Effect of Boundary Layer Mixing Devices on Hysteresis Behavior of Flow Past a Pitching Airfoil

Abstract

Wind tunnel experiments were carried out on NLF-0416 airfoil model to investigate the effect of boundary Layer mixing devices on the aerodynamic characteristics and hysteresis behavior associated with the pitching motion of the airfoil. Pressure measurements were carried out on the mid span of the airfoil and in the wake for quantitative information. The hysteresis behavior was observed in aerodynamic characteristics as a function of pitch rates and was found to increase with pitch rates. For the pitch rates covered in the present study, no large eddy vortex formation associated with deep dynamic stall was observed. The results show significant improvements in hysteresis behavior and aerodynamic characteristics by the application of boundary layer mixing devices. The data show that placing the vortex generators between 5% and 7% of the chord which corresponds to the approximate location of the pressure peak at lower angles of attack, increases the lift coefficient and hysteresis lift significantly while improving the drag characteristics. Vortex generators close to the pressure peak allows the mixing of the outer flow and boundary layer flow to occur before reaching adverse pressure gradient. At low angle of attack region vortex generators completely removes the hysteresis loops. In order to arrest the spanwise flow, boundary layer fences were also used which resulted in improvement of stall characteristics.
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