On the instability characteristics of a reattaching shear layer with nonlaminar separation

The instability characteristics of a reattaching shear layer with nonlaminar separation were investigated via controlled excitation. The most dominant structure of the flow, termed the step mode of instability, was at St h = 0.185, which was independent of the separating boundary layer's state. An explanation has been provided for whether the spectral energy for an excited flow will be concentrated at the excitation frequency f or at its higher harmonic component 2 f. The shear layer with transitional separation failed to support the growth of a disturbance. This was attributed to the high level of turbulence intensity in the initial boundary layer. The downstream evolotion of longitudinal velocity spectra showed that the step-mode instability peak quickly takes over the shear-layer instability peak at St θ ≊ 0.011 for the turbulent-separation case and prevails further downstream of the reattachment point.