Numerical Study Of The Unsteady Flow Field Of A Single Rotor And The Scale Effect

Based on three-dimensional unsteady and compressible N-S(Navier-Stokes)equations,the flow field of a single rotor is numerically simulated.Using Multiple Reference Frame Method and Sliding Mesh Method to describe rotational fluid field respectively,the calculation results of the two methods are in good agreement with experimental data,it proves that current mesh and numerical method can well capture the information of field near the rotor.To evaluate the turbulence model,imported to the simulation of the flow fluid,numerical verification of fluid field in hover state with eight degree of angle of attack is carried out.The SA(Spalart-Allmaras)turbulence model and Menter k-w SST turbulence model are used in the simulation,and the result shows that the simulation results by using these two turbulence models all agree well with the experiments,meanwhile,S A turbulence model has much higher computational efficiency.The flow field of the C-T(Caratonna-Tung)rotor and a 1/10 times scale reducing model of C-T rotor(which is called small rotor)in hover state is analyzed,the research focus on the impact on aerodynamic characteristic and flow structure of the scale effect.The simulation results demonstrates that the instantaneous lift coefficient of C-T rotor turns to be unstable over the cycle,whereas small rotor has less volatile and rotor speed has little influence on the average lift coefficient.The down-wash flow velocity of these two kinds of rotors first increases and then decreases in the radial direction,and the maximum velocity appears around the rotor tip.As being far away from the rotor disc,the local maximum velocity decreases.As the distance from the disk increase the maximum of longitudinal speed decrease.The flow field of the C-T rotor and small rotor in descent state is simulated and analyzed,and the evolution process of vortex ring state is captured.The effect of descent rate on aerodynamic performance,flow structure and pressure distribution,and meanwhile,scale effect is analyzed.Simulated results show that the instantaneous lift coefficient of C-T rotor can be very volatile,but small rotor has less volatile and rotor speed has a little influence on the average lift coefficient.The unsteady characteristic of C-T rotor is strong,whereas the lift coefficient of small rotor is more stable.The lift coefficient of C-T rotor decreases first and then increases with the increasing descent rate,whereas the lift coefficient of small rotor was increasing with the increasing descent rate.The descent rate has negative effect on aerodynamic characteristic of rotor in a certain range.When the descent rate exceeds a certain value,this effect is weakened or disappeared.The aerodynamic characteristic of C-T rotor is superior to small rotor on the whole,damage of vortex ring state to C-T rotor's aerodynamic characteristic is more destructive compared with small rotor.