ABSTRACT

Flow and mass transfer experiments are conducted to study the effect of wall shear on a two-dimensional turbulent boundary layer. The shear at the boundary is imparted by a moving belt, flush with the wall. In the first case (Case1), the wall shear is imparted along the direction of the flow. Velocity measurements are taken at 12 streamwise locations with four surface-to-freestream velocity ratios (0, 0.38, 0.52, 0.65) and a momentum-based Reynolds number between 770 and 1776. The velocity data indicate that the location of the "virtual origin" of the turbulent boundary layer "moves" downstream towards the trailing edge of the belt with increasing surface velocity. The highest belt velocity ratio (0.65) results in the removal of the "inner" region of the boundary layer. Measurements of the streamwise turbulent kinetic energy (TKE) show an inner-scaling at locations upstream and downstream of the belt and the formation of a new self-similar structure on the moving surface itself. Good agreement is observed for the variation of the shape factor (H) and the skin friction coefficient (c_f) with previous studies. Mass transfer measurements using naphthalene sublimation provide the variation of Stanton with Reynolds number on the plate downstream of the moving belt. It shows much reduced mass transfer due to removal of the inner region of the boundary at the highest belt velocity. In the second case (Case 2), the wall shear is in a direction opposite to the incoming flow. Boundary layer measurements are reported for four surface-to-freestream velocity ratios (0, -0.37, -0.50, -0.62) with the Reynolds number (based on the momentum thickness) between 922 and 1951. Velocity profiles downstream of the moving surface show an increased velocity deficit near the wall, which is more pronounced at higher (negative) belt velocity. Streamwise turbulence values downstream of the belt show the growth of a second peak in the logarithmic region of the boundary layer in addition to the normally-observed peak in the buffer region. This suggests the presence of larger length-scale turbulent eddies at locations away from the wall in the boundary layer. Spectral measurements indicate that the turbulent energy content is distributed over a wide portion of the logarithmic region. Mass transfer measurements for this case show little difference from the stationary belt case, which suggests that increased wall turbulence is balanced by an increase in the boundary layer thickness.