Numerical Investigation Of Aerodynamic Characteristics Based On The Flapping Airfoil

In this work,the immersed boundary-lattice Boltzmann method is adopted to study the influence of the motion trajectory and ground effect on the aerodynamic characteristics of a flapping airfoil.Inspired by small creatures and flapping MAVs,the investigation is conducted for NACA0012 airfoil at low Reynolds and small range of oscillation frequency.The plunging amplitude of all simulations is fixed at a chord length of airfoil c.?1?First,the airfoil synchronously performs plunging and pitching motions,named as the traditional motion.Through a great number of simulations,the optimal combinations of pitching amplitude and rotation axis location which correspond to the thrust and efficiency are?_m=30^o,x_p=c/4and?_m=45^o,x_p=c/2,respectively.Later,the surging motion in horizontal direction is added,and the motion trajectory can be modified by changing the relative surging frequency k,surging amplitude l_m and the phase difference?between pitching and surging motion.Compared with the traditional motion,it is observed that the figure-of-eight trajectory has a promotion for the mean thrust C_t,mean,mean and propulsive efficiency?_t.When k is an odd number,along with the special?,the mean lift C_l,mean,mean can also be improved.In addition,the phenomenon of multiple vortices and vortex capture appeares due to the added surging motion,which is helpful for the enhancement of aerodynamic characteristics.?2?The influence of ground effect on the aerodynamic characteristic is investigated when the foil executes the traditional motion,the pitching amplitude and rotation axis location are?_m=30^o and x_p=c/4,respectively.Meanwhile,the averaged height between ground and airfoil h_w is considered during some range.Compared with the results in free stream flow,the airfoil aerodynamic in ground effect have been improved for most cases.In particular,C_t,mean,mean and?_t at h_w=1.1c are significantly improved compared with those at h_w?1.2c.However,C_l,mean,mean becomes negative.For the special case of h_w=1.1c,the non-sinusoidal parameters SS is introduced to the plunging motion.Thus,C_l,mean,mean is changed by adjusting the plunging velocity of up and down strokes.It is revealed that C_t,mean,mean and?_t have a bit drop when SS is slightly smaller than 0.5,but C_l,mean,mean is changed from negative to positive.It is illustrated that SS can changes the aerodynamic characteristics of airfoil in ground effect.