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Ryan T. Reynolds
School of Engineering Sciences, University of Southampton Highfield, Southampton S017 1BJ, UK

Ian P. Castro
School of Engineering Sciences, University of Southampton, Southampton, SO17 1BJ, U.K.


Very rough 3D surfaces, typical of urban environments, have roughness elements which occupy a significant percentage of the developing boundary layer above. Laboratory wind tunnel measurements over staggered cube arrays have been undertaken to identify the influence of the relative roughness size on this flow. The surface flow around cubic roughness arrays shows strongly three-dimensional behavior, as seen with anisotropic invariant analysis. The strong contributions to the local shear stress from sweep events bringing high momentum fluid down into the cube canopy are not found in smaller roughness cases. The strong shear layer which exists at the top of the cubes (within the roughness sublayer) is strongly dependent on the boundary layer thickness to roughness height ratio. For deeper boundary layers, relatively, the increased range of scales of motion may lead to smaller peak turbulence intensities. Spatial correlation measurements have also shown the influence of the relative boundary layer thickness on the integral length scales in the near-wall region. The streamwise and wall-normal integral length scales derived from spatial correlations initially increase with height over the cubes and then remain constant for large portions of the outer boundary layer. In summary, the local roughness geometry and relative element size play an important role in determining the flow structure and turbulence characteristics over very rough surfaces.