Abstracts - KTH Mechanics

Very-rough-wall boundary layers
Ian P Castro ∗ and Ryan Reynolds ∗
Rough-wall boundary layers are ubiquitous. It is therefore striking that despite
their practical importance a number of the most fundamental questions concerning
the effects of surface roughness on the nature of the turbulent boundary layer remain
unanswered. The classical view was expounded by Townsend 1 , who argued that the
major effect is simply to change the surface stress, altering the value of the constant
in the usual log-law relation for the mean velocity profile, but without structural
changes in the turbulence. This amounts essentially to the assumption that the inner
and outer regions do not interact strongly. For small ratios of roughness height to
boundary layer depth, h/δ < 0.03,say,thereismuchevidencethatthisviewis
reasonable 2,3 . But it must inevitably become increasingly less so as h/δ rises and
thereisevidencethatforlargeenoughh/δ not only can structural features change
throughout the flow, but also that these differences (from the smooth wall boundary
layer) may depend on the topology of the roughness 4,5 . It remains generally unclear
exactly how and under what circumstances the roughness effects extend throughout
the flow.
In this presentation we will discuss these issues, provide further evidence that
roughness effects on the outer flow depend on h/δ and highlight the current uncertainties
about such flows. An example of current work is provided by figures 1(a) and
(b), which show PIV data in the boundary layer flow over a staggered array of cubes
(with h/δ = 14). Such data yield turbulence structural information and differences
from the usual smooth-wall behaviour will be emphasised.
∗ School of Engineering Sciences, University of Southampton, Southampton, UK
1 Townsend AA, The structure of turbulent shear flows, CUP
2 Jimenez J, Ann. Rev. Fluid Mech. 36, 173-196 (2004).
3 Raupach et al., Appl. Mech. Rev. 44, 1-25 (1997).
4 Krogstad P-A & Antonia RA, Expts. in Fluids, 27, 450-460 (1999)
5 Smalley et al., Expts. in Fluids, 33, 31-37 (2002)
Z(mm)
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CrossCorr: -0.20 -0.08 0.04 0.16 0.28 0.40 0.52 0.64 0.76 0.88 1.00
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Figure 1: (a) PIV ’snapshot’ of fluctuating velocity vectors behind cube in a staggered
array; (b) Spatial correlation contours of the axial mean velocity - fixed ’probe’ above
centre of cube.
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