Influence of Leading-Edge Vortex on Mean Thrust of Two Plunging Wings in Tandem under Different Spacing Distances

An experimental study was performed in a low Reynolds number water tunnel for two tandem wings undergoing a 2D sinusoidal plunging motion. To investigate the effect of the spacing distance between the forewing and hindwing on the mean thrust coefficients of the two wings, a three-component force sensor and a two-dimensional digital particle image velocimetry (DPIV) system were used to measure the aerodynamic forces of the wings and to observe the leading edge vortex (LEV) around them. When the phase difference between the forewing and hindwing was fixed, the effect of LEV around the hindwing on the forewing shows similarity at different spacing distances, namely, increasing or decreasing the mean thrust of the forewing all the time. The increase of the mean thrust coefficient of the forewing is due to the LEV of the hindwing enhancing the jet velocity behind the forewing and its effective angle of attack. As spacing distance increases, the effect of the LEV of the hindwing on the forewing declines sharply, resulting in the mean thrust coefficient of the forewing approaching to the value of the reference single wing quickly. When the phase difference was fixed, the effect of the forewing-shed vortex on the hindwing was large. The mean thrust of the hindwing may firstly increase and then decrease as the spacing distance increases. This may due to the spacing distance changes the timing of the forewing-shed vortex affecting the hindwing. When the forewing-shed vortex encounters the hindwing and has an opposite sense of rotation with the LEV around the hindwing, the effective angle of attack of the hindwing and the mean thrust of the hindwing increase. If the two vortices have the same sense of rotation, the value of the hindwing decreases.