Effect Of Rigid Bionic Pectoral Fin Shape On Propulsive Performance

Through hundreds of millions of years of natural selection,fish and other aquatic animals have evolved a variety of shapes and distinctive movement capabilities,which have high propulsion efficiency,high mobility,perfect fluid performance,low noise,good stealth,etc.,all of which are the goals pursued by current underwater robots.As an important part of thrust and shape control,it is particularly important to explore the motion law and shape of biomimetic fin propulsion.The pectoral fin with excellent shape will greatly improve the propulsion performance.In this paper,the propulsion performance of pectoral fins with different shapes is simulated by means of computational fluid dynamics.In order to realize the 3-DOF motion of pectoral fins,a fluid-structure coupling model of pectoral fins was established for calculation with the help of user-defined function and dynamic grid technology and three-dimensional unstructured tetrahedral grid.The changes of flow field and hydrodynamic performance around pectoral fins were analyzed,and the following results were obtained:(1)The planar pectoral fin with different aspect ratio moves in the same stroke law,and its propulsion performance is compared and analyzed from the hydrodynamic coefficient,pressure cloud diagram and vortex structure based on Q criterion.It is concluded that the planar pectoral fin with medium aspect ratio of 4.31 is the best in all experimental parameters and performance.Its thrust coefficient peak and lift coefficient peak are the largest,and the numerical values also dominate in most moments of the whole cycle,while the planar pectoral fin with small aspect ratio of 3.06 and the planar pectoral fin with large aspect ratio of 6.28 have poor performance.The general trend is that with the increase of aspect ratio within a reasonable range,the thrust and lift of pectoral fin will be improved,but after the increase of aspect ratio beyond a reasonable range,the thrust will decrease.In addition,the value of mid-span chord is larger and the range is wider than that of the low-pressure area on the pectoral fin surface.The vortex structure has good perpendicularity with the pectoral fin,and the main vortex structure is more robust.The pressure cloud and vortex structure of the other two pectoral fins are consistent with the results of hydrodynamic coefficient.In conclusion,the flat pectoral fin with aspect ratio of 4.31 has the best propulsion performance among all pectoral fins.Only when the aspect ratio is increased in an appropriate range can the thrust be positively affected.In order to further verify the validity of the conclusion and find out the planar pectoral fin with better propulsive performance,two sets of comparative experiments with aspect ratios between 4.31 and 6.28 were added.The results show that the pectoral fin propulsion performance deteriorates with the increase of aspect ratio,and the negative effects of transverse flow and tip effect increase with the increase of aspect ratio.(2)Concave pectoral fin has a special structure that can generate extra gain when pectoral fin moves,so the pectoral fin AR2 with the best propulsion performance is selected from the previous chapter,and it is bent in SolidWorks at three different angles,respectively 60°,90°and 120°.The concave fin is simulated and analyzed,and compared with the planar pectoral fin.The method is consistent with the previous chapter.In terms of hydrodynamic coefficient,the pectoral fin with a bending Angle of 60° has the best propulsion performance.Compared with the planar fin,the increase of thrust coefficient in the dynamic stroke stage is greater than that in the recovery stroke stage,which is bound to improve the propulsion efficiency of pectoral fin.In the pressure cloud,the pressure difference between the front and back of the pectoral fin with a bend Angle of 60° is larger.In the vortex structure diagram,its main vortex structure is thicker,but too large bending Angle will cause vortex structure disorder and poor verticality with pectoral fin plane,which affects the generation of thrust.This phenomenon is also manifested in the decrease of thrust coefficient and the decrease of pressure difference between front and rear of pectoral fin.In order to explore and further study the influence of different concave shapes on hydrodynamic performance,two planar pectoral fins with aspect ratios of 4.52 and 5.47 were subjected to the same bending treatment and simulated experiments were carried out.The analysis found that,An appropriate bending Angle of the concave fin is bound to have a positive impact on the pectoral fin propulsion performance,while too large bending Angle will affect the clarity of the flow field and the perpendicularity of the fin surface,thus reducing the pressure difference between the front and rear of the pectoral fin and the hydrodynamic performance...