TOPOLOGY OF THE NEAR-FIELD VORTICAL STRUCTURES IN A 3-DIMENSIONAL WAKE

Detailed three-dimensional phase-averaged measurements have been obtai ned 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 l ayer wind tunnel. Acoustic forcing at the fundamental roll-up frequenc y was used to phase-lock the initial formation and subsequent developm ent of the spanwise vortical structures. Phase-averaged measurements o f all three velocity components permitted the study of three-dimension al vorticity distributions without involving Taylor's hypothesis. The phase-averaged measurements show that the streamwise vorticity first a ppears just upstream of the spanwise vortex roll-up, in a region exper iencing streamwise stretching as rollers are formed. As spanwise rolle rs of opposite sign are shed alternately, a row of streamwise vortices is generated in an alternating sequence on either side of the wake. T he streamwise vortices go on to form vortex-loop structures, the ''leg s'' (ribs) of which connect adjacent spanwise rollers of opposite sign while the ''heads'' ride over the rollers. In addition, relatively st rong contributions to the streamwise vorticity are obtained from the r ollers as a result of their kinking in the streamwise direction. The c ontribution from the rollers is of opposite sign to that of the surrou nding rib. In terms of phase-averaged quantities, the maximum streamwi se vorticity in the initial ribs is equivalent to about 40% of the pea k spanwise vorticity and the rib circulation is equivalent to about 20 % of the spanwise roller circulation. (C) Elsevier Science Inc., 1996