A low-shear turbulent boundary layer

This paper describes experimental results measured in a low-shear turbulent boundary layer. The low-shear condition is exerted after the boundary layer reaches Re(theta)similar or equal to 2000 and has the effect of removing the inner layer; thus, these are the first results to show the behavior of an outer-layer-only turbulent boundary layer. The removal of the inner layer causes the gradual decay of the turbulent stresses over eleven boundary-layer thicknesses (roughly 20 large-eddy length scales) of streamwise distance, with the decay beginning at the wall and propagating into the outer flow with increasing downstream distance. However, the structure of the outer layer is little affected by the perturbation, as demonstrated by stress (anisotropy) ratios, quadrant analysis, and spectral measurements. Although the lack of near-wall production implies this flow must eventually decay into isotropic turbulence, this decay occurs relatively slowly because the dissipation is also greatly reduced with the decrease in near-wall shear. In addition, the outer-layer production is significant in maintaining the turbulence level. These results show that, once formed, the outer-layer characteristics are not explicitly dependent on the presence of the inner layer. These results are compared with similar studies of isotropic turbulence near a shear-free wall. Very close to the wall the two flows both show that the normal-stress components respond differently to the presence of the wall. However, away from the wall the isotropic results underpredict the distance to which the tangential stresses are damped by the impermeability condition at the wall. Finally, the results show general similarities to those in a boundary layer just downstream of reattachment, after a similar low-shear condition over the separation bubble. This raises the possibility that many of the important features of the reattaching flow can be captured by the present, simpler experiment. (C) 1996 American Institute of Physics.