Turbulent boundary layers over sparsely-spaced rod-roughened walls |
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| Institution: | 1. Department of Mechanical Engineering, KAIST, 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea;2. School of Mechanical and Nuclear Engineering, UNIST, 50 UNIST-gil, Eonyang-eup, Ulsan 689-798, Republic of Korea;1. Department of Mechanical Engineering, National Taiwan University of Science and Technology, Taipei 10672, Taiwan, Republic of China;2. Graduate Institute of Applied Science and Technology, National Taiwan University of Science and Technology, Taipei 10672, Taiwan, Republic of China;1. School of Aerospace Engineering, Guizhou Institute of Technology, Guiyang, 550003, China;2. Department of Mechanical Engineering, Harbin Institute of Technology, Harbin, 150001, China;3. Department of Mechanical Engineering, University of Minnesota, Minneapolis, MN 55455, USA;4. St. Anthony Falls Laboratory, University of Minnesota, Minneapolis, MN 55414, USA;1. Faculty of Aeronautics and Astronautics, Istanbul Technical University, 34469 Maslak, Istanbul, Turkey;2. School of Aeronautics, Universidad Politécnica de Madrid, 28040 Madrid, Spain |
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| Abstract: | Direct numerical simulations (DNSs) of spatially developing turbulent boundary layers (TBLs) over sparsely-spaced two-dimensional (2D) rod-roughened walls were performed. The rod elements were periodically arranged along the streamwise direction with pitches of px/k = 8, 16, 32, 64 and 128, where px is the streamwise spacing of the rods, and k is the roughness height. The Reynolds number based on the momentum thickness was varied from Reθ = 300–1400, and the height of the roughness element was k = 1.5θin, where θin is the momentum thickness at the inlet. The characteristics of the TBLs, such as the friction velocity, mean velocity, and Reynolds stresses over the rod-roughened walls, were examined by varying the spacing of the roughness features (8 ⩽ px/k ⩽ 128). The outer-layer similarity between the rough and smooth walls was established for the sparsely-distributed rough walls (px/k ⩾ 32) based on the profiles of the Reynolds stresses, whereas those are not for px/k = 8 and 16. Inspection of the interaction between outer-layer large-scale motions and near-wall small-scale motions using two-point amplitude modulation (AM) covariance showed that modulation effect of large-scale motions on near-wall small-scale motions was strongly disturbed over the rough wall for px/k = 8 and 16. For px/k ⩾ 32, the flow that passed through the upstream roughness element transitioned to a smooth wall flow between the consecutive rods. The strong influence of the surface roughness in the outer layer for px/k = 8 and 16 was attributed to large-scale erupting motions by the surface roughness, creating both upward shift of the near-wall turbulent energy and active energy production in the outer layer with little influence on the near-wall region. |
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| Keywords: | Direct numerical simulation Turbulent boundary layers Roughness Amplitude modulation |
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