Numerical Studies On Vortex Breakdown And Massive Separation During Dynamic Stall Near Critical Angle Of Attack

Stream-wise vortex breakdown and boundary layer separation are the two typical flow phenomena in the stall of aircraft with delta wing or low-sweep wing,which are characterized by multi-frequency and multi-scale.Dynamic stall is more common due to the variation of the angle of attack,in which the two flow structures present intensely non-linear hysteresis and are no longer just the function of the states but also extremely dependent on the motion process.In this paper,the static and dynamic stall flows with stream-wise vortex breakdown or boundary layer separation are numerically studied.Such flow features as stream-wise oscillations of vortex breakdown position or separation point,helical structures and vortex shedding are investigated.The correlations among the fluctuation of aerodynamic forces,pitching dynamic stability and the typical flow structures are analyzed.The main contents could be concluded as below:1.The rigid moving mesh method is developed and implemented in the in-house code UNITs(Unsteady NavIer–s Tokes solver).A new adaptive function is proposed and implemented into the scheme with adaptive dissipation by introducing a boundary layer“shielding function”to overcome the defect of early switching of the original adaptive function when the grid is excessively dense in the boundary layer.A new DDES method called DDES-AC(DDES with adaptive coefficient)is proposed to deal with the“grey area”problem,which is validated by computing the static and dynamic stall over a NACA0015 airfoil.The results indicate that the DDES-AC model could effectively alleviate the“grey area”,speed up the instability of the shear layer,accelerate the transition from RANS to LES,and improve the precision.2.The DDES(Delayed Detached Eddy Simulation)method is adopted to investigate the unsteady flows around an 80?/65?double-delta wing at 36?incidence or subjected to pitching motions of?=36?+6?sin(2k_pt).The concentrations are put on the behavior of vortex breakdown point,helical structures and their influences on the fluctuations of pressure and aerodynamic forces under stationary or pitching states.The effects of reduced frequency are discussed in detail.Pitching motion alters the increase rate of the helical structures in the stream-wise direction.The natural oscillation frequency of the breakdown location at 36?,St_n,is the critical value for the movement behavior of breakdown location and the pitching dynamic stability.When the pitching frequency is less than St_n,the upstream speed of BP is larger than that of the downstream,meanwhile,the variation of the pitching moment is ahead of the angle of attack and the pitching dynamic stability is lost.When the pitching frequency is larger than St_n,the situation is opposite.3.The DDES-AC model is implemented in the simulations of the static and dynamic stall over the NACA0015 airfoil.For the static stall,the attentions are paid to the stream-wise oscillation of the separation position,shear layer instability and shedding from the upper surface,the trailing edge vortex shedding,and their influence on the fluctuations of the pressure and aerodynamic forces.The effects of the angle of attack are also taken into account.For a typical deeply dynamic stall,the flow mechanism for the variation of the aerodynamic force is discussed,and the evolutions of the separation position,vortex structures and pressure on the surface are analyzed.For the static stall,it is observed that the stream-wise oscillation of the separation position presents a low frequency behavior,which changes the circulation around the entire airfoil,resulting in the large fluctuations in the lift.The oscillation of the separation position and fluctuations in the lift is nearly in-phase.The oscillation frequency of the separation position increases with the angle of attack.A concept called“dynamic balance in the flow flux”is used to comprehend the oscillation of separation position.A new dimensionless frequency is proposed and verified to be independent in the low frequency oscillation in the present and related literature,which is approximately 0.038.