Effects of continuously increasing pitching and plunging amplitudes on the aerodynamic forces of flapping airfoils

Recently, numerous studies have been conducted to clarify the effects of the increases in pitching and plunging amplitudes of flapping wings on thrust and lift generation. In the present study, the effects of continuously increasing pitching and plunging amplitudes on the aerodynamic performances of a two-dimensional (2D) flapping wing are investigated computationally. Continuously increasing pitching and plunging amplitudes have significant effects on the rate of leading-edge vortex (LEV) development and the time of LEV separation; as a result, the aerodynamic performance is influenced. Lift and thrust are gradually improved with increasing pitching and plunging amplitudes; however, higher amplitudes induce the production of drag forces. Furthermore, to compare the contributions of the pitching and plunging amplitudes, we conducted simulations with pure pitching or plunging amplitude increases while keeping the other factors constant. With the increase in pitching amplitude, the vortex on the upper surface becomes weaker during the downstroke and leads to the production of a vortex on the lower surface. During the upstroke, the effect of the increase in pitching amplitude on the vortex has a symmetric influence against the downstroke. The change in pitching amplitude has little effect on the lift and thrust but leads to the production of drag forces. When the plunging amplitude increases, the LEV and the second kind of vortex, the trailing-edge vortex (TEV), becomes stronger, which will cause a concurrent increase in lift and thrust. The increase in plunging amplitude greatly improves lift and can also enhance thrust.