EFFECTS OF STREAMWISE VORTICITY INJECTION ON A PLANE TURBULENT WAKE

Past now visualization investigations of plane wakes originating from laminar initial boundary layers have shown the presence of streamwise vortical structures riding among the spanwise (von Karman) vortices. Q uantitative measurements of the mean streamwise vorticity were recentl y obtained which shed more light on their formation and subsequent evo lution. Measurements made in the same wake with both initial boundary layers turbulent, however, showed that spatially stationary streamwise structures did not exist in this case, thus resulting in a distinctly different mean three-dimensional structure and streamwise evolution. In the present study, the effects of injecting streamwise vorticity on the structure and development of a wake originating from turbulent bo undary layers are investigated. A corrugated extension attached to the splitter plate trailing edge produced a relatively strong array of co unter-rotating pairs of streamwise vorticity which were injected into the turbulent wake. This resulted in large spanwise variations in the wake mean properties and Reynolds stress distributions. Although the m ean streamwise vorticity decreased with downstream distance, the spanw ise variations were found to persist into the far-field region (X/Thet a > 500) of the wake. In the near-held region (X/Theta < 500),the wake growth was increased, and the velocity defect was reduced, both becau se of increased entrainment due to the injected vorticity. However, in the far-field region, the forced wake growth rate and peak Reynolds s tresses were reduced significantly with respect to the undisturbed wak e. This result is attributed to the effect of the relatively strong st reamwise vorticity in making the spanwise structures more three dimens ional and, hence, less efficient at entraining surrounding fluid.