A robust scheme for free surface and pressurized flows in channels with arbitrary cross-sections

Flows in closed channels, such as rain storm sewers, often contain transitions from free surface flows to pressurized flows, or viceversa. These phenomena usually require two different sets of equations to model the two different flow regimes. Actually, a few specifications for the geometry of the channel and for the discretization choices can be sufficient to model closed channel flows using only the open channel flow equations. Transitions can also occur in open channels, like those from super- to subcritical flow, or vice versa. These particular flows are usually difficult to reproduce numerically and strong restrictions are imposed on the numerical scheme to simulate them. In this paper, an implicit finite-difference conservative algorithm is proposed to deal properly with these problems. In addition, a special flux limiter is described and implemented to allow accurate flow simulations near hydraulic structures such as weirs. A few computational examples are given to illustrate the properties of the scheme and the numerical solutions are compared with experimental data, when possible

3D LES computations of a shallow lateral expansion using an immersed boundary method

Proceedings of the Seventh International Conference on Hydroscience and Engineering, Philadelphia, PA, September 2006. http://hdl.handle.net/1860/732In environmental shallow flows, the phenomenon of flow separation often gives rise to large-scale turbulent structures (vortex shedding). In this study, 3D LES computations of three Shallow Lateral Expansion geometries are performed. The resolved large-scale turbulent structures are studied in detail in order to allow a comparison with laboratory experiments, carried out using the Particle Image Velocimetry (PIV) technique. When LES is applied for practical cases involving flow separation, immersed boundaries are often an essential part of the geometry. These boundaries can cause problems with respect to the Navier Stokes solver used, especially regarding the pressure correction module. A solution to this problem, known as Immersed Boundary Method (IBM), is found by using body forces to ensure the impermeability of internal boundaries. The classical IBM formulation, however, makes a systematic error regarding momentum transfer in the vicinity of solid walls. In this study an adjusted IBM is proposed, based on momentum fluxes instead of body forces. The adjusted model is applied to Shallow Lateral Expansion geometries of various aspect ratios. In order to analyze the real-time large-scale turbulent structures, the vector potential function of the velocity field is computed. This is a very suitable tool to detect large-scale flow structures. The turbulence features observed in the 3D LES computation are compared with the PIV data, especially regarding the vortex shedding behaviour. An analysis of Reynolds stresses and the downstream development of eddy length scales reveals the existence of two different regimes in the vortex shedding behaviour. The difference can be explained by the interaction of shed vortices with the primary and secondary recirculation cells that are present

On accurate momentum advection scheme for a z-level coordinate models

Abstract In this paper, we focus on a conservative momentum advection discretisation in the presence of zlayers. While in the 2D case conservation of momentum is achieved automatically for an Eulerian advection scheme, special attention is required in the multi-layer case. We show here that an artificial vertical structure of the flow can be introduced solely by the presence of the z-layers, which we refer to as the staircase problem. To avoid this staircase problem, the z-layers have to be remapped in a specific way. The remapping procedure also deals with the case of an uneven number of layers adjacent to a column side, thus allowing one to simulate flooding and drying phenomena in a 3D model