Reversal of nonlinear energy transfer direction in flows over low-aspect-ratio plates

This study investigates the energy exchange between coherent structures in flows over four low-aspect-ratio (low- ) plates using the tomographic particle image velocimetry dataset originally obtained by Zhu et al . (2024. J. Fluid Mech. 983 , A35). The chord-based Reynolds number is $5400$ , with fixed angle of attack $6 ^\circ$ . In this study, multiscale proper orthogonal decomposition is applied to extract the coherent structures, including those associated with the vortex-shedding frequency $St_1$ and its subharmonic counterpart. Subsequently, the coherent kinetic energy budget is analysed with a focus on inter-scale energy transfer. This study demonstrates that the energy transfer between the scales centred at $St_1$ and $0.5\,St_1$ can exhibit a reverse or forward direction, depending on the transformation pattern of the leading-edge vortices (LEVs). Specifically, different triadic interactions are excited during the LEV transformation, and manifest themselves during the formation of hairpin vortices downstream. Understanding this nonlinear energy transfer is essential for elucidating mechanisms underlying the development of turbulence in three-dimensional flows over low- plates.