Some Features of Unsteady Separating Turbulent Boundary Layers
A survey of the physical features of steady and unsteady separating turbulent boundary layers is presented for practical Reynolds numbers and reduced frequencies for helicopter and turbomachinery flows. Well upstream of separation there is little interaction between the periodic motions, so the flow away from the wall has little phase variation from the freestream. Near the wall between the viscous sublayer and the semi-logarithmic region, unexpected phase shifts of the velocity and turbulence oscillations occur.
Near separation and downstream more interaction occurs between the periodic and turbulent motions since the characteristic frequency of the large scale structures is much lower than upstream. Significant phase variations between the velocity and turbulence exist in the detached and back flows. For moderate oscillation amplitudes there is no effect of oscillation waveform on the mean flow features. Large amplitude oscillations affect the flow structure significantly.
KeywordsTurbulent Boundary Layer Eddy Viscosity Freestream Velocity Sinusoidal Waveform Large Amplitude Oscillation
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