MEASUREMENTS OF SPANWISE SCALE CHANGE IN A FORCED MIXING LAYER

Spanwise scale changes of the streamwise vortical structure in a plane forced mixing layer have been investigated through direct measurement s. Detailed three-dimensional phase-averaged measurements were obtaine d of the spanwise and streamwise vorticity in a forced mixing layer un dergoing three spanwise roller pairings. A two-stream mixing layer wit h a velocity ratio (U-2/U-1) of 0.6 and laminar initial boundary layer s was generated in a mixing-layer wind tunnel. Acoustic forcing, consi sting of a fundamental roll-up frequency and its first, second and thi rd subharmonics, was used to phase-lock the initial development and th e first three pairings of the spanwise rollers. Although the overall s panwise scale remained unchanged through the first two roller pairings , some (cyclic) 'readjustment' of the weaker streamwise structures was observed. The overall spanwise scale doubled during the third roller pairing. For the first time, one of the proposed mechanisms for the sc ale change has been identified and its details measured directly. The weakest (positive) streamwise vortex is split into two and displaced b y stronger neighbouring (negative) vortices. These two vortices (of th e same sign) then merge together, thus doubling the spanwise scale and circulation of the resulting streamwise vortical structure.