An experimental study has been conducted to investigate the three-dimensional structure of a plane, two-stream mixing layer through direct measurements. A secondary streamwise vortex structure has been shown to ride among the primary spanwise vortices in past flow visualization investigations. The main objective of the present study was to establish quantitatively the presence and role of the streamwise vortex structure in the development of a plane turbulent mixing layer at relatively high Reynolds numbers (Re(delta) approximately 2.9 x 10(4)). A two-stream mixing layer with a velocity ratio, U2/U1 = 0.6 was generated with the initial boundary layers laminar and nominally two-dimensional. Mean flow and turbulence measurements were made on fine cross-plane grids across the mixing layer at several streamwise locations with a single rotatable cross-wire probe. The results indicate that the instability, leading to the formation of streamwise vortices, is initially amplified just downstream of the first spanwise roll-up. The streamwise vortices first appear in clusters containing vorticity of both signs. Further downstream, the vortices re-align to form counter-rotating pairs, although there is a relatively large variation in the scale and strengths of the individual vortices. The streamwise vortex spacing increases in a step-wise fashion, at least partially through the amalgamation of like-sign vortices. For the flow conditions investigated, the wavelength associated with the streamwise vortices scales with the mixing-layer vorticity thickness. while their mean strength decays as approximately 1/X1.5. In the near field, the streamwise vortices grossly distort the mean velocity and turbulence distributions within the mixing layer. In particular. the streamwise vorticity is found to be strongly correlated in position, strength and scale with the secondary shear stress (u'w'BAR). The secondary shear stress data suggest that the streamwise structures persist through to what would normally be considered the self-similar region, although they are very weak by this point and the mixing layer otherwise appears to be two-dimensional.
