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initial field influences solution

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Old   May 7, 2014, 14:05
Default initial field influences solution
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Hello all,

I am new to OpenFoam and I have a problem with my first simulation. I have to simulate an external flow around an object and I noticed, that the solution depends on my initial field values of k and omega. I am using the k-omega-SST model and simplefoam. Does anybody has an idea, what could be the reason for that?

Thank you
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Old   May 7, 2014, 16:56
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Hi,

Have a look at this wiki page where the concept of free-stream turbulent boundary conditions is explained:

http://www.cfd-online.com/Wiki/Turbu...ary_conditions

These will have a significant effect on your solution if you are using RANS methods (this is what you're doing), less so for LES and none at all for DNS. Depends on what kind of simulation you're running.

Here is a pretty condensed read as to how turbulence modelling works. I'm sure it will do a better job at explaining this than me trying to do it in this post

http://www.cfd-online.com/W/images/3..._Beginners.pdf

Peace,

A
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Old   May 8, 2014, 06:19
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Thank you Artur.

I have already used the equations from the link you posted for my internalField values for k and omega. But then, the contours of the stream disturbance of 1% look pretty the same for different velocities(I attached two pictures to make clearly what I mean).

I found that a little bit strange so I tried out some things, which made me found the problem of my first post. But could it be right?

My boundary conditions for k and omega are:
inlet:turbulentIntensityKineticEnergyInlet/turbulentMixingLengthFrequencyInlet
outlet: zeroGradient
object: kqRWallFunction/omegaWallFunction
boundary: zeroGradient
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Old   May 8, 2014, 07:27
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As I said in the previous post, it is perfectly normal for the initial/free-stream k and omega values to affect your solution. It is then up to you to know what they should be in the actual flow you are simulating and to try and match them in CFD which is often non-trivial.

As for your BCs, I have never used these particular inlet ones so perhaps someone who has will be kind enough to comment on them.

Sharing some more information on your case could also be helpful: is it an external or internal flow, what the Reynolds number is, what is your mesh like, etc.

Peace,

A
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