| Summary: | The depletion of fossil fuel and the increase of fuel consumption globally create an increased demand for the use of renewable energy. Vertical axis tidal turbines are a promising renewable energy device which needs to be improved. One problem appears in its operation is the structural instability and noise coming from the vibration of the long slender vertical axis blades. The vibration is a result of fluid structure interaction between turbine blades and the unsteady tidal current. This interaction of the tides and the blade generates vortical features which can excite the turbine blades to vibrate and causes a tonal noise known as singing. The aim of this work is to predict the blade response and locked-in condition by controlling the vortex shedding. The vortex is controlled by modifying blade’s trailing edge shape. The modifications include truncated, sharp and rounded trailing edge shapes. The response is modeled by vibrations using a spring damper system. A 2D numerical model of a vertical axis tidal turbine blade is developed to resolve the vibration using OpenFOAM 2.2. The blade has 0.75 m chord length and 3.07×106 Re. The model employs the equivalence incoming velocity method which represents the actual unsteady tidal current by time varying velocity magnitude and angle of attack of the model incoming flow. The problem is examined by observing the force applied to a static blade, and a rotating three bladed vertical axis turbine primarily. This is to confirm that the mesh topology and selected boundary conditions are sufficient and robust to resolve the blade response model. The locked-in condition is clarified by the blade main frequencies, pressure distribution, displacement, and force coefficients. In addition to the reference trailing edge, three different trailing edge shapes were studied. From the results it can be seen that the response is sensitive to pitching motion, high blade initial angle of attack, high tidal velocity and low spring and damping constant blade material. The results also show that the blunt (conventional truncated) foil has the largest ability to control the turbine blade response which is demonstrated by the smallest amplitude and the least frequent turbine blade’s vibration. For all three trailing edge shapes, along with a more limited investigation of an asymmetric trailing edge all are shown to be able to shift the frequency of the resonant response. This will allow the designer to study the likely behaviour of their design. Overall, the developed methodology using a two-dimensional, three degree of freedom solution of the unsteady CFD around the foil is shown to provide useful insight to the tidal turbine designer at a reasonable computational cost.
|
|---|
