UNSTEADY-FLOW PAST AN AIRFOIL PITCHING AT A CONSTANT RATE

The unsteady flow past a NACA 0012 airfoil in pitching-up motion is experimentally investigated in a water towing tank using the particle image displacement velocimetry (PIDV) technique. The Reynolds number, based on the airfoil chord and the freestream velocity, is 5000. The airfoil pitching motion is from 0 to 30 deg angle of attack at a dimensionless pitch rate alpha of 0.131. Instantaneous velocity fields at different times have been acquired over the entire flowfield. Using the whole-field data, the out-of-plane component of vorticity is computed. The following features are observed. Boundary-layer separation near the airfoil leading edge leads to the formation of a vortical structure. The evolution of this vortex along the upper surface dominates the aerodynamic performance of the airfoil. Complete stall emerges when the boundary layer near the leading edge detaches from the airfoil, under the influence of the vortex. The vortex further triggers the shedding of a counter-rotating vortex near the trailing edge. A parallel computational study using the discrete vortex, random walk approximation has also been conducted. In general, the computational results agree very well with the experiment.