VORTICAL STRUCTURE MORPHOLOGY IN THE INITIAL REGION OF A FORCED MIXING LAYER - ROLL-UP AND PAIRING

Detailed three-dimensional phase-averaged measurements of the spanwise and streamwise vorticity formation and evolution in a forced mixing l ayer have been obtained. A plane two-stream mixing layer with a veloci ty ratio (U-2/U-1) Of 0.6, a maximum Reynolds number (Re-delta) of abo ut 3150 and laminar initial boundary layers was generated in a mixing layer wind tunnel. Acoustic forcing, consisting of a fundamental roll- up frequency and its subharmonic, was used to phase-lock the initial d evelopment and first pairing of the spanwise vortical structures. For the first time, phase-averaged measurements of all three velocity comp onents have been obtained on a three-dimensional grid, yielding the sp anwise and streamwise vorticity distributions without invoking Taylor' s hypothesis. The phase-averaged results show that the streamwise vort icity first appears in the form of 'ribs' just upstream of the first s panwise vortex roll-up. At the same time, the first spanwise roller be comes kinked, thus also contributing to the streamwise vorticity. As a result, in cross-stream cuts through the spanwise rollers, the stream wise vorticity appears in a 'three-tier' arrangement with opposite-sig ned vorticity in the centre. In terms of phase-averaged quantities, th e maximum streamwise vorticity in the initial ribs is equivalent to ab out 10-15% of the peak spanwise vorticity and the streamwise rib circu lation is equivalent to about 5-10% of the spanwise circulation. Furth er downstream, the peak streamwise vorticity decreases with increasing distance, while the average circulation remains approximately constan t. Downstream of the pairing, the streamwise vorticity levels in the s panwise rollers are reduced. However, the spanwise spacing of the stre amwise vortices does not increase within the measurement domain. Phase -averaged Reynolds stress measurements show that relatively high stres s levels (periodic and random) were generated in the cores of the span wise vortices.