Turbulence structure in a boundary layer with two-dimensional roughness
- 10 September 2009
- journal article
- research article
- Published by Cambridge University Press (CUP) in Journal of Fluid Mechanics
- Vol. 635, 75-101
- https://doi.org/10.1017/s0022112009007617
Abstract
Turbulence measurements for a zero pressure gradient boundary layer over a two-dimensional roughness are presented and compared to previous results for a smooth wall and a three-dimensional roughness (Volino, Schultz & Flack,J. Fluid Mech., vol. 592, 2007, p. 263). The present experiments were made on transverse square bars in the fully rough flow regime. The turbulence structure was documented through the fluctuating velocity components, two-point correlations of the fluctuating velocity and swirl strength and linear stochastic estimation conditioned on the swirl and Reynolds shear stress. The two-dimensional bars lead to significant changes in the turbulence in the outer flow. Reynolds stresses, particularly $\overline {{v'}^2} ^ +$ and $ - \overline {{u}'{v}'} ^ + $ , increase, although the mean flow is not as significantly affected. Large-scale turbulent motions originating at the wall lead to increased spatial scales in the outer flow. The dominant feature of the outer flow, however, remains hairpin vortex packets which have similar inclination angles for all wall conditions. The differences between boundary layers over two-dimensional and three-dimensional roughness are attributable to the scales of the motion induced by each type of roughness. This study has shown three-dimensional roughness produces turbulence scales of the order of the roughness heightkwhile the motions generated by two-dimensional roughness may be much larger due to the width of the roughness elements. It is also noted that there are fundamental differences in the response of internal and external flows to strong wall perturbations, with internal flows being less sensitive to roughness effects.
Keywords
This publication has 40 references indexed in Scilit:
- Comparison between experiments and direct numerical simulations in a channel flow with roughness on one wallJournal of Fluid Mechanics, 2008
- Reynolds Number Invariance of the Structure Inclination Angle in Wall TurbulencePhysical Review Letters, 2007
- Direct numerical simulation of the turbulent boundary layer over a rod-roughened wallJournal of Fluid Mechanics, 2007
- Simultaneous orthogonal-plane particle image velocimetry measurements in a turbulent boundary layerJournal of Fluid Mechanics, 2006
- Study of the near-wall-turbulent region of the high-Reynolds-number boundary layer using an atmospheric flowJournal of Fluid Mechanics, 2006
- Experimental support for Townsend’s Reynolds number similarity hypothesis on rough wallsPhysics of Fluids, 2005
- Characteristics of vortex packets in turbulent boundary layersJournal of Fluid Mechanics, 2003
- Surface Roughness Effects on Turbulent Boundary Layer StructuresJournal of Fluids Engineering, 2001
- Vortex organization in the outer region of the turbulent boundary layerJournal of Fluid Mechanics, 2000
- A wall-wake model for the turbulence structure of boundary layers. Part 1. Extension of the attached eddy hypothesisJournal of Fluid Mechanics, 1995