Hybrid Modelling of Flow Processes in Underground Pump Storage Reservoirs

Mini-Symposium: CFD in the Nearfield of Structure

CFD Simulation of Supercritical Flow in Narrow Channels Including Sediment Bypass Tunnels

Secondary currents, turbulence characteristics and bed shear stress distribution are the crucial parameters for specifying sediment transport. Hence, to understand the sediment transport in sediment bypass tunnels (SBTs), we performed computational fluid dynamics (CFD) simulation of supercritical flow in a narrow straight channel. The results of the steady state simulation (Froude number ≈ 1.8) were compared with experimental results. The commonly used two-equations linear eddy-viscosity models are limited to isotropic turbulence closure and hence could not replicate the desired secondary currents. Therefore, the LRR Reynolds Stress Model was used, and predicted the flow features and the secondary currents (three kinds in total) effectively. The secondary currents affected the longitudinal velocity and caused velocity dip. The existing atmEpsilonWallFunction was used to represent the dissipation of turbulent kinetic energy on the free surface. The obtained mean bed shear stress was 9.265 N/m2 which is lower than the measured value of 10.225 N/m2. This initial study contributes to the existing knowledge on the applications of CFD modelling in supercritical flow and SBTs. However, a comprehensive study considering different flow scenarios in narrow channels using improved LRR model and boundary conditions, over the used ones, is aimed to improve the accuracy in the estimation of flow field and bed shear stress

Hybride Modellierung der hydrodynamischen Prozesse in unterirdischen Pumpspeicherreservoirs

Currently energy storage systems are indispensable for the energy supply system. They are needed to ensure the required balance between production and demand of energy. As a short to medium term storage pumped storage plants have been used economically over a long period of time, but their expansion is limited locally. The reasons are in particular the required topography and the extensive human land use. Through the use of underground reservoirs instead of surface lakes expansion options could be increased. This work presents for the first time the hydrodynamic processes in underground pumped storage reservoirs. The knowledge of these processes is essential for a successful realization. In this thesis the reservoirs are branched caverns with ventilation, in which open channel flow occurs.Hydrodynamic processes in underground reservoirs occur due to plant operations and are influenced by the specific design. Based on hybrid modeling the processes are presented and analyzed. Hybrid means here the combination of experimental studies, numerical 3D simulations with OpenFOAM and numerical 2D simulations with TELEMAC-2D. By combining the multiple methods the hydrodynamic processes were analyzed qualitatively and quantitatively and calculation approaches were developed. The local flow processes partly show a great intensity, which is why the knowledge of the time-dependent averages of the flow variables (global effects) is not sufficient as a basis for a hydraulic calculation approach. The highest intensity of the local processes occurs during the filling of the reservoirs. Lower intensity occurs during the emptying of the reservoirs or for interruptions of these operations. Depending on operational and structural factors surge waves, undular bores and breaking bores occur. Independent of the wave type the calculation approaches allow the determination of the maximum water level. With this information the required height of the reservoirs can be sized, as well as the calculation of wave speeds that enable determining the duration of reservoir-oscillations, so that the operational characteristics can be found.The results that imply the basis for design criteria of underground pumped storage reservoirs go far beyond the current state of knowledge and have to be included in future plans and design developments of underground pumped storage plants

Reflection Phenomena in Underground Pumped Storage Reservoirs

Energy storage through hydropower leads to free surface water waves in the connected reservoirs. The reason for this is the movement of water between reservoirs at different elevations, which is necessary for electrical energy storage. Currently, the expansion of renewable energies requires the development of fast and flexible energy storage systems, of which classical pumped storage plants are the only technically proven and cost-effective technology and are the most used. Instead of classical pumped storage plants, where reservoirs are located on the surface, underground pumped storage plants with subsurface reservoirs could be an alternative. They are independent of topography and have a low surface area requirement. This can be a great advantage for energy storage expansion in case of environmental issues, residents’ concerns and an unusable terrain surface. However, the reservoirs of underground pumped storage plants differ in design from classical ones for stability and space reasons. The hydraulic design is essential to ensure their satisfactory hydraulic performance. The paper presents a hybrid model study, which is defined here as a combination of physical and numerical modelling to use the advantages and to compensate for the disadvantages of the respective methods. It shows the analysis of waves in ventilated underground reservoir systems with a great length to height ratio, considering new operational aspects from energy supply systems with a great percentage of renewable energies. The multifaceted and narrow design of the reservoirs leads to complex free surface flows; for example, undular and breaking bores arise. The results show excessive wave heights through wave reflections, caused by the impermeable reservoir boundaries. Hence, their knowledge is essential for a successful operational and constructive design of the reservoirs