Numerical Study On The Effects Of The Attached Flexible Plate On The Propulsion Of A Traveling Wavy Foil

In statistics,85 percent of the fish in the world use BCF(body and/or caudal fin)as their main mode of locomotion,fish swing their bodies and caudal fins that interact with the surrounding fluids to push themselves forward.Therefore,it is particularly important to investigate the role of the fish body and caudal fin in the swimming process.Lattice Boltzmann method and immersed boundary method are used to simulate fish swimming in this paper.The body of the fish is described by the NACA0012 airfoil,and the deformable caudal fin is modeled by a passively flapping flexible plate attached to the end of the airfoil.This simplified model is used to study the coupling effect of the caudal fin and the body on fish swimming.The flexibility of the caudal fin is measured by the bending stiffness of the attached plate,and the chordwise body flexibility is expressed by the wavelength ? of its travelling wave.This article analyzes the hydrodynamics under the coupling of the caudal fin and the body in the swimming process.This research provides a new perspective for understanding why fishes with different morphologies in nature choose specific motions.This work is also instructive for the design of the bionic robot fish.The influence of the length and stiffness of the attached flexible plate on the propul-si on performance is studied first.When the wavelength is ?=1.0,the flexible plate with the length l varying from 0.1 to 0.3 and the bending stiffness KB varying from 0.01 to 250 are simulated.The impact of the caudal fin on hydrodynamics and propulsion per-formance is discussed.Both the flexibility of the attached plate and the tail-to-body length ratio play important roles in thrust generation and propulsion efficiency,and a parameter range with good propulsion performance is obtained.The results show that at the wavelength ?=1.0,when the tail-to-body ratio is greater than 0.2,and the bending stiffness of the attached plate is greater than 1.0,the reverse von-Karman vortex street appears in the wake,and the force generated due to the pressure distribution points from downstream to upstream while the propulsion efficiency changes from negative to pos-itive.The combined effect of the chordwise flexibility of the airfoil and the attached plate on the propulsion performance is also studied.When tail-to-body is 0.25,the propulsion performance of the airfoil movement is examined at a wavelength of 0.8 ???2.0.The results show that the propulsion performance is the optimized at 1.0???1.25,and the higher propulsion efficiency is achieved with less power consumption.Also,the propulsion performance is examined by changing the tail-to-body ratio for different motion wavelengths ?=1.0,1.25,2.0.It is found that when the airfoil fluctuates greatly and the chordwise flexibility is strong,there exists a optimized parameter range 0.25?l?0.3.When the motion of the airfoil has a large wavy motion,the power consumption is significantly reduced.When the tail-to-body ratio is large enough,higher propulsion efficiency can be obtained with much smaller power consumption than that of wavy motion.Through the hydrodynamic study,it is found that the thrust force is mainly generated by the attached plate,while the lift force is mainly by the airfoil.