Research On The Propulsive Performance Of The Non-sinusoidal Flapping Bionic Foils

The propulsion performance of bionic flapping foil is the current vehicle propulsion research hot issues in the new field of air and underwater vehicle have received widespread attention at home and abroad.To explore the propulsion performance of flapping foils in different motion,consider the non-sinusoidal motion mode that is rarely studied in recent research,and use the commercial software Star CCM+ based on the CFD solver to perform numerical simulation calculations.This paper,from simple to complex,sequentially studied the propulsion performance of single foil under non-sinusoidal pure heave motion,the propulsion performance of single foil under non-sinusoidal heave and pitch motion,the propulsion performance of tandem double-foils under non-sinusoidal pure heave motion,and the propulsion performance of tandem double-foils under non-sinusoidal heave and pitch motion based on the overlapping grid method.The main contents are as follows:(1)According to the research needs of the research group,a two-dimensional model of single or multiple flapping foils of a standard airfoil under non-sinusoidal motion is proposed.After the instantaneous hydrodynamic foil flapping at different mesh density and time comparing step,convergence validated numerical model described herein.At the same time,a motion equation determined by non-sinusoidal coefficients is established,in which the motion form includes the heave motion of the flapping foil and the pitch motion rotating around the center of rotation of the flapping foil.(2)By changing the Strouhal number representing the frequency of the flapping foil,different working conditions are designed,and the propulsion performance of a single foil under non-sinusoidal pure heave motion is analyzed from the hydrodynamic performance and propulsion performance.The study found that,compared with sinusoidal motion,non-sinusoidal motion will cause irregular changes in hydrodynamic force and angle of attack,increase the average thrust and power,and reduce the propulsion efficiency.Observing the vortex structure on the surface of the flapping foil and its surroundings at a certain time,it is found that the vortex shedding modes produced by different non-sinusoidal modes are different.(3)The single foil’s motion increases from single-degree-of-freedom to two-degree-of-freedom,and the force of single flapping-foil under non-sinusoidal compound motions under different working conditions is designed,and the propulsion performance is analyzed from the hydrodynamic and propulsion performance.The study found that the hydrodynamic force and angle of attack of the flapping foil after increasing the degrees of freedom strengthened the unsystematicness under the influence of non-sinusoidal mode shapes.Compared with sinusoidal motion,the thrust and power obtained by most non-sinusoidal compound motion single foil increase,and the propulsion efficiency decreases.The vortex structure of non-sinusoidal flapping wings can be observed to change the vortex shedding mode with the change of motion.(4)By changing the global phase difference in the tandem double-foils system,design the force situation of the flapping foil under non-sinusoidal motion under different working conditions,and analyze the propulsion performance of the tandem double-foils under non-sinusoidal pure heave motion and non-sinusoidal compound motion.Studies have shown that the hydrodynamic force of the tandem double-foils is greater than that of the single foil under the same working condition,and the maximum thrust of the front and rear flapping foils is different.The non-sinusoidal coefficient may enhance the irregular changes in the hydrodynamic force.At the same time,the vortex shedding mode,number of vortexes and wake vortex splitting phenomenon of tandem double-foils are closely related to the non-sinusoidal mode of vibration.In addition,the propulsion performance of the front wing will be affected by the distance between the front and rear wings,but the effect is not significant.