Effect of Reynolds number and airfoil thickness on the leading-edge suction in unsteady flows

Determining the behavior of the leading-edge suction force, represented non-dimensionally by the leading-edge suction parameter (LESP), can reliably help indicate the state of flow over the airfoil and therefore the force and moment characteristics. The current work aims at studying the variations in the LESP, forces, and pitching moment with freestream Reynolds number and airfoil thickness in unsteady flows. Computational data for the NACA 0012, 0015, and 0018 airfoils undergoing a baseline pitching motion over a range of freestream Reynolds number conditions are analyzed. The critical LESP, which is the instantaneous value of LESP at leading-edge vortex initiation, is observed to first decrease and subsequently increase with Reynolds number. This behavior can be correlated to the rate at which leading-edge flow curvature increases with Reynolds number. Thicker airfoils are observed to sustain a larger amount of suction force prior to breakdown and ensuing leading-edge vortex (LEV) shedding. Lift, drag, and moment are found to be dependent on thickness and Reynolds number prior to LEV shedding due to differences in the boundary layer characteristics, but independent after suction breakdown due to the similarity in LEV dynamics. These findings serve to support the development of a more generalized definition of a suction-force parameter that is independent of flow conditions and airfoil geometry.