Hydrodynamic Study On Biomemetic Fish Flapping Caudal Finpropulsion System

Fish and cetaceans have an outstanding capability to swim efficiently and locate preciselyunder water by flow control. By contrary, the traditional propulsion systems used in UnmannedUnderwater Vehicles (UUVs) always have many screw propellers distributed in variousdirections for both propulsion and maneuvering, which take large room and consume highenergy. In recent years, many kinds of bio-propulsion systems have been develped. Amongthese bio-propulsion systems, the BCF (Body and Caudal Fin) mode is widely used. Therefore,fish swimming in BCF mode are paid more attention by researchers. This paper presents studyon hydordynamic performance of biomemetic fish flapping caudal fin propulsion system. Inthe study, Computational Fluid Dynamics (CFD) method is applied to analyze propulsionperformance of the flapping caudal fin propulsion system.Firstly, in order to study the propulsion perfromance of the hydrofoil in complex flowfield, hydrodynamic performance of a two-dimensional flapping hydrofoil behind anoscillating semi cylinder is calculated based on RANS equations. Interaction modes betweenincoming vortices and foil vortices are analyzed. The results show that incoming vorticesmake large influence on hydrodynamics of the hydrofoil. The hydrofoil could absorb energyfrom incoming vortices so that the propulsion efficiency is enhanced.Secondly, effects of caudal fin shapes and flexibility are studied. First of all, thepropulsion performance of flapping caudal fins with three different shapes (the tuna caudal fin,the dolphin caudal fin and the whale caudal fin) is calculated by both the unsteady panel methodand the RANS equations based method. The comparison between calculation results andexperimental results indicates that the two numerical methods are both validated. It is alsoshown that the caudal fin shape especially as far as its projected area has a crucial influence onthe propulsion performance of a flapping caudal fin. The tuna caudal fin produces the smallestvortices distribution and has the largest propulsion efficiency. Then, the propulsion performanceof a flapping caudal fin with chordwise flexibility is studied. The effects of chordwise deflectionphase angle on propulsion performance are discussed. It is figured out that some chordwisedeflection phase angles make import power smaller, propulsion efficiency higher. Energycould be saved by adjusting chordwise deflection phase angle.Thirdly, effects of asymmetric motions are studied. Numerical simulations on a flapping caudal fin under asymmetric motions are performed by the panel method and RANSequations based method. Four kinds of asymmetric motions are discussed, includingasymmetric pitch motion, asymmeric heave motion, flapping motion with pitch bias andflapping motion with heave bias. The study indicates that under asymmetric motions,unsymmetrical hydrodynamics in one motion period can be produced. It is indicated that thepitch bias makes the mean lateral force non-zero. Meanwhile, hydrodynamics varying inmotion period is not continuous under heave bias. Thus, Modifications are done so as to getrid of the jump of hydrodynamics. Besides that, shedding vortices are also influenced byasymmetric motions.Finally, the self-propelled swimming of a bio-mimetic UUV called “FangSheng-I” iscalculated by RANS equations based method. An algorithm of fluid-motion coupling isdesigned and implemented in the CFD code. Spring based dynamic mesh and remeshtechniques are used to keep grid quality. The comparison between calculating results andexperimental results shows that the numerical method is validated. After that, it is indicatedthat that motion parameters including motion frequency, heave amplitude and pitch amplitudemakes an important effects on cruising velocity, hydrodynamic performance and sheddingvortices.