Numerical investigation of the aerodynamic performance of dragonfly-like flapping foil in take-off flight

In this study, the aerodynamic performance of a dragonfly-like flapping foil is investigated in take-off flight using two-dimensional numerical simulations. Two parameters, foil spacing L* (L/c) and the phase shift between forefoil and hindfoil ψ, which characterize the flow in the tandem configuration, are varied to explore their impact on the resulting aerodynamic performance. Both the vertical and thrust forces generated in one cycle are found to be strongly dependent on ψ and relatively weakly dependent on L*. The tandem configuration is beneficial for the vertical force generation in the range of 0° ≤ ψ ≤ 30°, and further, it aids the thrust force generation for 40° ≤ ψ ≤ 110°. The flow interactions between the forefoil and hindfoil can increase the vertical force generated in a cycle by a maximum of ∼1.2 times when ψ is close to 0° relative to the no foil–foil interaction case (additive effect of two single foils). These interactions can also increase the thrust force generated in a cycle by a maximum of ∼2.8 times when ψ is close to 80°. The vortex structures reveal that the enhancement in thrust force mainly depends on the timing of interactions and how the hindfoil utilizes the vortex detached from the forefoil to benefit from it. Further, we find that dragonflies use in-phase stroking pattern (ψ = 0°) with a large downstroke angle of attack, α_D (α_D = 86 °), in the initial phase of take-off as it aids in generating additional vertical force. Our findings can significantly contribute to the design of micro aerial vehicles.