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Pressure Inlet Boundary Conditions

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Old   March 11, 2016, 13:19
Question Pressure Inlet Boundary Conditions
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Silvia S.
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Good day, everyone.

This is my first time writing in this forum (in any forum, as a matter of fact), so i hope i won't be at fault posting this in a new thread...

I'm learning how to use Fluent for my thesis but i'm actually stuck on (among other things...For now ) the BCs definition for my study case - admittedly because i have some difficulties understanding how pressures are treated in Fluent.
I'd be quite grateful to anyone who could explain to me what i'm getting wrong.

I'm trying to simulate a case of external aerodynamics: a helicopter rotor in hover to be precise. Since i only know the rotational speed of the blades, i'm using (beside symmetry and periodic BCs) pressure inlet and pressure outlet as upper and lower boundaries.

Now, i thought i understood the difference between gauge and operative pressure (GP= absoluteP - OP), so i initially set my operating pressure equal to an athmosferic 101325Pa and thought i would set a null gauge pressure.

My difficulties stem from the input required for the Pressure Inlet Bc, which asks for Gauge Total Pressure and Initial/Supersonic Gauge Pressure.
I did look up the definitions in the Fluent User's Guide (before coming here to disturb you ), but i'm not sure i understood them completely...

- GAUGE TOTAL PRESSURE
It says that "the total pressure value is the gauge pressure with respect to the operating pressure", with "total pressure defined as:
totalP=staticP+0.5ro*v^2"
(for incompressible fluid; otherwise the expression obviously changes to account for the Mach number).
Does this mean that i have to use staticP=operatingP=101325Pa (an equivalence that leaves me a bit perplexed...) and make an hypothesis about my freestream velocity? If yes, should i use an arbitrary low value (e.g. 3m/s), considering the flow field unperturbed far from the blade?

-INITIAL/SUPERSONIC GAUGE PRESSURE
The manual says that's another term for the static pressure, which will be necessary if the solution will be initialized based on the pressure inlet BC (exactly what i was planning to do...At least for now..). It also says to "remember that the static pressure value you enter is relative to the operating pressure"....Sssooooo....I should put it equal to what? Zero?

Truly madly deeply thank you in advance to whoever will try to help me
(I have a feeling that left on my own i'll grow old before managing to get an acceptable solution )
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Old   March 15, 2016, 02:33
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Lucky Tran
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A lot of people seem to get tripped up by the gauge pressure, surprisingly. I guess it's because they expect Fluent to eventually become inconsistent, which it doesn't.

The operating pressure is 101325 Pa by default. If you want your pressure to be this pressure anywhere (inlet, outlet, etc) then you should specify 0 Pa (again a gauge pressure). End of story. If it helps your understanding, then just specify the operating pressure to be 0 Pa and do everything in absolute units.

All pressures in fluent are gauge pressures, always!

For stagnation pressure. The local static pressure need not be equal to your operating pressure. You are considering the one specific case where they are equal and not the general case where the static pressure can take arbitrary values. Again, a lot of people get confused here for some strange reason. You get some number X for the absolute stagnation pressure, I don't care what it is. Subtract the operating pressure from X and specify that as the gauge stagnation pressure.

You should know what the stagnation pressure is, otherwise your problem hasn't been fully defined and you need to go dig up what it is.
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Old   March 16, 2016, 14:24
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Silvia S.
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Once again, thank you for your reply LuckyTran : I do think it helped me understand how to consider pressures in Fluent. Also, I decided to change the operative pressure to zero and work with only absolute pressures.

My only problem now is that I have no reference for my simulation in particular from where to dig up my stagnation pressure at inlet, so I'll need to make an "educated guess".

In your opinion, would it be all right to look at it this way?

- I put the boundaries of my external domain (the stationary zone of fluid) sufficiently far from the blade to be considered zones of free-stream flow.
(AT LEAST I hope... I have yet to find precise information about that issue in literature. At the moment I'm keeping 5 blade radii from the inlet and from the domain axis, and 18 blade radii to the outlet so as not to cut the wake)

-for a rotor in Hover, the asymptotic velocity upstream of the rotor is null

-I could therefore use a stagnation pressure equal to the static pressure at sea level (101325Pa ; same value I'm going to give to the outlet pressure)
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