Experimental study on mean thrust of two plunging wings in tandem

The phase difference and spacing distance between two plunging wings in tandem affect the interaction of the vortices between the forewing and hindwing. An experimental study is performed in a low Reynolds number water tunnel to investigate how this interaction changes the mean thrust coefficient and propulsive efficiency. A threedimensional force sensor and two-dimensional digital particle image velocimetry are used to measure the wing thrust, lift force, and leading edge vortex around the wings. The mean thrust coefficient of the forewing nearly changes sinusoidally with the phase difference in the range of 0-360 deg. The increase in the mean thrust coefficient of the forewing is caused by the leading edge vortex and stagnation region of the hindwing, enhancing the jet velocity behind the forewing and its effective angle of attack. The curve of the mean thrust coefficient of the hindwing has a V-shaped bottom. The decrease in the mean thrust coefficient of the hindwing is caused by the vortex shed from the forewing restraining the leading edge vortex formation of the hindwing, reducing its effective angle of attack. The propulsive efficiency of the hindwing shows a large variation range compared to that of the forewing.