| Current study presents a numerical investigation on the effect of parameters such as frequency and amplitude as well as the asymmetry of angle of attack profile on the flow behavior around a pitching foil. First, a simulation is carried out on a stationary airfoil for validation purposes and the results are in good agreement with the experiments. In the pitching foil, following the angle of attack profile of a Vertical Axis Wind Turbine (VAWT), it was found that production of negative lift and drag increases with increasing frequency at different amplitudes. Thus, the mean force along the airfoil, which is the force component of torque production in VAWT applications, increases. In addition, as the pitching amplitude increases the angle of attack profile becomes more asymmetric which also affects the generated forces and results in higher hysteresis in lift and drag forces. In fact, the pitching frequency and amplitude affect the vorticity field around the airfoil. Comparison of generated forces as well as the vorticity field of all the cases of current study shows that there are similar behaviors for cases with different pitching frequency and amplitude. It was found that changes in these parameters affect the velocity and acceleration that airfoil experiences during increasing and decreasing angle of attack. To be able to compare all the cases with different pitching amplitude, frequency and angle of attack profile, one should compare the velocity and acceleration that airfoil experiences in different cases. It was found that the higher the velocity and acceleration of leading edge and trailing edge is during the pitch up interval, the higher the generated forces along the airfoil and more compact the leading edge and trailing edge vortex will be. The evolution of vorticity field and the inherent pressure distribution around the airfoil is also discussed for all the cases. Although increasing velocity and acceleration improves the force component of torque, it was observed that the average thrust is negative for all the cases and we cannot identify a particular trend for thrust production. |
