CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > CFX

Is it possible to get value of a variable at given coordinates without a point?

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

Reply
 
LinkBack Thread Tools Display Modes
Old   March 22, 2010, 17:58
Question Is it possible to get value of a variable at given coordinates without a point?
  #1
New Member
 
Join Date: Mar 2010
Posts: 4
Rep Power: 7
technocrat is on a distinguished road
Is it possible to get value of a variable at given coordinates without crating a point there?
Maybe it can be done via user fortran? How?
I want to implement my own boundary conditions (like d^2u/dx^2=0) that is why I need values of velocity like velocity and pressure at points with arbitrary coordinates inside domain.
technocrat is offline   Reply With Quote

Old   March 23, 2010, 06:37
Default
  #2
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,929
Rep Power: 85
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
You can create monitor points where ever you like. But keep in mind they just return the value of the element they are in, they do not interpolate between the nearby nodes.

But CFX has built in gradient functions for first order derivatives, not sure about second order derivatives. If available you should use those functions.
ghorrocks is offline   Reply With Quote

Old   March 23, 2010, 14:19
Default
  #3
Member
 
Dmitry Volkind
Join Date: Jan 2010
Location: Chelyabinsk, Russia
Posts: 58
Rep Power: 7
dvolkind is on a distinguished road
Greetings!

What kind of boundary condition is that? As far as I understand, d^2u/dx^2=0 yields a flat speed profile. Probably if you could estimate du/dx somehow, it might be easier to specify the velocity as a linear function of coordinate via expression. I also doubt that second order derivative can be used as a boundary condition for it's neither first-type nor second-type.
dvolkind is offline   Reply With Quote

Old   March 23, 2010, 16:20
Default
  #4
New Member
 
Join Date: Mar 2010
Posts: 4
Rep Power: 7
technocrat is on a distinguished road
Thank you for your answers.

Quote:
Originally Posted by ghorrocks View Post
... You can create monitor points where ever you like...
Monitor points can not be used as locations in expressions (to set up my BC in CFX-Pre I need an expression that will give u values). It is possible to use Source points as locations, but I'll have to create hundreds of them (one per each hexahedra in two layers, closest to boundary) to approximate 2-nd derivative. This is possible (with perl script), but creating so much points is too slow according to my experience.

Quote:
Originally Posted by ghorrocks View Post
...But CFX has built in gradient functions for first order derivatives...
I didn't find a way to use this in CFX-Pre. For example this expression
Code:
probe(Velocity u.Gradient)@Source Point 1
produces error
Code:
The function 'probe' referenced in parameter 'Expression Value' in object '/FLOW:Flow Analysis 1/OUTPUT CONTROL/MONITOR OBJECTS/MONITOR POINT:Monitor
 Point 1' has an invalid argument, 'Velocity u.Gradient'. The solver does not support operator-suffixed variables here.

Last edited by technocrat; March 23, 2010 at 16:55.
technocrat is offline   Reply With Quote

Old   March 23, 2010, 16:42
Default
  #5
New Member
 
Join Date: Mar 2010
Posts: 4
Rep Power: 7
technocrat is on a distinguished road
Quote:
Originally Posted by dvolkind View Post
...
What kind of boundary condition is that? As far as I understand, d^2u/dx^2=0 yields a flat speed profile.
I think it doesn't. Normal vector to outlet boundary plane in my case is \{1,0,0\}. \frac{\partial^2}{\partial x^2}u=0 does'nt imply any relation between u values at different y coordinates (if you meant that). This condition just makes speed change smoother near outlet.
technocrat is offline   Reply With Quote

Old   March 23, 2010, 18:05
Default
  #6
New Member
 
Join Date: Mar 2010
Posts: 4
Rep Power: 7
technocrat is on a distinguished road
Maybe I can get what I want via user fortran?
I'm now trying to find appropriate data acquisition routin in manual, but hasn't succeed yet. Probably I'm looking at what I need and don't see it?



Quote:
The locale is always the locale for which the User CEL routine is called. It is not possible to call USER_GETVAR for non-local information (for example, using the face gradients on BOUNDARY:Inlet for the boundary condition specification of a variable on BOUNDARY:Outlet).
technocrat is offline   Reply With Quote

Reply

Tags
cfx, user fortran

Thread Tools
Display Modes

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 Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Point coordinates via orinial point address after decomposing diana OpenFOAM Running, Solving & CFD 0 August 6, 2008 10:27
error in COMSOL:'ERROR:6164 Duplicate Variable' bhushas Main CFD Forum 1 May 30, 2008 04:35
How to edit point coordinates in ICEM? Alexey CFX 6 December 24, 2007 11:31
Gmsh and samplesurface touf Open Source Meshers: Gmsh, Netgen, CGNS, ... 2 December 10, 2007 03:27
Replace periodic by inlet-outlet pair lego CFX 3 November 5, 2002 21:09


All times are GMT -4. The time now is 22:07.