A High Reynolds Number Turbulent Boundary Layer with Regular ‘Braille-Type’ Roughness

A new high Reynolds number wind-tunnel facility at New Mexico State University (NMSU) was fitted with a roughened surface consisting of sheets of paper embossed by a Braille printer. The resulting roughness distribution was regular, three-dimensional and relatively sparse in the spanwise direction. Careful hot-wire studies show that the near-wall peak in turbulence intensity is reduced by the roughness, as expected. Comparisons with smooth-wall data indicate that turbulence is only affected by the roughness in the near-wall region, again as expected. However, analysis of the energy spectra showed an unexpected result: large-scale structures are significantly influenced by the roughness elements, despite the elements being 2 orders of magnitude smaller in size than these large-scale motions. This result excites the possibility of manipulating large-scale flow features with tiny protrusions.