Experimental and Computational Investigation of the Knoller–Betz Effect

Abstract

The ability of a sinusoidally plunging airfoil to produce thrust, known as the Knoller–Betz or Katzmayr effect, isinvestigated experimentally and numerically.Water-tunnel experiments are performed providing flow visualizationand laser Doppler velocimetry data of the un steady wakes formed by the plunging foils. Vortical structures andtime-averaged velocity profiles in the wake are compared with numerical computationsfrom a previously developedinviscid, unsteady panel code that utilizes a nonlinear wake model. Qualitative and quantitative comparisons areexcellent over a broad range of reduced frequencies and Strouhal numbers, indicating that the formation andevolution of the thrust-indicative wake structures are primarily inviscid phenomena. Results at Strouhal numbersgreater than about 1.0(based on plun ge amplitude)demonstrate nonsymmetric, deflected wake patterns, whereboth an average thrust and an average lift are produced. These highly nonlinear wake formations are generatedreproducibly, both experimentally and numerically.