[doug]

Hi,

I've seen several airfoil cases where a solution will "completely" converge nicely, but if left to continue to run, it suddenly becomes unstable and diverges. Has anyone seen this?

Here is an example using the SA model on a NACA 0012 airfoil at 10 deg alpha, Re=440000, y+=1 at the wall. The 2D mesh has 40,000 cells.

I realize this is somewhat of a coarse mesh, but I've seen the identical phenomenon with 2D mesh sizes up to 160,000 cells, y+=1 at the wall.

Here is the config file:
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% SU2 configuration file %
% Case description: 2D Airfoil (compressible) %
% Author: CloudFoil %
% File Version 3.2.1 "eagle" %
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

% CloudFoil Case UUID: 799D989D-D2C9-4415-8588-A66904EB4BF5

% ------------- DIRECT, ADJOINT, AND LINEARIZED PROBLEM DEFINITION ------------%
PHYSICAL_PROBLEM= NAVIER_STOKES
KIND_TURB_MODEL= SA
MATH_PROBLEM= DIRECT
RESTART_SOL= NO
LOW_MEMORY_OUTPUT= NO

% -------------------- COMPRESSIBLE FREE-STREAM DEFINITION --------------------%
MACH_NUMBER= 1.1544113012229539E-01
AoA= 1.0000000000000000E+01
FREESTREAM_PRESSURE= 7.7716899999999994E+04
FREESTREAM_TEMPERATURE= 2.7385519999999997E+02
REYNOLDS_NUMBER= 4.4000000000000000E+05
REYNOLDS_LENGTH= 2.0000000000000001E-01

% ---------------------- REFERENCE VALUE DEFINITION ---------------------------%
REF_ORIGIN_MOMENT_X = 5.0000000000000003E-02
REF_ORIGIN_MOMENT_Y = 0.0000000000000000E+00
REF_ORIGIN_MOMENT_Z = 0.0000000000000000E+00
REF_LENGTH_MOMENT= 2.0000000000000001E-01
REF_AREA= 2.0000000000000001E-01
REF_PRESSURE= 7.7716899999999994E+04
REF_TEMPERATURE= 2.7385519999999997E+02
REF_DENSITY= 9.8787499999999995E-01

% -------------------- BOUNDARY CONDITION DEFINITION --------------------------%
MARKER_HEATFLUX= ( airfoil, 0.0 )
MARKER_FAR= ( farfield, outlet )
MARKER_PLOTTING= ( airfoil )
MARKER_MONITORING= ( airfoil )

% ------------- COMMON PARAMETERS DEFINING THE NUMERICAL METHOD ---------------%
NUM_METHOD_GRAD= WEIGHTED_LEAST_SQUARES
CFL_NUMBER= 1.0000000000000000E+00
MAX_DELTA_TIME= 1E10
CFL_ADAPT= NO
CFL_ADAPT_PARAM= ( 1.5, 0.5, 1.0, 100.0 )
RK_ALPHA_COEFF= ( 0.66667, 0.66667, 1.000000 )

% ----------------------- SLOPE LIMITER DEFINITION ----------------------------%
REF_ELEM_LENGTH= 2.0000000000000004E-02
LIMITER_COEFF= 0.3
LIMITER_ITER= 99999
SHARP_EDGES_COEFF= 3

% ------------------------ LINEAR SOLVER DEFINITION ---------------------------%
LINEAR_SOLVER= FGMRES
LINEAR_SOLVER_PREC= LU_SGS
LINEAR_SOLVER_ERROR= 1E-6
LINEAR_SOLVER_ITER= 5

% -------------------------- MULTIGRID PARAMETERS -----------------------------%
MGLEVEL= 3
MGCYCLE= W_CYCLE
MG_PRE_SMOOTH= ( 1, 2, 3, 3 )
MG_POST_SMOOTH= ( 0, 0, 0, 0 )
MG_CORRECTION_SMOOTH= ( 0, 0, 0, 0 )
MG_DAMP_RESTRICTION= 7.5000000000000000E-01
MG_DAMP_PROLONGATION= 7.5000000000000000E-01

% -------------------- FLOW NUMERICAL METHOD DEFINITION -----------------------%
CONV_NUM_METHOD_FLOW= JST
SPATIAL_ORDER_FLOW= 2ND_ORDER_LIMITER
SLOPE_LIMITER_FLOW= VENKATAKRISHNAN
AD_COEFF_FLOW= ( 0.15, 0.5, 0.02 )
TIME_DISCRE_FLOW= EULER_IMPLICIT

% -------------------- TURBULENT NUMERICAL METHOD DEFINITION ------------------%
CONV_NUM_METHOD_TURB= SCALAR_UPWIND
SPATIAL_ORDER_TURB= 2ND_ORDER_LIMITER
SLOPE_LIMITER_TURB= VENKATAKRISHNAN
TIME_DISCRE_TURB= EULER_IMPLICIT
CFL_REDUCTION_TURB= 1.0

% --------------------------- CONVERGENCE PARAMETERS --------------------------%
EXT_ITER= 10000
CONV_CRITERIA= RESIDUAL
RESIDUAL_REDUCTION= 1.0000000000000000E+01
RESIDUAL_MINVAL= -1.0000000000000000E+01
STARTCONV_ITER= 10
CAUCHY_ELEMS= 100
CAUCHY_EPS= 1E-6
CAUCHY_FUNC_FLOW= DRAG

% ------------------------- INPUT/OUTPUT INFORMATION --------------------------%
MESH_FILENAME= mesh.su2
MESH_FORMAT= SU2
DIVIDE_ELEMENTS= NO
CGNS_TO_SU2= NO
MESH_OUT_FILENAME= mesh_out.su2
SOLUTION_FLOW_FILENAME= solution_flow.dat
SOLUTION_ADJ_FILENAME= solution_adj.dat
OUTPUT_FORMAT= PARAVIEW
CONV_FILENAME= history
RESTART_FLOW_FILENAME= restart_flow.dat
RESTART_ADJ_FILENAME= restart_adj.dat
VOLUME_FLOW_FILENAME= flow
VOLUME_ADJ_FILENAME= adjoint
GRAD_OBJFUNC_FILENAME= of_grad.dat
SURFACE_FLOW_FILENAME= surface
SURFACE_ADJ_FILENAME= surface_adjoint
WRT_SOL_FREQ= 10000
WRT_CON_FREQ= 10

[awendorff]

I would recommend trying to turn off multigrid and see if that solves your problem. If you really want to use multigrid, try switching to Green-Gauss instead of Weighted Least Squares.