Unsteady Vortex Flows Produced by Trailing Edge Articulation

The unsteady vortex flows produced by biologically inspired tail articulation are investigated. The application is to provide active means of reducing tonal noise due to upstream wake interaction with downstream propellers on underwater vehicles. By reducing the wake velocity defect, the periodic unsteady propeller blade pressure fluctuations that are the source of the noise should be reduced. Accordingly, experiments to measure the flows produced by an upstream stator fitted with a movable trailing edge were carried out in a water tunnel for Reynolds numbers in the range $75,000<Re<300,000$⁠. A stator model with a hinged flapping trailing edge section operated at frequencies up to $21Hz$ corresponding to a range of Strouhal number $0.0<St<0.18$⁠. Velocity measurements of the articulating stator wake were carried out by laser Doppler velocimetry (LDV) and particle image velocimetry (PIV). Reduced mean and rms LDV data show that trailing edge articulation generates vortex structures with dependence on both Strouhal number and articulation amplitude. Estimates of the time mean stator drag that were obtained by integrating the mean wake profiles were used to estimate optimal Strouhal numbers in terms of wake elimination. Instantaneous phase-averaged measurements via PIV show a transition in the unsteady stator wake flow regimes as St is increased, from a deflected vortex sheet to a series of rolled up, discrete vortices. Measurements of the wake highlight the characteristics of the vortex structures and provide a means to estimate the impact on downstream propellers.