New perspectives on the impulsive roughness-perturbation of a turbulent boundary layer

Abstract

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.

Dynamic roughness perturbation of a turbulent boundary layer

“…The turbulent boundary layer experiments were performed in the 2 ft × 2 ft wind tunnel at Caltech, previously described in Jacobi & McKeon (2011), where a zero-pressure-gradient was maintained by an adjustable ceiling which limited the spatial variation in pressure coefficient C p 0.01 over the range of streamwise measurement locations; thus irrotational effects are assumed negligible. The boundary layer was tripped at the leading edge, far upstream of the position of the dynamic perturbation.…”

“…The oscillation was not perfectly sinusoidal due to slippage and frictional non-uniformities in the slots through which the armature reciprocated. The flow field downstream of the roughness impulse was measured by hot-wire anemometry and particle-image velocimetry (PIV), details of which, including validation of the unperturbed flow, have been previously reported by Jacobi & McKeon (2011).…”

“…The dynamically perturbed case refers to a perturbation by the roughness patch when the patch was actuated by the motor; the statically perturbed case refers to the previous study by Jacobi & McKeon (2011) with the identical roughness elements Dynamic roughness perturbation of a turbulent boundary layer 265 fixed in position at a roughly equivalent amplitude. The streamwise growth rate of the boundary layer thickness was approximately the same for both the static and dynamically perturbed flows, and both perturbed boundary layers grew more quickly than the unperturbed boundary layer.…”

“…More recently, Jacobi & McKeon (2011) reconsidered the case of a short impulse of static roughness on an otherwise smooth flat plate, and were able to place a number of the key observations of previous experimenters into a simple and physically motivated framework. By building on the idea of the 'stress bore' introduced by Smits et al (1979) in the context of abrupt bends in otherwise flat plates, it was shown that the boundaries of the internal layers generated by a static impulse of roughness roughly demarcate a region of the downstream flow field which behaves as a bore of shear stress.…”

Experimental manipulation of wall turbulence: A systems approach

“…The essential details of the experimental study follow (full details of the facility, operating conditions and measurement techniques can be found in Jacobi and McKeon 24,64 ). The experiments were performed in a low free-stream turbulence, incompressible, zero-pressure-gradient turbulent boundary layer.…”