Behavior of an Unsteady Turbulent Boundary Layer

Measurements on an unsteady turbulent boundary layer with well-defined initial and boundary conditions are reported in this paper. The test boundary layer is a standard, steady, flat-plate turbulent boundary layer at the entrance to the unsteady region, and is subjected to a linearly decreasing freestream velocity in a time-dependent manner. The experiments were conducted in a specially designed water tunnel with the frequency of the imposed oscillations ranging from quasisteady (/"^O) up to the "bursting frequency" in turbulent boundary layers. It was found that the mean velocity and turbulence intensity profiles were unaffected by the imposed oscillations. The amplitude ratio of the periodic component, although as much as 1.7 for quasisteady oscillations, becomes unity over the outer region of the boundary layer at higher frequencies. Both the boundary-layer thickness and the Reynolds stress distribution across the boundary layer become frozen over the oscillation cycle at high frequencies.