CFD Analyses On The Interaction Flowfield And Aerodynamic Characteristics Of Helicopter For Nap-of-the-earth Flight

The helicopter always flies near the ground during taking off, landing and carrying out special tasks. Due to the block effect of ground on the rotor wake, the rotor wake rolls up after impinging on the ground, which brings in obvious variations of the aerodynamic forces and flight control of helicopter. Therefore, deep investigations on the aerodynamic characteristics of helicopter rotor in ground effect have important theoretical and practical significance. The key of research relies on solving the interaction flowfields of fuselage/rotor/tail-rotor/ground high-efficiently and accurately. A CFD method based on momentum source model has been developed, and a hybrid trimming method has been proposed and used in the trimming analyses of helicopter. Using the methods mentioned, the influences of different advance ratios and different flight heights on the interaction flowfields and aerodynamic characteristics of helicopter are calculated, and the variation law of aerodynamic characteristics and control of the helicopter rotor is explored. The concrete research contents are as follows:In Chapter 1, as the premise and research background, the important significance, research difficulties and developments on investigations of the helicopter in ground effect(IGE) are introduced briefly at first, and the advantages of CFD method in numerical simulation of IGE and the necessity of considering helicopter trim analysis are described. Finally, the research method and contents of this paper are given.In Chapter 2, the characteristics of different types of mesh are compared, and considering the computational efficiency, accuracy and the characteristics of the interaction flowfield at the In-Ground-Effect flight, triangular prism meshes are generated near the fuselage and tetrahedron meshes are generated in far-field region, and an unstructured hybrid mesh generation method is established for the numerical simulation of the flowfield. Using this method, a hybrid mesh system including rotor, helicopter and ground is generated.In Chapter 3, taking N-S equations as governing equations, a CFD method is established for predicting the interaction flowfield of helicopter fuselage/rotor/ground, and in this method, the effects of rotor and tail rotor on flowfield are simulated by a momentum source model, the viscosity effect is calculated by one equation S-A turbulence model. Jameson second-order central difference scheme is employed in spatial discretization, and the five-step Runge-Kutta method is adopted in temporal discretization. On this basis, the numerical simulations of the flowfield of the isolated rotor, isolated fuselage, rotor/fuselage interference and rotor/ground interference are carried out, which verifies the validity of the present CFD method.In Chapter 4, firstly, the helicopter flight dynamics model is established, and the trimming equations of the conventional helicopter is given. On this basis, the genetic algorithm/quasi Newton iteration hybrid trimming algorithm is proposed and established, combined with high precision CFD method established in Chapter 3, the hybrid trimming analysis coupled CFD method is constructed. Using the hybrid trimming method, taking ROBIN rotor/fuselage interference model and the UH-60 A helicopter as numerical examples, trimmed calculations are compared with the experimental data, the sudden increase of rotor power required in low-speed forward flight near the ground is simulated well, and it is demonstrated the trimming analysis method is effective to analyze the interaction flowfields.In Chapter 5, based on CFD method and trimming method, the analysis method of nap-of-the-earth flight interference flowfield and the aerodynamic characteristics of helicopter is established. Firstly, the method is used to calculate the rotor thrust increment in ground effect. The calculated results are compared with the experimental data, and the validity of the method is verified. Then, the influences of the moving ground and inclined ground on the ground effect are investigated, and some meaningful results are obtained. On this basis, the flowfields of rotor/ground and rotor/fuselage/tail rotor/ground in different flight heights and different advance ratios have been calculated respectively, and the parametric variation laws of the aerodynamic characteristics and control of helicopter rotor have been analyzed. Some valuable conclusions are obtained.