Topology of the near-field vortical structures in a three-dimensional wake

Detailed three-dimensional phase-averaged measurements have been obtained of the spanwise and streamwise vorticity formation and near-field development in a forced plane wake. The plane wake with nominally two-dimensional laminar initial boundary layers was generated in a shear layer wind tunnel. Acoustic forcing at the fundamental roll-up frequency was used to phase-lock the initial formation and subsequent development of the spanwise vortical structures. Phase-averaged measurements of all three velocity components permitted the study of three-dimensional vorticity distributions without involving Taylor's hypothesis. The phase-averaged measurements show that the streamwise vorticity first appears just upstream of the spanwise vortex roll-up, in a region experiencing streamwise stretching as rollers are formed. As spanwise rollers of opposite sign are shed alternately, a row of streamwise vortices is generated in an alternating sequence on either side of the wake. The streamwise vortices go on to form vortex-loop structures, the ''legs'' (ribs) of which connect adjacent spanwise rollers of opposite sign while the ''heads'' ride over the rollers. In addition, relatively strong contributions to the streamwise vorticity are obtained from the rollers as a result of their kinking in the streamwise direction. The contribution from the rollers is of opposite sign to that of the surrounding rib. In terms of phase-averaged quantities, the maximum streamwise vorticity in the initial ribs is equivalent to about 40% of the peak spanwise vorticity and the rib circulation is equivalent to about 20% of the spanwise roller circulation. (C) Elsevier Science Inc., 1996