Turbulence structures over irregular rough surfaces

Turbulent flow in a channel with irregular two-dimensional rough surfaces is analysedthrough wall-resolving large eddy simulation (LES). Both walls of the channel areroughened through the superimposition of sinusoidal functions having random amplitudeand four different wavelengths. The downward shift of the velocity profile in thelog region due to the roughness, known as roughness function, is well captured in thesimulations. The spanwise and wall-normal turbulence intensities are found to increasewith the roughness height, while the streamwise component decreases. The analysisof the Reynolds stress anisotropy tensor highlights a tendency towards isotropisation,confirmed by the vorticity rms. The analysis of the statistics shows that the effects ofthe roughness on the turbulent flow are greatly related to the increase of the heightof the maximum peaks of the corrugations. Although the inner layer is dramaticallyaffected by the wall irregularities, the outer layer appears not affected by the specificwall shape and a collapse of the turbulence statistics in smooth- and rough-wall conditionsis observed. Much of the present results are consistent with the observationsmade over regular roughness, nevertheless the turbulence structures and the turbulentkinetic energy production analysis shows that the turbulence dynamics are directlyaffected by the local shape of the irregularities. The spatial inhomogeneities of theheight of the roughness peaks and of the cavity region locally modifies the structuresintroducing variable length-scales. Overall the irregular roughness destroys the coherentpattern of the flow selectively, depending on the spatial distribution of the higherroughness peaks.