Core instability of the spanwise vortices in a plane mixing layer

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Abstract
A spanwise core instability of the primary vortices in a plane mixing layer has been identified as a viable mechanism for the generation of streamwise vortical structures far downstream of the flow partition. The core instability is excited by a time‐harmonic wave train with spanwise phase variation, synthesized by a mosaic of individually controlled surface heaters flush mounted on the flow partition. The flow is visualized in planes of constant cross‐stream elevation by means of a schlieren system. As a result of the forcing, the primary vortices undergo spanwise deformations that induce secondary vortical structures, the shape and strength of which depends on the magnitude of the prescribed spanwise phase variation. The appearance of small‐scale structures within the large coherent vortices suggests that core instability is an important contributor to mixing.