Conditional spatially averaged turbulence and dispersion characteristics in flow over two-dimensional dunes

Conditional turbulence and dispersion characteristics in flow over a series of two-dimensional dunes are analyzed by applying the spatial averaging methodology. To this end, the flow data measured over an array of points on the vertical central plane were used. The manifestation of the wake-interference flow over dunes, including the boundary layer diffusion, flow separation, and flow reattachment, is demonstrated by overlapping the vorticity contours on the velocity vector diagram. The vertical distributions of the spatially averaged (SA) streamwise velocity, Reynolds shear and normal stresses, dispersive shear and normal stresses, turbulent kinetic energy (TKE) fluxes, and dispersive kinetic energy (DKE) fluxes are analyzed. It is recognized that the dominance of the conditional SA streamwise velocity associated with the temporal sweep events causes the flow to accelerate temporally. The conditional SA Reynolds stresses and TKE fluxes associated with the temporal ejection events are dominant above the dune crest, whereas those associated with the temporal sweep events are the governing mechanism below the crest. An examination of the conditional dispersive normal stresses and the DKE fluxes reveals that within the upper portion of the roughness sublayer, the dispersive ejection events are the key flow dispersion mechanism, whereas near the dune trough, the dispersive outward interaction events prevail. Furthermore, the temporal ejection events are found to be more persistent, but less frequent, than the temporal sweep events. Besides, the frequencies of the occurrences of the temporal bursting events are higher below the crest than those above it, suggesting the flow to be more turbulent below the crest.