Study On Mechanism And Performance Of Batoid Fishes Swimming Under Self-Propulsion

Aquatic creatures such as fish have fast,efficient propulsion capability and flexible maneuverability,which is the goal that underwater vehicle is constantly pursuing.The research motivation of fish propulsion mechanism is to develop bionic underwater vehicle.The underwater vehicle can imitate some key characteristics of fish morphology and kinematics,and achieve the propulsion performance equivalent to that of real fish.The batoids have a flat body,large pectoral fins that generate forward thrust,low noise when swimming,excellent maneuverability and the ability to glide forward.The bionic batoid underwater vehicle has the characteristics of strong concealment,high maneuverability,stable navigation near the bottom,and long voyage.At present,the research on the propulsion mechanism based on the morphology and kinematics of real fish is not deep enough to support the development of bionic underwater vehicles.The wake field of the caudal fin and pectoral fin oscillating have been analyzed by large number of studies,and the characteristics of the anti-Kaman vortex street in the caudal fin wake and the 3D double-ring vortex structure in the pectoral fin wake were obtained.However,the key vortex structures in the undulating and semi-oscillating motions of batoid have not been well studied.This study takes real batoid(including undulatory stingrays and semi-oscillatory raja porosa)as the research object,adopts computational fluid dynamics method,studies its self-propelled hydrodynamic performance and swimming mechanism.Aiming at the problem that the fish body model is simple in the research of stingray propulsion mechanism at present,a scheme based on the fish body reconstructed from the data of live fish 3D laser scanner as the calculation model is proposed.The first-order Fourier series is used to simulate the wave of stingray and the binary first-order function is used to obtain the undulatory amplitude of each point of the fish body.The scheme restores the geometry and active flexible deformation of real stingray to the greatest extent possible.Then,combined with computational fluid dynamics,the self-propulsion numerical simulation method is established.On this basis,according to the characteristics that the existing undulating fins only undulate vertically,the hydrodynamic performance of stingray without lateral movement is studied.Then,the hydrodynamic performance of stingray with lateral movement is studied,and the formation and evolution of the main vortex structure on the surface of stingray and the pressure distribution on the fish body are analyzed in detail.Finally,the effects of undulatory frequency on the hydrodynamic performance and vortex structure are studied.In nature,some batoids live near the substrate,whose swimming is often affected by the substrate.In order to explore the hydrodynamic performance of undulatory swimming near the substrate,the self-propelled swimming of stingrays near the substrate and unbounded fluid was studied.The changes of instantaneous velocity,forward force,lift,instantaneous loss power,power efficiency and Froude efficiency with gap ratio were analyzed in detail.Then by analyzing the pressure distribution and vortex structure at typical instant with different gap ratios,the mechanism behind the change of hydrodynamic parameters caused by ground effect was explained.Aiming at the problem of asymmetric pressure distribution on the stingray body in the simulation,the lateral motion mode of the stingray was improved.The lateral movement mode of the pectoral fin of the stingray changed from single propagation to symmetrical propagation.Stingray swimming under fast and slow frequency cases were studied,and the swimming performance such as velocity,thrust,power loss,power efficiency and Froude efficiency were given in both cases.The vortex structure and pressure distribution on the surface of the stingray were analyzed in detail,and the formation and characteristics of the main vortex structures were analyzed.On this basis,swaying and yawing degrees of the stingray are released,and the swimming performance of the stingray under three degrees of freedom was studied.In order to study the propulsion mechanism of fish at the semi-oscillation motion mode,which falls between the two motion modes of undulation and oscillation,morphological and kinematic data of the raj a porosa were obtained using a similar scheme.Numerical simulation of the self-propulsion process of the raj a porosa at fast and slow frequency cases was carried out,and its swimming performance under a single degree freedom was obtained.The propulsion mechanism of the semi-oscillating motion was explored by analyzing the vortex structure around the fish body and the pressure distribution on the fish body.Finally,the influence of the semi-oscillating frequency of the raja porosa on its swimming performance were discussed.The comprehensive numerical simulation results of stingray and raja porosa show that the research scheme proposed in this study explores the hydrodynamic performance and propulsion mechanism of batoid,with certain accuracy and reliability.The morphological and kinematic characteristics of batoid allow it to maintain high Froude efficiency during swimming,and the average swimming speed during steady state has a linear relationship with the frequency.In addition,the hydrodynamic performance of stingray at not transverse motion,transverse single propagation motion,transverse symmetric propagation motion under single degree of freedom,and transverse symmetric propagation under three degrees of freedom was compared.The results show that the propagation mode of transverse motion has great influence on hydrodynamic performance of stingray.The presence or absence of transverse motion is one of the reasons for the difference of swimming velocity between bionic undulating fin and real fish.