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

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.

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

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