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Title: Geometric Effects of Shallow Dimples in Turbulent Channel Flows at Reτ ≈ 180: A Vorticity Transport Perspective
Authors: NG JEE HANN 
Tay Chien Ming,Jonathan 
Keywords: Shallow dimples
Vorticity transport
Turbulent drag reduction
Direct numerical simulation
Issue Date: 1-Jun-2020
Publisher: Springer
Citation: NG JEE HANN, RAJEEV KUMAR JAIMAN, LIM TEE TAI, Tay Chien Ming,Jonathan, KHOO BOO CHEONG (2020-06-01). Geometric Effects of Shallow Dimples in Turbulent Channel Flows at Reτ ≈ 180: A Vorticity Transport Perspective. Flow, Turbulence and Combustion 105 (1) : 83-122. ScholarBank@NUS Repository.
Abstract: Shallow dimples have shown a great potential to reduce turbulent drag in several experiments. However, the mechanism underlying their effects on turbulence dynamics remains a subject of deliberation. In this numerical study, a vorticity transport perspective is adopted to elucidate the geometric effects of shallow dimples on the turbulence dynamics in a fully-developed turbulent channel flow. The geometric modifications considered in the present direct numerical simulation at a bulk Reynolds number of 2800 include changing the planform design and modulating the streamwise wall slope. The planform design had a significant impact on the mean flow topology and the drag property. Out of the five planform designs, diamond (shape) dimples produced the highest amount of total drag reduction of ≈7%, whereas symmetric circular dimples caused the greatest increase in total drag by ≈6%. Results revealed that geometric modifications of shallow dimples mainly affect the vorticity redistribution process that predominates in the inner region of a turbulent boundary layer, and in turn the generation of Reynolds stress. The drag property of a dimpled surface was found to scale with the velocity–vorticity correlation term representing the physical process of vorticity stretching and reorientation. The occurrence of flow separation near the leading edge of the dimple altered the vorticity dispersion process in the outer region, thereby modifying the generation of Reynolds stress in the bulk flow. On the other hand, the vortex dispersion process in the inner layer can be altered by modulating the streamwise wall slope. Accordingly, the vorticity redistribution process was modified but the near-wall generation of Reynolds stress was unaffected. Thus, only the form drag varied with the streamwise wall slope, while the viscous drag remained largely the same.
Source Title: Flow, Turbulence and Combustion
ISSN: 1386-6184
DOI: 10.1007/s10494-020-00112-6
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