Dynamics of multi-scale vortical structures behind a barchan dune

In this study, multi-scale vortical structures and vortex dynamics around a fixed-bed barchan dune have been experimentally investigated based on the particle image velocimetry technique, wavelet transform, and the finite-time Lyapunov exponent method. It was found that the dynamic characteristics of a dune wake are predominated by large- and intermediate-scale coherent structures. Quadrant analysis of the Reynolds-stress distribution for the corresponding wavelet components revealed that ejection and sweep events are the main contributors to the whole field, while outward and inward interaction events just dominate the region near the dune crest. In addition, the process of ejection and sweeping motions associated with the turbulent bursting sequence can also be captured by applying proper orthogonal decomposition analysis of the decomposed velocity field for the different wavelet components. Finally, a continuous development process of the different wavelet scale structures in the shear layer was visualized in the Lagrangian framework. The small-scale waves grow exponentially and gradually develop into larger-scale vortices when convected downstream until the reattachment point, and larger-scale vortices break into the smaller ones.

Automated summary

This study investigates the multi-scale vortical structures and vortex dynamics around a fixed-bed barchan dune using particle image velocimetry technique, wavelet transform, and the finite-time Lyapunov exponent method. The results show that large- and intermediate-scale coherent structures dominate the dynamic characteristics of the dune wake. Ejection and sweep events contribute the most to the entire field, while outward and inward interaction events mainly occur near the dune crest. The development process of different wavelet scale structures in the shear layer is visualized in the Lagrangian framework, where small-scale waves grow into larger-scale vortices downstream until the reattachment point.