VORTICAL FLOW IN THE WAKE OF A PITCHING AIRFOIL

The wake structure behind an airfoil pitching at small amplitude and large values of reduced frequency is investigated using flow visualization and laser Doppler velocimetry. Results. show that at a sufficiently high oscillation frequency a momentumless wake is obtained. Increas- ing the frequency further leads to a jet-like behavior corresponding to thrust conditions. The wake structure can be modified by manipulating the shape of the oscillation waveform. An example with simultaneous wake and jet structures is presented. The instantaneous streamwise velocity profiles in the wake are constructed using phase averaging techniques. These profiles show the signature of the passage of isolated and concentrated patches of vorticity. The behavior of each flow field described here is explained in terms of the unusual arrangement of its vorticity field. Comparisons with recent full Navier-Stokes calculations of this flow field are discussed