postulate that the streamwise vortices generated by passive dimples can achieve the same drag reduction effect as those generated by active oscillating wall or transverse jets. The objective of the present paper is to establish such drag reduction using streamwise pressure measurement and to investigate the possible role of these streamwise vortices on the effect of drag reduction using hot-wire measurement.…”

An Experimental Investigation on the Characteristics of Turbulent Boundary Layer Flows Over a Dimpled Surface

“…As a result, hot gas-contacting turbine blades have to be cooled intensively by using various cooling techniques, such as internal convective cooling [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] and film cooling [21][22][23][24] on the blade exterior, in order to ensure a good structural integrity of the turbine blades. Dimple arrays, which can be placed throughout the entire internal cooling passage of turbine blades incorporating with other augmentation methods (such as impingement holes, rib turbulator, and pin-fins), have become a magnet in forced convective heat transfer studies in recent years [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]. Due to its good heat transfer enhancement performance with comparatively smaller pressure loss penalties in comparison to other types of heat transfer augmentation devices such as pin-fins and rib tabulators, a dimpled surface (i.e., a flat surface with dimple arrays) provides a desirable alternative for the...…”

“…A number of experimental and numerical studies have been conducted in recent years to investigate heat transfer augmentation performance of dimpled surfaces [3][4][5][6][7][9][10][11][12][13][15][16][17][18][19]. It is found that the heat transfer performance of a dimpled surface is greatly affected by the configuration of the dimples, including dimple diameter, dimple depth ratio, distribution pattern, and shape of the dimples.…”

“…They found that, as the channel height H/D decreases, both the shedding of unsteady vortex structures and local Nusselt numbers become greater. Mitsudharmadi et al [5] investigated the effects of round edged dimple arrays on the development of boundary layer flows over dimpled surfaces with depth ratios of 4%, 8%, and 12%. Their measurements revealed that the flow separation observed for the test case with deeper edged dimples has similar flow structures as those of the cases with sharp dimple edge.…”

“…The experimental study was performed in a low-speed, open-circuit wind tunnel located at the Department of Aerospace Engineering of Iowa State University. Similar as those used in previous studies [1][2][3][4][5], a rectangular channel with a small aspect ratio was designed to simulate the channel flow inside the internal cooling channel of gas turbine blades. The tunnel has an optically transparent test section of 20 mm  152 mm in cross section (H/W ¼ 0.132) with a corresponding hydraulic diameter (D h ) of 35.4 mm (i.e., D h ¼ 35.4 mm).…”