[pro_]

Im trying to initialize my unsteady aeroelastic simulation with a steady solution
1. The steady solution uses the Euler solver for an airfoil at 1 degrees.
2. Dual time step method is used, therefore 2 'solution_flow' steady solutions were used.
3. EVERY single time i try to restart the solution always diverges
4. References here, https://arc.aiaa.org/doi/10.2514/1.J058663 and https://www.eccomas2016.org/proceedings/pdf/6597.pdf manage to 'initialize the unsteady solution from a steady state solution'. So im wondering what am i doing wrong?

This is the steady cfg

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%  STEADY STATE SOLUTION                                                       %
% SU2 configuration file                                                       %
% Case description: inviscid NACA64A010 flutter                                %
% Author: Santiago Padron                                                      %
% Institution: Stanford University                                             %
% Date: 07-09-15                                                               %
% File Version 7.0.3 "Blackbird"                                                %
%                                                                              %
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

% ------------- DIRECT, ADJOINT, AND LINEARIZED PROBLEM DEFINITION ------------%
%
% Physical governing equations (EULER, NAVIER_STOKES,
%                               WAVE_EQUATION, HEAT_EQUATION, FEM_ELASTICITY,
%                               POISSON_EQUATION)
SOLVER= EULER
%
% Mathematical problem (DIRECT, CONTINUOUS_ADJOINT)
MATH_PROBLEM= DIRECT
%
% Read binary restart files (YES, NO)
READ_BINARY_RESTART= YES
%
%
% -------------------- COMPRESSIBLE FREE-STREAM DEFINITION --------------------%
%
% Mach number (non-dimensional, based on the free-stream values)
MACH_NUMBER= 0.87
%
% Angle of attack (degrees, only for compressible flows)
AOA= 1.0
%
% Free-stream pressure (101325.0 N/m^2, 2116.216 psf by default)
FREESTREAM_PRESSURE= 101325.0
%
% Free-stream temperature (288.15 K, 518.67 R by default)
FREESTREAM_TEMPERATURE= 288.15

% ---------------------- REFERENCE VALUE DEFINITION ---------------------------%
%
% Reference origin for moment computation (m or in)
REF_ORIGIN_MOMENT_X = -0.5
REF_ORIGIN_MOMENT_Y = 0.00
REF_ORIGIN_MOMENT_Z = 0.00
%
% Reference length for pitching, rolling, and yawing non-dimensional
% moment (m or in)
REF_LENGTH= 1.0
%
% Reference area for force coefficients (0 implies automatic
% calculation) (m^2 or in^2)
REF_AREA= 1.0
%
% Flow non-dimensionalization (DIMENSIONAL, FREESTREAM_PRESS_EQ_ONE,
%                              FREESTREAM_VEL_EQ_MACH, FREESTREAM_VEL_EQ_ONE)
REF_DIMENSIONALIZATION= DIMENSIONAL

% -------------------- BOUNDARY CONDITION DEFINITION --------------------------%
%
% Euler wall boundary marker(s) (NONE = no marker)
MARKER_EULER= ( airfoil )
%
% Far-field boundary marker(s) (NONE = no marker)
MARKER_FAR= ( farfield )

% ------------------------ SURFACES IDENTIFICATION ----------------------------%
%
% Marker(s) of the surface in the surface flow solution file
MARKER_PLOTTING = ( airfoil )
%
% Marker(s) of the surface where the non-dimensional coefficients are evaluated.
MARKER_MONITORING = ( airfoil )

% ------------- COMMON PARAMETERS DEFINING THE NUMERICAL METHOD ---------------%
%
% Numerical method for spatial gradients (GREEN_GAUSS, WEIGHTED_LEAST_SQUARES)
NUM_METHOD_GRAD= GREEN_GAUSS
%
% CFL number (stating value for the adaptive CFL number)
CFL_NUMBER= 25.0
% Adaptive CFL number (NO, YES)
CFL_ADAPT= YES
%
% Parameters of the adaptive CFL number (factor down, factor up, CFL min value,
%                                        CFL max value )
CFL_ADAPT_PARAM= ( 0.1, 2.0, 1.0, 1e10 )
%

% ------------------------ LINEAR SOLVER DEFINITION ---------------------------%
%
% Linear solver or smoother for implicit formulations (BCGSTAB, FGMRES, SMOOTHER)
LINEAR_SOLVER= FGMRES
%
% Preconditioner of the Krylov linear solver (ILU, LU_SGS, LINELET, JACOBI)
LINEAR_SOLVER_PREC= ILU
%
% Minimum error of the linear solver for implicit formulations
LINEAR_SOLVER_ERROR= 1E-4
%
% Max number of iterations of the linear solver for the implicit formulation
LINEAR_SOLVER_ITER= 2

% -------------------------- MULTIGRID PARAMETERS -----------------------------%
%
% Multi-grid levels (0 = no multi-grid)
MGLEVEL= 0
%
% Multi-grid cycle (V_CYCLE, W_CYCLE, FULLMG_CYCLE)
MGCYCLE= W_CYCLE
%
% Multi-grid pre-smoothing level
MG_PRE_SMOOTH= ( 1, 2, 3, 3 )
%
% Multi-grid post-smoothing level
MG_POST_SMOOTH= ( 0, 0, 0, 0 )
%
% Jacobi implicit smoothing of the correction
MG_CORRECTION_SMOOTH= ( 0, 0, 0, 0 )
%
% Damping factor for the residual restriction
MG_DAMP_RESTRICTION= 0.75
%
% Damping factor for the correction prolongation
MG_DAMP_PROLONGATION= 0.75

% -------------------- FLOW NUMERICAL METHOD DEFINITION -----------------------%
%
% Convective numerical method (JST, LAX-FRIEDRICH, CUSP, ROE, AUSM, HLLC,
%                              TURKEL_PREC, MSW)
CONV_NUM_METHOD_FLOW= JST
%
% Spatial numerical order integration (1ST_ORDER, 2ND_ORDER, 2ND_ORDER_LIMITER)
MUSCL_FLOW= YES
%
% Slope limiter (VENKATAKRISHNAN, BARTH_JESPERSEN)
SLOPE_LIMITER_FLOW= VENKATAKRISHNAN
%
% Entropy fix coefficient (0.0 implies no entropy fixing, 1.0 implies scalar
%                          artificial dissipation)
ENTROPY_FIX_COEFF= 0.001
%
% 2nd and 4th order artificial dissipation coefficients
JST_SENSOR_COEFF= ( 0.5, 0.02 )
%
% --------------------------- CONVERGENCE PARAMETERS --------------------------%
%
%
% Convergence criteria (CAUCHY, RESIDUAL)
%
CONV_CRITERIA= RESIDUAL
%
%
% Min value of the residual (log10 of the residual)
CONV_RESIDUAL_MINVAL= -8
%
% Start convergence criteria at iteration number
CONV_STARTITER= 0
%
% Number of elements to apply the criteria
CONV_CAUCHY_ELEMS= 100
%
% Epsilon to control the series convergence
CONV_CAUCHY_EPS= 1E-10
%

% ------------------------- INPUT/OUTPUT INFORMATION --------------------------%
%
% Mesh input file
MESH_FILENAME= 0_mesh.su2
%
% Mesh input file format (SU2, CGNS)
MESH_FORMAT= SU2
%
% Restart flow input file
SOLUTION_FILENAME= solution_flow.dat
%
% Output file format (TECPLOT, TECPLOT_BINARY, PARAVIEW,
%                     FIELDVIEW, FIELDVIEW_BINARY)
TABULAR_FORMAT= CSV
%
% Output file convergence history (w/o extension) 
CONV_FILENAME= history
%
% Output file restart flow
RESTART_FILENAME= solution_flow.dat
%
% Output file flow (w/o extension) variables
VOLUME_FILENAME= flow
%
% Output file surface flow coefficient (w/o extension)
SURFACE_FILENAME= surface_flow
%
% Writing solution file frequency
WRT_SOL_FREQ= 1000
%
% Writing solution file frequency for physical time steps (dual time)
WRT_SOL_FREQ_DUALTIME= 1000
%
% Writing convergence history frequency
WRT_CON_FREQ= 1
%
% Writing convergence history frequency (dual time, only written to screen)
WRT_CON_FREQ_DUALTIME= 1
%
% Screen output
SCREEN_OUTPUT= (TIME_ITER, INNER_ITER, RMS_DENSITY, RMS_ENERGY, LIFT, DRAG_ON_SURFACE, PLUNGE, PITCH) 
%
%
VOLUME_OUTPUT= PRIMITIVE, GRID_VELOCITY
%
HISTORY_OUTPUT= (ITER, TIME_DOMAIN, RMS_RES, AERO_COEFF, AEROELASTIC)
%
OUTPUT_FILES= (RESTART, CSV, PARAVIEW, SURFACE_PARAVIEW)

This is the unsteady cfg code

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%                                                                              %
% SU2 configuration file                                                       %
% Case description: inviscid NACA64A010 flutter                                %
% Author: Santiago Padron                                                      %
% Institution: Stanford University                                             %
% Date: 07-09-15                                                               %
% File Version 7.0.3 "Blackbird"                                                %
%                                                                              %
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

% ------------- DIRECT, ADJOINT, AND LINEARIZED PROBLEM DEFINITION ------------%
%
% Physical governing equations (EULER, NAVIER_STOKES,
%                               WAVE_EQUATION, HEAT_EQUATION, FEM_ELASTICITY,
%                               POISSON_EQUATION)
SOLVER= EULER
%
% Mathematical problem (DIRECT, CONTINUOUS_ADJOINT)
MATH_PROBLEM= DIRECT
%
% Read binary restart files (YES, NO)
READ_BINARY_RESTART= YES
%
RESTART_SOL= YES
%
RESTART_ITER= 2
%
RESTART_STEADY_STATE= YES
% -------------------- COMPRESSIBLE FREE-STREAM DEFINITION --------------------%
%
% Mach number (non-dimensional, based on the free-stream values)
MACH_NUMBER= 0.87
%
% Angle of attack (degrees, only for compressible flows)
AOA= 1.0
%
% Free-stream pressure (101325.0 N/m^2, 2116.216 psf by default)
FREESTREAM_PRESSURE= 101325.0
%
% Free-stream temperature (288.15 K, 518.67 R by default)
FREESTREAM_TEMPERATURE= 288.15

% ---------------------- REFERENCE VALUE DEFINITION ---------------------------%
%
% Reference origin for moment computation (m or in)
REF_ORIGIN_MOMENT_X = -0.5
REF_ORIGIN_MOMENT_Y = 0.00
REF_ORIGIN_MOMENT_Z = 0.00
%
% Reference length for pitching, rolling, and yawing non-dimensional
% moment (m or in)
REF_LENGTH= 1.0
%
% Reference area for force coefficients (0 implies automatic
% calculation) (m^2 or in^2)
REF_AREA= 1.0
%
% Flow non-dimensionalization (DIMENSIONAL, FREESTREAM_PRESS_EQ_ONE,
%                              FREESTREAM_VEL_EQ_MACH, FREESTREAM_VEL_EQ_ONE)
REF_DIMENSIONALIZATION= DIMENSIONAL

% ------------------------- UNSTEADY SIMULATION -------------------------------%
%
% Time domain
TIME_DOMAIN=YES
%
% Unsteady simulation (NO, TIME_STEPPING, DUAL_TIME_STEPPING-1ST_ORDER, 
%                      DUAL_TIME_STEPPING-2ND_ORDER, SPECTRAL_METHOD)
TIME_MARCHING= DUAL_TIME_STEPPING-2ND_ORDER
%
% Time Step for dual time stepping simulations (s)
TIME_STEP= 0.0006283185307
% 36 steps per period, based on the pitch natural frequency
%
% Total Physical Time for dual time stepping simulations (s)
MAX_TIME= 0.1884955592
%
% Number of internal iterations (dual time method)
INNER_ITER= 30

% ----------------------- DYNAMIC MESH DEFINITION -----------------------------%
SURFACE_MOVEMENT= AEROELASTIC
%
% Motion mach number (non-dimensional). Used for initializing a viscous flow
% with the Reynolds number and for computing force coeffs. with dynamic meshes.
MACH_MOTION= 0.87
%
% Moving wall boundary marker(s) (NONE = no marker, ignored for RIGID_MOTION)
MARKER_MOVING= ( airfoil )

% -------------- AEROELASTIC SIMULATION (Typical Section Model) ---------------%
% Activated by GRID_MOVEMENT_KIND option
%
% The flutter speed index (modifies the freestream condition in the solver)
FLUTTER_SPEED_INDEX = 0.65
%
% Natural frequency of the spring in the plunging direction (rad/s)
PLUNGE_NATURAL_FREQUENCY = 100
%
% Natural frequency of the spring in the pitching direction (rad/s)
PITCH_NATURAL_FREQUENCY = 100
%
% The airfoil mass ratio
AIRFOIL_MASS_RATIO = 60
%
% Distance in semichords by which the center of gravity lies behind
% the elastic axis
CG_LOCATION = 1.8
%
% The radius of gyration squared (expressed in semichords)
% of the typical section about the elastic axis
RADIUS_GYRATION_SQUARED = 3.48
%
% Solve the aeroelastic equations every given number of internal iterations
AEROELASTIC_ITER = 3

% -------------------- BOUNDARY CONDITION DEFINITION --------------------------%
%
% Euler wall boundary marker(s) (NONE = no marker)
MARKER_EULER= ( airfoil )
%
% Far-field boundary marker(s) (NONE = no marker)
MARKER_FAR= ( farfield )

% ------------------------ SURFACES IDENTIFICATION ----------------------------%
%
% Marker(s) of the surface in the surface flow solution file
MARKER_PLOTTING = ( airfoil )
%
% Marker(s) of the surface where the non-dimensional coefficients are evaluated.
MARKER_MONITORING = ( airfoil )

% ------------- COMMON PARAMETERS DEFINING THE NUMERICAL METHOD ---------------%
%
% Numerical method for spatial gradients (GREEN_GAUSS, WEIGHTED_LEAST_SQUARES)
NUM_METHOD_GRAD= GREEN_GAUSS
%
% CFL number (stating value for the adaptive CFL number)
CFL_NUMBER= 25.0
% Adaptive CFL number (NO, YES)
CFL_ADAPT= YES
%
% Parameters of the adaptive CFL number (factor down, factor up, CFL min value,
%                                        CFL max value )
CFL_ADAPT_PARAM= ( 0.1, 2.0, 1.0, 1e10 )
%

% ------------------------ LINEAR SOLVER DEFINITION ---------------------------%
%
% Linear solver or smoother for implicit formulations (BCGSTAB, FGMRES, SMOOTHER)
LINEAR_SOLVER= FGMRES
%
% Preconditioner of the Krylov linear solver (ILU, LU_SGS, LINELET, JACOBI)
LINEAR_SOLVER_PREC= ILU
%
% Minimum error of the linear solver for implicit formulations
LINEAR_SOLVER_ERROR= 1E-4
%
% Max number of iterations of the linear solver for the implicit formulation
LINEAR_SOLVER_ITER= 2

% -------------------------- MULTIGRID PARAMETERS -----------------------------%
%
% Multi-grid levels (0 = no multi-grid)
MGLEVEL= 3
%
% Multi-grid cycle (V_CYCLE, W_CYCLE, FULLMG_CYCLE)
MGCYCLE= W_CYCLE
%
% Multi-grid pre-smoothing level
MG_PRE_SMOOTH= ( 1, 2, 3, 3 )
%
% Multi-grid post-smoothing level
MG_POST_SMOOTH= ( 0, 0, 0, 0 )
%
% Jacobi implicit smoothing of the correction
MG_CORRECTION_SMOOTH= ( 0, 0, 0, 0 )
%
% Damping factor for the residual restriction
MG_DAMP_RESTRICTION= 0.75
%
% Damping factor for the correction prolongation
MG_DAMP_PROLONGATION= 0.75

% -------------------- FLOW NUMERICAL METHOD DEFINITION -----------------------%
%
% Convective numerical method (JST, LAX-FRIEDRICH, CUSP, ROE, AUSM, HLLC,
%                              TURKEL_PREC, MSW)
CONV_NUM_METHOD_FLOW= JST
%
% Spatial numerical order integration (1ST_ORDER, 2ND_ORDER, 2ND_ORDER_LIMITER)
MUSCL_FLOW= YES
%
% Slope limiter (VENKATAKRISHNAN, BARTH_JESPERSEN)
SLOPE_LIMITER_FLOW= VENKATAKRISHNAN
%
% Entropy fix coefficient (0.0 implies no entropy fixing, 1.0 implies scalar
%                          artificial dissipation)
ENTROPY_FIX_COEFF= 0.001
%
% 2nd and 4th order artificial dissipation coefficients
JST_SENSOR_COEFF= ( 0.5, 0.02 )
%
% Time discretization (RUNGE-KUTTA_EXPLICIT, EULER_IMPLICIT, EULER_EXPLICIT)
TIME_DISCRE_FLOW= EULER_IMPLICIT

% ------------------------ GRID DEFORMATION PARAMETERS ------------------------%
%
% Linear solver or smoother for implicit formulations (FGMRES, RESTARTED_FGMRES, BCGSTAB)
DEFORM_LINEAR_SOLVER= FGMRES
%
% Preconditioner of the Krylov linear solver (ILU, LU_SGS, JACOBI)
DEFORM_LINEAR_SOLVER_PREC= LU_SGS
%
% Number of smoothing iterations for mesh deformation
DEFORM_LINEAR_SOLVER_ITER= 500
%
% Number of nonlinear deformation iterations (surface deformation increments)
DEFORM_NONLINEAR_ITER= 1
%
% Print the residuals during mesh deformation to the console (YES, NO)
DEFORM_CONSOLE_OUTPUT= NO
%
% Minimum residual criteria for the linear solver convergence of grid deformation
DEFORM_LINEAR_SOLVER_ERROR= 1E-14
%
% Type of element stiffness imposed for FEA mesh deformation (INVERSE_VOLUME, 
%                                          WALL_DISTANCE, CONSTANT_STIFFNESS)
DEFORM_STIFFNESS_TYPE= INVERSE_VOLUME
%
% Visualize the surface deformation (NO, YES)
VISUALIZE_SURFACE_DEF= NO
%
% Visualize the volume deformation (NO, YES)
VISUALIZE_VOLUME_DEF= NO

% --------------------------- CONVERGENCE PARAMETERS --------------------------%
%
% Number of total iterations
TIME_ITER= 360
%
% Convergence criteria (CAUCHY, RESIDUAL)
%
CONV_CRITERIA= RESIDUAL
%
%
% Min value of the residual (log10 of the residual)
CONV_RESIDUAL_MINVAL= -6
%
% Start convergence criteria at iteration number
CONV_STARTITER= 0
%
% Number of elements to apply the criteria
CONV_CAUCHY_ELEMS= 100
%
% Epsilon to control the series convergence
CONV_CAUCHY_EPS= 1E-10
%

% ------------------------- INPUT/OUTPUT INFORMATION --------------------------%
%
% Mesh input file
MESH_FILENAME= 0_mesh.su2
%
% Mesh input file format (SU2, CGNS)
MESH_FORMAT= SU2
%
% Restart flow input file
SOLUTION_FILENAME= solution_flow.dat
%
% Output file format (TECPLOT, TECPLOT_BINARY, PARAVIEW,
%                     FIELDVIEW, FIELDVIEW_BINARY)
TABULAR_FORMAT= CSV
%
% Output file convergence history (w/o extension) 
CONV_FILENAME= history
%
% Output file restart flow
RESTART_FILENAME= restart_flow.dat
%
% Output file flow (w/o extension) variables
VOLUME_FILENAME= flow
%
% Output file surface flow coefficient (w/o extension)
SURFACE_FILENAME= surface_flow
%
% Writing solution file frequency
WRT_SOL_FREQ= 1000
%
% Writing solution file frequency for physical time steps (dual time)
WRT_SOL_FREQ_DUALTIME= 1000
%
% Writing convergence history frequency
WRT_CON_FREQ= 1
%
% Writing convergence history frequency (dual time, only written to screen)
WRT_CON_FREQ_DUALTIME= 1
%
% Screen output
SCREEN_OUTPUT= (TIME_ITER, INNER_ITER, RMS_DENSITY, RMS_ENERGY, LIFT, DRAG_ON_SURFACE, PLUNGE, PITCH) 
%
HISTORY_OUTPUT= (ITER, TIME_DOMAIN, RMS_RES, AERO_COEFF, AEROELASTIC)
%
OUTPUT_FILES= (RESTART, CSV, PARAVIEW, SURFACE_PARAVIEW)

I have tried many times to restart the original aeroelastic config template as well as play around with the convergence criteria, mesh deformation parameters and solver parameters but NOTHING works. I hope to see some form of documentation or even better a tutorial from SU2 concerning aeroelastic simulations

This is the error,

 CSysSolve::FGMRES(): system solved by initial guess.


Error in "void CSysSolve<ScalarType>::ModGramSchmidt(int, std::vector<std::vector<ScalarType> >&, std::vector<CSysVector<ScalarType> >&) const [with ScalarType = double]":
-------------------------------------------------------------------------
FGMRES orthogonalization failed, linear solver diverged.
------------------------------ Error Exit ----------------------------

[hlk]

I'm not sure what the problem is here, but I might be able to offer suggestions on additional information to provide for troubleshooting, such as:

- whether this error code occurs on the 1st iteration, or after multiple, of both the inner and outer iterations.
- if you've tried increasing the number of inner iterations
- if the steady problem is converged, and how many orders of magnitude the residuals are reduced, prior to restarting
- you mention that you change the solver settings, but since the error is listing the linear solver, have you tried changing the linear solver method settings?



It looks like you suspect (with reason) that this is an issue with the code - if you continue to have this problem after checking the suggestions above, and/or after someone else is able to weigh in on this question, please go ahead and post this to the github issue tracker:
https://github.com/su2code/SU2/issues


Please check first whether someone has reported a similar issue, and comment on the existing one if present.
Good luck

[pcg]

I fabricated two time domain restart files from the steady results for this problem (manually adding 0 grid velocities), the case also diverges, so the problem is not with steady state restarts.

We have this long running issue https://github.com/su2code/SU2/issues/828 that the Euler wall boundary condition might not be appropriate for time domain (for reasons unknown) I suspect this might be why you cannot replicate those results from years ago, maybe try going back to version 6.0.1...

[pro_]

Originally Posted by pcg

I fabricated two time domain restart files from the steady results for this problem (manually adding 0 grid velocities), the case also diverges, so the problem is not with steady state restarts.

We have this long running issue https://github.com/su2code/SU2/issues/828 that the Euler wall boundary condition might not be appropriate for time domain (for reasons unknown) I suspect this might be why you cannot replicate those results from years ago, maybe try going back to version 6.0.1...

Thanks for looking into the case, just as a note also:
- v7.0.0 onwards causes divergence
- v6.2 leads to divergence as well

Will do, Ill have a go with v6.0.1

[pro_]

Originally Posted by pcg

I fabricated two time domain restart files from the steady results for this problem (manually adding 0 grid velocities), the case also diverges, so the problem is not with steady state restarts.

We have this long running issue https://github.com/su2code/SU2/issues/828 that the Euler wall boundary condition might not be appropriate for time domain (for reasons unknown) I suspect this might be why you cannot replicate those results from years ago, maybe try going back to version 6.0.1...

If I got this right, if I were to run the simulation WITHOUT the steady restart for say a period of 1000 time steps. Wherein I see a periodic response (as attached), would this solution be valid contrast to a solution from a steady state initialized solution? (What im trying to study is periodic and not something like an impulse (short time period)).

Essentially, what im trying to say is that, the steady state initialization is only but 'speed up' the solution. Is this right to say?

[pro_]

- whether this error code occurs on the 1st iteration, or after multiple, of both the inner and outer iterations.

The divergence occurs at several outer iterations, the convergence parameter was set to 10^-8 RMS_DENSITY. I wonder if I could change this to the RMS_ENERGY? If so what would be the command in the config file for this?

- if you've tried increasing the number of inner iterations

Yes, Ive increased it up to 100 sub iterations, but since my objective is low fidelity modelling im trying to make the computational cost as cheap as possible. Early divergence would usually occur for low mach numbers (M=0.7) with high flow flutter speed indexes (FLUTTER_SPEED_INDEX= 1.2), which makes sense in the physics as this is a highly unstable system. But the aim is to try achieve of a solution for a set time step before any divergence occurs.

- if the steady problem is converged, and how many orders of magnitude the residuals are reduced, prior to restarting

I set the RESIDUAL (RMS_DENSITY) value to -20 in the steady state solution, but for all variations (speed/flutter speed index), the max convergence value was to the power of -13. Im not entirely sure, but this would be a deeply converged solution would it not?

- you mention that you change the solver settings, but since the error is listing the linear solver, have you tried changing the linear solver method settings?

Yes, in the LINEAR_SOLVER_ITER (increased) and ERROR. Unfortunately only a slight difference

[pcg]

Yes I think that is fair to say (as I mentioned in a previous reply), if you have a repeating cycle it does not matter how you got there (I'm sure there are caveats for special cases, but this should not be one of them).