New results on the structure of turbulence in a mixing layer with and without swirl

• We study the near field of a round jet at Re ≈ 200,000, with and without swirl.
• The dynamics of streamwise vortices inducing mixing is analyzed with TR-PIV.
• Vortices of alternate sign are organized as radial arrays, instead of azimuthal.
• Swirl acts by tilting this structure, leading to enhanced injections and ejections.
• Dynamical scenarios are proposed to explain the origin of this spatial organization.

The near field of a 2.14 × 105 Reynolds number, low-Mach-number, cylindrical jet with and without swirl has been investigated by high-speed stereo PIV in a transverse plane, two diameters downstream of the jet exit. Using spatiotemporal correlations, we investigate the dynamics of streamwise vorticity in the shear layer, responsible for the mixing properties of the jet, and show how swirl affects this vorticity. A dynamical scenario is proposed, which explains how the mean shear and the azimuthal m = 0 vortices contribute to the spatial organization observed.