CFD Online Discussion Forums

CFD Online Discussion Forums (
-   CFX (
-   -   Changing velocity in CFX5 (

Dimitris May 2, 2002 05:48

Changing velocity in CFX5
Hi everyone

I have a peculiar problem. The velocity inside the domain increases over 60% compared to the inlet velocity and it's not because of a smaller domain or things "in the way" inside the domain that causes is. Any ideas why I have this phenomena?

cfd guy May 2, 2002 09:45

Re: Changing velocity in CFX5
Did you take a look at your pressure field? If you have some high gradients of pressure in certain regions you can expect some local increases of velocity.
cfd guy

Dimitris May 2, 2002 09:49

Re: Changing velocity in CFX5
I have a reference pressure of 20 bar and an static pressure of 0 at the exit...but already after a few centimeters into the domain the velocity increases drastically. I'm confident that it hasn't anything to do with the domain getting smaller or something.....I've even put symmetry planes all around and still have the same behaviour....I can't understand....can't find anything wrong with the geometry as well...

steve May 2, 2002 15:54

Re: Changing velocity in CFX5
That's a pretty large pressure gradient (depending on the domain length); I would expect a large acceleration of the flow. Are you sure you want to set your pressure at 20 bar and not 20 Pa. And are you sure you want to set the static pressure of 0 at the outlet rather than specifying a relative pressure.


Dimitris May 2, 2002 16:54

Re: Changing velocity in CFX5
Well...I'm simulating the flow in a quenching chamber and a pressure of 20 bar is very normal considering the circumstances... When I used CFX4 I modeled in the same way but without this strange acceleration after the inlet...

steve May 2, 2002 17:20

Re: Changing velocity in CFX5
With zero static pressure at the outlet?

Dimitris May 2, 2002 18:48

Re: Changing velocity in CFX5
Yes....that's the way I've always would you model a rectangular domain with an inlet velocity, a reference pressure inside the domain and an outlet? What would you put on the outlet instead of 0 static pressure?

steve May 2, 2002 19:05

Re: Changing velocity in CFX5
So is that 0 relative pressure rather than an absolute pressure?

Astrid May 5, 2002 04:20

Re: Changing velocity in CFX5
Dimitris, some questions:

- What kind of BC's do you have on your inlet?

- Is it an inlet or an opening?

- Does your temperature drop?

- Do you have a uniform inlet profile and a developed velocity profile downstream (than it would be really simple)?


Dimitris May 6, 2002 06:23

Re: Changing velocity in CFX5
The inlet B.C. is "inlet".....I don't know what you mean...

Astrid May 6, 2002 06:56

Re: Changing velocity in CFX5
- It could have been an opening.

- What did you specify on the inlet? Normal velocity?

- Make a plane in Post that crosses the inlet. Is there sometihing peculiar on pressure and temperature?

- Have you checked the manual on the BC's you can specify on an inlet?


Dimitris May 6, 2002 07:16

Re: Changing velocity in CFX5
I'm doing it the exactly same way as I've been doing with my other simulations but it doesn't want to work. I have no idea of what to do. Perhaps I can e-mail you my .db and .def file and you can take a look at it if you have time and want to...

Astrid May 17, 2002 02:21

Re: Changing velocity in CFX5

Sorry, I lost your e-mail but have the files attached. So, I used your original def-file, while solving ALL equations and found nothing strange. I obtained a decent velocity and temperature field.

I remember from the e-mail that you made some variations on this def-file. Can you elaborate on these variations? Did you use the obtained velocity field with different temperatures (thus, FLUIDS = FALSE and TKE AND EPS = FALSE) ?


All times are GMT -4. The time now is 22:08.