3-DIMENSIONAL STRUCTURE OF STRAIGHT AND CURVED PLANE WAKES

The formation and evolution of the three-dimensional structure of stra ight and mildly curved (b/(R) over bar < 2 %) flat plate wakes at rela tively high Reynolds numbers (Re-b = 28 000) have been studied through detailed measurements of the mean and fluctuating velocities. In both cases, the role of initial conditions was examined by generating wake s from untripped (laminar) and tripped (turbulent) initial boundary la yers. The curved wake was affected by the angular momentum instability such that the inside half of the wake was unstable, whereas the outsi de half was stable. In both the straight and curved untripped wakes, l arge spanwise variations, in the form of 'pinches' and 'crests', were observed in the contours of mean velocity and Reynolds stresses. Well- organized, 'spatially stationary' streamwise vorticity was generated i n the near-field region in the form of quadrupoles, to which the spanw ise variations in the velocity contours were attributed. The presence of mean streamwise vorticity had a significant effect on the wake grow th and defect decay rates, mainly by providing additional entrainment. In the straight wake, the mean streamwise vorticity decayed on both s ides of the wake such that it had decayed completely by the far-field region. However, in the curved case, the mean streamwise vorticity on the unstable side decayed at a rate significantly lower than that on t he stable side. Despite the decay of mean streamwise vorticity, the sp anwise variations persisted into the far wake in both cases. The effec ts of curvature were also apparent in the Reynolds stress results whic h showed that the levels on the unstable side were increased significa ntly compared to those on the stable side, with the effect much strong er in the initially laminar wake. With the initial boundary layers tri pped, spatially stationary streamwise vortex structures were not obser ved in either the straight or curved wakes and the velocity contours a ppeared nominally two-dimensional. This result further confirms the st rong dependency of the three-dimensional structure of plane wakes on i nitial conditions.