Turbulence structures over irregular rough surfaces

Turbulent flow in a channel with irregular two-dimensional rough surfaces is analysed through wall-resolving large eddy simulation (LES). Both walls of the channel are roughened through the superimposition of sinusoidal functions having random amplitude and four different wavelengths. The downward shift of the velocity profile in the log region due to the roughness, known as roughness function, is well captured in the simulations. The spanwise and wall-normal turbulence intensities are found to increase with the roughness height, while the streamwise component decreases. The analysis of the Reynolds stress anisotropy tensor highlights a tendency towards isotropisation, confirmed by the vorticity rms. The analysis of the statistics shows that the effects of the roughness on the turbulent flow are greatly related to the increase of the height of the maximum peaks of the corrugations. Although the inner layer is dramatically affected by the wall irregularities, the outer layer appears not affected by the specific wall shape and a collapse of the turbulence statistics in smooth-and rough-wall conditions is observed. Much of the present results are consistent with the observations made over regular roughness, nevertheless the turbulence structures and the turbulent kinetic energy production analysis shows that the turbulence dynamics are directly affected by the local shape of the irregularities. The spatial inhomogeneities of the height of the roughness peaks and of the cavity region locally modifies the structures introducing variable length-scales. Overall the irregular roughness destroys the coherent pattern of the flow selectively, depending on the spatial distribution of the higher roughness peaks.