Particle Image Velocimetry Based Trailing Edge Force Estimation for a Symmetric Airfoil

Unsteady forces on the trailing edge of a lifting surfaces are driven by the interaction between the convecting boundary layer and the trailing edge discontinuity. Predictions of these forces typically rely on combining empirical pressure spectra with Amiet or Howe's pressure scattering formulation. Here these forces were evaluated in a water-tunnel facility for a two-dimensional NACA0016 airfoil operating at a chord based Reynolds number of 9.8x105. By using a water tunnel, the airfoil could be tested at high Reynolds numbers and low velocities, which is a regime typically unaccounted for in empirical models developed in wind tunnel testing. Due to the complexity though of pressure and force measurements in water tunnels, which are typically affected by high-frequency facility noise, this study instead employed optical based force estimations for evaluating the trailing edge force spectrum. This was accomplished by combining Howe's vortex-force analogy with high-speed (10 kHz) planar particle image velocimetry measurements. The force spectra was compared with predictions using existing empirical pressure models. The results highlight the benefit of optical-based force estimations for extending model development. In addition the comparison to existing models identified areas for improvement in both the measurement technique as well as empirical pressure spectrum model development.