An experimental study of organized motions in the turbulent plane mixing layer

Large-scale coherent structures in a large, single-stream plane mixing layer of air have been investigated experimentally. The unforced, initially fully turbulent mixing layer rolls up into organized structures whose average passage frequency f m at any downstream distance x from the lip depends on x . These structures are detected for the entire length of the measurement, i.e. up to x = 3 m or 5000θ e . The Strouhal number St θ (= f m θ/ U e ) is observed to be a constant (≈ 0.024) at all x . θ e and θ are, respectively, the exit and local momentum thicknesses of the mixing layer, and U e is the free-stream velocity. (The entrainment velocity on the zero-speed side is found to be 0.032 U e .) The coherent-structure properties are educed in the developing and self-preserving regions of the mixing layer using an optimized conditional-sampling method, triggered on the peaks of a local reference ũ-signal obtained from the high-speed edge of the layer. Sectional-plane contours of the properties of the structure such as coherent vorticity, Reynolds stress and production reveal that the structure formation and evolution are complete by x ≅ 500θ e , beyond which the structure achieves an ‘equilibrium’ state as defined by the structure properties.