某外X形外形
本案例采用的外X形外形来自NASA-TM X-3070报告。该外X形模型由机身、十字型鸭翼和十字型尾翼组成。模型长细比约为22,头部为半球形。该模型的试验在兰利8英尺跨声速风洞中进行。风洞试验段截面为2.44 m×2.44 m的正方形,马赫数范围为0.20~1.30。研究人员开展了外X形模型的测力试验,获得了力和力矩特性曲线。本文以该外X形外形为测试算例,检验SU2对于复杂外形流场的模拟能力。
图 2 某外X形模型风洞试验照片
表 1某外X形模型几何和流场参数
计算网格
本次计算所采用的网格为非结构网格,网格单元约为405万,网格点约为102万。模型表面为三角形网格,边界层区域采用三棱柱,空间区域采用四面体网格填充。模型周围及尾流区网格适当加密以捕捉空间涡结构。
图 3某外X形网格
SU2求解器设置
下面以马赫数为0.8、攻角为10°计算工况为例,介绍本算例的参数设置。
(1)问题定义
% ------------- DIRECT, ADJOINT, AND LINEARIZED PROBLEM DEFINITION ------------%%% Physical governing equations (EULER, NAVIER_STOKES,% WAVE_EQUATION, HEAT_EQUATION, LINEAR_ELASTICITY,% POISSON_EQUATION)PHYSICAL_PROBLEM= NAVIER_STOKES%% Specify turbulence model (NONE, SA, SA_NEG, SST)KIND_TURB_MODEL= SA%% Mathematical problem (DIRECT, CONTINUOUS_ADJOINT, DISCRETE_ADJOINT)MATH_PROBLEM= DIRECT%% Restart solution (NO, YES)RESTART_SOL= NO%% Regime type (COMPRESSIBLE, INCOMPRESSIBLE)REGIME_TYPE= COMPRESSIBLE
(2)自由来流参数设置
% Mach numberMACH_NUMBER= 0.80%% Angle of attackAoA=10%% Free-stream temperatureFREESTREAM_TEMPERATURE= 2.94326E+02%% Reynolds numberREYNOLDS_NUMBER= 6.56000E+06%% Reynolds lengthREYNOLDS_LENGTH= 1.0
(3)参考值设置
% Reference origin for moment computationREF_ORIGIN_MOMENT_X = 0.0REF_ORIGIN_MOMENT_Y = 0.0REF_ORIGIN_MOMENT_Z = 0.0%% Reference length for pitching, rolling, and yawing non-dimensional momentREF_LENGTH= 0.9427%% Reference area for force coefficients (0 implies automatic calculation)REF_AREA= 0.001408%% Flow non-dimensionalization (DIMENSIONAL, FREESTREAM_PRESS_EQ_ONE,% FREESTREAM_VEL_EQ_MACH, FREESTREAM_VEL_EQ_ONE)REF_DIMENSIONALIZATION= DIMENSIONAL
(4)边界条件设置
% Navier-Stokes wall boundary marker(s) (NONE = no marker)MARKER_HEATFLUX= (wall, 0.0 )%% Farfield boundary marker(s) (NONE = no marker)MARKER_FAR= ( inlet, outlet )%% Symmetry boundary marker(s) (NONE = no marker)% MARKER_SYM= ( left )%% Marker(s) of the surface to be plotted or designedMARKER_PLOTTING= (wall )%% Marker(s) of the surface where the functional (Cd, Cl, etc.) will be evaluatedMARKER_MONITORING= (wall )
(5)数值求解通用参数
% Numerical method for spatial gradients (GREEN_GAUSS, WEIGHTED_LEAST_SQUARES)NUM_METHOD_GRAD= GREEN_GAUSS%% Courant-Friedrichs-Lewy condition of the finest gridCFL_NUMBER= 5.0%% Adaptive CFL number (NO, YES)CFL_ADAPT= NO%% Parameters of the adaptive CFL number (factor down, factor up, CFL min value,% CFL max value )CFL_ADAPT_PARAM= ( 1.5, 0.5, 5.0, 20.0 )%% Number of total iterationsEXT_ITER= 10000
(6)限制器设置
% Coefficient for the limiterVENKAT_LIMITER_COEFF= 0.05%% Coefficient for the sharp edges limiterADJ_SHARP_LIMITER_COEFF= 3.0%% Reference coefficient (sensitivity) for detecting sharp edges.REF_SHARP_EDGES= 3.0%% Remove sharp edges from the sensitivity evaluation (NO, YES)SENS_REMOVE_SHARP= NO
(7)迭代参数
% Linear solver for implicit formulations (BCGSTAB, FGMRES)LINEAR_SOLVER= FGMRES%% Preconditioner of the Krylov linear solver (JACOBI, LINELET, LU_SGS)LINEAR_SOLVER_PREC= ILU%% Linaer solver ILU preconditioner fill-in level (0 by default)LINEAR_SOLVER_ILU_FILL_IN= 0%% Minimum error of the linear solver for implicit formulationsLINEAR_SOLVER_ERROR= 1E-10%% Max number of iterations of the linear solver for the implicit formulationLINEAR_SOLVER_ITER= 5
(8)流场计算数值格式
% Convective numerical method (JST, LAX-FRIEDRICH, CUSP, ROE, AUSM, HLLC,% TURKEL_PREC, MSW)CONV_NUM_METHOD_FLOW= ROE%% Monotonic Upwind Scheme for Conservation Laws (TVD) in the flow equations.% Required for 2nd order upwind schemes (NO, YES)MUSCL_FLOW= YES%% Slope limiter (VENKATAKRISHNAN, MINMOD)SLOPE_LIMITER_FLOW= VENKATAKRISHNAN%% Time discretization (RUNGE-KUTTA_EXPLICIT, EULER_IMPLICIT, EULER_EXPLICIT)TIME_DISCRE_FLOW= EULER_IMPLICIT%% Relaxation coefficientRELAXATION_FACTOR_FLOW= 0.9
(9)湍流计算数值格式
% Convective numerical method (SCALAR_UPWIND)CONV_NUM_METHOD_TURB= SCALAR_UPWIND%% Monotonic Upwind Scheme for Conservation Laws (TVD) in the turbulence equations.% Required for 2nd order upwind schemes (NO, YES)MUSCL_TURB= NO%% Slope limiter (VENKATAKRISHNAN, MINMOD)SLOPE_LIMITER_TURB= VENKATAKRISHNAN%% Time discretization (EULER_IMPLICIT)TIME_DISCRE_TURB= EULER_IMPLICIT%% Relaxation coefficientRELAXATION_FACTOR_TURB= 0.9
(10)收敛准则
% Convergence criteria (CAUCHY, RESIDUAL)CONV_CRITERIA= RESIDUAL%% Residual reduction (order of magnitude with respect to the initial value)RESIDUAL_REDUCTION= 10%% Min value of the residual (log10 of the residual)RESIDUAL_MINVAL= -12%% Start convergence criteria at iteration numberSTARTCONV_ITER= 10%% Number of elements to apply the criteriaCAUCHY_ELEMS= 100%% Epsilon to control the series convergenceCAUCHY_EPS= 1E-10%% Direct function to apply the convergence criteria (LIFT, DRAG, NEARFIELD_PRESS)CAUCHY_FUNC_FLOW= DRAG%% Adjoint function to apply the convergence criteria (SENS_GEOMETRY, SENS_MACH)CAUCHY_FUNC_ADJFLOW= SENS_GEOMETRY
(11)输入输出设置
%MESH_FILENAME= Woeber_Pointwise_HLCRM_FullGap_HexPrismPyrTets_Medium.cgnsMESH_FILENAME=missile.su2%% Mesh input file format (SU2, CGNS, NETCDF_ASCII)MESH_FORMAT= SU2%% Change the scale of the numerical grid (useful to change the length units% or to re-scale the grid)% MESH_SCALE_CHANGE= 0.001% Mesh output fileMESH_OUT_FILENAME= mesh_out.su2%% Restart flow input fileSOLUTION_FLOW_FILENAME= restart_flow.dat%% Restart adjoint input fileSOLUTION_ADJ_FILENAME= solution_adj.dat%% Output file format (TECPLOT, TECPLOT_BINARY, PARAVIEW,% FIELDVIEW, FIELDVIEW_BINARY)OUTPUT_FORMAT= TECPLOT_BINARY%% Output file convergence history (w/o extension)CONV_FILENAME= history%% Output file restart flowRESTART_FLOW_FILENAME= restart_flow.dat%% Output file restart adjointRESTART_ADJ_FILENAME= restart_adj.dat%% Output file flow (w/o extension) variablesVOLUME_FLOW_FILENAME= flow%% Output file adjoint (w/o extension) variablesVOLUME_ADJ_FILENAME= adjoint%% Output objective function gradient (using continuous adjoint)GRAD_OBJFUNC_FILENAME= of_grad.dat%% Output file surface flow coefficient (w/o extension)SURFACE_FLOW_FILENAME= surface_flow%% Output file surface adjoint coefficient (w/o extension)SURFACE_ADJ_FILENAME= surface_adjoint%% Writing solution file frequencyWRT_SOL_FREQ= 500%% Writing convergence history frequencyWRT_CON_FREQ= 1%% Output residual values in the solution filesWRT_RESIDUALS= NO%% Output limiters values in the solution filesWRT_LIMITERS= NO%% Output the sharp edges detectorWRT_SHARPEDGES= NO
结果分析
大攻角流场特征
图4展示了10°攻角外X形流场的空间涡结构及及表面压力分布。可以看到,两侧鸭翼和尾翼均产生了稳定的涡结构。旋涡在前缘翼根处开始形成,流经翼面背风区,最终消失在尾迹区。在旋涡的诱导下,翼面与旋涡之间的气流得到加速,使背风区压力进一步下降,从而获得所谓的“涡升力”。
图 4 外X形空间涡结构及及表面压力分布
攻角影响
图5展示了不同攻角下外X形流场空间涡结构特征。可以看到,小攻角(0°≤α≤4°)下,气流附着在翼面上,翼面后方没有发现模型的涡结构。攻角增大到6以后,尾翼和鸭翼先后脱落出稳定的涡结构。随着攻角的增加,旋涡的尺度和强度也随之增强。
图 5 不同攻角下外X形流场空间涡结构
力和力矩系数
(a) 升力系数
(b) 轴向力系数
(c) 俯仰力矩力系数
图 6力和力矩系数计算结果与试验结果对比
图6展示了SU2计算的升力、轴向力和俯仰力矩系数曲线与试验结果的对比,计算结果与试验结果基本吻合。值得注意的是,传统翼型升力系数随着攻角的增加成线性增长,而该外X形升力系数则呈现非线性增长趋势,其中的非线性部分主要由“涡升力”贡献。
结论
1)采用SU2计算了外X形流场,计算得到的升力、轴向力和俯仰力矩系数曲线和试验结果基本吻合,表明SU2具备模拟外X形等复杂外形流场的能力。
2)大攻角下,外X形两侧鸭翼和尾翼将产生稳定的涡结构。在旋涡的诱导下,翼面与旋涡之间的气流得到加速,使背风区压力进一步下降,从而获得所谓的“涡升力”。
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