A Numerical Study on the Influence of the Rear Wing Size on the Thrust Performance of the Two Dimensional Tandem Flapping Wings

In this paper, the thrust performance of the two-dimensional elliptic tandem flapping wings is numerically investigated on five different rear wings of chord length varying between 0.5C1 to 1.5C1 at an interval 0.25C1. Here, C1 is the chord length of the front wing that is kept constant. The aspect ratio (i.e. the ratio of the chord length and thickness of the wing) of all the wing is fixed as 8. The Strouhal number and Reynolds number based on the chord length of the front wing is set as 0.32 and 5000, respectively. The starting phase angle between the two wings is maintained at 0 deg for in-phase flapping. It is found that the identical rear wings in a tandem flapping mode (in the presence of front wing) produce a significantly large amount of thrust force as compared to in a single flapping mode (in the absence of front wing). From the thorough investigation of the flow regime and vortex structure, it is observed that the constructive interaction of the shear layers of the front wing and the leading edge vortex of the rear wing results in higher thrust performance in tandem flapping mode. The transient peak thrust is noticed for the rear wing having the smallest size. The correlation of the flow regime and transient thrust coefficient is also discussed in the paper.