Analysis of Velocity Structures in a Transitionally Rough Turbulent Boundary Layer
Studies on rough turbulent boundary layers have shown that the surface roughness alters the velocity field near the wall, leading to an increased skin friction. This effect is in particular felt in the roughness sub-layer (3k − 5k, where k denotes the cha
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Analysis of Velocity Structures in a Transitionally Rough Turbulent Boundary Layer Ali Doosttalab, Suranga Dharmarathne, Murat Tutkun, Ronald Adrian, and Luciano Castillo
5.1 Introduction Studies on rough turbulent boundary layers have shown that the surface roughness alters the velocity field near the wall, leading to an increased skin friction. This effect is in particular felt in the roughness sub-layer (3k 5k, where k denotes the characteristic roughness height) and may extend across the boundary layer for k=ı < 50 as mentioned in [9] (note that ı represents the boundary layer thickness). Knowledge of the effects of roughness on turbulent structures is vital for modelling purposes and flow control strategies which can then reduce skin friction drag in turbulent boundary layers by modifying the structures of turbulence. Flores et al. [5] performed direct numerical simulation (DNS) of a moderately rough channel flow and found that the attached vortical structures were independent of the details of the wall. This was attributed to memory of the structures as the structures “forget their origin” quickly. Wu and Christensen [15] experimentally studied the impact of realistic roughness on the spatial structures of wall turbulence at moderate Reynolds numbers and reported shortening of streamwise velocity coherent structures. Their proper orthogonal decomposition (POD) analysis revealed that the structures with large streamwise dimension are more sensitive
A. Doosttalab () • S. Dharmarathne • L. Castillo Department of Mechanical Engineering, Texas Tech University, Lubbock, TX 79409, USA e-mail: [email protected] M. Tutkun Department of Process and Fluid Flow Technology, IFE, Kjeller, Norway Department of Mathematics, University of Oslo, Blindern, 0316 Oslo, Norway R. Adrian School for Engineering of Matter, Transport and Energy, Arizona State University, P.O. Box 876106, Tempe, AZ 85287-6106, USA © Springer International Publishing Switzerland 2017 A. Pollard et al. (eds.), Whither Turbulence and Big Data in the 21st Century?, DOI 10.1007/978-3-319-41217-7_5
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to surface roughness when compared to the structures which have shorter extent in streamwise direction. Lee et al. [10] conducted DNS of turbulent flows over a smooth wall, and a rough wall with either two-dimensional or three-dimensional roughness elements. The results showed that the packet of coherent structures was more inclined over the rough surface geometry compared to the smooth one. Townsend [13] proposed the Reynolds number similarity hypothesis, which states that at sufficiently high Reynolds number and outside the roughness sub-layer (3k 5k), the characteristics of turbulent flow are independent of surface roughness. Doosttalab et al. [4] showed that the Townsend’s wall similarity hypothesis is valid for single-point statistics for both velocity and thermal fields for k=ı > 71. However, validity of the Townsend’s hypothesis for multi-point statistics and structures of the velocity field still needs to be addr
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