Dynamic Behavior of an Unsteady Turbulent Boundary Layer

Summary

This paper reports experiments on an unsteady turbulent boundary layer. The upstream portion of the flow is steady (in the mean). In the downstream region, the boundary layer sees a linearly decreasing free-stream velocity. This velocity gradient oscillates in time, at frequencies ranging from zero to approximately the bursting frequency. Considerable detail is reported for a low-amplitude case, and preliminary results are given for a higher amplitude sufficient to produce some reverse flow. For the small amplitude, the mean velocity and mean turbulence intensity profiles are unaffected by the oscillations. The amplitude of the periodic velocity component, although as much as 70% greater than that in the free stream for very low frequencies, becomes equal to that in the free stream at higher frequencies. At high frequencies, both the boundary layer thickness and the Reynolds stress distribution across the boundary layer become frozen. The behavior at larger amplitude is quite similar. Most importantly, at sufficiently high frequencies the boundary layer thickness remains frozen at its mean value over the oscillation cycle, even though flow reverses near the wall during a part of the cycle.