CFD Analysis of Oscillating Blades in Dynamic Stall Conditions

摘要

Blades have variety of applications such as propellers, helicopter's rotors, wind turbines and turbomachines. Aerodynamic investigation of blades is required for designing blades with good performance. Such investigations are typically performed assuming rigid blades subjected to steady state flows. However, blades are not rigid and their flexibility can cause considerable pitching and plunging oscillations of the blades. Such oscillations create unsteady aerodynamic conditions and play a significant role on the blades overall aerodynamic performance. In this study, the aerodynamic characteristics of three oscillating airfoils are studied in a two-dimensional viscous flow to show the importance of the unsteady behavior of the blades. These airfoils are E387, FX 63-137 and SD2030 which can be used in radio control Unmanned Aerial Vehicles (UAVs), Micro Air Vehicles (MAVs), human powered aircrafts and small Horizontal Axis Wind Turbines (HAWTs) in a wide range of Reynolds numbers between 100,000 to 750,000. A complete set of dynamic CFD simulations are used to find lift and drag forces acting on the airfoils in different unsteady conditions such as changes in amplitude and frequency of pitching oscillations, and changes in Reynolds number. Results of this study show that although changes in the frequency of oscillation and Reynolds number of the flow cause significant changes in the mean lift and drag coefficients (C_l and C_d), the change in the amplitude of oscillations has a larger influence on the C_l and C_d of the oscillating airfoils. The occurrence of dynamic stall is the most important result of the oscillating blades in the unsteady conditions. Also, decreasing the lift and increasing the drag forces are excessive after separation of the boundary layer and occurrence of the dynamic stall. The reattachment of the flow to the surface of the airfoils needs time, and meanwhile C_l stays in its minimum value before the flow reattaches to the surface of the airfoil. Moreover, the lift reduction will decrease the efficiency of the blades and the drag increment can cause structural damages in the blades. Based on the simulations for the three airfoils, FX63-137 shows better stability and performance in unsteady conditions. For FX63-137 airfoil the dynamic stall is shallow for a wide range of changes in reduced frequency and Reynolds number, and it shows less instability and moderate separation of the boundary layer from the upper surface of the airfoil even for larger amplitude of oscillations, compared to the other two airfoils.