Study of vortex shedding from a vibrating hydrofoil
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The world energy consumption has been increasing significantly as the living standard of the human beings increases. The energy market is more challenging and changing faster than before. Large amount of this energy has been used to generate electricity and mostly are still from primary energy source like fossil fuels. Therefore, hydropower will play an important role in a bid to meet this increasing demand in a sustainable way. Due to the fast-changing electricity demand, the hydropower plants have been facing technical challenges such as the cracking of the turbine blade. The purpose of this assignment is part of the main goal to understand the problem and main reason behind the blade failure in the Francis turbine. A blade cascade test rig resembling that of a Francis turbine runner has been designed and set up in the Waterpower Laboratory (VKL) at the Norwegian University of Science and Technology (NTNU). The scope of the investigation for this assignment has been narrowed down in order to reduce the other affecting parameters that will increase the complexity. The focus will be studying the vibration frequencies of the hydrofoil caused by vortex shedding at the various water flow velocities by measuring and analysing the pressure pulsation. Both measurements without and with external excitation will be carried out in this experiment. Thus, the effect of vibrating hydrofoil on self-excitation frequency of the hydrofoil by the vortex shedding can be studied. The quartz dynamic pressure sensors are installed at the wake of the hydrofoil for measuring the pressure pulsations caused by the vortex shedding. For the excitation of the hydrofoil, two patches of piezoceramic materials based actuators will be used. The vibrations at the trailing edge of the hydrofoil will be measured by using the Laser-Doppler Vibrometer (LDV). The measurements are post processed using mainly FFT function in MATLAB. The results show that there are small deviations in the natural frequencies but the direct influences from the flowing water are difficult to determine as the deviations are within the measurement uncertainty range. The applied external excitations do not affect the lock-in range of the vortex shedding in the experiment. However, the dominant of the excitation frequency over the natural frequency can be observed. The hydrofoil vibrates with higher amplitude at the excitation frequency when the applied frequency near its natural frequency.