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Old   July 10, 2010, 11:23
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yeah , and that solves the problem, thanks.
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Old   August 26, 2010, 16:07
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A LITTLE PROBLEM WHICH MIGHT BE LESS RELEVANT TO LATTER PROBLEMS IS fieldAverage FOR ME , HOW SHOULD I DICTATE THE SOLVER TO COMPUTE fieldAverage S ON EVERY TIME STEP ?
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Old   September 3, 2010, 06:40
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aren't the averages calculated everytime you write the data ?
I thought so.
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Old   September 3, 2010, 07:03
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Hi All!
I believe Archy is wright, i saw the files in the folders created when running the simulation.

Marta
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Old   September 6, 2010, 04:02
Default Outlet definition ?
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Hi all,

I am having problems defining the outlet of my system. It should be a 0 Pascal vacuum outlet, but there is no "nice" pressure drop in front of the outlet ( outlet is on bottom of picture, wall is on left, center od axis-symmetry is the edge on the right ). I am not sure if i did everything right.

Here are the definitions of the 0/ folder concerning my outlet:

boundaryT
type fixedValue;
value uniform 300;

boundaryU
type calculated;

dsmcRhoN
type fixedValue;
value uniform 0;

dsmcSigmaTcRMax
type zeroGradient;

fD
type zeroGradient;

iDof
type zeroGradient;

internalE
type zeroGradient;

linearKE
type zeroGradient;

momentum
type zeroGradient;

q
type zeroGradient;

rhoM
type fixedValue;
value uniform 0;

rhoN
type fixedValue;
value uniform 0;

Did i make an obvious mistake there ?

thanks for your help,
Archy
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Old   September 6, 2010, 05:53
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Dear Archy, I am experiencing the same difficulty with an outlet at a very low pressure.
Now i'm trying to verify the reason of this behaviour, which i have seen by looking at the densities.
I would also like to ask you how you managed to look into the pressure values, because this is not a physical parameter calculated by default in dsmcFoam.

Thank you in advance!

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Old   September 6, 2010, 05:59
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I've set up a filter to calculate the pressure

Go to
Filters > Data Analysis > Calculator

Here I've enterered
rhoNMean * 1.38065e-23 * 300 (sometimes paraview is bitchy and the Mean value is not accepted, take rhoN instead )

boltzmann constant, Temperature 300 K assumed.
Then I've plotted the resulting array.

It should be possible to calculate the local temperature from the internalE value, shouldn't it ?
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Old   September 6, 2010, 06:39
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Thank you so much for you quick answer Archy!


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Old   September 6, 2010, 07:02
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can you post your outlet definitions , so we could take a look at it?
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Old   September 6, 2010, 08:59
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I've defined the boundary temperature of the outlet to be 300 Kelvin and I've run a simulation with 1e-12 K where I could see a "sucking" effect as expected from an vacuum outlet.

I think defining a temperature for an opening as an outlet cannot be right !
I would like to use:

boundaryT
type zeroGradient;

because I cannot know the temperature of the gas at the outlet.
But zeroGradient or calculated types are not accepted by dsmcFoam.

So how to define an unkown gas temperature ??

Last edited by archymedes; September 6, 2010 at 09:43.
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Old   September 6, 2010, 09:47
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At the outlet I have used a zeroGradient boundary type too, for temperature.

At the moment i'm trying to run a case where i have only the mass flow rate specified at the inlet (mass and number densities, velocity are the only parameters defined). The rest of the boundaries is 'zeroGradient' or 'calculated' type.

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Old   September 6, 2010, 11:30
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Strange... in the boundaryT file I cannot set a zeroGradient type for the outlet

--> FOAM FATAL ERROR:
Zero boundary temperature detected, check boundaryT condition.



From function Foam::FreeStream<CloudType>::inflow()
in file lnInclude/FreeStream.C at line 166.

FOAM aborting

#0 Foam::error:rintStack(Foam::Ostream&) in "/opt/openfoam170/lib/linuxGccDPOpt/libOpenFOAM.so"
#1 Foam::error::abort() in "/opt/openfoam170/lib/linuxGccDPOpt/libOpenFOAM.so"
#2 Foam::FreeStream<Foam:smcCloud<Foam::dsmcParcel> >::inflow() in "/opt/openfoam170/lib/linuxGccDPOpt/libdsmc.so"
#3
in "/opt/openfoam170/applications/bin/linuxGccDPOpt/dsmcFoam"
#4
in "/opt/openfoam170/applications/bin/linuxGccDPOpt/dsmcFoam"
#5 __libc_start_main in "/lib/tls/i686/cmov/libc.so.6"
#6
in "/opt/openfoam170/applications/bin/linuxGccDPOpt/dsmcFoam"

[4]+ Aborted

how did you apply it ? did you define a rhoN / rhoM at the outlet ? ( I set it to zero for pure vacuum ).

Thanks,
Archy

Last edited by archymedes; September 6, 2010 at 11:51.
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Old   September 6, 2010, 16:20
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Hi Archy, here is the folder with the input parameters. Anyway i'm not sure about it, i'm just trying to find out the correct way to impose boundary conditions, at the moment.

What i would like to do is to verify and plot the pressure drop due to gas flow inside a tube with a specific geometry.
I have a 0 Pa pressure at the outlet and a fixed mass flow rate at the inlet. How can i specify these parameters without using too many constraints for the system in order to avoid over determining the problem?


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Old   September 7, 2010, 03:37
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Hi Marta,

not sure how to do this. I think density and velocity at the inlet is the only way to determine a certain massflow rate. As for the outlet you have set everything to zeroGradient, I don't understand how dsmcFoam can derive a 0 Pa outlet from that.

These are just guesses:

1) I think at least one of the particle file ( rhoN, rhoM, dsmcRhoN ) has to determine 0 particles at outlet, just to make sure it really is 0 Pa.

2) The walls absorb a particle and re-emit it with a cosine distribution right ? if this is really the case that boundaryU and momentum should be (0 0 0) at all walls. I don't understand how this conforms with slip on the walls, but zeroGradient there whould mean full speed along the walls, wouldn't it ?

what are the foro's ? freestream boundaries ? walls ?

the simulation runs with these definitions ? (I get errors when set boundaryT at outlet to zeroGradient, mavbe I am overconstrained.. )

cheers,
archy
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Old   September 7, 2010, 05:47
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Hi Archy, thank you very much for your reply.
I will try to modify the input files as you recommend, and then i'll let you know how it works.

The 'foros' are the inlet sections for mass flow rate. Lateral is a part of the wall towards the discharge section of the vacuum chamber next to the outlet section.

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Old   September 7, 2010, 09:17
Default dsmcFoam: there is no outlet
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An outlet DOES NOT exist in dsmcFoam.

This is my conlusion after studying the examples and effects on changing the input files.

there are only two inflowBoundarModels

FreeStream and noFlow

with FreeStream you can define a particle density in the constant/dsmcProperties file
This will affect ALL OPENINGs of the system, as this model was made for the free stream around a wing or spacecraft.

To simulate a dsmc flow within a vacuum chamber with inlet/outlet, I think we can use the FreeStream for the opening ( boundaryT and boundaryU to define the flow and temperature ) but for the outlet I see no legal way to define it.
Only to set the boundaryT Temperature to almost zero Degree eg. 1e-10. In this way almost no particles are injected there ( almost no energy ) but they can still leave the system as it is an opening.
As soon as the chamber got filled with particles from the inlet side, the fact that they have some energy will make them overrule the "floating" particles from the outlet.

U think this could be true ? or am I complete lost here?

thanks for your comments,
archy
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Old   September 20, 2010, 06:59
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Dear Archy, sorry for not having replied for so long, but i had to clarify some topics in my mind.

To run our simulations we probably need to modify the inflow method in dsmcCloud.C, in particular, we shall consider the possibility to modify:

this->inflowBoundary().inflow()

To be able to initialise a different number of particles in different areas of your domain, you'll need to modify:

void Foam:smcCloud<parcelType>::initialise()

here each single cell is filled with particles, so that it is possible (if you add some specifications) to apply different densities and so on to some specific cell regions.

If you need a different way to control the boundary conditions with respect to the freeStream model (which is actually the only implemented scheme in dsmcFoam), then it might be necessary to implement your own model.

To do this, have a look at : src/lagrangian/dsmc/submodels/InflowBoundaryModel

Hope this helps,

Marta
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Old   September 20, 2010, 11:55
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hi all.

there is one other thing i would wanna ask here, why doesnt my simulation reach a steady state independent of how many time steps is elapsed ?
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Old   September 20, 2010, 16:28
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Hi Maneshi,
I think something is going wrong with your simulation, but i cannot answer more precisely, since i do not know what your test case is about, nor which boundary you have used.
If you have constant boundaries, then i think it's impossible not to reach a steady state condition, if everything is set up properly.

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Old   September 21, 2010, 03:24
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my simulations is a little diffrent with yours but with the same process. A molecular dynamic Mdfoam simulation of a nano channel with walls on 4 sides and a cyclic inlet to oulet in which the flow is pressure driven. is there any hints you can mention ?
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