Experimental and computational investigation of the Knoller-Betz effect

The ability of a sinusoidally plunging airfoil to produce thrust, known as the Knoller-Betz or Katzmayr effect, is investigated experimentally and num erically. Water-tunnel experiments are performed providing flow visualizati on and laser Doppler velocimetry data of the unsteady wakes formed by the p lunging foils. Vortical structures and time-averaged velocity profiles in t he wake are compared with numerical computations from a previously develope d inviscid, unsteady panel code that utilizes a nonlinear wake model. Quali tative and quantitative comparisons are excellent over a broad range of red uced frequencies and Strouhal numbers, indicating that the formation and ev olution of the thrust-indicative wake structures are primarily inviscid phe nomena. Results at Strouhal numbers greater than about 1.0 (based on plunge amplitude) demonstrate nonsymmetric, deflected wake patterns, where both a n average thrust and an average lift are produced. These highly nonlinear w ake formations are generated reproducibly, both experimentally and numerica lly.