Numerical Investigation On Aerodynamic Characteristics And Flow Field Of Biplane Airfoils For High Angle Of Attack Tilting Transition

Modern aircrafts need to adapt to diverse application scenarios,thus the structures and performances consequently pursue light-weight,high mobility and high-speed and other advantages.Among them,vertical takeoff or landing(VTOL)has aroused the interest of academic and engineering circles.Biplane VTOL MAV was designed as a late compound aircraft which not only possessed VTOL modes,but also can achieve fast cruise like fixed wings.Four modes contain vertical takeoff,tilting transition,cruise and vertical landing.Especially in the tilting transition mode,biplane wings easily encounter unsteady flow at high angles of attack,leading to high nonlinear aerodynamics.And severe aeroelastic issues are exposed such as buffeting and lock-in phenomenon,which reduce flight stability even cause structural damages.Many efforts have been made for the research topic that the airfoil undergoes flow at high angles of attack,but biplane configuration also introduces aerodynamic interference,therefore,the same issue of biplane configuration should be explained by more complicated physical mechanism.Unfortunately,it remains elusive,with no reasonable description forthcoming.In consideration of the design process and service life of this aircraft,numerical investigation on unsteady dynamics of 2D NACA 0012 biplane airfoils at high angles of attack had been carried out in this paper.Based on U-RANS method,simulations of tilting transition modes were presented utilizing ANSYS-Fluent.Time-averaged and fluctuating aerodynamic characteristics were calculated and analyzed under different spacing ratios,respectively.Besides,we continued to perform a series of studies on the limit cycle oscillation and lock-in of oscillating biplane airfoils,then systematic explanations were given to elaborate the mechanism of lift coefficient variation of biplane airfoils from details of flow fields.In addition,we constructed integral form of effective angle of attack for tilting transition of biplane configuration.The main research contents of this paper are listed as follows:(1)Towards the high angles of attack(30°?90°)complex flow,we took various spacing ratios into consideration to numerically study averaged aerodynamics of 2D biplane airfoils,and classified these results and discussed the developing tendency.First of all,simulations were used to analyze the variation law of mean aerodynamic loads as well as instantaneous fluctuation characteristics of lift and drag.What's more,extensive and in-depth research about part and total unsteady aerodynamics of biplane airfoils impacted by different spacing ratios.For flow structures particularly wake distributions,we had carried out detailed induction and classification according to trailing-edge vortices growth and evolution,including integrated vortex,transiting vortex and coshedding vortex.Biplane configuration with spacing ratio ?=1.5 provided better lift-to-drag ratio and the fluctuating lift curves quickly converged,therefore,it is likely to be the optimal spacing.(2)High angle of attack is one of the main sources of buffeting.We investigated spacing effect and Reynolds number effect on the buffeting frequencies of biplane airfoils.Predicted conclusion was similar to the best spacing ratio in(1),however,increasing Reynolds number benefited to buffeting mitigation.We qualitatively analyzed the distribution of stagnation point regions of the upper and lower airfoils for the sake of further explanation of buffeting characteristics.Additionally,there were limit cycle oscillation of two parts if biplane airfoils underwent low-amplitude pitching motion under different spacing ratios.An interesting finding was that 8-shaped hysteresis loops originating from evolution of flow structures.More importantly,we could confirm when beating emerged or disappeared through Fast Fourier Transform,and drew the V-shaped locking regions only considering spacing ratios.(3)We revealed the effect of pivot location on dynamic loads of biplane airfoils.Assuming the upstroke or downstroke of high-amplitude pitching motion simulating tilting transition,the optimal pivot points were found during each stroke.Besides,flow fields at the same angle of attack but in opposite direction were discussed and the viewpoint was put forward that the farther from leading-edge point the pivot was,the more flow field development lagged.Based on this,we derived the integral equation of effective angle of attack for biplane airfoils and verified its rationality.The studies above deepened the understanding of complicated unsteady flow of biplane airfoils in tilting transition modes,and would provide theoretical guidance for control optimization of biplane configuration in engineering practice.(4)We carried out computational simulations on biplane airfoils with optimal spacing ratio A=1.5,in particular,from two tilting transition modes:from vertical takeoff to cruise(First Tilting Transition)and from cruise to vertical landing(Second Tilting Transition).A quarter-circle-shaped flight path was designed and thus it was convenient for biplane airfoils to undergo uniform or accelerating circular motion in freestream.In addition,we compared wake vorticity variation during two tilting transition modes,and plotted lift coefficient curves as well as their summation.Particular attention to critical state of wake from vortex shedding to stable distribution.