Application of the LANS-alpha Model to Gravity Currents

Abstract

This thesis aims to investigate how HERCULES performs when running a lock-exchange gravity current case. The LANS-alpha model with stratification is also tested as a subgrid model in HERCULES using the same gravity current case. Gravity currents have been studied using both direct numerical simulation (DNS) and large eddy simulation (LES). On the other hand, the LANS-alpha model has only been applied to several test cases which mainly focus on isotropic turbulence and wall-bounded unstratified flows. We begin by reviewing the characteristics of the turbulent structures in the gravity currents and the motivation to use the LANS-alpha model. This is followed by the implementation of the model in HERCULES with both grid-dependent alpha and flow-dependent alpha. For the numerical study, a gravity current is generated using a lock release in a horizontal channel. With a fine grid, the front location and the three-dimensionality of the gravity current can be resolved accurately using HERCULES. When the grid resolution is coarse, the LANS-alpha model can improve the results considerably using grid-dependent alpha with both subgrid terms. The flow-dependent alpha requires modification in its definition as the grid-dependent alpha outperforms it in resolving the front location and the small-scale, three-dimensional structures.