Newbee with simple problem I think

[Don]

Hello Everyone,

I've downloaded and installed Elmer, and am trying to figure out how to run a simple simulation.

I have a large object at an ambient temperature of 299 Kelvin
It is placed in a large room which is 327.6 Kelvin for an extended period of time. (eventually I will simulate this temperature fluctuating over time, but for now this is not necessary)
I need to simulate how long it takes the object to reach a uniform temperature.

The object is a 300 gallon cube shaped container filled with liquid, I've chosen glycerol to start with, and am not trying to incorporate convection at this point.

I’ve tried my best to set up and run the simulation, I have loaded all 20 frames in the postprocessor, and the temperature gradient shows up in the render, but it doesn’t change over time no matter which frame I look at.

(My setup and the Sif printout are below my signature)
Any help or directions will be appreciated

Thank you
Don

My geometry is a cube with rounded corners, all boundaries merged into a single boundary. (A single body with a single boundary)

SETUP:
Max output level 4
Steady State max Iter 1
Timestepping method BDF
BDF Order 2 (tried 3 as well, same result)
Simulation Type “Transient”
Timestep intervals 20
Output intervals 1
Timestep sizes 1800 (is this seconds, I’ve tried different values from 5 to 1800000)
HEAT EQUATION:
Active checked (applied to Body 1)
MATERIAL:
Glycerol applied to Body 1
BODY FORCE:
None
INITIAL CONDITION:
Heat Equation – Temperature 299.8167
BOUNDARY CONDITION
Temperature 327.5944 Applied to Boundary 1
Tried setting External Temperature to the same value, no effect
SET BOUNDARY PROPERTIES
Checked
------------------------------------------------------------------------------
The Sif results are below
------------------------------------------------------------------------------
Header
CHECK KEYWORDS Warn
Mesh DB "." "."
Include Path ""
Results Directory ""
End
Simulation
Max Output Level = 4
Coordinate System = Cartesian
Coordinate Mapping(3) = 1 2 3
Simulation Type = Transient
Steady State Max Iterations = 1
Output Intervals = 1
Timestepping Method = BDF
BDF Order = 3
Timestep intervals = 20
Timestep Sizes = 5
Solver Input File = case.sif
Post File = case.ep
End
Constants
Gravity(4) = 0 -1 0 9.82
Stefan Boltzmann = 5.67e-08
Permittivity of Vacuum = 8.8542e-12
Boltzmann Constant = 1.3807e-23
Unit Charge = 1.602e-19
End
Body 1
Target Bodies(1) = 1
Name = "Body 1"
Equation = 1
Material = 1
Initial condition = 1
End
Solver 1
Equation = Heat Equation
Procedure = "HeatSolve" "HeatSolver"
Variable = -dofs 1 Temperature
Exec Solver = Always
Stabilize = True
Bubbles = False
Lumped Mass Matrix = False
Optimize Bandwidth = True
Steady State Convergence Tolerance = 1.0e-5
Nonlinear System Convergence Tolerance = 1.0e-8
Nonlinear System Max Iterations = 20
Nonlinear System Newton After Iterations = 3
Nonlinear System Newton After Tolerance = 1.0e-3
Nonlinear System Relaxation Factor = 1
Linear System Solver = Iterative
Linear System Iterative Method = BiCGStab
Linear System Max Iterations = 500
Linear System Convergence Tolerance = 1.0e-8
Linear System Preconditioning = ILU0
Linear System ILUT Tolerance = 1.0e-3
Linear System Abort Not Converged = False
Linear System Residual Output = 1
Linear System Precondition Recompute = 1
End
Equation 1
Name = "Equation 1"
Active Solvers(1) = 1
End
Material 1
Name = "Glycerol (room temperature)"
Heat Conductivity = 0.28
Heat Capacity = 2400.0
Density = 1261.0
Viscosity = 1.49
Sound speed = 1904.0
Heat expansion Coefficient = 0.610e-3
End
Initial Condition 1
Name = "InitialCondition 1"
Temperature = 299.8167
End
Boundary Condition 1
Target Boundaries(1) = 1
Name = "BoundaryCondition 1"
Temperature = 327.5944
End

[petroo]

Hi Don,

I have a large object ...

Timestep intervals = 20
Timestep Sizes = 5

Those settings are somewhat contradictory: If you simulate a large object with heat dissipation generated by a driving force of only 30 K you certainly will need more than 100 sec to see any effect.

Or did I miss something in this analysis?

Regards,

Peter

[Don]

Hi Don,

I have a large object ...

Timestep intervals = 20
Timestep Sizes = 5

Those settings are somewhat contradictory: If you simulate a large object with heat dissipation generated by a driving force of only 30 K you certainly will need more than 100 sec to see any effect.

Or did I miss something in this analysis?

Regards,

Peter

Peter,

Those were just the last settings I had tried.
My initial settings were 1800 seconds per timestep. (I've tried all the way from 5 seconds to 1800000 seconds per timestep and the results are the same)

SETUP:
Max output level 4
Steady State max Iter 1
Timestepping method BDF
BDF Order 2 (tried 3 as well, same result)
Simulation Type “Transient”
Timestep intervals 20
Output intervals 1
Timestep sizes 1800 (is this seconds, I’ve tried different values from 5 to 1800000)

I just did one with convection included, the velocity and pressure are animating, but the temperature is exactly the same no mater what I do.
I must be missing something in the heat equation, but just can't figure out what it is?

Or, could my installation be broken?

Thank you very much
Don

[petroo]

Hello Don,

Those were just the last settings I had tried.
My initial settings were 1800 seconds per timestep. (I've tried all the way from 5 seconds to 1800000 seconds per timestep and the results are the same)

Hmm - with those very large timesteps there should be some effect visible.

Anyhow, I'd suppose to change a few things just to get the simulation convinced to do something perceivable:

• Change outer temperature to 500 K (you don't have to stand next to the [simulated] barrel, do you? )
• Only take the sides, and not the lid and the bottom, as heat transfer areas. This should induce a rather hefty convection.
• Allow for more calculated timesteps: I did not work with convection myself so far, but I'd expect it to have to develop over some series of explicitly targeted time steps.
• Maybe break the symmetry, e.g. by even leaving out one of the four barrel sides w/r/to heat coupling: Ideal symmetry as it is calculated in a simulation is contrary to our physical observation where it practically never occurs. Maybe set the fourth wall to a defined lower temperature even.

Or, could my installation be broken?

Wouldn't think so: If the calculations work, on a purely technical basis, the system should behave well. This does not exclude errors in the general formulation of the solver, of course, which is very hard for us to trace. Actually, I am somewhat apostatizing from the heat equation myself in regard of my recent wall calculations (see other thread on dysfunct physics). But I still believe that my effect is produced "between my headphones"

So I am still waiting to be proselytized again ...

Regards,

Peter

[raback]

Hi

In your Equation block where you want to account for convective heat transfer you should define

Code: Select all

Convection = computed

Maybe you missed this.

-Peter

[Don]

petroo,

Thank you for your suggestions, I will test them out today and comment back on the results.

raback,

I did test convection=calculated in several different attempts. Convection does produce velocity and pressure changes over time. But even in these simulations, with fluid moving around inside the container, the heat distribution looks exactly the same, which makes no sense at all.
There is a gradient, it seems to be span only the first layer of tetrahedrons, it does not change over time, and appears exactly the same no mater how I setup the simulation, even with noticible convection.

I do have to say that other than this little problem, I am very impressed with Elmer. We can not afford to purchase the "name brand" FEM packages, and I have been looking for an opensource solution for a long time. My expectation was if I found one, it would be dificult to learn, but curent issues aside, I have found Elmer much easier than expected.

Thank you both for your help
Don

[Don]

What are the standard mesh units?
I'm assuming it's meters, looking at the output of opening my project, the bounding box is as follows:

Code: Select all

Bounding box: [ -590.713, -489.113, -489.113] x [ 590.713, 489.113, 489.113]

Could that be my whole problem? My container is 38 inches wide X 46 in long X 38 in tall, that would make it span from [-482.6,-584.2,-482.6] to [482.6,584.2,482.6] in millimeters
However if the standard unit is meters that would be HUGE!

If this in the problem, is it possible to set the scale in ElmerGUI to 0.001?
I don't see an obvious place to set the scale?

Thanks
Don

[petroo]

What are the standard mesh units?
I'm assuming it's meters, looking at the output of opening my project, the bounding box is as follows:

Code: Select all

Bounding box: [ -590.713, -489.113, -489.113] x [ 590.713, 489.113, 489.113]

Hey, this really is a big container. The unit is meters!

If this in the problem, is it possible to set the scale in ElmerGUI to 0.001?

Yes, there is, and it is quoted in some other thread, but I don't find it right now. With ElmerGrid you can use the command -scale <factor>, but in newer Elmer versions you may define a scaling factor directly in the SIF file, but I don't remember the correct way how to do it. I'd rather go for a recreation of the geometry from scrap, so to have everything directly correct.

Regards,

Peter

[Don]

Hey, this really is a big container. The unit is meters!

Good Guess huh?

I've already re-modeled my container in meters, and made sure the STEP export was in Meters. HOWEVER..... the bounding box info when opening the (new) project it still x1000

I am using the GUI not the command line but have entered "size = 0.001" in the [Initial Conditions>Mesh Update>Mesh Update 1] parameter, the project built ok and the simulation is curently processing.

I've turned convection on, made my mesh finer, and added more time steps, so there's about an hour left of the run.

I'll let you know how it turns out.
Don

[petroo]

I've already re-modeled my container in meters, and made sure the STEP export was in Meters. HOWEVER..... the bounding box info when opening the (new) project it still x1000

Yes - STEP is a matter of its own. We had this discussion just some days ago in another thread, and I had this effect myself about a year ago.

Hope your current calculation is performing well!

Regards,

Peter