Detached-Eddy-Simulation Approaches And The Application In Civil Aircraft Stall

The prediction of the aerodynamic characteristics of commercial aircraft at the boundary of the flight envelope is a challenge.The stalling characteristics are important indicator for evaluating aircraft safety.In the stall state,the aerodynamic characteristics of the aircraft are obviously non-linear,and the large-scale flow separation on the wing caused serious safety problems for flight and became one of the important causes of flight accidents.In order to ensure flight safety and improve flight quality,it is necessary to study the aerodynamic characteristics of civil aircraft under stall conditions.However,the prediction of flow separation in stall imposes high requirements on numerical simulation.The Reynolds Averaged Navier-Stokes(RANS)method is still insufficient for the prediction of stall separation flow.In addition,the required computing resources are obviously not satisfied for Large-Eddy Simulations(LES)in current.So the RANS-LES hybrid method is a relatively good choice for stall prediction.As a representative method in the RANS-LES interface hybrid model,the detached eddy simulation(DES)exhibits great potential in complex separation flows with the simple defination and wide application scope.The present work aims at high-precision numerical simulation of civil aircraft stalls.For the shortage of the traditional RANS method for the prediction of stall separation,the global and zonal DES is constructed based on different turbulence models.By applying the DES-type method in different cases,the calculation strategy is discussed,the effects of numerical dissipation are analyzed,and the performance of different DES methods in complex separation flow problems is compared.Furthermore,the numerical simulation of civil aircraft stall was performed using the DES method.Through the analysis of the stall characteristics,the separation characteristics of the flow field after stall are deeply understood,which provides reference for the improving the methods of civil aircraft stall assessment.The research work includes the following aspects:1.Numerical simulation of the Stall for the airfoil and the typical configuration of the civilian aircraft was performed using RANS method,and the performance of the RANS in stall is analyzed.The computational results show that RANS has a good accuracy in the lift linear segments,but the disadvantages are highlighted as the angle of attack increases and the separation occurs.The RANS provides a higher stall angle of attack than actual angle and is not sensitive to the changes in separation.2.The hybrid RANS-LES method is used to overcome the shortcomings of the RANS method in the prediction of the stall.According to the different hybrid mechanism between RANS and LES,the main classification of the RANS-LES hybrid method is reviewed,and the position in which of the DES-type method were analyzed.The DES-type method can be divided into global and zonal DES method according to different approaches of action.In addition,the computational strategies of the DES are discussed,and the computational requirements of the DES on grids and numerical schemes are discussed.For smaller numerical dissipation,the Roe scheme based on fifth-order WENO interpolation is constructed.3.The application of global DES in complex flow separation is studied.Global DES method based on different turbulence models is constructed firstly,and the reliability is verified through the calculation of the flow over backward facing step.Then the calculation of the flow over NACA0021 airfoil in a deep stall,supersonic base flow and the the flow over multi-element airfoil is performed using the different global DES method,and the numerical dissipation effects of the different spatial schemes and the performance of different global DES method is studied.The computational results indicate that 1)The fifth-order WENO-Roe scheme provides better resolution than the third-order MUSCL scheme for small-scale structures.The numerical dissipation has great influence on the average flow field.2)IDDES can provide better prediction in the near-wall region,MDDES presents a deviation between the calculation and the experiment,and the transition from RANS to LES mode is overly delayed by DDES-SA causing the overly dissipation.4.The zonal DES method was studied in order to overcome the delayed transition between RANS to LES mode in the DDES method,which is constructed based on the k-? SST model.Based on the different reasons that trigger flow separation,several DES modes are contained in ZDES and a new sub-grid scale definition is used.The flow over the multi-element airfoil and unsteady shock oscillation is numerically simulated using present ZDES,and the computational results show that 1)ZDES method enables rapid transition from RANS to LES mode avoiding the delay in DDES.2)ZDES can more accurately calculate the low frequency characteristics of shock oscillations than DDES and IDDES.5.The stall of the civil aircraft is studied using ZDES method coupled with a low-dissipation Roe scheme,and numerical simulation of the stall for the airfoil and the typical configuration of the civilian aircraft was performed.1)ZDES can accurately predict the aerodynamic characteristics of airfoil stall.The URANS method coupled with high-precision spatial scheme can capture some large flow structures,but results in the weak instability in excessive separation area,and the stall angle of attack is smaller than experiment.2)Aeroelastic deformation on the main wing suppresses the flow separation to some extent and delays the stall.After stalling,the induction of the low pressure zone on inner wing causes lateral flow on the main wing,the main wing wake interferes with the flow in the empennage region due to the downwash effect,and the peak of the lift coefficient spectrum moves to the low frequency region as the angle of attack increases.The stall separation feature of the configuration is deeply understood through analyzing the computational results.As a result,it provides a reference for improving the calculating and evaluating methods of the stall characteristics of the civil aircraft.