Streamwise Variation of Mean Velocity Field for the Turbulent Boundary Layer interacting with Controlled Longitudinal Vortex Arrays.

Abstract

Detailed mean flow measurements have been made in a planar turbulent boundary layer interacting with artificially generated trailing vortex arrays in a free stream. Two parameters, namely, spanwise periodicity of the longitudinal vortex arrays (L/S ; here L denotes the spanwise distance between neighboring airfoils, S airfoil span) and ratio of the longitudinal vortex scale b to boundary layer scale δ0, were independently varied for five cases. Within the boundary layer, there exists a spanwise transport of streamwise momentum due to pairs of counterrotating secondary flows. As a result, spanwise periodic variation corresponding to the spanwise spacing of airfoil elements arises in the mean flow properties, and is persistently maintained in the far downstream region. The value of L/S plays a dominant role under the distortion process in this flow field. In the case of L/Sg1, the spanwise periodic variations of mean flow profiles are significantly large, and these spanwise periodicities are persistently maintained in the streamwise direction. In the case of L/S<1, although the spanwise variations are small in magnitude, the spanwise periodicities of mean flow profiles change in the streamwise direction, while the effect of b/δ0 is caused as a local Reynolds number effect.