Direct measurement of thrust and efficiency of an airfoil undergoing pure pitching

Abstract We experimentally investigate the thrust and propulsive efficiency of a NACA 0012 airfoil undergoing oscillating pitching motion at a Reynolds number of $1.7\times 10^{4}$ . While previous studies have computed thrust and power indirectly through measurements of momentum deficit in the object’s wake, we use a pair of force transducers to measure fluid forces directly. Our results help solidify a variety of experimental, theoretical and computational answers to this classical problem. We examine trends in propulsive performance with flapping frequency, amplitude and Reynolds number. We also examine the measured unsteady forces on the airfoil and compare them with linear theory dating from the first half of the 20th century. While linear theory significantly overpredicts the mean thrust on the foil, its prediction for the amplitude and phase of the time-varying component is surprisingly accurate. We conclude with evidence that the thrust force produced by the pitching airfoil is largely insensitive to most wake vortex arrangements.