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Gui July 25, 2007 16:05

Defining a surface from points or edges in CFXpost
I'd like to know the mass flow in a particular location of my domain : through a square hole. So I'd just like to define a surface from point or edges in CFX-post. But in user surface you can only specify offset from surface or intersection or some other options.

Thank you very much and sorry for all these questions but I need to have results very soon.

Robin July 25, 2007 16:27

Re: Defining a surface from points or edges in CFX
Hi Gui,

You don't need to define a surface from edges to do this. Just do the following:

1. create a plane that cuts through your hole, 2. right click on the plane at the center of the hole and choose "Set Plane Center" from the contextual menu, 3. Set the plane bounds to be Circular with a radius slightly larger than your hole but small enough not to cut into the rest of your domain.

Regards, Robin

P.S. When posting to the forum, try to explain why you are trying to do something. The answer may not always be what you expect ;-)

Gui July 25, 2007 17:37

Re: Defining a surface from points or edges in CFX
Thanks a lot !

Wooster July 26, 2007 10:16

Re: Defining a surface from points or edges in CFX
you know, I've been trying to do this in Post forever. all it takes is a right click. The menu for creating the plan doesn't allow me (that I've seen) to offset the plane center from a particular axis.


Robin July 26, 2007 10:37

Re: Defining a surface from points or edges in CFX
Actually, it does. Change the plane definition to point and normal and select the point by clicking in the center of your plane.


Wooster July 26, 2007 11:19

Re: Defining a surface from points or edges in CFX
Yes, I believe I have seen that option. The problem with it is that if your axis zero point is at a random place in the model, you have issues getting it to work. The answer to this (and something I perpetually forget to do) is to setup an alternate coordinate system manually in Pre and base the Post results on that coordinate frame.

thanks -W

Robin July 26, 2007 11:44

Re: Defining a surface from points or edges in CFX
The axis zero point shouldn't be an issue. What problem do you run into?

Wooster July 26, 2007 14:04

Re: Defining a surface from points or edges in CFX
first of all, sorry for taking the post Gui.

Anyway, my problem is in the setup of the plane, you have to figure out where exactly the axis zero is (not a problem because you can get it to show that). Then once you do that, you have to come up with a way of expressing where the hole (holes in my case) all are. So its a bit like "finding" the point where your plan should be. ex: say I'm interested in some XZ cross section of my model. The axis zero is 4 inchs up and to the left of the model (because I've once again forgotten to setup my Coordinates in Pre). Now I have to figure out how far down the plane should rest (done by trial and error or a calculator with measurements). That's a simple example but if you create, say, 28 holes all in a strange non-circular pattern, and tightly packed (so you really have to find the center) this can turn into a nightmare of numbers just to find the center point of the hole referenced to the coordinate axis. If there is some ridiculously simple way of clicking on any one of the holes (not individually called out mind you) and getting a plane, I do not know it. I'm self taught here so that feature would be new to me (and VERY helpful). I can detail my specific problems in another post if you would like. -W

Robin July 26, 2007 14:31

Re: Defining a surface from points or edges in CFX
Hi Wooster,

If you can display the edge or surface around the hole you can start by picking a point on the hole and estimating the direction. I don't see why you need to worry about the axis zero though, since the normal direction is simply a vector and doesn't change with a translational transformation (i.e. if the hole is aligned with the z axis at a point 4 inches up and over the normal direction is still 0,0,1). The point is the point at which the center of the plane is.

If you holes are arbitrarily aligned and it is difficult to find the plane normal, just use the "from 3 points" option. If you can see the sides of the hole, click on any three points aligned with the plane that you want (if you pick from the edge instead of the surface it will work better), then re-center the plane with the right click option. If you want to move the plane in the normal direction, just use the quick-edit option or click and drag the plane in the viewer.

Regards, Robin

Wooster July 26, 2007 15:12

Re: Defining a surface from points or edges in CFX
I think my troubles begin with finding that point then reproducing it at a fixed Z with different XY coordinates. I haven't tried some of the right clicking techniques and have been constructing my planes with numbers instead of actually picking points from the views. My computer is dead at the moment (the technician did a very tear filled eulogy) but once I'm back up and running I'll try out some of your suggestions. I'm thinking that you're most likely solving my problem and telling me about something I missed, I just need to get my hands on the Post processor again to confirm this.


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