Numerical simulation of three-dimensional flows in water storage tanks - Application to Norra Ugglarps Reservoirs in South Sweden

Sammanfattning: The main purpose of this thesis was to simulate the three-dimensional flow patterns in tanks for water storage. The practical intentions of the simulations were to identify water-quality problems related to insufficient water exchange. The water storage reservoirs at Norra Ugglarp in South Sweden were investigated as a special case study. In this thesis, the open source CFD software OpenFoam was chosen to simulate flow features in tanks. The first part of this thesis focused on developing an understanding of the case study; that is, the distribution of water in West Skåne and especially the functioning of Norra Ugglarps reservoirs. An overview of possible mixing problems in the tank was detailed and presented. As modelling was chosen to identify mixing problems, mechanisms and equations about flow modelling were first discussed before going into the OpenFOAM software itself. The modelling strategy in OpenFoam do not differs from other CFD software; nevertheless, setting up the case and post processing the results were quite unusual and discussed in some detail. When it came to the modelling itself, in order to become familiar with the software, some simple test setups were first explored in two-dimensional tanks of simple geometry. Then, the simulations were extended to three-dimensional situations, still for rectangular tanks. Finally, the software was employed to simulate the flow in the reservoirs at Norra Ugglarp. From the modelling results, it was possible to derive the following conclusions. The incoming velocity is a key parameter as injection of water should be powerful enough to reach the surface and renew the upper layer water. Water level range is another important parameter to renew more volume in the tanks. All conclusions tend to prove that the reservoirs at Norra Ugglarp suffer from insufficient mixing, mainly due to both the weak incoming velocity of new water and the limited water level range in use. Thus, solutions to alleviate poor mixing are presented, such as increasing the water level range or monitoring existing valves to ameliorate the filling regarding water circulation.