Turbulent drag reduction by anisotropic permeable substrates-analysis and direct numerical simulations
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Abstract
We explore the ability of anisotropic permeable substrates to reduce
turbulent skin-friction, studying the influence that these substrates have on
the overlying turbulence. For this, we perform DNSs of channel flows bounded by
permeable substrates. The results confirm theoretical predictions, and the
resulting drag curves are similar to those of riblets. For small
permeabilities, the drag reduction is proportional to the difference between
the streamwise and spanwise permeabilities. This linear regime breaks down for
a critical value of the wall-normal permeability, beyond which the performance
begins to degrade. We observe that the degradation is associated with the
appearance of spanwise-coherent structures, attributed to a
Kelvin-Helmholtz-like instability of the mean flow. This feature is common to a
variety of obstructed flows, and linear stability analysis can be used to
predict it. For large permeabilities, these structures become prevalent in the
flow, outweighing the drag-reducing effect of slip and eventually leading to an
increase of drag. For the substrate configurations considered, the largest drag
reduction observed is
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1469-7645