Research On Numerical Model And Mechanism Of Deformable Bionic Structure-fluid Interaction System

To explore the ocean,the development of marine equipment has to be given priority.As a new star in the area of marine equipment,underwater robot is playing an increasingly important role in the applications of marine development,marine management and marine safety.Specifically,underwater robot is widely used either for the purpose of national economy and national defense,such as in ocean observations,seabed topography scanned,underwater pipeline maintenance,emergency,weapons delivery and objection detection.To compensate for the weakness in conventional underwater robots propelled by propellers,fish and other marine lives are usually taken for the bionic propulsion prototype for their natural advantages,including excellent motion flexibility,environmental adaptability and target concealment.So this promising area has been attracting more and more attentions,and diversity of fish-inspired robots have been developed in these years.However,these bio-inspired swimming robots are still dwarfed when compared with their vivos in the natural environment.The mainly cause may be attributed to the lack of understanding the motion mechanism accurately of the bionic prototype.Therefore,the effective scientific solution to this contradiction is that deeply revealing the motion mechanisms of these marine lives based on fluid dynamics.Essentially,the motions of the underwater bionic prototypes are the results of the dynamics interactions between flexible structures and the surrounding fluid in nature.For the purpose of revealing such interactions,this thesis developed an efficient numerical algorithm,and based on which three typical physical problems abstracted from the bionic motion were numerically investigated.Firstly,a fluid-structure interaction algorithm based on heterogeneous parallel strategy was presented.In this algorithm,the entire flow field is decomposed into three numerical domains,including fluid domain,structure domain and boundary interaction domain.Among them,the fluid domain is solved by the lattice Boltzmann method,the boundary interaction domain is solved by immersed boundary method,and the structure domain is solved by direct-forcing scheme,finite element method or finite difference method which depended on its motion type.To verify and validate the presented algorithm,several typical benchmark problems were simulated and visualized,including flow pass a circular cylinder and passively flapping of a flexible plate.and vortex-induced vibration of a circular cylinder.It was confirmed that this algorithm is suitable for the simulation of fluid structure interaction system in two or three dimensional.Secondly,the interaction between two flexible plates in a three-dimensional uniform flow was numerically investigated based on the presented algorithm mentioned above.In the tandem configuration,the anomalous hydrodynamic drafting of the two flexible plates was discovered,specifically,the flapping amplitude and drag force of the downstream plate was higher than the upstream one.In the side-by-side configuration,three coupled flapping modes were identified,including isolated-like flapping mode,symmetrical flapping mode and independent flapping mode.By extracting the vortical structures,it was found that there are two vortex interaction modes in the tandem configuration,including merged vortex shedding and isolated vortex shedding.Similarly,there are there vortex interaction modes in the side-by-side configuration,which are highly corresponding to their coupling motions.The underlying interaction mechanisms were elucidated by the time history analysis of the flow field.Thirdly,the interaction between a flexible plate and an elastically-mounted circular cylinder in tandem configuration was numerically investigated based on the presented algorithm.Three typical vibration modes of the cylinder were discovered not matter the flexible plate was fixed upstream or downstream,including vibration strengthened,vibration weaken and vibration suppressed.However,the motion of the flexible plate was found to be highly depended on the configuration.As the plate was put on the downstream,three flapping modes were observed,including normal flapping,inverted flapping and inclined flapping.Also,constructive interaction mode and destructive interaction mode of the plate with incoming vorticity were identified.Particularly,the plate could gain well vibration weaken and drag reduction effect under the two-row type “2S”wake characteristic.As the plate was put on the upstream,the bi-stability was observed,including stretch-straight and stable flapping with high amplitude.Correspondingly,three interaction modes between the cylinder with the plate’s wake were identified,including immersed mode,slaloming mode and interception mode.Among these interaction modes,the slaloming mode is beneficial for amplifying the cylinder’s vibration amplitude and reducing the drag force,the peak value of the increasing and decreasing rate could reach to7.84% and 12.91%,respectively.Moreover,the rest two interaction modes were found to be helpful for the cylinder’s drag reduction and vibration weaken,the peak value of which could reach to 57.95% and 88.26%,respectively.The underlying interaction mechanisms in this system were elucidated by the time history analysis of the flow field.Finally,the mechanism of jellyfish-like propulsion was numerically investigated based on the presented algorithm mentioned above.It was found that jellyfish’s thrust force and displacement could be improved as the Reynolds number or wave-form coefficient increases,the maximum displacement per cycle is about 2.13 times the length of the body,and the maximum propulsion efficiency is about 17.63%.The jellyfish could effectively swim forward no matter under the “faster contraction”mode or “slower contraction”mode.The asymmetry time cost between the contraction and expansion phase could promote the jellyfish’s propulsive performance.Moreover,the existing of coasting phase among the entire deformation cycle was found be beneficial for the jellyfish’s propulsion,such as improving the thrust displacement.Through the time history analysis of the flow field,the physical connection between the jellyfish’s deformation behavior and the flow characteristics was elucidated.Then,the propulsion mechanism of the jellyfish was revealed in the view of fluid dynamics,which could be used for further optimization.