CFD Online Discussion Forums

CFD Online Discussion Forums (http://www.cfd-online.com/Forums/)
-   CFX (http://www.cfd-online.com/Forums/cfx/)
-   -   Coordination Frame Problem - strange (http://www.cfd-online.com/Forums/cfx/78697-coordination-frame-problem-strange.html)

Luk_Fiz July 30, 2010 03:00

Coordination Frame Problem - strange
 
Hi,
Since I am preparing parts to bigger simulation I am checking small things which I have never performed in CFX before. This is a strange problem related to coordination frames.
My problem is multi-domain with interfaces. One of them is a simple domain, which is (during time dependent simulation) twisted and squeezed accordingly to some prescribed functions, in time. These functions are related to "Total Mesh Displacement X", "theta" and other CFX functions. After careful geometrical considerations I managed to describe the movement accurately and everything is ok. And now:
In the other part of my entire arrangement is a second domain of exactly the same size and initial shape. This domain is shifted and turned comparing to the first one however it performs the same movement and deformations during the simulation.
And here is the problem:
Because of similarity of domains and motions I wanted to use the same functions to describe the movement, but prepare second coordinate frame that way, that this second domain is in the same position relatively to second coords as the first on in the first (global) coords. In this purpose I have created second coords, set up second domain to them but... It does not work!! These formulas creates some strange movement of second domain and it looks like it was working in the old one coords.

Can somebode be that kind and describe how to overcome this. In shortcoming I want to know how to force CFX expression to hold in local coordinate system.

Luk

To be more specific: assume that we move the domain and the local coords the same way (the domain is always in the same position relatively to local coords):
1- If I translate relatively to coord 0 - everything is ok
2- If I rotate relatively to coord 0 - it fails

Luk_Fiz July 30, 2010 05:42

Ok,
After few trials I have found that the problem was related to Total Mesh Displacement.
I decided to modify my functions describing domain squeezing in such a way to not to use any build-in CFX functions. During this I have found that while "r" and "theta" seems to be local (I mean x and y that are used in them are local x and y) than Total Mesh Displacement x uses global x. I was not aware of and I cannot find in documentation is it proper on not.

Sorry for answering my own questions but before I got it I was working two days on this problem.
Luk

stumpy July 30, 2010 08:20

That sounds correct. "x", "y" and "z" are the only variables that are transformed into local coordinate frames. There is a note in the doc about this:
"A variable or CEL expression that is used in the specification of some quantity is evaluated with x, y, z treated as being local, and all other non-scalar variables (such as u, v, w) treated as being global. Non-scalar CEL functions (such as torque, force) are treated as being local. The quantity that was specified by the variable or CEL expression, once evaluated, will be treated as being local, if applicable (that is, if it is a vector or tensor quantity, or component thereof)."


All times are GMT -4. The time now is 18:23.