Experimental-numerical investigation of a pitching airfoil in deep dynamic stall

The results of a comprehensive experimental campaign are compared to computational fluid dynamics simulations results to assess the modelling capabilities for a NACA 23012 pitching airfoil in deep dynamic stall regime. The experimental campaign involved fast unsteady pressure measurements and particle image velocimetry. Two-dimensional simulations were carried out with EDGE, developed by FOI. The investigated test case consists in a sinusoidal pitching motion with a 10 degrees amplitude and a reduced frequency of 0.1 around a mean angle of attack of 10 degrees. The behaviour of the experimental lift and pitching moment coefficients is in close agreement with the two-dimensional simulations results, also during the downstroke motion where the flow field is characterised by severe unsteadiness conditions. A three-dimensional numerical model was built to evaluate the relevance of three-dimensional effects on the experiments. Three-dimensional simulations were carried out using the commercial code FLUENT. During upstroke motion, three-dimensional simulations results are in better agreement with the experiments, in particular in terms of the lift coefficient curve slope and of the pitching moment coefficient peak. The flow fields evaluated by particle image velocimetry surveys show strong vortical structures moving on the airfoil upper surface during the downstroke motion that are captured only by the three-dimensional model; then, the flow fields comparison demonstrates the importance of three-dimensional effects for a deep dynamic stall condition.