Properties of the large structure in a slightly heated turbulent mixing layer of a plane jet

Abstract

The convection speed and average inclination of the large structure in the turbulent mixing layer of a plane jet have been obtained using different techniques. Ensemble-averaged shapes of the signatures of velocity and temperature associated with this structure are also presented. An array of cold wires is used to simultaneously observe the temperature fluctuation θ at several points in space. A cold-wire/X-wire combination is used to measure θ, the streamwise u and the normal v velocity fluctuations. The front of the structure moves faster than the back and the average inclination of this structure to the jet axis is approximately 40°. A detection scheme, based on positive peaks of the cubed values of the temperature derivative, is used to identify the large structure and obtain average signatures of θ, u, v, and of the products uv, θ and vθ. These signatures are consistent with a vortex-like description of the large structure. They also reflect the existence of a thin interconnecting region or ‘braid’ between two consecutive vortical structures. This region is identified by the sharp changes in temperature and is interpreted as part of the large structure. In the central part of the mixing layer, at least 80% of the Reynolds shear stress and 50% of the average normal heat flux appear to be contributed by the large structure.