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RAMAN6089 November 26, 2009 07:35

Cel expressions
i'm new user for CFX 11.0 and is founding difficulty in creating CEL expressions. can anybody send me the guide or tutorilas for the same.

Rui November 26, 2009 11:08

I don't think you're a new user. Do ALL the CFX Tutorials, and then you'll be a new user.
(you can also find them here: )

RAMAN6089 November 27, 2009 04:34

rui, i have already done the tutorilas to get the basic understanding.
MY ACTUAL QUESTIONS wAS That wat are the basic conditions we should always use as CEL expressions to get the convergance easily.
i mean see if u can help me this way.
for example Avgarea expressions could be taken to make the solver solve the problem easily.....i guess

and also., thanks for sending me the link.

Rui November 27, 2009 05:00

In your previous post you said you had difficulties in creating CEL expressions and you asked for tutorials. That's why I said you should do all the CFX tutorials, and posted that link.

Now, I have no idea how you can use CEL expressions to get easy convergence. Maybe you could tell us a bit about what you have in mind.

Rui November 27, 2009 05:56

If your fluid is incompressible, the outlet pressure doesn't change anything (apart from adding a fixed value to the pressure everywhere), does it?
And yes, impose a velocity, or mass flow, at the Inlet and pressure at the Outlet.

RAMAN6089 November 27, 2009 06:02

i need ur experience while solving a fluid problem relating to air ducts.
i have designed one ducting Plenum(air box), i'm checking the feasibilty for the same.

conditions are:
Air is supplied to the ducting system by fan at 9.14 m/s (it's a big) to fulfill the capacity of 38000 Cfm(or 64600 m3/hr)
my conditions for Pre are:
normal speed = 9.14 m/s

at outlet i'm making it as opening and static pressure= 250 Pa or 25mmWc

i'm doubtful how this static pressure is gonna effect coz there is sepration in the ducting afterwards. and if i give velocity at opening than it will forcefully withdraws the air through it. suggest

RAMAN6089 November 27, 2009 06:03

u are nearby my problem,......
i have given the velocity at inlet and pressure at outlet.....
suggest me how to calculate pressure at outlet...
with some example

Rui November 27, 2009 06:13

If you gave the pressure at the outlet, you don't need to calculate it.

RAMAN6089 November 27, 2009 06:21

i calculated the pressure drop by considering the bends and friction drop due to the wall.

how u calculate pressure drop???

Rui November 27, 2009 06:31

To me it sounds a good approximation.

ghorrocks November 28, 2009 06:13

If the simulation is incompressible then the outlet pressure is not important, it will just offset the whole pressure field. The only important thing is the relative pressure. So set the outlet pressure to zero and the simulation will calculate the inlet pressure based on the flow.

RAMAN6089 November 30, 2009 07:05

for compressible fluid
it good to hear from you for your consideration on incompressible fluid
but wat if we are using a compressible fluid like air, than by setting the outlet pressure to be zero is suitable.
and if not wat wud be the particular conditions suitable for such kind of problem??

ghorrocks November 30, 2009 17:34

The plenum air flow you describe does not sound like something you would need a compressible flow model for. Why are you considering compressibility?

If you do want to use a compressible flow model then yes, you need to set an absolute pressure level. In this case you simply have to put the outlet boundary somewhere you know what the pressure is.

RAMAN6089 December 1, 2009 00:17

it sound to me that providing the relative pressure to zero might solve my problem as any how in plenum the air has a every chance to escape from the plenum as there are vents, so the pressure will not rise much.......

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