Abstract: The ability of a sinusoidally plunging airfoil to produce thrust, known as the Knoller-Betz or Katzmayr effect, is investigated experimentally and numerically. Water tunnel experiments are performed providing flow visualization and laser Doppler velocimetry data of the unsteady wakes formed by the plunging foils. Vortical structures and time-averaged velocity profiles in the wake are compared with numerical computations from a previously developed inviscid, unsteady, panel code which utilizes a non-linear wake model. Qualitative and quantitative comparisons are excellent over a broad range of Strouhal numbers indicating that the formation and evolution of the thrust-producing wake structures is essentially an inviscid phenomenon. Results at Strouhal numbers greater than about 1.0 (based on plunge amplitude) demonstrate non-symmetric, deflected wake patterns, where both an average-thrust and an average-lift are produced. These highly non-linear wake formations are generated reproducibly, both experimentally and numerically.

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