Numerical Investigation Of Fluid And Flexible Structure Interaction Based On Immersed Boundary Method

The Fluid-Structure Interaction is a nonlinear and multi-physical phenomenon,which exists widely in nature and engineering fields.In this paper,two kinds of fluid-structure coupling problems are numerically studied by immersion boundary method,including soft-tail drag reduction problem and propulsion of flexible pitching plate.The main work and conclusions of this paper are as follows:The problem of"soft-tail drag reduction"is studied.The so-called"soft-tail drag reduction"means that in the flow past a rigid plate with a trailing closed wire of a small bending modulus acting as a flexible afterbody,which can achieve the drag reduction effect of the whole system.The influence of the Reynolds number and the length of the flexible wire are investigated.We found that with the action of fluid load,the flexible wire presents five typical modes,which are static deformation(SD)mode,micro-vibration(MV)mode,multi-frequency vibration(MFV)mode,periodic pitching(PF)mode and non-periodic pitching(NPF)mode.A certain length of flexible wire can change the resistance of the system effectively.With the increase of the length of flexible wire,the maximum drag reduction of approximately 20%.By analyzing the wake and the pressure distribution near the soft tail,the physical mechanism of changing the dynamic characteristics of the system through the soft tail is revealed.The dynamic characteristics of pitching flexible plates in uniform flow are studied.The effects of St number,pitching amplitude(A_L)and bending stiffness(K)on wake modes and push-drag transition characteristics were systematically investigated.It is found that for pitching plates with different flexural stiffness,the wake structure presents three modes in St-A_L parameter space,namely:Kármán street,Reserve Kármán street and asymmetric street.And we give the wake mode distribution and push-drag transition boundary curve in the St-A_Lphase plane.Then,by introducing the parameters describing the deformation and motion characteristics of the pitching plate,a scaling method of push-drag transition is given.It is also found that the flexible plate has the best propulsive performance when the system in resonance state.The peak values of trailing edge amplitude,thrust,power and efficiency coincide at the same frequency ratio at different pitching frequencies.The results are helpful for us to understand the internal mechanism of biological dynamic performance improvement through flexible deformation.