CFD Analyses On The Aerodynamic Interaction Flowfield Of Tiltrotor Aircraft In Ground Effect

Tiltrotor aircrafts combine the advantages of traditional helicopters and fixed-wing aircrafts,namely the traditional helicopters’ VTOL and hover capability,fixed-wing aircrafts’ high-speed cruise capability,which are widely used in military and civil fields.However,due to the combination of conventional helicopters and fixed-wing aircrafts,the interaction flowfield of tiltrotor aircrafts is more complicated.At the same time,near-ground flight is an inevitable process,the flowfield of the tiltrotor will change significantly due to the blocking of the ground.Therefore,it is of great practical significance and theoretical value to study the interaction flowfield between tiltrotor fuselage / rotor / ground.In this paper,through the research of the grid generation method,the momentum source is adopted to simulate the flowfield of the rotors,and a CFD method considering accuracy and efficiency is developed,which is also suitable for simulating the interaction folwfield of tiltrotor aircrafts when Nap-of-the-Earth flight occurs.The numerical simulations of interaction flowfield of tiltrotor aircraft and ground under different parameters such as advance ratio and ground height have been carried out,and the characters of interaction flowfield are explored.The main contents of this paper include the following aspects:In Chapter 1,the uniqueness of the ground effect phenomenon of tiltrotor aircraft is introduced,as well as the domestic and foreign research status and difficulties of calculating tiltrotor aircraft’s interaction flowfield.The research progress of momentum source method and the development status of interaction flowfield simulation of tiltrotor aircraft and ground are expounded.The main research methods adopted in this paper are proposed.In Chapter 2,a grid system for the calculation of interaction flowfield of tiltrotor aircraft IGE is established.Firstly,by comparing the advantages and disadvantages of different kinds of grids,the grid technology used in this paper is selected.In order to improve the computational accuracy and reduce the computation time,triangular prism grids are generated in the boundary layer near the fuselage,and the other regions are filled with tetrahedral grids.According to the calculation requirement,the dense grid area and sparse grid area are divided.At the same time,density of the grids near the rotor disk and near the ground are increased to capture the flowfield characteristics near the rotor and the ground.The Descartes background grid system is introduced to realize the efficient search for the contribution unit of the momentum source item.In Chapter 3,based on the grid generation method set up above,the Navier-Stokes equations have been taken as governing equations,the Spalart-Allmaras model is used as turbulence model to simulate viscous effect near the fuselage,the discretization of convective fluxes and the temporal discretization are completed by Jameson scheme and five-step Runge-Kutta iteration respectively.A tripping method consisting of BEMT and SRMS aerodynamic models and Newton iteration method is proposed.The effect of rotors on flowfield is simulated by momentum source term.In this paper,different examples are given to verify the effectiveness of the method proposed.The numerical simulations are carried out for the isolated rotor,isolated fuselage and rotor / ground interaction flowfield.In Chapter 4,the numerical simulation of interaction flowfield of the tiltrotor aircraft/ground in hover is carried out.The height above ground,rotor / wing spacing,wing configuration and other parameters are selected.The interaction flowfield under different conditions are simulated respectively,the effect of different parameters such as height above ground on the interaction flowfield of the tiltrotor aircraft/ground is analyzed,and some conclusions about the flowfield characters are obtained.In Chapter 5,the numerical simulations of interaction flowfield of the tiltrotor aircraft/ground in low-speed flight are carried out.And the interaction phenomenon of the tiltrotor/ground and the tiltrotor aircraft/ground are analyzed respectively.The flowfields of different forward flight speed are compared and the effects of speed on the flowfield are obtained.At the same time,the formation and changes of ground vortex are observed.The flowfields of different ground height,rotor/wing spacing and wing configuration are calculated and different parameters’ effects on flowfield are analyzed.