Dynamic large eddy simulation of surging airfoils with moderate to large streamwise oscillations

Abstract

Lift force is compared between experiments and LES. Overall, a good agreement is obtained for the lift force in all cases. Additionally, for all cases, flowfields from LES are examined at different phases of the surging cycle. For the moderate advance ratio cases, a similar flow pattern is observed between the two advance ratios and no distinct vortex is shed from the airfoil. However, in all three high advance ratio cases a distinct vortex is shed from the suction side near the geometric leading edge. This prominent leading-edge vortex (LEV) is shed as the minimum velocity is reached in the surging cycle and advects downstream in the horizontal direction with about the mean free-stream velocity. The relative shedding position of the LEV, with respect to the airfoil, varies significantly between the three high advance ratio cases. In the case with the highest advance ratio of 1.2, a reversed flow condition is present and the LEV crosses to be ahead of the (geometric) leading edge before sweeping over the airfoil.

A dynamic large eddy simulation (LES) based numerical investigation is carried out for flow over two surging airfoils with moderate to large streamwise oscillations. In each case, the airfoil is set at a fixed angle of attack and is subjected to a sinusoidal surging motion (i.e., in the streamwise direction). The amplitude of oscillation is characterized by the advance ratio of the blade section, which is defined by the ratio of the maximum relative velocity in excess of the mean relative velocity, or the freestream velocity, to the mean relative velocity. The relative airfoil velocity is defined with respect to the ambient fluid. In this study, the amplitude of the sinusoidal oscillations is varied within a moderate and high range. For the moderate range, a NACA 0018 airfoil at a mean Reynolds number of 300,000 and 4° angle of attack is considered. Two advance ratios of 0.34 and 0.51 are considered to match with the experiments of Strangfeld et al.. For the high range, a NACA 0012 airfoil at a mean Reynolds number of 40,000 and 6° angle of attack is considered at three advance ratios of 0.8, 1.0 and 1.2 to compare with the experiments of Granlund et al.. These cases span a range of operating conditions, including a reversed flow condition.