Influence Of Uniform Unsteady Incoming Flow On Hydrodynamic Force And Moment

Incoming flow has a great influence on the hydrodynamic force and moment of airships,bionic aircraft,underwater vehicles and offshore platforms.When the relative velocity of the motion body is same,the study of the hydrodynamic transformation relationships will greatly reduce the times of calculation in hydrodynamic analysis.It also provides a theoretical basis for simplifying water tunnel and wind tunnel experimental equipment and avoiding the use of dynamic mesh model in CFD by using relative motion principle.Most of the research results of traditional relative motion principle and the effect of unsteady flow on hydrodynamic force and moment are based on the assumption of inviscid flow,which are rarely proved strictly and systematically.In this study,the influence of uniform unsteady incoming flow on hydrodynamic force and moment is analyzed based on the viscous flow,and the conversion relationships at the same relative velocity is obtained,that is,the relative motion principle in the case of uniform unsteady incoming flow.Firstly,a virtual intermediate flow field is introduced to connect the incoming flow field and the non-incoming flow field,and the conversion relationship between the vorticity,velocity and particle acceleration distribution in the two flow fields is derived based on the vorticity transfer equation.The parameters of the incoming flow field are decomposed into the corresponding part of the non-incoming flow field and the part related to the incoming flow.Because it is not convenient to compare the pressure in the two flow fields,a new expression of hydrodynamic force and moment is proposed based on the theorem of vorticity moment.This expression only needs the distribution of velocity and vorticity in the flow field,and it can be used to compare the force and moment conveniently.Finally,on the basis of the conversion relationship of flow field parameters,the conversion methods of hydrodynamic force and moment in uniform unsteady flow are obtained by using this expression,which is applicable to both viscous flow and non-viscous flow.In order to verify the correctness of the conclusion,it is compared with the existing theory,and the NACA0012 airfoil is selected as the research object for numerical example verification.Compared with the existing theory of inviscid flow,the relative motion principle presented in this study is not only compatible with the existing theory of inviscid flow when the object is not rotating,but also applicable to the case that the object is rotating.The moment reference point can be arbitrarily selected,and the conversion hydrodynamic moment relationship obtained in this study is more general than the existing theory.The linear superposition relation of particle acceleration and the transformation relation are verified by the self-made panel method.When the incoming flow is unsteady,the correctness of the hydrodynamic force and moment conversion relationship is verified by combining theoretical analysis and numerical calculation.In the case of viscous flow,the CFD simulation results show that the velocity,vorticity and particle acceleration distribution in the two flow fields conform to the parameter conversion method.The difference between the hydrodynamic force and moment of the two flow fields is in good agreement with the theoretical values.