Comparison Study of Non Sinusoidal Pitch over Sinusoidal Pitch at Higher Angle of Attack

Abstract

A comparison study between sinusoidal and non-sinusoidal pure pitching motions of a NACA0012 airfoil at low Reynolds number (Re = 500) is undertaken using 2D Navier-Stokes simulations. The study is carried out for reduced frequency, k = 8 16 and pitching amplitude, θo = 10°58°. The non-sinusoidal pitching motion follows the same instantaneous angle of attack time history of a sinusoidal plunging motion. For both pitching motions, the mean thrust coefficient does not continuously increase with θo as predicted by inviscid flow theory. To generate thrust from a sinusoidal pitching airfoil, kθo has to be atleast 1.4; CTmean increases until kθo = 6.3 after which CTmean decreases. Within the periodic flow regime, nonsinusoidal pitching is found to give better thrust performance over sinusoidal pitching motion. A maximum of 55% increase in CTmean is found at k = 8 and θo = 30°. But in the chaotic flow regime, non-sinusoidal pitching produces drag where sinusoidal motion is still able to generate thrust. Increasing θo keeps the non-sinusoidal pitching airfoil at their peak amplitudes for longer duration in a cycle. As a result, the trailing edge vortex creates larger suction pressure on the back side of the airfoil and hence larger drop in thrust is experienced for non-sinusoidal pitching. The interactions of the leading edge vortex with the leading edge and previously shed leading edge vortex during the stroke reversal also reduce thrust by some extent and airfoil experiences drag at higher pitching amplitude.