Response of unsteady loads and flow structures of pitching airfoil and cycloidal propeller in a surging inflow Available to Purchase

Most of vertical-axis turbines and underwater propulsion systems operate in a uniform inflow, but the response of performance and flow structures in a non-uniform inflow is not understood well. Thus, in this work, the unsteady loads and flow structures of pitching airfoils and cycloidal rotors in a surging inflow are studied deeply, with a consideration of three important parameters including the surging amplitude σ, phase lag τ, and frequency ratio N_a. Compared with that in a uniform inflow, the mean and instantaneous performance of pitching airfoils fluctuate a lot and the flow field becomes more disordered, especially at large σ and high reduced frequency k_p. Evidently, increasing σ leads to an intensive flow separation and earlier occurrence of stall, which is more obvious than τ. When N_a is smaller than 1, the mean drag coefficient changes with a sine-type function at various τ, but the mean lift coefficient only varies at N_a=1 and 2. When it comes the cycloidal rotor, increasing σ significantly increases the side and propulsive force coefficients, as well as the thrust-based efficiency, and the maximal increment is about 7% at λ = 0.62. Similarly, changing τ could also enhance the performance, but the maximal increment is relatively low than σ. Although changing τ can enhance the efficiency, it only appears at τ = 0° and 180°. Both changing σ and τ can modify the stagnation point location by increasing/decreasing the instantaneous inflow velocity, further varying the instantaneous performance of the single blade and rotor, but the influence of σ is more apparent. Then, at large N_a, both the mean side and propulsive force coefficients can maintain a fixed value with the change of τ, which indicating that performance stability of the cycloidal rotor has a great improvement when the surging frequency of the inflow is much smaller than the pitching frequency of the blade, which is advisable practically.