Experimental Study On Fluid Structure Interaction Of Flexible Wing With High Aspect Ratio

Near space refers to the area which is 20 to 100 kilometers above the ground.This area is between aviation and aerospace areas and has received little attention for a long time.High altitude and long endurance aircraft can fly in the near space,which has the functions of reconnaissance and early warning.People have paid more attention to it in recent years.The requirement of long endurance of such vehicle determines that it should have a high aspect ratio and light structure.This structure makes it flexible,and flutter may occur due to fluid-structure interaction during flight,which will seriously affect the safety of the aircraft.Therefore,it is very important to study this phenomenon.According to the above background,a flexible wing with high aspect ratio is designed.Its bending stiffness is 13.5N·m~2,torsional stiffness is 5.2N·m~2,aspect ratio is 8,and airfoil is NACA 0012.The wing can flutter during flight.Wind tunnel experiments were conducted on the wing under the conditions of angle of attack?=0°?90°and Reynolds number Re_c=8.2×10~4?1.9×10~5,and the lift,drag,displacement,vortex and flow field were measured.It is found that some characteristics of the flexible wing with high aspect ratio are similar to those of the rigid wing under the same conditions.For example,the variation of the time average value of lift coefficients and drag coefficients with Reynolds number and angle of attack under the conditions of?=0°?90°,Re_c=8.2×10~4?1.4×10~5and the variation of nondimensional vortex shedding frequency St with Reynolds number under the conditions of?=30°?90°,Re_c=8.2×10~4?1.4×10~5are similar to those of the rigid wing under the same conditions.The difference is that the flexible wing is twisted due to the aerodynamic force under the conditions of?=8°?20°,Re_c=8.2×10~4?1.4×10~5,and the effective angle of attack increases,so the lift of the flexible wing is larger than that of the rigid wing under the same conditions.It is also found that flutter occurs under the conditions of?=10°?16°,Re_c=1.9×10~5,and the motion can be divided into two types:small amplitude flutter occurs under the conditions of?=10°,Re_c=1.9×10~5 and large amplitude flutter occurs under the conditions of?=12°?16°,Re_c=1.9×10~5.The small amplitude flutter has no effect on the aerodynamic characteristics of the wing,while the large amplitude flutter leads to a sharp increase in the drag and a sharp drop in the lift,which deteriorates the aerodynamic characteristics of the wing.In terms of energy,the energy absorbed by the system in a period of large amplitude flutter is much more than that in the case of small amplitude flutter,which leads to larger bending and torsional motion amplitude,more chaotic vibration and higher nonlinearity of the wing in large amplitude flutter.In terms of flow field,the shear layer will develop from near the suction surface of the wing to curl up at the leading edge,then form large-scale vortex structure,and continue to spread and dissipate towards the wake in a period of large amplitude flutter,while in a period of small amplitude flutter,the shear layer will always flow close to the suction surface of the wing,and it will separate from the wing only in some phases and then attach to the wing surface,Their huge differences in flow field lead to different flutter types.