|dc.description.abstract||When fluid flows over an object, vortexes can develop, depending on the geometry of the object and conditions in the flow. Unwanted vortexes can be generated around both stay vanes and guide vanes in a hydropower plant, and they may cause high-frequent noise, vibrations, and other phenomena in the vane cascade, and also when they traverse into the turbine runner. Several phenomena can induce vortex structures, but in this master thesis it is primarily the occurrence of a Kármán vortex street that has been analyzed. Behind bluff bodies, for example a cylinder, alternating vortexes may develop. In a hydropower plant, these vortexes are generated at high frequencies, and may introduce flow-induced vibration of a vane. If the shedding frequency coincides with the natural frequency of the vane, vibrations can be significantly amplified and put structural integrity at risk.
The purpose of the master thesis was to investigate if a truncated guide vane with a retrofitted modification could mitigate the onset of a Kármán vortex street, and to investigate the modified design with FSI simulations. This was done by establishing a numerical methodology that will serve as a framework for future work related to this thesis. Both the truncated and modified design were analyzed with the software ANSYS. The simulations are based on the prototype test rig for 1 GV cascade flow that is assembled at the Hydropower Laboratory at the Norwegian University of Science and Technology. Additionally, an experimental lab measurement was designed for future work, with the purpose of reproducing and validating numerical results obtained in this thesis.
CFD results indicates that the retrofitted modification has a positive influence on the wake, seemingly breaking up the Kármán vortex street. The lift force on the GV with a truncated edge was characterized by oscillations, due to vortex shedding. In contrast, the lift force on the modified GV was significantly stabilized, and similar observations were made for velocity fields and the turbulent kinetic energy in the wake. Transient two-way FSI simulations were carried out to confirm that the modified GV would mitigate flow-induced vibration, but without success. The FSI simulations were characterized by numerical instability, and difficult to set up correctly. The numerical methodology needs further work and validation through experiments, but results presented in this master thesis shows that the technology has a very interesting and promising potential for mitigating the presence of Kármán vortexes.||en