Numerical Study On Gliding Performance Of Biomimetic Flying Fish

Aerial-aquatic vehicles incorporate the advantages of aircrafts,surface ships and submarines,with high-speed,low-delectability and high-threatening.They are distinguished from the conven-tional underwater vehicles,particularly in their multifunctional features.It is very challenging to design an aerial-aquatic vehicle with good performance in both water and air,due to their signif-icant difference in density.As a kind of aerial-aquatic animals in the nature,flying fish,or ex-ocoetidae,can locomote in both air and water,transforming efficiently between these two media.Their unique aerial-aquatic ability inspires people in designing the artificial aerial-aquatic vehicles.In this thesis,we focus on the gliding performance of flying fish.Following the geometrical con-figuration of the Cypselururs hirarii(a special specie of flying fish),we build three-dimensional geometrical models,on which computational fluid dynamic simulations and three degrees of free-dom(3-DOF)simulations are performed.Here,the open source code OpenFOAM,which is based on the finite-volume method,is employed for the CFD simulations,with the DDES model involved for the turbulence modeling.In the CFD simulations,three different flying fish models arc consid-ered,between which their aerodynamic characteristics,such as lift,drag and moment coefficients are compared.The comparisons between the current numerical results and the previous wind tun-nel experiments have also been made,showing a good agreement between them.We find that the aerodynamic performance of the flying fish is comparable to hawk,petrel and wood duck,which also operate at low Reynolds numbers.In specific,by analyzing the flow topologies in the wake,we find that the flow attaches well to the surfaces of the pectoral fins in the range of low angle of attacks(AoAs),resulting in a linear relationship between the lift coefficient and the AoA,while for the high AoAs,i.e.,as AoA is larger than 10 degrees,the flow separates from the trailing edge,forming large-scale vortices,which converge to the symmetric plane as they travel downstream Furthermore,we build a 3-DOF dynamic model to predict the trajectories within the longitudinal plane.This 3-DOF model is tested by using the aerodynamic coefficients from the previous wind tunnel experiments.To find the optimal initial condition,i.e.the initial AoA and path angle,for the flying fish to achieve the maximum horizontal distance or the maximum height,a plenty of cases with different initial conditions are considered.The same 3-DOF model has also been ap-plied to predict the gliding trajectories of the artificial flying fish models proposed in the current study,based on the aerodynamic coeff cients obtained from our CFD simulations.The results show that the maximum horizontal distance of 45 meters,and the maximum height of 13 meters can be achieved,according well with the previous field observations,implying that the artificial models can reduplicate the gliding performance of the real flying fish.