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constants in K-e turbulence model

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Old   June 22, 2001, 07:21
Default constants in K-e turbulence model
  #1
Hu
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Hi, all

I am new to PHOENICS. The version of PHOENICS I'm using is 3.2 on Windows NT platform. I got a few questions here. I tried to find out the entries where I can modify the constants such as C1e, C2e, Cmu in the main menu of the VR editor but I cannot get it. Could anyone tell me how to do that via VR editor, or I must do it manually in Q1 file?

The other question is that I wanted to simulate the ground roughness in the domain. What I did was I specified a global roughness, say 0.01m in the Sources page of the main menu, and using Fully-Rough option. Then I put a Plate on the bottom of the domain as the ground. I was wondering if the global roughness parameter would affect the auto wall function employed by Earth. Because I also put some cubic objects simulating the buildings and the material was 198 with smooth wall friction. I mean if the Earth will use the Fully-Rough wall function to calculate the flows near the cubic objects, instead of using default log-law wall function?

Please tell me how to or show me where I can get more information. Thank you for your time.

Hu
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Old   June 26, 2001, 12:20
Default Re: constants in K-e turbulence model
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Mike Malin
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The turbulence model coefficients can only be modified in Subroutine GROUND of the EARTH code, and so you must have a re-compilable version of PHOENICS.

A global roughness height should be used everywhere, except in so far as there was an error in the fully-rough wall-function sequence used by EGWF (Earth-Generated Wall Functions) which meant that a constant friction factor of 1.e-3 was used instead of the fully-rough wall function. I forget on which version this was corrected, but I believe the error may be present in V3.2.
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Old   June 27, 2001, 13:29
Default Re: constants in K-e turbulence model
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Hu
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Hi, Mike

Thanks for your help. One more thing I want to make it clear. Did you mean the global roughness height is applied everywhere in the computing domain once it has been specified? Therefore what I should do if I want to specifed two different roughness height to different objects? If the fully-rough wall function contains an error in the EGWF, is there another way to simulate the ground roughness? I am using PHOENICS to simulate atmospheric boundary layer, so I need a proper boundary condition for the earth's surface. Is it possible to specify that boundary condition via VR editor?

Thanks again.

Hu
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Old   July 5, 2001, 12:09
Default Re: constants in K-e turbulence model
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Mike Malin
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Hi,

Did you mean the global roughness height is applied everywhere in the computing domain once it has been specified?

Yes.

Therefore what I should do if I want to specifed two different roughness height to different objects?

You use the alternative methods of SPEDAT PIL commands and props-file entries, that are explained on CHAM's website at:

http://www.cham.co.uk/phoenics/d_pol...d/enc_tu84.htm

If EGWF=T, which is the default, then the software error I mentioned in the last email will render all fully-rough friction invalid at the surfaces of blockages. However, the 'plate' object will OK. It is just 'blockage' objects that are affected.

Since the fully-rough wall function contains an error in the EGWF, is there another way to simulate the ground roughness?

If the 'ground' is represented by a PLATE object then the fully-rough wall function will be OK. There should be no problem.

If you are using fully-rough wall functions on internal blockages, then you will have to put EGWF=F and use the SPEDAT commands (see website documentation as mentioned above)to define the fully-rough roughness height if it differs from the global setting used on the ground plate.

This ground plate can also use a SPEDAT command if it necessary, as a SPEDAT setting overides the global setting.

Regards, Michael Malin
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