Abstract
Numerically reproducing wind tunnel experimental tests of flow behavior around a pitching airfoil is a challenge, especially under the occurrence of dynamic stall at relatively high angles of attack. This not only requires the application of advanced turbulence models, but also asks for examining the influence of the most relevant model parameters in detail. This contribution presents the results of an extensive computational study of the unsteady flow around a pitching NACA0012 airfoil at a Reynolds number of 1.35 × 105, as obtained by first performing 2D Unsteady Reynolds-Averaged Navier-Stokes (URANS) simulations whereby the flow characteristics are simulated by the Transition SST turbulence model. The influence of a large number of parameters on the numerical results is investigated, namely the blockage ratio, computational grid resolution and y+ value, time step size, inlet freestream turbulence properties, airfoil trailing edge detailing and turbulence model. Integral aerodynamic force coefficients and detailed flow patterns are analyzed and compared with measurements from wind tunnel experiments presented in the literature. For the best-performing parameters, an adequate agreement with the experimental tests is obtained for the upstroke phase, while for the downstroke phase some differences appear. These differences are investigated in more detail by considering the results from a 2.5D Large Eddy Simulation (LES), which provides deep and complementary insights into the flow behavior during dynamic stall at the selected Reynolds number.
Highlights
Airfoils undergoing pitching motion experience a stall behavior that is distinctly different from the behavior at fixed angles of attack (AOA)
The starting vortex generated at the onset of pitching motion near the leading edge contributes to the variation at the fist cycle in both the 2D Unsteady Reynolds-Averaged Navier-Stokes (URANS) and 2.5D Large Eddy Simulation (LES) simulations, due to the impulsive rotation of the airfoil (Panda and Zaman, 1994; Ol et al, 2009)
(5) The current work shows that 2.5D LES is able to predict force coefficients in deep dynamic stall with reasonable accuracy, a significant improvement over the 2D URANS simulation results in terms of the agreement with experimental data is lacking
Summary
Airfoils undergoing pitching motion experience a stall behavior that is distinctly different from the behavior at fixed angles of attack (AOA). A second vortex, which counter-rotates with respect to the LEV, develops at the suction side close to the leading edge (McCroskey, 1981; Gharali and Johnson, 2013) This secondary vortex interacts with the LEV and both are shed into the wake. The wake flow during the downstroke is characterized by an alternating vortex pattern originating from shed LEVs and TEVs. The integral forces and moments experience substantial variations and large hysteresis, resulting in a significant increase in stresses that make the blades more susceptible to fatigue damage. The main physical features involved in dynamic stall are discussed by McCroskey (1981) and more recently by Lee and Gerontakos (2004)
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