# height & displacement

 Register Blogs Members List Search Today's Posts Mark Forums Read

 February 1, 2005, 06:25 height & displacement #1 azmir Guest   Posts: n/a from physical point of view the roughness height and displacement option for wall BC would be in what dimension?

 February 1, 2005, 07:33 Re: height & displacement #2 Richard Guest   Posts: n/a Both are in metres.

 February 2, 2005, 06:23 Re: height & displacement #3 azmir Guest   Posts: n/a Thanks. And if the geometry scale for the mdl is in cm, a 0.01 value for the height would be 0.01cm? Or still 0.01m?

 February 2, 2005, 07:04 Re: height & displacement #4 Richard Guest   Posts: n/a No, the scale factor when writing the .geom file applies only to vertex coordinates. It doesn't apply to any of the physical models or boundary conditions. Roughness height etc will be still be in metres. Similarly, velocities will always be in m/s, never (cm)/s.

 February 2, 2005, 08:58 Re: height & displacement #5 azmir_isa Guest   Posts: n/a Interesting. The result doesnt seem to show it would was in meters though. Flow near the wall (y < 0.01m) is moving. If roughness height was 0.01m and indeed not 0.01cm, flow would have been stuck in this y<0.01m region. Would this have s'thing to do with my 3.15 old and 3.15 new. In the new 3.15, there is a unit dimension for roughness but don't think there was in the old 3.15. No, not yet using 3.22 still...

 February 2, 2005, 10:09 Re: height & displacement #6 Richard Guest   Posts: n/a You shouldn't be in a position to judge what the velocity is at the roughness height or displacement thickness: the roughness is a sub-grid model. If your mesh is fine enough to have near-wall cells the size of the roughness parameters, you shouldn't be using the roughness model. In fact, I think star might ignore the roughness model when this is the case.

 February 3, 2005, 07:08 Re: height & displacement #7 azmir Guest   Posts: n/a Thanks for your inputs. I am obviously confused. Yet, your last statement is confirmed not true. I saw my mis-dimensioned roughness model somehow influenced the result, i.e. the roughness model was not ignored by Star (kw-SST). The details of its influence from the vector plots got me into thinking about the dimensions of roughness & displacement at the beginning. Well, can't burden u too much with my little problem. Thanks for your comments so far. I'd investigate this further. Cheers!

 February 3, 2005, 08:59 Re: height & displacement #8 Richard Guest   Posts: n/a It's worth mentioning also the roughness model is a means of modifying the wall shear stress. It doesn't directly fix the velocity components. If you were able to look at the velocity at the roughness height (and, as I said, you shouldn't) you wouldn't see u=0.

 February 4, 2005, 05:54 Re: height & displacement #9 azmir Guest   Posts: n/a Thanks again. If u feel there is anything more I should know about implementation of roughness in Star, please give me pointers. Online references for example.

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are On Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post Guillaume CFX 0 July 29, 2008 09:38 Chin Lim FLUENT 4 July 4, 2008 04:39 rodi FLUENT 2 January 15, 2008 08:56 gh FLUENT 0 June 1, 2005 10:42 Michael Heim FLUENT 0 July 6, 2004 14:08

All times are GMT -4. The time now is 06:38.