Turbulent boundary layer flow with a step change from smooth to rough surface |
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| Institution: | 1. College of Civil Engineering, Hunan University, Changsha 410082, China;2. Dept. of Architecture and Civil Engineering, City University of Hong Kong, Hong Kong;3. Key Laboratory of New Technology for Construction of Cities in Mountain Area (Ministry of Education), School of Civil Engineering, Chongqing University, Chongqing 400045, China;1. Beijing''s Key Laboratory of Structural Wind Engineering and Urban Wind Environment, School of Civil Engineering, Beijing Jiaotong University, Beijing, 100044, China;2. China Mobile Group Beijing Company Limited, Beijing, 100007, China;3. National Wind Institute, Department of Civil, Environmental and Construction Engineering, Texas Tech University, Lubbock, TX, 79409, USA;4. School of Civil Engineering, Chongqing University, Chongqing, 400044, China;1. Department of Mechanical Engineering, University of Utah, 201 Presidents’ Cir, Salt Lake City, UT, 84112, USA;2. Department of Mechanical & Materials Engineering, Portland State University, 1825 SW Broadway, Portland, OR, 97201, USA |
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| Abstract: | A direct numerical simulation (DNS) dataset of a turbulent boundary layer (TBL) with a step change from a smooth to a rough surface is analyzed to examine the characteristics of a spatially developing flow. The roughness elements are periodically arranged two-dimensional (2-D) spanwise rods, with the first rod placed 80θin downstream from the inlet, where θin denotes the inlet momentum thickness. Based on an accurate estimation of relevant parameters, clear evidence for mean flow universality is provided when scaled properly, even for the present roughness configuration, which is believed to have one of the strongest impacts on the flow. Compared to previous studies, it is shown that overshooting behavior is present in the first- and second-order statistics and is locally created either within the cavity or at the leading edge of the roughness depending on the type of statistics and the wall-normal measurement location. Inspection of spatial two-point correlations of the streamwise velocity fluctuations shows a continuous increase of spanwise length scales of structures over the rough wall after the step change at a greater growth rate than that over smooth wall TBL flow. This is expected because spanwise energy spectrum shows presence of much energetic wider structures over the rough wall. Full images of the DNS data are presented to describe not only predominance of hairpin vortices but also a possible spanwise scale growth mechanism via merging over the rough wall. |
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| Keywords: | Turbulent boundary layer Direct numerical simulation Surface roughness Hairpin vortex |
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