Effects of stroke deviation on hovering aerodynamic performance of flapping wings

In this paper, the typical normal-hovering mode with different surging motions is numerically simulated by solving two-dimensional unsteady Navier-Stokes equations with the aim of investigating the effects of stroke deviation on aerodynamic performance. An elliptic wing model with 2% thickness is employed, conducting a horizontal motion (plunge), a vertical motion (surge), and a rotating motion (pitch). A low Reynolds number of 100 is adopted. The various surging motion in each half-stroke is defined by a half-sine or full-sine waveform, while the pitching and plunging motions are fixed for 16 patterns. The details of the aerodynamic force histories, vortex dynamics, induced jet effects, and time-averaged aerodynamics are systematically analyzed. The results show that for most patterns, stroke deviation plays a negative role in reducing lift and increasing energy consumption, which results in a decline of lifting efficiency. The forward surging motion that commences up the horizontal stroke plane attenuates the wake capture mechanism and reinforces the delayed stall mechanism. Compared to the typical normal-hovering pattern with no deviation, the resulting lift in pattern E decreases at the beginning of stroke and increases at the midstroke. The downward surging motion shows an opposite effect on the aerodynamics. The minimum power (-10.2%) is consumed in pattern F, although the minimum lift is generated in the meantime. In addition, the maximum lift augmentation of 8.7% is produced in pattern I along with the characteristic of power economy. Our study can provide advice on utilizing stroke deviation to increasing lift production and decreasing power consumption.