Effect of different shaped transverse grooves on a zero pressure gradient turbulent boundary layer

An experimental study has been performed to investigate the response of a turbulent boundary layer to different shaped transverse grooves. Square, semicircular and triangular grooves with widths of 5, 10 and 20mm and depth to width (d/w) ratio of unity were investigated. The ratios of the groove depth to the oncoming boundary layer thickness (d/S₀) are 0.067, 0.133 and 0.267, respectively, for d = 5, 10 and 20mm. The experiments were performed at two freestream velocities (U₀) of 2.0 and 5.5m/s, corresponding to Reynolds number (R₀) based on the momentum thickness just upstream of the groove of 1000 and 3000, respectively. The turbulence parameters were measured using hot-wire anemometry, and measurements were performed at several locations downstream of the groove. The development of the wall shear stress and the internal layer downstream of the groove was also investigated. The ejection and sweep events and the bursting frequency were estimated based on the quadrant decomposition method. -- The development of the turbulent boundary layer downstream of the grooves was compared with results from the corresponding smooth-wall case. For all groove shapes, the 20mm groove has the most pronounced effect on all turbulence parameters. In general, the effect of the square groove on the turbulent characteristics is more pronounced than the effects of the semicircular and triangular grooves. The wall shear stress (Շw) was estimated from the slope of mean velocity at the wall. An increase in Շw just downstream of the groove for all groove shapes and sizes was observed. The increase in Շw is followed by a small decrease in Շw below the smooth-wall value before it relaxes back to the corresponding smooth-wall value at approximately x/S₀ = 3. The increase in Շw is more pronounced at R₀= 3000 than that at R₀ = 1000. At R₀= 1000, the spectrum of wall-normal turbulence intensity (v') downstream of the groove shows a significant increase at the higher wave number (k₁), while the increase in the spectrum of streamwise turbulence intensity (u') is not discernible at the same k₁ range. There is an increase in the bursting frequency (ƒB⁺) on the grooved-wall compared to the smooth-wall case. The variation of ƒB⁺ downstream of the groove is somewhat similar to the variation of Շw