Roughness- and buoyancy-triggered secondary flows in gravity currents

This study uses large eddy simulations with a mixture model to investigate how secondary flows (SFs) in gravity currents (GCs), which are triggered by spanwise heterogeneous roughness or unstable buoyancy convection, influence their layer structures. These processes are analogous to those governing density-driven flows in stratified river and estuary systems. We introduce a double-averaged methodology to separate the contributions of SFs and bed roughness to the spatial fluctuations within GCs. Our results show that the spanwise locations of low and high momentum paths for GCs are locked at the crests and valleys of a rough impermeable bed, respectively, while a rough permeable boundary reverses these locations. Strong Rayleigh-Taylor instabilities developing in bed pores can eliminate the roughness-triggered SFs within GCs and generate new buoyancy-driven ones with an opposite rotation. Asymmetric boundary shear creates a barrier layer of GCs that prevents the SFs from penetrating their jet region, which continuously intensifies the rolls but restricts their vertical growth. On rough impermeable beds, these SFs sustain as a coexistence of the first and second kinds, with the first kind generated by streamwise vortex stretching. On rough permeable beds, the second kind dominates as unsteady buoyancy convection breaks the skewing of the mean shear induced by the spanwise pressure gradient. In the mean flow field, energy-transfer terms related to the SFs and bed roughness alleviate and exacerbate the uneven distribution of mean kinetic energy, respectively. In the dispersive field, the SFs-related component transfers dispersive kinetic energy from the lower part of SFs to their upper part, while the bed-roughness-related one makes an inverted transfer with a relatively small contribution. In the turbulent field, transfer terms related to the SFs and bed roughness both tend to suppress the homogenization of turbulent distribution within GCs. These findings provide insight into complex flow-bed interactions relevant to component transport and mixing processes in estuaries and oceans.