Experimental Study On The Effect Of Angle Of Attack On The Flow Around The Airfoil

In recent years,with the development of unmanned aerial vehicles,micro air vehicles and wind turbines,the aerodynamic characteristics of airfoils at low Reynolds numbers have attracted increasing attention.Under the condition of low Reynolds number,the flow around the airfoil is usually laminar flow,and laminar separation bubbles always exist.The laminar boundary layer has poor resistance to adverse pressure gradients,which can easily lead to transition or even stall.In order to study the derivation characteristics of the airfoil boundary layer separation with the angle of attack,PIV experiments are carried out on the SD7003 airfoil,NACA63₄-021 airfoil and the improved NACA63₄-021 airfoil.For the SD7003 airfoil,the flow field information at three angles of attack of 4°,6°,and 8°is measured by the Time-Resolved Particle Image Velocimetry（TR-PIV）system in the recirculation wind tunnel.The distribution of mean velocity streamlines,Reynolds shear stress and pulsating velocity root mean square at different angles of attack are compared and analyzed,and it is found that the transition occurs near the reattachment point in each working condition.With the increase of the angle of attack,the separation bubble and the reattachment point move to the leading edge of the airfoil,and the thickness of the separation bubble increases,and the velocity pulsation and shear motion in the flow field also increase.The Proper orthogonal decomposition（POD）results show that the flow field is mainly composed of vortexes and flow direction oscillation structures.After the fluid transitions downstream of the laminar separation bubble,periodic alternating positive and negative vortexes appear.The POD modal coefficient power spectrum curves show that the distribution of energy in the flow field shifts from high frequency to low frequency as the angle of attack increases.Both POD decomposition results and two-point cross-correlation results of the flow field space behind the airfoil show that the flow structures scale in the flow field are bigger and bigger with the increase of the angle of attack.For the smooth airfoil based on NACA63₄-021 and the improved concave-convex leading edge airfoil,the flow field information under the condition of 14°～24°angle of attack is measured by the PIV system in the recirculation wind tunnel.The distributions of the mean velocity streamlines and the vectors of the basic airfoil and the concave-convex leading edge at different angles of attack are compared,and it is found that the leading edge convex hulls change the flow separation state of the airfoil.The basic airfoil stalls at an angle of attack of 20°,and the airfoil with convex hulls on its leading edge still maintains a wide range of adhesion flow at an angle of attack of 24°,which effectively improves the stall angle of attack.The airfoil with convex hulls on its leading edge presents an asymmetrical flow state with unilateral stall.The leading-edge flow separation occurs first at the trough,forming a laminar separation bubble.The flow field at one convex peak maintains a long attached flow,and the flow field at the other convex peak has an early stall which occurs at an angle of attack of 16°.In order to further study the control mechanism of how the convex hulls on the leading edge affect the airfoil boundary layer separation,the distribution of the Reynolds stress,the root mean square of the velocity fluctuation and the vorticity of two typical conditions of 16°angle of attack before stall and 20°angle of attack after stall are compared and analyzed.Then,the flow field is reconstructed using the modes with 90%energy after POD decomposition.The Linear Stochastic Estimation（LSE）of the structures in the conditional events such as Q2/Q4 events and the vortex cores of the clockwise vortexes in the reconstructed flow field is carried out.It is found that the convex hulls on the leading edge make the boundary layer transition in advance by enhancing the pulsation and shear motion in the flow field around the airfoil,and the ability of the transition turbulent boundary layer to resist the adverse pressure gradient is enhanced.The convex hulls on the leading edge of the airfoil enhance the intensity of the Q4 events in the flow field in the stall area,and introduce energy into the boundary layer,so that the improved airfoil still maintains a longer attached flow in the stall area,thereby improving the aerodynamic performance of the airfoil with convex hulls on its leading edge.