Turbulent flow over a rough backward-facing step |
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| Affiliation: | 1. School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore 639798, Singapore;2. Hydrology Technical Group, Pacific Northwest National Laboratory, Richland, WA 99352, USA;1. Department of Mechanical Engineering, Celal Bayar University, 45140 Manisa, Turkey;2. Department of Mechanical Engineering, Technology Faculty, F?rat University, 23119 Elaz??, Turkey;1. Department of Mechanical Engineering, University of Anbar, Ramadi 31001, Iraq;2. Department of Mechanical Engineering, KBU, 47800 Selangor, Malaysia;3. Department of Mechanical Engineering, Faculty of Engineering, Islamic University in Medinah, Prince Naif Ibn Abdulaziz, Al Jamiah, Medinah 42351, KSA;4. DNV GL, Engineering Department, Las Vegas, NV 89146, USA;1. Department of Mechanical Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia;2. Department of Mechanical Engineering, University of Thi-Qar, 64001 Nassiriya, Iraq;3. Young Researchers and Elite Club, Mashhad Branch, Islamic Azad University, Mashhad, Iran;4. School of Mechanical and Mining Engineering, The University of Queensland, QLD 4072, Australia;1. Department of Mechanical Engineering, KBU International College, 47800 Petaling Jaya, Selangor, Malaysia;2. Department of Thermofluids, Faculty of Mechanical Engineering, Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor Bahru, Malaysia;3. FABE, Limkokwing University of Creative Technology, Jalan Teknokrat 1/1, 63000 Cyberjaya, Selangor, Malaysia;4. Department of Mechanical Engineering, University of Anbar, 31001 Anbar, Iraq |
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| Abstract: | This work characterizes the impacts of the realistic roughness due to deposition of foreign materials on the turbulent flows at surface transition from elevated rough-wall to smooth-wall. High resolution PIV measurements were performed in the streamwise-wall-normal (x–y) planes at two different spanwise positions in both smooth and rough backward-facing step flows. The experiment conditions were set at a Reynolds number of 3450 based on the free stream velocity U∞ and the mean step height h, expansion ratio of 1.01, and the ratio of incoming boundary layer thickness to the step height, δ/h, of 8. The mean flow structures are observed to be modified by the roughness and they illustrate three-dimensional features in rough backward-facing step flows. The mean reattachment length Xr is significantly reduced by the roughness at one PIV measurement position while is slightly increased by the different roughness topography at the other measurement position. The mean velocity profiles at the reattachment point indicate that the studied roughness weakens the perturbation of the step to the incoming turbulent flow. Comparisons of Reynolds normal and shear stresses, productions of normal stresses, quadrant analysis of the instantaneous shear-stress contributing events, and mean spanwise vorticity reveal that the turbulence in the separated shear layer is reduced by the studied roughness. The results also indicate an earlier separation of the turbulent boundary layer over the current rough step, probably due to the adverse pressure gradient produced by the roughness topography even before the step. |
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| Keywords: | Turbulence Rough-wall turbulent flow Backward-facing step flow |
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