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Pressure, mass or velocity boundary condition?

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Old   October 21, 2019, 12:15
Default Pressure, mass or velocity boundary condition?
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Abdullah Arslan
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Good day,

I have inlet condition Tin 793K, Pin 1 atm, Vin 0.1 m/s and inlet size is 3.1X1mm.

The mixture is multi-component and its density is 227g/m3. Corresponding mass flow rate is 7.0565E-8 kg/m3.

For both velocity and mass flow rate inlet (Min) conditions, velocity profile is same with <5% error. But pressure is around 1 Pa in CFX post.

Why cannot I get ~ 1atm pressure?

P.S Absolute pressure is set 1 atm (it is just reference) and in 'material' tab, reference temperature and pressure are STP (also just reference as CFX theory guide).
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Old   October 21, 2019, 13:20
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It seems you are looking for absolute pressure, correct?

What variable are you looking at in CFD-Post ? Pressure, or Absolute Pressure?

Not certain what you meant by setting Absolute Pressure to 1 atm, there is no panel to set Absolute Pressure anywhere. Did you mean to set the domain-based Reference Pressure to 1 atm?
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Old   October 22, 2019, 06:35
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Quote:
Originally Posted by Opaque View Post
It seems you are looking for absolute pressure, correct?

What variable are you looking at in CFD-Post ? Pressure, or Absolute Pressure?

Not certain what you meant by setting Absolute Pressure to 1 atm, there is no panel to set Absolute Pressure anywhere. Did you mean to set the domain-based Reference Pressure to 1 atm?
No I am not looking for absolute pressure.
I am looking for pressure (to be around 1 atm but getting 1-2 Pa just)
Yes, there is none and yes I meant domain-based reference pressure.

So what is pressure in CFX-post? gauge pressure?
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Old   October 22, 2019, 07:16
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The variable Pressure in Post is the deviation from your Reference Pressure.
If your reference pressure is 1 atm, and your Pressure says 1 Pa, then this means that the absolute Pressure is 1atm + 1 Pa
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Old   October 22, 2019, 07:36
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Quote:
Originally Posted by Gert-Jan View Post
The variable Pressure in Post is the deviation from your Reference Pressure.
If your reference pressure is 1 atm, and your Pressure says 1 Pa, then this means that the absolute Pressure is 1atm + 1 Pa
Yes, It is very clear from previous posts in forum.

My dilemma is: I am using ideal gas species at STP. So solution should give at least closer to that value. Or that is gauge pressure?

However, at inlet using velocity and not pressure.
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Old   October 22, 2019, 08:31
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Static Pressure can be considered as Gauge Pressure, provided you have a reference pressure of 1 atm

You mentioned: "My dilemma is: I am using ideal gas species at STP. So solution should give at least closer to that value."

Closer to what value? To Standard Temperature and Pressure? No. If you have a reference pressure of 1 atm, then the variable Pressure will be Static Pressure, relative to reference Pressure.

You can get the variable Pressure to be close to STP (1 atm), but then you need to set the reference pressure to vacuum = 0Pa. This is also possible. Results won't change, it is just an off set.
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Old   October 22, 2019, 08:54
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Quote:
Originally Posted by Gert-Jan View Post
Static Pressure can be considered as Gauge Pressure, provided you have a reference pressure of 1 atm

You mentioned: "My dilemma is: I am using ideal gas species at STP. So solution should give at least closer to that value."

Closer to what value? To Standard Temperature and Pressure? No. If you have a reference pressure of 1 atm, then the variable Pressure will be Static Pressure, relative to reference Pressure.

You can get the variable Pressure to be close to STP (1 atm), but then you need to set the reference pressure to vacuum = 0Pa. This is also possible. Results won't change, it is just an off set.
yes to STP or 1 atm value (though this is just reference as T will not be around 298K but what is set at inlet).

I will try 0 Reference pressure but tried before and simulation didn't start.
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Old   October 22, 2019, 08:58
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In principle, Opaque already pointed you in the right direction. The STP that you are looking for is in the variable "Absolute Pressure" = static pressure + reference Pressure
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