Numerical Study On Self-Propulsive Characteristics Of Flexible Body With Variable Stiffness

The self-propulsion of organisms is a typical flow-structure interaction problem,which contains complex fluid dynamical mechanism.In this thesis,the self-propelled model of flexible flapping plate and FSI algorithm are used to numerically study the influence of the fluid environment around the flexible body,its own configuration and structural materials on its propulsion characteristics,and reveal the mechanical principle of biological motion.The corresponding results and conclusions are briefly given as follows:(1)Taking a model of a self-propelled flapping plate,we have numerically studied the locomotion of the plate in wakes of two tandem cylinders.By adjusting the distance between circular cylinders,a variety of typical wake patterns are constructed to simulate the complex hydrodynamic environment,and then the self-propulsion characteristics of flexible plate in complex vortex wake is discussed.Based on the numerical simulation analysis,it is found that there are three motion modes of the flexible plate:hold stationary,drift upstream,or drift downstream.The results show that the motion mode of the flexible plate mainly depends on the initial position and flapping amplitude,and the holding stationary mode is more likely to appear in the medium amplitude and appropriate initial position.Compared with the case directly in a uniform flow,the energy consumption of the flapping plate in the wake patterns is less and the minimum amplitude required for holding stationary is inversely proportional to the shedding vortex strength.Meanwhile,the effect of releasing style is also investigated in detail.By analyzing the temporal and spatial evolution of the equilibrium position of the plate and the vortical structure around the plate under different releasing style,the functional relationship between the pre-flapping of the flexible plate and the vortex shedding phase of the cylinder is revealed.Moreover,the equilibrium position spacing of the flapping plate under different vortex shedding patterns is accurately predicted.(2)The effect of non-uniform chordwise stiffness distribution on the self-propulsive performance of three-dimensional flexible plates is studied numerically.Some typical stiffness distributions,including uniform,declining and growing distribution,are considered.First,the normalized bending stiffness (?) is derived,which can well represent the overall bending stiffness of the non-uniform plates.For different non-uniformly distributed plates with the same (?),the maximum displacement d-ifference between the trailing and leading edges of the plate during the flapping is almost identical.There exists a common optimal (?) at which all the plates achieve their optimal performance,i.e.,the highest cruising speed and efficiency.Second,we reveal what kind of non-uniform distribution could be the best at a specific (?) in terms of the propulsive performance.The force analysis indicates that a larger bending deformation in the anterior part for the growing distribution leads to a larger thrust.Hence,the large local slope along the anterior flexible plate is preferred to enhance the propulsive performance.In addition,the effects of mass ratio and aspect ratio on the deformation and propulsion characteristics of the plate are discussed in detail.The results obtained in this study may shed some light on a better understanding of the hydrodynamic effect on the self-propulsion of the non-uniform stiffness wings or fins of animals.(3)The locomotion of a flapping flexible plate with different shapes and non-uniform chordwise stiffness distribution in the stationary fluid is studied.The normalized bending stiffness K* of 3D plates with arbitrary stiffness distribution and shape parameters is derived,and the overall bending stiffness characteristics of non-uniform plates with different shapes are reasonably described.It is found that the propulsion characteristics of the flapping plate mainly depend on the effective bending stiffness.In the range of medium flexibility,the flapping plate with differ-ent stiffness distribution and shape parameters shows higher propulsion speed and efficiency.At the same time,increasing the area moment and the anterior flexibility can significantly improve the propulsion characteristics and bending energy of the plate.The evolution of vortical structures and the pressure distribution in the near field and on the top and bottom surfaces of the plate are studied.The relationship between the stiffness distributions,shape parameters and the local deformation,normal force of the flapping plate is revealed.These findings are of great significance to the optimal design of propulsion systems with flexible fins.