Large Eddy Simulation of Near-Bed Flow Over Bottom Roughness in Open Channel: Fid-Bau Portal

Large Eddy Simulation of Near-Bed Flow Over Bottom Roughness in Open Channel

Xu, Bowen / Samuel Li, S.

Channel-bed roughness has the influence on near-bed flow, with implications in hydraulic engineering design. Previous studies suggest that small-scale structures of turbulence are an important element for near-bed flow. However, very little research has dealt with turbulence structures of flow in the vicinity of roughness elements at the channel bed. The reasons for this include the high costs and technical difficulties to obtain fine resolution measurements of near-bed flow from natural and laboratory channels. Therefore, Computational Fluid Dynamics (CFD) models become an attractive tool for the investigation of near-bed flows. The purpose of this paper is to use Large Eddy Simulation (LES) as a mathematical model to predict eddy motions, including small-scale motions near the bed and around roughness elements. In this study, LES runs are performed under conditions matching a series of laboratory experiments. The LES runs use OpenFOAM as a numerical solver. The simulations numerically solve one-phase incompressible flow using the finite volume method and yield snapshots of instantaneous velocity and pressure fields. The simulations use the rigid lid approximation at the upper boundary of the model channel and apply cyclic conditions between its upstream and downstream lateral open boundaries. A significant challenge in LES is to resolve small-scale motions of dynamic importance. This paper handles the challenge by resolving the viscous sublayer. This treatment permits the use of no-slip condition, which is realistic. The ensemble averages of instantaneous velocities are validated using experimental data. The LES results are further analyzed to reveal eddy motions and turbulent flow patterns. The results provide details of turbulence structures near the bed with the presence of roughness elements.