[CFX] How to avoid negative temperatures?
 

Hi folks,

I´m having negative temperatures like -6.10352E-06 [ºC], using ANSYS CFX 12, as a lower limit, with these boundary conditions (please, see below). Is there anything wrong with my set-up in ANSYS CFX Pre 12.0? The files were stored in http://rapidshare.com/files/35064994...tures.rar.html or at http://www.4shared.com/file/22215781..._temperat.html. The initial temperatures were 0.0 [ºC] and the heat flux condition was only 1.0 [W m^-2]. The computational domain was a solid.


MATERIAL: k10
Material Group = CHT Solids,Particle Solids
Option = Pure Substance
Thermodynamic State = Solid
PROPERTIES:
Option = General Material
EQUATION OF STATE:
Density = 14900 [kg m^-3]
Molar Mass = 1.0 [kg kmol^-1]
Option = Value
END
SPECIFIC HEAT CAPACITY:
Option = Value
Specific Heat Capacity = 332.94 [J kg^-1 K^-1]
END
THERMAL CONDUCTIVITY:
Option = Value
Thermal Conductivity = 43.1 [W m^-1 K^-1]
END
END
END
END
FLOW: Flow Analysis 1
SOLUTION UNITS:
Angle Units = [rad]
Length Units = [m]
Mass Units = [kg]
Solid Angle Units = [sr]
Temperature Units = [K]
Time Units = [s]
END
ANALYSIS TYPE:
Option = Transient
EXTERNAL SOLVER COUPLING:
Option = None
END
INITIAL TIME:
Option = Automatic with Value
Time = 0 [s]
END
TIME DURATION:
Option = Total Time
Total Time = 110 [s]
END
TIME STEPS:
Option = Timesteps
Timesteps = 0.22 [s]
END
END
DOMAIN: metalduro
Coord Frame = Coord 0
Domain Type = Solid
Location = METAL
BOUNDARY: conv
Boundary Type = WALL
Location = SURFACE_ INFERIOR,SURFACE_ LATERAIS,SURFACE_ SUPERIOR
BOUNDARY CONDITIONS:
HEAT TRANSFER:
Option = Adiabatic
END
END
END
BOUNDARY: fluxo
Boundary Type = WALL
Location = SURFACE_ FLUXO
BOUNDARY CONDITIONS:
HEAT TRANSFER:
Heat Flux in = 1 [W m^-2]
Option = Heat Flux
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 = 0 [C]
END
END
END
SOLID DEFINITION: k10
Material = k10
Option = Material Library
MORPHOLOGY:
Option = Continuous Solid
END
END
SOLID MODELS:
HEAT TRANSFER MODEL:
Option = Thermal Energy
END
THERMAL RADIATION MODEL:
Option = None
END
END
END
OUTPUT CONTROL:
MONITOR OBJECTS:
MONITOR BALANCES:
Option = Full
END
MONITOR FORCES:
Option = Full
END
MONITOR PARTICLES:
Option = Full
END
MONITOR POINT: Monitor Point 1
Cartesian Coordinates = 0 [mm], 3.95 [mm], 2.129 [mm]
Option = Cartesian Coordinates
Output Variables List = Temperature
END
MONITOR RESIDUALS:
Option = Full
END
MONITOR TOTALS:
Option = Full
END
END
RESULTS:
File Compression Level = Default
Option = Standard
END
TRANSIENT RESULTS: Transient Results 1
File Compression Level = Default
Include Mesh = On
Option = Selected Variables
Output Variables List = Temperature,Total Temperature
OUTPUT FREQUENCY:
Option = Time Interval
Time Interval = 0.22 [s]
END
END
END
SOLVER CONTROL:
ADVECTION SCHEME:
Option = High Resolution
END
CONVERGENCE CONTROL:
Maximum Number of Coefficient Loops = 100
Minimum Number of Coefficient Loops = 1
Timescale Control = Coefficient Loops
END
CONVERGENCE CRITERIA:
Residual Target = 0.000001
Residual Type = RMS
END
TRANSIENT SCHEME:
Option = Second Order Backward Euler
TIMESTEP INITIALISATION:
Option = Automatic
END
END
END
END
COMMAND FILE:
Version = 12.0.1
Results Version = 12.0
END
SIMULATION CONTROL:
EXECUTION CONTROL:
EXECUTABLE SELECTION:
Double Precision = On
END
INTERPOLATOR STEP CONTROL:
Runtime Priority = Standard
MEMORY CONTROL:
Memory Allocation Factor = 1.0
END
END
PARALLEL HOST LIBRARY:
HOST DEFINITION: unifei
Host Architecture String = winnt-amd64
Installation Root = C:\Program Files\ANSYS Inc\v%v\CFX
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
Partition Weight Factors = 0.250, 0.250, 0.250, 0.250
END
END
RUN DEFINITION:
Run Mode = Full
Solver Input File = \
C:\0_IHTC_2010_04_02_2010\05_ferramentasemrevest\0 1semrevferramenta1s\
emfuro.def
END
SOLVER STEP CONTROL:
Runtime Priority = High
EXECUTABLE SELECTION:
Double Precision = On
END
MEMORY CONTROL:
Memory Allocation Factor = 1.0
END
PARALLEL ENVIRONMENT:
Number of Processes = 4
Start Method = HP MPI Local Parallel
Parallel Host List = unifei*4
END
END
END
END

Thanks for helping me!

This sounds like a normal boundedness problem. I think "Computational Fluid Dynamics" by Roache has a discussion about this.

Have you done a mesh, time step and convergence sensitivity check? If not then do it - these are the sort of problems which come up when you take short-cuts.

Is this book: Roache, P. J. Computational Fluid Dynamics. Albuquerque: Hermosa, 1972. 446 p.?

How to do that (convergence sensitivity check)?

Rogerio

Yes, that's the book. It is the classic reference text for things relating to CFD accuracy. It explains the mesh sensitivity issue in great detail. But the simple version is you keep refining the mesh until the output parameter you are interested in converges to an accuracy tolerance which is acceptable for your analysis.