Abstract :
[en] For over 15 years, our research group at the University of Liege has been conducting basic and applied research on flow and transport phenomena involved in environmental and civil engineering applications. The research topics cover a wide range of multiphase flow, including aerated flow and sediment-laden flow. The research group has developed the modelling system WOLF, which performs 1D, 2D-horizontal, 2D-vertical and 3D simulations of free surface and pressurized flow, coupled with air- or sediment-transport as well as morphodynamic simulations [9]. An original depth-averaged k- model is used to evaluate the eddy viscosity and diffusivity [5]. In a composite modelling approach [e.g., 2, 8], the numerical simulations have systematically been carried out in parallel with experimental research in flumes and scale models of hydraulic structures [3, 4, 7].
A particularly challenging issue in multiphase flow modelling is the need to handle accurately and efficiently a wide range of time scales involved in the relevant phenomena (e.g., sediment transport). Therefore, the modelling system provides a series of complementary numerical schemes designed to be combined for covering the whole range of relevant time scales.
This presentation will provide examples of analysis of aerated flow, such as on stepped spillways and in penstocks [6], and of reservoir sedimentation. In particular, recent experimental findings have revealed that the flow pattern in rectangular shallow reservoirs is considerably modified at a macro-scale when suspended load is added to the flow [1]. This sudden change in flow pattern has been observed to take place quickly after the beginning of the experiment, so that it is very unlikely to result from morphodynamic changes of the reservoir bottom. Therefore, the numerical model WOLF has been used to investigate the effect of two other possible feedback mechanisms of sediment deposits and suspended load on the overall flow pattern, namely (i) increased bottom roughness and (ii) turbulence damping. From the simulations results, it can be argued that turbulence intensity is probably the main cause for the flow pattern to change at a macro-scale, as suspended load exerts a turbulence damping effect at a micro-scale in the two-phase flow (water-sediment mixture).
Finally, the analysis of flow and sediment transport in shallow rectangular reservoirs also highlights the need, when structures are designed based on numerical simulations, to carefully check the stability of the computed flow fields needs by conducting sensitivity analyses, not only with respect to the modelling parameters but also with respect to the initial conditions.
