Heat radiation

General discussion about Elmer
BDesco
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Heat radiation

Post by BDesco » 28 Jun 2020, 19:42

Hello,
I'm new to Elmer.
I'm trying to solve this simple problem with Elmer.
U-shaped 3D object. One end is heated. There is heat transfer through the legs. ok. I get that results.
But as the legs are not that far apart: there should be radiation transfer as well from one leg to the other.
Yet I see no change in temperature on the inside of the 'cold ' leg.

I've attached the project.
Thank you.

raback
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Re: Heat radiation

Post by raback » 29 Jun 2020, 01:09

Hi

Have you defined radiation boundary conditions between the legs? You need "diffuse grey" model with "radiation boundary open".

-Peter

BDesco
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Re: Heat radiation

Post by BDesco » 30 Jun 2020, 10:29

Thanks Raback,
Yes I defined the radiation boundary condition between, but I selected idealized and not open. The model was just to test Elmer.
I adapted it to "diffuse grey" model with "radiation boundary open".

I get an error (see below). I read something of the need to define the Gebhardt factors when using diffuse grey. Doens't Elmer calculate those?

Solver output:
ELMER SOLVER (v 8.4) STARTED AT: 2020/06/30 09:25:55
ParCommInit: Initialize #PEs: 1
MAIN:
MAIN: =============================================================
MAIN: ElmerSolver finite element software, Welcome!
MAIN: This program is free software licensed under (L)GPL
MAIN: Copyright 1st April 1995 - , CSC - IT Center for Science Ltd.
MAIN: Webpage http://www.csc.fi/elmer, Email elmeradm@csc.fi
MAIN: Version: 8.4 (Rev: Release, Compiled: 2020-04-08)
MAIN: Running one task without MPI parallelization.
MAIN: Running with just one thread per task.
MAIN: Lua interpreted linked in.
MAIN: =============================================================

MAIN:
MAIN:
MAIN: -------------------------------------
MAIN: Reading Model: case.sif
LoadInputFile: Scanning input file: case.sif

LoadInputFile: Loading input file: case.sif

LoadInputFile: Number of BCs: 4
LoadInputFile: Number of Body Forces: 0
LoadInputFile: Number of Initial Conditions: 0
LoadInputFile: Number of Materials: 1
LoadInputFile: Number of Equations: 1
LoadInputFile: Number of Solvers: 1
LoadInputFile: Number of Bodies: 1

Loading user function library: [HeatSolve]...[HeatSolver_Init0]
LoadMesh: Base mesh name: ./.

LoadMesh: Elapsed REAL time: 0.0300 (s)

MAIN: -------------------------------------
AddVtuOutputSolverHack: Adding ResultOutputSolver to write VTU output in file: case

Loading user function library: [HeatSolve]...[HeatSolver_Init]
Loading user function library: [HeatSolve]...[HeatSolver_bulk]
Loading user function library: [HeatSolve]...[HeatSolver]
RadiationFactors: ----------------------------------------------------
RadiationFactors: Computing radiation factors for heat transfer
RadiationFactors: ----------------------------------------------------
RadiationFactors: Total number of Radiation Surfaces 112 out of 688

'ViewFactors' is not recognized as an internal or external command,
operable program or batch file.

ERROR:: systemc: Command exit status was 1

RadiationFactors: Message
RadiationFactors: All done time (s) 2.9000E-02
RadiationFactors: ----------------------------------------------------

OptimizeBandwidth: ---------------------------------------------------------

OptimizeBandwidth: Computing matrix structure for: heat equation...done.
OptimizeBandwidth: Half bandwidth without optimization: 2141
OptimizeBandwidth:

OptimizeBandwidth: Bandwidth Optimization ...done.
OptimizeBandwidth: Half bandwidth after optimization: 391
OptimizeBandwidth: ---------------------------------------------------------

Loading user function library: [ResultOutputSolve]...[ResultOutputSolver_Init]
Loading user function library: [ResultOutputSolve]...[ResultOutputSolver_bulk]
Loading user function library: [ResultOutputSolve]...[ResultOutputSolver]
ElmerSolver: Number of timesteps to be saved: 1
MAIN:
MAIN: -------------------------------------
MAIN: Steady state iteration: 1
MAIN: -------------------------------------
MAIN:
SingleSolver: Attempting to call solver
SingleSolver: Solver Equation string is: heat equation
HeatSolver: Solving the energy equation for temperature

HeatSolve:
HeatSolve:
HeatSolve: -------------------------------------
HeatSolve: TEMPERATURE ITERATION 1
HeatSolve: -------------------------------------
HeatSolve:
HeatSolve: Starting Assembly...
HeatSolve: Assembly:


Program received signal SIGSEGV: Segmentation fault - invalid memory reference.

Backtrace for this error:
#0 0xffffffff
#1 0xffffffff
#2 0xffffffff
#3 0xffffffff
#4 0xffffffff
#5 0xffffffff
#6 0xffffffff
#7 0xffffffff
#8 0xffffffff
#9 0xffffffff
#10 0xffffffff
#11 0xffffffff
#12 0xffffffff
#13 0xffffffff
#14 0xffffffff
#15 0xffffffff
#16 0xffffffff
#17 0xffffffff
#18 0xffffffff
#19 0xffffffff
#20 0xffffffff
#21 0xffffffff

raback
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Re: Heat radiation

Post by raback » 30 Jun 2020, 12:41

Hi

It is indeed better to start with "idealized". That should usually work easily. However, that will give you no heat exchange between bodies.

You don't seem to have the ViewFactors binary in your path.

-Peter

BDesco
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Re: Heat radiation

Post by BDesco » 03 Jul 2020, 04:16

Thanks again,
the complete U shape is one body.
Viewfactors.exe is in the bin directory.
I also found: viewtopic.php?f=16&t=6999.
I've added the files.
Attachments
New folder (6).zip
(181.15 KiB) Downloaded 2 times

BDesco
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Re: Heat radiation

Post by BDesco » 03 Jul 2020, 09:42

Maybe a more fundamental question.
Is there documentation that explains things a bit more thoroughly than those on the official Elmer website? (Or am I missing something).

For example:
- how to know what units the entered data is in? e.g. weight in kg? (I kinda figured out that it is what one would expect, but no clear statement about this makes me uneasy).
- how to know about your idealized explanation? I found in the models manual: "Radiation String: The type of radiation model for this boundary, one of: None, Idealized, Diffuse Gray. Note that, when using the diffuse gray radiation model, the file containing the Gebhardt factors must be given in the simulation section." This is very very limited. Is there more thorough explanation?
- or "Radiation Target Body Integer: This flag may be used to set the direction of the outward pointing normal. This is used when
computing viewfactors. A body identification number must be given. The default is that the normal points to less dense material or outward on outer boundaries." In some files on this forum I found it can equal 1 or -1.

With this little information it is a really, really steep learning curve.

raback
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Re: Heat radiation

Post by raback » 03 Jul 2020, 10:29

Hi

Have you read the chapter for HeatSolver in Elmer Models Manual? I know the documentation is limited but >700 pages still something. It just reflects the width of the application fields. Heat transfer is just one of many.

There are no units specified since the user can choose whatever consistent unit system. We usually use SI units. Noteworthy for radiation is that it likes Kelvins (or other unit starting from absolute zero) as you can see it being proportional to T^4.

You don't need to give the target body unless the direction is ambiguous. Good way to specify this easily is to give emissivity in material section. Normal will point out of the body where emissivity is found.

There are also ~10 consistency tests involving radiation.

-Peter

BDesco
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Re: Heat radiation

Post by BDesco » 04 Jul 2020, 05:38

Hello,
Yes, I quoted from this manual (though mine is only 342 pages - http://www.nic.funet.fi/pub/sci/physics ... Manual.pdf).

For example: "here are no units specified since the user can choose whatever consistent unit system." Where did you learn this?
For example: "Radiation String: The type of radiation model for this boundary, one of: None, Idealized, Diffuse Gray. Note that, when using the diffuse gray radiation model, the file containing the Gebhardt factors must be given in the simulation section." Is this last part needed? Or only if the factors are already available, if not they are computed by Elmer? And how would one give "the file containing the Gebhardt factors" in "the simulation section".
These 'explanations' in the manual give me more questions than answers.

I did specify the emissivity in the material section. Did you find time to have a look at my file?

"There are also ~10 consistency tests involving radiation". I'm sorry but I don't understand this.

Could you please specify how you learned to work with Elmer? How do you solve an issue? How would you go about figuring out the quotes from this and my previous message?

Thanks.

raback
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Re: Heat radiation

Post by raback » 04 Jul 2020, 18:29

BDesco wrote:
04 Jul 2020, 05:38
Could you please specify how you learned to work with Elmer? How do you solve an issue?
Well, I coded large parts of the code. If I have an issue I code some more ;-)

This is a small open source project and this support is pro bono. Commercial codes are there for a purpose as they can have an army of people giving support and writing documentation.

-Peter

BDesco
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Re: Heat radiation

Post by BDesco » 07 Jul 2020, 06:59

Thank you Peter, I understand that.
Are you willing to help me?

Thank you.

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