Experimental and numerical investigation of density current over rough bottoms and uneven bedforms

Density currents, caused by density differences within a fluid, which occur frequently in both natural and industrial flows, has been recognized as an important fluid phenomenon with rich features. The density difference between two different fluids is caused by salt, temperature or other conservative scalars. The effect of the boundary over which the density current passes has been investigated during the past years. But the behavior of the density current over rough bottoms, especially over the irregularly arranged rough bottom is quite unusual and cannot be investigated and interpreted by using the usual numerical models. In this study, rough and uneven bottoms are considered in both laboratory experiments and large eddy simulations (LES). The configuration of our investigation is the lock-exchange flow. In the experiment, high speed video cameras and an Acoustic Doppler Velocimeter (ADV) are used to capture the development of density currents. The large eddy simulations (LES) are conducted using the open source computational fluid dynamics code OpenFOAM. One of challenges is how to represent the geometry of the rough bottoms. A new immersed boundary method (IBM) has been developed based on the OpenFOAM platform to represent the rough elements. A experimental and numerical combined method is proposed to study the behaviors of density currents over rough bottoms and analyze the effects of the rough bottoms. It is found that, both of the roughness size and their arrangement have significant effects on the dynamics of the currents. The entrainment of the currents over large size roughness cannot be predicted by the existed entrainment laws. This study also focuses on the effects of boundary conditions and initial depth of the dense fluid. The differences in energy dissipation and overall front development in wall-bounded and open channels are examined. Through direct numerical simulations (DNS), it is evident that with the decrease of initial release depth ratio, the effect of the top boundary becomes less important. It is found that the energy dissipation distribution in the bottom layer is similar for cases with the same initial depth ratio regardless the top boundary condition. The simulation results also reveal that for low Reynolds number cases, the viscous diffusion cannot be neglected in the energy budget. To reflect the real dynamics of density current, the dimensionless Froude number and Reynolds number should be by the release depth.