New perspectives on the impulsive roughness-perturbation of a turbulent boundary layer
The zero-pressure-gradient turbulent boundary layer over a flat plate was perturbed by a short strip of two-dimensional roughness elements, and the downstream response of the flow field was interrogated by hot-wire anemometry and particle image velocimetry. Two internal layers, marking the two transitions between rough and smooth boundary conditions, are shown to represent the edges of a 'stress bore' in the flow field. New scalings, based on the mean velocity gradient and the third moment of the streamwise fluctuating velocity component, are used to identify this 'stress bore' as the region of influence of the roughness impulse. Spectral composite maps reveal the redistribution of spectral energy by the impulsive perturbation – in particular, the region of the near-wall peak was reached by use of a single hot wire in order to identify the significant changes to the near-wall cycle. In addition, analysis of the distribution of vortex cores shows a distinct structural change in the flow associated with the perturbation. A short spatially impulsive patch of roughness is shown to provide a vehicle for modifying a large portion of the downstream flow field in a controlled and persistent way.
© 2011 Cambridge University Press. Received 7 September 2010; revised 27 December 2010; accepted 9 February 2011; first published online 26 April 2011. This work is supported by the Air Force Office of Scientific Research Hypersonics and Turbulence portfolio, under grant #FA9550-08-1-0049 (Program Manager J. Schmisseur). The authors wish to thank C. Gonzalez of California Polytechnic University Pomona for assistance with the PIV setup, as well as M. Guala of the Graduate Aerospace Laboratories at the California Institute of Technology for assistance in preparing the wind tunnel for the current experiments. In addition, the authors thank the reviewers for their very helpful comments, which significantly improved this manuscript.
Published - Jacobi2011p14644J_Fluid_Mech.pdf