The response of turbulent boundary layers to small turbulence levels in the external free stream

The influence of grid-generated wind tunnel turbulence on the development of a turbulent boundary layer was studied. The free stream turbulence level was varied in the range of 0.05 to 1 percent and velocity profiles were measured in sidewall boundary layers for different distances from the nozzle exit at approximately zero pressure gradient in the flow direction. For a fixed transition, the shape parameter, skin friction coefficient, and the wake component were estimated from the boundary layer profiles measured at different free stream turbulence levels. It was found that for external turbulence levels less than 1 percent the boundary layer shape parameter and wall shear stress vary with the square of the turbulence intensity rather than in a linear way.