Streamwise vortex structure in plane mixing layers

TLDR

The study investigates three aspects of the secondary streamwise vortex structure in a plane mixing layer. Experimentally, the authors characterized the spanwise vortex instability that generates the secondary structure by measuring critical Reynolds numbers and wavelengths, visualized the counter‑rotating vortices, proposed a winding vortex‑line model, and quantified its impact on spanwise entrainment in a helium–nitrogen mixing layer. The secondary streamwise vortex structure is present over the primary spanwise vortices, its critical Reynolds number varies with velocity ratio, density ratio, and shear‑layer profile while its normalized wavelength remains constant, and it creates a distinct spanwise entrainment pattern whose scale grows downstream.

Abstract

The development of three-dimensional motions in a plane mixing layer was investigated experimentally. It is shown that superimposed on the primary, spanwise vortex structure there is a secondary, steamwise vortex structure. Three aspects of this secondary structure were studied. First, the spanwise vortex instability that generates the secondary structure was characterized by measurements of the critical Reynolds number and the spanwise wavelength at several flow conditions. While the critical Reynolds number was found to depend on the velocity ratio, density ratio and initial shear-layer-profile shape, the mean normalized wavelength is independent of these parameters. Secondly, flow visualization in water was used to obtain cross-sectional views of the secondary structure associated with the streamwise counter-rotating vortices. A model is proposed in which those vortices are part of a single vortex line winding back and forth between the high-speed side of a primary vortex and the low-speed side of the following one. Finally, the effect of the secondary structure on the spanwise concentration field was measured in a helium–nitrogen mixing layer. The spatial organization of the secondary structure produces a well-defined spanwise entrainment pattern in which fluid from each stream is preferentially entrained at different spanwise locations. These measurements show that the spanwise scale of the secondary structure increases with downstream distance.