Flow Stability And Drag Reduction Performance Of The Porous Flexible Skin Made Of Micro Floating Raft Arrays

With the development of national defense,bionic flexible surface drag reduction technology has gradually become a research hotspot in the field of fluid drag reduction.At present,the parameters of drag reduction structures used by researchers usually come from the specific continuous geometry of animal and plant surfaces in nature,which leads to the low abstraction of drag reduction surfaces and the difficulty of modeling,which is not convenient for the in-depth study of drag reduction characteristics of research objects.In this paper,by simulating the dolphin skin structure and combining the vibration reduction mechanism of floating raft with the porous flexible drag reduction wall,a porous flexible skin structure made of micro floating raft arrays for drag reduction on the surface of underwater vehicle is constructed,and the boundary layer flow stability and drag reduction performance of the skin are studied.This article mainly carried out the following work:(1)The flow stability characteristic equation of porous flexible skin is established.Firstly,the physical model of the porous flexible skin is constructed;then the structural control equation of the skin is established based on the plate and shell theory,and the equivalent form of the structural control equation is obtained by Fourier transform;finally,the support impedance function and fluid impedance function of the micro floating raft element in the equation are determined respectively,and then the flow stability characteristic equation is obtained,which lays a foundation for the subsequent stability analysis.(2)The flow stability of porous flexible skin boundary layer is analyzed.Based on the time model,the stability characteristic equation is numerically solved,and the curve of the growth rate of the flow instability in the boundary layer with the vibration frequency of the disturbance wave is obtained.The analysis results show that the specific structural parameters and surface parameters of the micro floating raft array skin can effectively suppress the time growth rate of the flow instability and make the flow process in the boundary layer stable,and then delay the transition from laminar flow to turbulent flow.(3)A numerical solver based on immersed boundary method is established.Firstly,the governing equation of the immersed boundary method is obtained based on the N-S equation.Then,the governing equation is discretized in space and time by using Cartesian grid and second-order Runge Kutta method.Based on the discretized governing equation,the flow chart of fluid structure interaction numerical calculation of flexible skin is established.Finally,the accuracy of the numerical solver is verified by an example of flow around a cylinder.The results show that the solver used in this paper has good accuracy,which lays a foundation for the subsequent numerical calculation.(4)The numerical calculation and analysis of drag reduction performance of porous flexible skin are completed.Based on the immersed boundary method,the simulation model of porous flexible skin is established.The relationship curve between the skin structure parameters and the resistance coefficient is obtained by numerical calculation.The results show that the maximum drag reduction ratio of the skin to the rigid plate can reach 15%within the specific unit spacing and specific flow velocity range.In addition,appropriately increasing the stiffness ratio,mass ratio,and surface permeability coefficient can reduce the fluid velocity near the skin,reduce the resistance of the skin,and improve the drag reduction performance of the skin.