A combined visualization-anemometry study of the turbulent drag reducing mechanisms of triangular micro-groove surface modifications

Flow visualization and hot-film anemometry studies have been conducted to investigate the influence of small-scale (15 viscous wall units) streamwise surface riblets on the structure, surface drag, and bursting characteristics of a turbulent boundary layer. Visualization studies indicate that the familiar low-speed streak structure forms above the riblet surface, but that spanwise streak spacing is increased by 40 percent over flat plate flows. Fluid within the riblets is observed to move very slowly, with lateral transport of fluid becoming negligible near the riblet surface. Profiles of mean velocity and turbulence statistics indicate that the overall character of the turbulent flow over the riblet surface is similar to that over a flat plate. A momentum balance indicates that the surface drag for the riblet surface is approximately 25 percent lower than a flat plate flow, with more momentum retained in the logarithmic region above the grooved plate than above the flat plate. Comparative examination of turbulence bursting using the VITA technique revealed no difference in the burst frequency for the riblet surface; however, ensemble averaged velocity behavior indicates the presence of a more organized bursting behavior above the riblet surface. A model suggesting how the riblets affect momentum transport in the near-wall is hypothesized.