Investigation of boundary condition effects on the propagation of density current using direct numerical simulations

The propagation of density current in a channel has been studied extensively using theoretical, experimental and numerical tools. For high resolution numerical method, such as direct numerical simulations (DNS), the boundary conditions on the bottom and top of the channel are usually assumed to be no-slip and no-penetration. This study aims to investigate the effects of various boundary conditions encountered in reality, such as shear-stress free top boundary in an open channel, wind shear, suction/blowing bottom due to groundwater flow. The DNS code used in the research implements a revised Kleiser and Schumann (1980) influence-matrix method to treat the Robin type velocity boundary conditions and the related "tau" error corrections. This revised method broadens the applicability of the original Kleiser and Schumann method and is ideal for the purpose of this research. Comparisons of the simulation results reveal that the boundary conditions changes the turbulent flow field and therefore the propagation of the front. The effects on some of the parameters (such as front speed) are investigated and quantified. Further study need to address the scale effects when the vertical scale of the density current is small than or comparable with the channel depth.