Dynamics of an Unsteady Turbulent Boundary Layer

Abstract

The response of a well-defined, steady, flatplate turbulent boundary layer subjected subsequently to two types of oscillations of the freestream velocity in the test section is studied in this investigation. In the first case, the free-stream velocity decreased linearly in the test section in a time-dependent manner such that the magnitude of the streamwise gradient of the free-stream velocity varied sinusoidally from zero to a maximum value at frequencies ranging from zero to approximately the bursting frequency. In the second case, the mean free-stream velocity decreased in the test section in a nonlinear fashion independent of the oscillation frequency, while the amplitude and phase of the free-stream oscillations varied along the streamwise direction in an arbitrary manner, depending upon the oscillation frequency. Detailed measurements are reported for the first case. It was found that the mean velocity and mean turbulence intensity profiles in the boundary layer were unaffected by the imposed oscillations. The ratio of the amplitude of the periodic velocity component in the boundary layer to that in the free-stream, although as much as 1.7 for quasi-steady oscillations, becomes unity over the outer region of the boundary layer at high frequencies. Also, at high frequencies of imposed oscillations, both the boundary layer thickness and the Reynolds stress distribution across the boundary layer becomes frozen over the oscillation cycle at their mean values. Only the mean velocity profile measurements were carried out in the second case. It was found that, despite the large streamwise variations of the amplitude and phase of the free-stream oscillations, the mean velocity profile in the boundary layer once again remained independent of the imposed oscillation frequency so long as the mean free-stream velocity distribution remained unchanged.