Patrik Andreasson

CFD-Modelling and Validation of Free Surface Flow During Spilling of Reservoir in Down-Scale Model

Abstract Fully three dimensional modelling of the spilling from a reservoir with relatively complex geometry were performed and compared to experimental results from a physical scale model with the aim to advance the science of numerical modelling of free surface flow of real reservoirs. In the set-up in focus the water was spilled from the reservoir through three gates that could be manoeuvred separately. In the first case two of the gates were closed and the third gate was partly opened. In this experimental set-up the water surface in the reservoir was close to horizontal. Therefore it was here meaningful to compare a rigid lid modelling approximation to the more computational heavy method of Volume of Fluids. In the second case, all three gates were open, resulting in a nonhorizontal varied flow surface profile in the reservoir upstream critical sections at the spillway crests. This case was simulated with Volume of Fluids and the position of the air-water interface was derived for two turbulence models, the standard k-ε and SSG. Water levels, velocities and the shape of the water surface were compared to experiments. The simulation results capture qualitative features such as a vortex near the outlet and show good quantitative agreement with the experiments regardless of method used to simulate the free surface. In general, simulations with the standard k-ε and the more advanced SSG turbulence models give the same results with respect to the averaged quantities measured.

A study of the location of the entrance of a fishway in a regulated river with CFD and ADCP

Simulation-driven design with computational fluid dynamics has been used to evaluate the flow downstream of a hydropower plant with regards to upstream migrating fish. Fieldmeasurements with an Acoustic Doppler Current Profiler were performed, and the measurements were used to validate the simulations. The measurements indicate a more unstable flow than the simulations, and the tailrace jet from the turbines is stronger in the simulations. A fishway entrance was included in the simulations, and the subsequent attraction water was evaluated for two positions and two angles of the entrance at different turbine discharges. Results show that both positions are viable and that a position where the flow from the fishway does not have to compete with the flow from the power plant will generate superior attraction water. Simulations were also performed for further downstream where the flow from the turbines meets the old river bed which is the current fish passage for upstream migrating fish. A modification of the old river bed was made in the model as one scenario to generate better attraction water. This considerably increases the attraction water although it cannot compete with the flow from the tailrace tunnel.

A mathematical simulation of energy conversions in a fully developed channel flow

A detailed analysis of energy conversions in a fully developed channel flow on a sloping plane is provided. It is the lowering of the centre of mass that releases potential energy which, of course, ultimately will leave the domain as a boundary heat flux. A mathematical model, based on the conservation laws for heat and momentum and a one-equation turbulence model, is used for the purpose. As the calculations include the viscous region, the turbulence model is extended to include low-Reynolds number effects. Verification studies are carried out by comparing predictions with data from laboratory experiments. The model is thereafter used to analyse the conversions of energy from its first appearance as a source term in the mean kinetic energy equation till it finally leaves as a boundary heat flux. The model provides a clear and consistent picture of all the details in the budgets and fluxes involved.

Energy conversions in turbulent wind-induced countercurrent flow

A detailed analysis of energy conversions and fluxes in wind-induced countercurrent flow is provided. At the free surface mechanical energy is supplied to the waterbody by the wind. Via stages of pressure-volume work and kinetic energy (divided into a mean and a fluctuating part), the energy will ultimately leave the flow as a surface heat flux. The analysis is based on mathematical modelling, supported by order-of-magnitude estimates and comparisons with reported experimental studies. A Reynolds-stress model, extended into the viscous sublayers, was required to predict adequately the mean velocity and turbulence structure of countercurrent flow.

Effect of spatial resolution of rough surfaces on numerically computed flow fields with application to hydraulic engineering

Abstract In numerical simulations of flow over rough surfaces, the roughness is often not resolved but represented by a numerical model. The validity of such an assumption is investigated in this paper by Reynolds-Averaged Navier-Stokes simulations of flow over a surface with a large roughness. The surface was created from a high-resolution laser scanning of a real rock blasted tunnel. By reducing the geometrical resolution of the roughness in two steps, the importance of an appropriate surface description could be examined. The flow fields obtained were compared to a set-up with a geometrical flat surface where the roughness was represented by a modified form of the Launder and Spalding wall-function. The flow field over the surface with the lowest resolution was substantially different from those of the two finer resolutions and rather close to the results from the set-up with the wall-function. The results also yield that the finer the resolution is the more vorticity is formed close to the rough surface and more turbulence is generated.

Inlet Blockage Effects in a Free Surface Channel With Artificially Generated Rough Walls

When considering free surface flow in channels, it is essential to have in-depth knowledge about the inlet flow conditions and the effect of surface roughness on the overall flow field. Hence, we h ...