[Transonic]

Hi, when is it more appropriate to use a Pressure far field boundary condition over a pressure outlet boundary condition please?

What does ‘pressure outlet’ actually mean too?

Thanks in advance

[LuckyTran]

Pressure outlet applies a static pressure at the outlet boundary.


Pressure far field is meant to be used as an inlet boundary condition. It is almost never appropriate to use a far field as an outlet condition unless you have a very special case.

[Transonic]

Hi LuckyTran, but why is it appropriate to have the downstream edge of the mesh boundary as an inlet though? Wouldn’t the flow exit from that edge and enter through the front?

I think I’m getting confused with how the boundary conditions actually work here?

Thanks in advance

[NickFL]

Originally Posted by Transonic

why is it appropriate to have the downstream edge of the mesh boundary as an inlet though?

Generally you do not want flow entering at downstream location since you don't know what are the conditions coming back into the domain. Often we will extend the outlet to a location where we know the flow is leaving. The pressure-far-field condition just means it is far away from the body, and going further will have no influence on the solution.

Originally Posted by Transonic

Wouldn’t the flow exit from that edge and enter through the front?

No this would only happen if you coupled the boundary conditions for a situation like a heat exchanger where you would simulate one rod of 1000s of rods.

Originally Posted by Transonic

I think I’m getting confused with how the boundary conditions actually work here?

Yes, I think you better review the documentation before you continue. Transonic flow is difficult and there are many nuances that are unique to this flow regime.

[Transonic]

Hi Nick, thanks for trying to clear that up!

Do you know any good resources on the basics of how boundary conditions work please? I’m still confused about why the pressure far field is usually used for the whole domain

I’ve been looking at the Ansys user guide, but it explains things from an experienced point of view in my opinion with formulas

Thanks in advance

[NickFL]

There are lots of websites and books out there, from theoretical to more practical. Without knowing your background it will be difficult to find a great match. But one I still refer to is at bakker.org. It is more oriented to mixing problems.

[Transonic]

Hi Nick, I’ll take a look at it.

So I’m just starting out with Ansys, been trying to learn it for upcoming projects at uni

I more want to understand what boundary conditions mean, how they work and why we use them

Thanks again

[LuckyTran]

For supersonic problems you don't need to provide anything at an outlet. For subsonic problems you need to supply a downstream static pressure condition, hence almost always it is a pressure outlet.

A far field BC is a derived form of a pressure (inlet BC) that allows the pressure to vary but limited to matching the same (upstream) characteristic (i.e. the Riemann invariant). This generally means that the static pressure at the boundary will be whatever it needs to be to match the (upstream) stagnation pressure, stagnation temperature, and mach number. If you knew what the pressure should be, you would just use a pressure inlet/outlet and assign the pressure. The issue arises from when (because people are lazy) don't know what this pressure is. Then they rely on the far field BC to do this work for them.

Simulations of airfoils has a known upstream and downstream stagnation pressure and temperature (the ambient conditions) so it popular to use a farfield BC everywhere in such scenarios. Using a farfield BC as an outlet here also creates its own problems (there is no free lunch) but can often be ignored.

So to reemphasize, you always want to use a pressure outlet condition and clamp the static pressure at the outlet, except when you can't. So if you see a case where a far field is used at an outlet you should ask why they didn't use a pressure outlet? Could they have known? Should they have known? Or are they perhaps, just bad?

[Transonic]

Hi luckyTran, thanks for that explanation!

Why is it that for supersonic flows I don’t need to specify anything at the outlet please? Is it because the pressure would change in like transonic/supersonic speeds, so that why it is right to use a pressure far field!

Thanks for the help understanding this!

[LuckyTran]

It's supersonic, which means the flow is moving faster than the downstream information can propagate upstream. Pressure disturbances travel at the sound speed.

[Transonic]

Hi LuckyTran, I’ve never thought about it like that- thank you so much for that!

[HectorRedal]

Originally Posted by Transonic

Hi LuckyTran, I’ve never thought about it like that- thank you so much for that!

Hi,


Sorry to jump into the discussion.
In regards with setting an outlet pressure for a flow travelling horizontally (x-axis), is it valid when you have gravity force acting vertically (trasverse flow)? I mean flow in the x-direction and gravity force in the y-direction.


If you consider pressure as a sum of p + p', where p' takes into account the term related to gravity (m * g * h, m = mass, g = gravity, h=height). I would only set p constant, the other term of the pressure.

I would set the pressure constant at the exit, but not constant the term that accounts for the hidrostatic pressure.



Would this be valid? Thanks.


Best regards,

[LuckyTran]

For pressure outlets (and inlets also) you are specifying p=whatever is your condition. p' does not need any specification because the properties of the gases on the boundary + the specification of g takes care of it unambiguously.

So yes it's perfectly valid if you know your stuff.

Note that the pressure that Fluent solves for and when you plot pressure is also p and p' is removed.

[svantevid]

I'm having some issues while using pressure outlet and inlet for a simulation of the centrifugal fan. I'm using the MRF method (rotation of the rotational part of the domain that encloses the rotor of the fan and 2 stationary domains as the inlet and outlet volume).

I defined 0 Pa gauge total pressure for the pressure inlet (so the ambient pressure) and 0 Pa gauge static pressure (also ambient pressure) for the pressure outlet.

The problem is that the pressure values at the outlet of the rotor are strange, as the static pressure at the outlet of the rotor cannot be negative.

Am I prescribing wrong pressure at the outlet?