Numerical Study On Self-propulsive Performance Of Single Or Multiple Flexible Plates

Phenomenon of fluid-structure interaction exists widely in the nature,the in-vestigation of fluid-structure interaction is significant in engineering and technology.The self-propulsive performance of single or multiple flexible plate in three typical fluid-structure interaction problems is investigated by an immersed boundary-lattice Boltzmann-nonlinear finite element method.The main results and conclusions are given briefly as follows:The free locomotion of a two-dimensional flapping flexible plate near the flat ground is studied numerically.The mechanisms underlying the ground effect are elucidated.It is found that an appropriate density ratio between the plate and surrounding fluid(M)can improve the propulsive performance of the plate.When M is relatively small,the lateral force is enhanced,and the input work is increased when the plate is near the ground;when M is large,the deformation of the plate is inhibited and the input work is decreased when the plate is close to the ground.Usually the closer the plate flapping is to the wall,the more efficient the propulsion is.On the other hand,when the plate is close enough(within a critical lowest distance),the efficiency reaches a plateau with the highest efficiency.The vortices pattern and pressure field are also analyzed to explore the mechanism.The effect of trailing-edge shape on the self-propulsive performance of three-dimensional flexible plates is studied numerically.In our study,the trailing edges of the plates are symmetric chevron shapes,and the trailing-edge angle ? varies from 30°(concave plate)to 150°(convex plate).Under different bending stiffnesses K,three regimes of the propulsive performance in terms of propulsive velocity U and efficiency ? as a function of ? are identified.When K is small,moderate and large,the square,con-vex and concave plate achieves the best performance,respectively.Analyses of vortical structures and velocity fields show that usually the jet behind the plate with the best performance is longest.Besides,the inclination angle of the jet may be small.The different propulsive performances at small and moderate K are mainly attributed to the phase lag of the trailing edge.The force acting on the plate is analysed and it is found that the thrust force is mainly contributed by the normal force.If U,? and K are rescaled by the normal force and the area moment of the plate,the curves for different? almost,collapse into a single curve when the bending stiffness coefficient is small or moderate.The scaling confirms that the normal force should be the characteristic fluid force at small or moderate K and the ? effect is governed by the area moment.The self-propulsion of multiple side-by-side flexible plates is studied numerically.The results show that a series of typical configurations of the schooling is formed spon-taneously under different transverse spacing and flapping phases between plates.In the case of the plates flapping in-phase,for N=2,the configuration changes from Alternative-Leading to Non-Interfering mode with the increase of the transverse spac-ing,meanwhile the propulsive metrics of the schooling increase monotonically to be con-sistent with those of the single plate;for N>2,the steady configuration do not emerge when the transverse spacing is small,then the stagger-periodic and stagger-stable mode orderly emerges with the increase of the transverse spacing.In the antiphase scenarios,when the number of plates is even,the propulsive velocity and efficiency of the schooling decrease monotonically with the increase of lateral spacing;when the number of the plates is odd,a single plate is in front of the schooling meanwhile the propulsive perfor-mance of schooling is not great,with the increase of transverse spacing,two adjacent plates flapping reversely lead the schooling which improves the propulsive performance of the schooling.