STUDY ON WIND TUNNEL WALL INTERFERENCE IN DYNAMIC AIRFOIL TEST

The aerodynamic nonlinear characteristics of airfoil caused by dynamic stall are prominent, and the coupling effect with wall interference brings great uncertainty to wind tunnel test data. In view of this, the mechanism and influence law of the wall effect in the airfoil dynamic test are revealed by means of experiment and numerical method. The results show that:compared to the static test, due to the existence of the wall interference, both the total pressure and the static pressure distribution of the wake zone during the dynamic test of airfoil are even more inhomogeneous. The wind tunnel wall interference of dynamic airfoil test is more serious. The differences of the velocity and pressure distribution on the middle section and the end section of airfoil are more obvious. Compared with the pressure surface, the two-dimensional flow of the suction surface becomes worse. The side wall interference suppresses the flow separation near the middle section of airfoil, and induces a spanwise separation flow near the end. The unsteady pressure coefficient of the upper and lower wall versus real-time attack angle also shows a hysteresis effect, the direction of the hysteresis loop is opposite, and the main characteristic frequency of the wall pressure fluctuation equals to the oscillation frequency of airfoil pitching. Influenced by the wind tunnel wall interference, the three-dimensional dynamic stall vortex structure of the airfoil is "Omega" type. In the positive stroke, the lift coefficient and the lift line slope are increased by the upper and lower wall of the wind tunnel, and the airfoil is induced to be stalled in advance. In the negative stroke, the lift coefficient of airfoil is reduced. The sidewall interference induces a spanwise flow on airfoil surface, reduces the chord-orientation flow velocity of airfoil, causes the reduction of lift coefficient in the negative stroke, but the interference effect is less in the positive stroke, besides the dynamic stall is delayed. The upper and lower wall interference of the dynamic airfoil test in the FL-11 wind tunnel should be a dominant factor. However, the sidewall interference cannot be ignored, especially at the high angle of attack and negative stroke during the oscillation period of airfoil.