On the stabilization of leading-edge vortices with different reduced frequencies over finite-aspect-ratio pitch-up wings

An implicit large-eddy simulation method is used to numerically simulate the flowfield around a finite-span rectangular wing in pitch motion. The effect of the reduced frequency (k) on the stability of the leading-edge vortex (LEV) is studied. The stability of LEVs is considered to include adhesion and structural stability. The wing is pitched at a position of around 1/4c (chord length), and the flow has a Reynolds number of 9053. Reduced frequencies of 0.2, 0.4, 0.8, and 1.6 are studied. For k = 0.2, the calculated results are in good agreement with experimental measurements, which demonstrates the reliability of the calculation method. The results show that the structural stability of LEVs can be significantly enhanced by increasing k. Larger values of k reduce the spatial scale of the LEVs, as well as prevent them from growing too fast and bursting. Moreover, a larger reduced frequency also delays the decrease in the LEV circulation, allowing the LEV structure to maintain stability at a larger angle of attack (α). In addition, a larger value of k also helps to enhance the adhesion stability of the LEVs. Numerical simulation results show that smaller values of k encourage the formation of secondary vortices. These accelerate the backflow between the LEVs and the wing, thus promoting the upward movement of the LEVs and reducing their adhesion stability. However, analysis of the convection terms in the vortex dynamic equations indicates that lower k is beneficial for the vorticity in the LEV to be transported outwards, but very small k will cause the reverse transport of vorticity, which is harmful to the stability of the LEV.