Investigations On CFD Simulation And Active Flow Control For Unsteady Dynamic Stall Of Helicopter Rotor

The dynamic stall of helicopter rotor has important effects on the aerodynamic and vibration characteristics of rotor, and then limits the flight envelope, flight performance and vibration level of helicopter. The investigations on exploring the impact mechanisms of dynamic stall on unsteady flowfield and aerodynamic characteristics of rotor, suppressing adverse effects of dynamic stall and improving the aerodynamic performance of rotor are always the key and difficult points of helicopter aerodynamics. In this paper, based on the new generation method of moving-embedded grid system around rotor, the high-efficiency and high-accuracy CFD methods for solving unsteady flowfield of rotor are investigated, and the numerical analyses about rotor unsteady dynamic stall are conducted in emphases. Moreover, employing the synthetic jet technology, variable droop leading-edge(VDLE) method and new combinational control strategy, the numerical analyses for active flow controls on rotor and airfoil dynamic stall are carried out, and the experimental investigations about airfoil stall control via synthetic jet are conducted. The major constructions of the investigated paper are as follows:As background of the present work, the developments and applications of the CFD methods in rotor flowfield simulation, rotor dynamic stall analyses and rotor active flow control are described firstly. Then, the importances of numerical investigations on rotor dynamic stall and mechanism analyses for rotor active flow control are pointed out. Also, the problems in present researches and corresponding methods which will be used in this paper are briefly introduced.Considering the complex aerodynamic shape of rotor blade, based upon the solution of Poisson equations, the parametric generation methods of grid around rotor blade are established in Chapter 2, and the moving-embedded grid system of rotor is generated with combination of the Cartesian background grid. Aiming at addressing the problems about hole cells identification and donor cells searching among the embedded grid system, the novel “disturbance diffraction” method and the new “minimum distance” strategy are proposed respectively.In Chapter 3, taking N-S equations as governing equations, the rotor CFD methods with high efficiency and high accuracy are established to solve the unsteady flowfied of rotor. In order to reduce the non-physical dissipation, the Roe and AUSM type scheme are employed to calculate the convective flux respectively. To simulate the unsteady characteristics of rotor flowfield, the dual-time method is adopted and the implicit LU-SGS method without matrix storage is used at the pseudo-time iteration. According to the coexistence of the incompressible and compressible flows in rotor flowfield, the coppresponding preconditioning method is developed. To further improve the efficiency of rotor CFD method, the Open-MP parallel computation technology and agglomeration multigrid algorithm are conducted. In addition, the one equation S-A turbulence model and two equations k-ω SST turbulence model are employed respectively to accurately simulate the large flow separation on retreating blade of rotor. By systematic example validations, it is demonstrated that the present CFD methods satisfy the high efficiency and high accuracy requirements on simulating the vortex flow characteristics and aerodynamic characteristics of rotor, which provides favorable foundations for further investigations on rotor unsteady dynamic stalls and their active flow control analyses.Based upon the present CFD methods, the formation and convective motions of dynamic stall vortex in dynamic stall process of rotor airfoil are numerically simulated in Chapter 4, and the influences of shedded vortex on aerodynamic characteristics of airfoil are analyzed. The parametric analyses on dynamic stall of rotor airfoil are carried out, including reduced frequency, mean angle of attack, and amplitude of the oscillatory airfoil. Then, considering three-dimensional flow effects in forward flight, the flow separation, characters of dynamic stall vortex and aerodynamic characteristics of rotor in dynamic stall are analyzed in emphases, and comparisons are made with the corresponding 2-D airfoil cases. Furthermore, effects of axial-induced velocity and spanwise flow on aerodynamic characteristics of rotor are analyzed, and the impact mechanisms of rotor forward flight velocities on unsteady aerodynamic characteristics of rotor are investigated.In Chapter 5, based on the assumption of the blowing/suction velocity over the orifice of synthetic jet actuator, and according to the rotational speed and inflow velocity in forward flight of rotor, the jet control equations are established for rotor(airfoil) active flow control. Firstly, the flowfield of the isolated synthetic jet and interactions between jet and main flow are simulated, and the effectiveness of the boundary conditions are verified by taking TAU0015 and VR-7B airfoils as numerical cases. Then, the influence rules of synthetic jet parameters(momentum coefficient, jet angle, jet location and excitation frequency) on static stall and dynamic stall control effects of rotor airfoil are investigated, and mechanisms of the jet on postponing flow separation and unsteady aerodynamic characteristics of airfoil are analyzed. Some disagreements in present studies are explained by the parametric analyses. Additionally, the investigations of jet control effects on improving the unsteady aerodynamic characteristics of rotor are carried out, and some valuable conclusions are obtained, which can provide theoretical foundations for further applications of synthetic jet on rotor active flow control.In Chapter 6, firstly, the control effects of variable droop leading-edge(VDLE) on dynamic stall of rotor airfoil are numerically simulated, and the influence rules of VDLE on control effects are achieved. Then, the multi-objective optimization of VDLE are conducted, and the optimal results indicate that brief “upward” of leading edge is a better strategy for enhancing the aerodynamic performance of rotor airfoil. Subsequently, three dimensional VDLE control on rotor dynamic stall are numerically investigated, and some rules of VDLE control on improving rotor aerodynamic characteristics are obtained. Furthermore, the novel control strategy with the combination of synthetic jet and VDLE is proposed, and the new control strategy on the active flow control of rotor and airfoil is simulated, as a result, the advantages of the new combinational control method are found compared with the individual flow control method.Finally, the wind tunnel tests are designed and conducted for the synthetic jet control on stall of rotor airfoil in Chapter 7. Based upon the Particle Image Velocimetry(PIV) measurements of flowfield around airfoil, measurements of model aerodynamic forces and velocity profiles in boundary layer of airfoil, the systematic investigations are carried out to obtain the control effects of synthetic jet on improving the maximum lift coefficient and stall angle of airfoil with different parameters. Some conclusions are obtained which correlate well with the numerically simulated results. In addition, the quantitative control functions of synthetic jet on delaying stall of rotor airfoil are innovatively proposed for the first time. Furthermore, the dual synthetic jet array is experimentally analyzed for its influences on improving active flow control effects of airfoil stall. Some new conclusions are obtained in the respect of influences of synthetic jet arrays on the control effects of airfoil stall, such as phase difference and relative jet angle of jet arrays.