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-   -   increasing mesh quality is leading to poor convergence (http://www.cfd-online.com/Forums/cfx/64243-increasing-mesh-quality-leading-poor-convergence.html)

 tippo May 4, 2009 17:27

increasing mesh quality is leading to poor convergence

Hello!

I am trying to make a mesh quality study.
Therefore, i'd like to calculate the average heat transfer coeficient of a wall.
(attached you can find a jpeg about the geometry, the meshing and the momentum and mass chart)
http://img13.imageshack.us/img13/977/geometry2.th.jpg
http://img164.imageshack.us/img164/9657/meshing.th.jpg
http://img355.imageshack.us/img355/4...rgence2.th.jpg

The fluid domain is air at 400°C in a very narrow duct with an overall lenght of about 6m, the solid domain is steel.
I'm using a SST - model .
Inlet condition: 21.8m/s, 400°C
Outlet condition: Average Static Pressure 0 Pa (i also tried a velocity of 21.8m/s but nothing changes)

On both walls of the fluid i am using inflation.

As long as i keep the overall number of elements below 700 000, the simulation reaches a convergence of 1.0e-004 within 35-40 time steps.
The result for the heat transfer coeficcient looks reasonably correct.

Everytime i am increasing the the mesh quality by decreasing the min. edge length at default face spacing, the simulations has a very very poor convergence.
That means, it eaven don't reaches 1.0e-003, as you can see in the attached chart!
(eg. from 500 000 to 1 400 000 nodes),

Can you see any obvious mistakes in my simulation?
Are there some general tips how to solve such problems (narrow duct which is very long)?
This is one of my first cfx simulations and i am waiting for my instruction course.

Many thanks in advance,
tippo

Ps: i know that a convergence with 1.000e-004 is also not good, but that is as far as i get in the moment.

Outputfile with bad convergence:
This run of the CFX-11.0 Solver started at 21:25:51 on 4 May 2009 by
user Andreas on NB-ANDREAS (intel_p3.sse_winnt5.1) using the command:

"C:\Programme\ANSYS Inc\v110\CFX\bin\perllib\cfx5solve.pl"
-stdout-comms -batch -ccl - -P 35 -sharedlic-port 1177

Setting up CFX Solver run ...

+--------------------------------------------------------------------+
| |
| CFX Command Language for Run |
| |
+--------------------------------------------------------------------+

LIBRARY:
MATERIAL: Air at 25 C
Material Description = Air at 25 C and 1 atm (dry)
Material Group = Air Data, Constant Property Gases
Option = Pure Substance
Thermodynamic State = Gas
PROPERTIES:
Option = General Material
Thermal Expansivity = 0.003356 [K^-1]
ABSORPTION COEFFICIENT:
Absorption Coefficient = 0.01 [m^-1]
Option = Value
END
DYNAMIC VISCOSITY:
Dynamic Viscosity = 1.831E-05 [kg m^-1 s^-1]
Option = Value
END
EQUATION OF STATE:
Density = 1.185 [kg m^-3]
Molar Mass = 28.96 [kg kmol^-1]
Option = Value
END
REFERENCE STATE:
Option = Specified Point
Reference Pressure = 1 [atm]
Reference Specific Enthalpy = 0. [J/kg]
Reference Specific Entropy = 0. [J/kg/K]
Reference Temperature = 25 [C]
END
REFRACTIVE INDEX:
Option = Value
Refractive Index = 1.0 [m m^-1]
END
SCATTERING COEFFICIENT:
Option = Value
Scattering Coefficient = 0.0 [m^-1]
END
SPECIFIC HEAT CAPACITY:
Option = Value
Specific Heat Capacity = 1.0044E+03 [J kg^-1 K^-1]
Specific Heat Type = Constant Pressure
END
THERMAL CONDUCTIVITY:
Option = Value
Thermal Conductivity = 2.61E-02 [W m^-1 K^-1]
END
END
END
MATERIAL: Steel
Material Group = CHT Solids, Particle Solids
Option = Pure Substance
Thermodynamic State = Solid
PROPERTIES:
Option = General Material
EQUATION OF STATE:
Density = 7854 [kg m^-3]
Molar Mass = 55.85 [kg kmol^-1]
Option = Value
END
REFERENCE STATE:
Option = Specified Point
Reference Specific Enthalpy = 0 [J/kg]
Reference Specific Entropy = 0 [J/kg/K]
Reference Temperature = 25 [C]
END
SPECIFIC HEAT CAPACITY:
Option = Value
Specific Heat Capacity = 4.34E+02 [J kg^-1 K^-1]
END
THERMAL CONDUCTIVITY:
Option = Value
Thermal Conductivity = 60.5 [W m^-1 K^-1]
END
END
END
END
FLOW:
SOLUTION UNITS:
Angle Units = [rad]
Length Units = [m]
Mass Units = [kg]
Solid Angle Units = [sr]
Temperature Units = [K]
Time Units = [s]
END
SIMULATION TYPE:
Option = Steady State
EXTERNAL SOLVER COUPLING:
Option = None
END
END
DOMAIN: Fluid
Coord Frame = Coord 0
Domain Type = Fluid
Fluids List = Air at 25 C
Location = B4
BOUNDARY: Default Fluid Solid Interface Side 1
Boundary Type = INTERFACE
Location = F1762.4
BOUNDARY CONDITIONS:
HEAT TRANSFER:
Option = Conservative Interface Flux
END
WALL INFLUENCE ON FLOW:
Option = No Slip
END
END
END
BOUNDARY: Fluid Default
Boundary Type = WALL
Location = F1306.4,F981.4,F985.4
BOUNDARY CONDITIONS:
HEAT TRANSFER:
END
WALL INFLUENCE ON FLOW:
Option = No Slip
END
END
END
BOUNDARY: inlet
Boundary Type = INLET
Location = Inlet
BOUNDARY CONDITIONS:
FLOW REGIME:
Option = Subsonic
END
HEAT TRANSFER:
Option = Static Temperature
Static Temperature = 400 [C]
END
MASS AND MOMENTUM:
Normal Speed = 21.82 [m s^-1]
Option = Normal Speed
END
TURBULENCE:
Option = Medium Intensity and Eddy Viscosity Ratio
END
END
END
BOUNDARY: outlet
Boundary Type = OUTLET
Location = Outlet
BOUNDARY CONDITIONS:
FLOW REGIME:
Option = Subsonic
END
MASS AND MOMENTUM:
Option = Average Static Pressure
Relative Pressure = 0 [Pa]
END
PRESSURE AVERAGING:
Option = Average Over Whole Outlet
END
END
END
DOMAIN MODELS:
BUOYANCY MODEL:
Option = Non Buoyant
END
DOMAIN MOTION:
Option = Stationary
END
MESH DEFORMATION:
Option = None
END
REFERENCE PRESSURE:
Reference Pressure = 1 [atm]
END
END
FLUID MODELS:
COMBUSTION MODEL:
Option = None
END
HEAT TRANSFER MODEL:
Option = Thermal Energy
END
Option = None
END
TURBULENCE MODEL:
Option = SST
END
TURBULENT WALL FUNCTIONS:
Option = Automatic
END
END
INITIALISATION:
Option = Automatic
INITIAL CONDITIONS:
Velocity Type = Cartesian
CARTESIAN VELOCITY COMPONENTS:
Option = Automatic
END
EPSILON:
Option = Automatic
END
K:
Option = Automatic
END
STATIC PRESSURE:
Option = Automatic
END
TEMPERATURE:
Option = Automatic with Value
Temperature = 400 [C]
END
END
END
END
DOMAIN: Solid
Domain Type = Solid
Location = B874
Solids List = Steel
BOUNDARY: Default Fluid Solid Interface Side 2
Boundary Type = INTERFACE
Location = F1765.874
BOUNDARY CONDITIONS:
HEAT TRANSFER:
Option = Conservative Interface Flux
END
END
END
BOUNDARY: Solid Default
Boundary Type = WALL
Location = F1641.874,F1779.874,F843.874,F845.874,F847.874
BOUNDARY CONDITIONS:
HEAT TRANSFER:
END
END
END
DOMAIN MODELS:
DOMAIN MOTION:
Option = Stationary
END
MESH DEFORMATION:
Option = None
END
END
INITIALISATION:
Option = Automatic
INITIAL CONDITIONS:
TEMPERATURE:
Option = Automatic with Value
Temperature = 400 [C]
END
END
END
SOLID MODELS:
HEAT TRANSFER MODEL:
Option = Thermal Energy
END
Option = None
END
END
END
DOMAIN INTERFACE: Default Fluid Solid Interface
Boundary List1 = Default Fluid Solid Interface Side 1
Boundary List2 = Default Fluid Solid Interface Side 2
Interface Type = Fluid Solid
INTERFACE MODELS:
Option = General Connection
FRAME CHANGE:
Option = None
END
PITCH CHANGE:
Option = None
END
END
MESH CONNECTION:
Option = Automatic
END
END
OUTPUT CONTROL:
RESULTS:
File Compression Level = Default
Option = Standard
END
END
SOLVER CONTROL:
Option = High Resolution
END
CONVERGENCE CONTROL:
Maximum Number of Iterations = 80
Physical Timescale = 2 [h]
Solid Timescale Control = Auto Timescale
Timescale Control = Physical Timescale
END
CONVERGENCE CRITERIA:
Residual Target = 1.E-4
Residual Type = RMS
END
DYNAMIC MODEL CONTROL:
Global Dynamic Model Control = On
END
END
EXPERT PARAMETERS:
topology estimate factor = 1.3
END
END
COMMAND FILE:
Version = 11.0
Results Version = 11.0
END
EXECUTION CONTROL:
INTERPOLATOR STEP CONTROL:
Runtime Priority = Standard
EXECUTABLE SELECTION:
Double Precision = Off
END
MEMORY CONTROL:
Memory Allocation Factor = 1.0
END
END
PARALLEL HOST LIBRARY:
HOST DEFINITION: nbandreas
Remote Host Name = NB-ANDREAS
Installation Root = C:\Programme\ANSYS Inc\v%v\CFX
Host Architecture String = intel_p3.sse_winnt5.1
END
END
PARTITIONER STEP CONTROL:
Multidomain Option = Independent Partitioning
Runtime Priority = Standard
EXECUTABLE SELECTION:
Use Large Problem Partitioner = Off
END
MEMORY CONTROL:
Memory Allocation Factor = 1.0
END
PARTITIONING TYPE:
MeTiS Type = k-way
Option = MeTiS
Partition Size Rule = Automatic
END
END
RUN DEFINITION:
Definition File = C:/Dokumente und \
Einstellungen/Andreas/Desktop/Stroemi/Stroemi1.def
Interpolate Initial Values = Off
Run Mode = Full
END
SOLVER STEP CONTROL:
Runtime Priority = Standard
EXECUTABLE SELECTION:
Double Precision = Off
END
Preferred License = 35
Shared License Port = 1177
END
MEMORY CONTROL:
Memory Allocation Factor = 1.0
END
PARALLEL ENVIRONMENT:
Number of Processes = 1
Start Method = Serial
END
END
END

+--------------------------------------------------------------------+
| |
| Solver |
| |
+--------------------------------------------------------------------+

 ghorrocks May 4, 2009 20:09

Have a look here for some tips:

http://www.cfd-online.com/Wiki/Ansys...gence_criteria

In your case I bet the better mesh refinement has lead to transient features being resolved. If the flow really is steady state then larger physical time steps should help. If it is truly transient then there is no alternative but to do transient simulations.

Glenn Horrocks

 tippo May 5, 2009 10:55

Hello,

thank you, i will read through this paper.

best regards
tippo

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