[WanderleyGrespan]

Hello,

Can anyone tell me the difference between the two boundary conditions; the STATIC PRESSURE boundary condition and the OPENING PRESSURE boundary condition.

I have tried to understand them from the explaination in CFX HELP but couldnt get the idea clear. Given examples for both cases will make it easier for me to understand it. Thanx alot for your help and time. Best Regards

[ghorrocks]

Static pressure defines simply the pressure variable.

Opening pressure uses static pressure when the flow is leaving the domain but total pressure when it is entering it. To explain the important of this: If the flow starts at rest and is isentropically accelerated to some velocity (and there are no losses), its static pressure will be reduced by the Bernoulli effect but its total pressure will be the same as the original resting pressure. If the velocity changes the static pressure changes but the total pressure remains the same.

So using total pressure for flows entering the domain is often a good idea as it means that if the velocity at the boundary changes then the corresponding pressure change is automatically handled.

[WanderleyGrespan]

Ghorrocks, thanks for the reply.

But maybe you can help me with the problem itself. Its a problema very simple, its just a pipe transporting Natural Gas to the atmosfere.

Here is the question, i have a pipe in T form, 2" diameter.

At inlet (point 1), i know the value for the manometric pressure for the Natural GAS, and the two outlets(points 2 and 3) goes to the atmosfere.

So, whats the best choise for my BC??

[ghorrocks]

It depends on the geometry. Please post an image of the geometry.

[WanderleyGrespan]

That's the problem. Very simple.

[ghorrocks]

The image is not attached. See FAQ: https://www.cfd-online.com/Wiki/Ansy...n_the_forum.3F

[WanderleyGrespan]

post.jpg.jpg

let me know if you are able to see the image now.

[WanderleyGrespan]

My goal is to find out what is the mass flow out, and how is the curve Manometric Pressure x Mass Flow.

[ghorrocks]

The exits should be outlets with zero relative pressure. If you know the manometer pressure at 1 then it sounds like you know the static pressure. If that is the case then point 1 is a static pressure boundary (not the default opening). But be aware that this boundary condition configuration is not very numerically stable and you may have convergence difficulties (see the documentation on this, advice for obtaining convergence).

[WanderleyGrespan]

Yes, i know that is not a robust case.

But here is my final question.
if i choose TOTAL PRESSURE in point 1, and use the valur of manometric pressure. is that a wrong BC??

thank you very much for the answers

[ghorrocks]

Your manometer probably measured static pressure. So if you apply that pressure as a total pressure your pressure will be wrong by the dynamic head component. Whether that is significant of not will depend on your exact configuration. So it may or may not be important.

[WanderleyGrespan]

Thank you very much.

[Antanas]

Set expected mass flow rate at 1. Then vary it to get average relative static pressure at 1 close to your manometer pressure.

[WanderleyGrespan]

I'll try this way too. Thanks for the tip!