Effect of Unsteady Wake Passing Frequency on Boundary Layer Transition, Experimental Investigation, and Wavelet Analysis

Abstract

Detailed experimental and theoretical investigations were carried out to study the effect of unsteady wake passing frequency on the boundary layer transition along the concave surface of a curved plate under a zero longitudinal pressure gradient. Periodic unsteady flow with different passing frequencies is generated utilizing an unsteady flow research facility with a rotating cascade of rods positioned upstream of the curved plate. Extensive unsteady boundary layer measurements are carried out. The data are analyzed using conventional and wavelet-based methods. Local time scales are defined as those of the most energetic fluctuations, and are calculated from wavelet transforms of the velocity signals. The dominant time scales are mapped as functions of the distance to the plate, the downstream location, and the phase relative to the wake-passing. Furthermore, conditional sampling is applied, laminar and turbulent time scales are calculated and the effects of wake passing frequency on these scales are shown.