On the Response of Planar Wakes to Asymmetric Initial Conditions

This work reports an experimental investigation on the response of a planar wake generated by a profiled flat plate to various upstream flow conditions. A tripping wire was placed on the upper side of the flat plate just downstream of the leading edge of the plate that resulted in asymmetric separating shear layers at the trailing edge. The near wake asymmetry is compared to the symmetrical case at two different Reynolds numbers. Two asymmetric initial conditions resulted, namely, laminar boundary layer on the lower side and a turbulent boundary layer on the upper side, and a turbulent boundary layer on the lower side and tripped turbulent boundary layer on the upper surface. The near wake dynamics were investigated under the effects of the degree of asymmetry using hot-wire anemometry and flow visualizations. The measurements showed when one of the two boundary layers was tripped, the wake shifted towards the tripped side and wake spreading was found to be larger than in the case of the symmetrical wake with the effect being more pronounced in the asymmetric laminar wake. Self-similarity of the asymmetrical wakes was established by properly selecting appropriate similarity variables however, the similarity of the wake was less evident in the tripped laminar boundary layer case. Convection velocity, U_c, of the Von Karman large eddies, estimated using processed flow visualization images seemed to increase with increased Reynolds number and with increased upstream momentum thickness. In the symmetric laminar wake, U_c/U_∞ increases from 0.2 and reached an asymptotic value of about 0.85 further downstream. In the presence of perturbation, U_c/U_∞ attained a constant value of about 0.83 further downstream compared to the symmetric case. For the turbulent wake, however, asymmetry of the turbulence levels was found to increase the convection speed compared to both the laminar wake and the symmetric turbulent wake reaching a constant value nearly at the same downstream position for both the symmetric and asymmetric turbulent wake.