Pressure and Velocity boundary conditions
I've presented my last results last week for some teachers and one of them has made me a question which I cannot answer.
I've been simulating a flow in a straight pipe (100 D) and in the same straight pipe with one and two bends upstream of it. I've been using ANSYS CFX v.13. Boundary conditions have been inlet uniform velocity and outlet average static pressure to all cases.
I've been questioned about what would be the boundary conditions for pressure at the inlet and velocity at the outlet. This teacher has experience in CFD but is not used to CFX modeling. He defends the idea that any mathematical condition for both velocity and pressure is necessary at all domain boundaries. He also said that, probably, CFX has already the condition of full developed flow at the outlet. Once the two bend case simulated has not presented fully developed flow at the outlet he questioned if thah result was correct.
Could anyone of you guys tell me if he is right or wrong and how CFX treats boundary conditions?
Do I have to extend the outlet condition downstream in order to find fully developed condition?
If only inlet velocity and outlet pressure conditions are necessary, how it works on CFX? How the solver finds and treats pressure and velocity at inlet and outlet, respectively.
Ramon Silva Martins
P.S.: Sorry if it's already been discussed. I couldn't find.
The flow boundaries which are implemented in CFX are fundamentally of two types - velocity and pressure. Which ever is defined, the solver works out the other. CFX does not give you the option to specify both at a single boundary. But read the documentation - when you specify pressure it applies a zero normal gradient constraint on the momentum so you are putting constraints on both pressure and velocity.
If you want the fully developed condition then it sounds like you need to go further downstream. But you can also do this using a periodic pair with a defined flow rate, so then you can get the fully developed flow with a short domain.
Hello Glenn. Thank you very much for your answer.
I could not find any information about the constraint on the momentum equation when average pressure is specified at the outlet in the documentation. Could you point me that?
Still, since we started the discussion, I would like to reformulate my question. Actually, I would like to know if it is OK if the domain outlet is located where fully developed flow is not achieved yet? How does CFX treat or calculate it then? What are the mathematical conditions to the momentum equation at this boundary?
I'm still searching and studying to try to understand that...
I appreciate your attention again.
Ramon Silva Martins
The outlet condition (in pressure mode) simply applies the specified pressure and zero normal gradients on most other variables. This is accurate in fully developed flow but does cause an issue where the flow is not fully developed (ie the normal gradients are not zero). But having said that for most engineering calculations this is a minor consideration and the accuracy loss is insignificant.
This is all described in the documentation.
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