Effects of load fluctuation on hydropower tunnels
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This dissertation presents a study of the main challenges and consequences of imposing rapid hydropeaking production patterns on unlined hydropower tunnels. It contains a thesis part and four research papers. The main contributions are two-fold: physical effects observed, and predictions or monitoring methods developed. Close collaboration with hydropower companies and inspection of over 100 km of hydropower tunnels in use has revealed that mechanical wear caused by sediment transportation in Norwegian hydropower plants has increased due to hydropeaking. The operational costs related to such damages are considerable. It was found that when compared to non-hydropeaked systems, hydropeaked systems experience an increase in the frequency and volume of rock falls. To facilitate analysis of the hydrodynamics arising in hydropower tunnel system several useful prediction and monitoring methods have been tested. A novel scanning technique that describes the complex surface geometry of drill and blast tunnels is introduced. The technique provides unique opportunities to reduce the uncertainty in hydraulic calculations, and an improved method to estimate the roughness of rock blasted tunnels based on scan data is proposed. The surface meshes generated by scanning of the prototype were used to perform 3D CFD simulations. The numerical investigations demonstrate that combining these two technologies yields a better understanding of the fluid-structure interaction, and provides the possibility for spatially-distributed estimates of the velocities, pressure, level of turbulence, shear forces and the volume fraction of water/air occurring in a critical component of a HPT system. 3D simulations not only contribute to reducing the uncertainty of 1D simulation, they can also serve to calibrate discharge measurements.