Mass flow inlet and pressure outlet issue

nikhil · July 12, 2011, 13:49

Hello,

I am analzying following case in Fluent.
-> 3D
-> Transient
-> Mass flow inlet BC (Transient Mass flow, Always positive)
-> Pressure Outlet BC (Static Pressure = 0 Pa)

With the above setup, when i monitor the static pressure at the inlet, i am getting negative static pressure at inlet.
Thus there is a negative pressure at inlet and 0 Pa at outlet. Technically, there should be a reverse flow, but Fluent shows no reverse flow.

Also, as my inlet mass flow bc always applies postive value for mass flow rate, static pressure at the inlet should always be higher than the outlet pressure, shouldn't it?. But i am getting it completely opposite.

Any comments why this is happening?

famerfamer · February 17, 2012, 17:16

static pressure is not the driven force for the flow but it is the dynamic pressure. If you set outlet pressure as 0, the inlet pressure can be negative because of the pgh, the hydraulic pressure. But it depends on the difference of the hydraulic pressure and the inlet dynamic pressure.

Hopefully it helps.

delaneyluke · February 18, 2012, 08:51

Hi Nikhil
What is it you are analyzing?
Regards
Luke

banty · March 16, 2012, 00:17

Hi,

Mass flow rate allow the total pressure to vary in response to the interior solution. on the other hand the pressure inlet BC, total pressure is fixed and the mass flux varies. A mass flow inlet is used when it is more important to match a prescribed mass flow rate than to match the total pressure of the inflow stream. so, if u are more interested with pressure then using pressure inlet BC may be good idea.

Quote:

Originally Posted by nikhil

Hello,

I am analzying following case in Fluent.
-> 3D
-> Transient
-> Mass flow inlet BC (Transient Mass flow, Always positive)
-> Pressure Outlet BC (Static Pressure = 0 Pa)

With the above setup, when i monitor the static pressure at the inlet, i am getting negative static pressure at inlet.
Thus there is a negative pressure at inlet and 0 Pa at outlet. Technically, there should be a reverse flow, but Fluent shows no reverse flow.

Also, as my inlet mass flow bc always applies postive value for mass flow rate, static pressure at the inlet should always be higher than the outlet pressure, shouldn't it?. But i am getting it completely opposite.

Any comments why this is happening?

MDB · December 11, 2013, 12:16

Hi all,

I know I am joining quite late, but think that I can still contribute a bit.

You should check the total pressure (static + dynamic), and recall that the dynamic pressure is $\frac{1}{2}\,\rho\,\cdot\,{u_{\infty}}^2$ , hence whenever the mean velocity is high, the dynamic pressure is so.

With mass-flow-inlet and pressure outlet, Fluent will work out the pressure at the inlet for your flow to be possible. Verify that the mass-flow out matches the flow rate you are imposing, and should the computed pressure at the inlet not match your known pressure at the inlet, update the imposed pressure at the outlet (static and gauge) and keep your simulation running.

Hope it helps,

MDB

Zaktatir · December 11, 2013, 12:30

Manuel has said already what i usually suggest to my folks:
-Check the total pressure / Mass flow
-Check Convergence
-Check your reference pressure (mean for incomp/ zero or mean for compr)

July 12, 2011, 13:49	Mass flow inlet and pressure outlet issue	#1
nikhil New Member Nik Join Date: Mar 2010 Location: Michigan Posts: 25 Rep Power: 16	Hello, I am analzying following case in Fluent. -> 3D -> Transient -> Mass flow inlet BC (Transient Mass flow, Always positive) -> Pressure Outlet BC (Static Pressure = 0 Pa) With the above setup, when i monitor the static pressure at the inlet, i am getting negative static pressure at inlet. Thus there is a negative pressure at inlet and 0 Pa at outlet. Technically, there should be a reverse flow, but Fluent shows no reverse flow. Also, as my inlet mass flow bc always applies postive value for mass flow rate, static pressure at the inlet should always be higher than the outlet pressure, shouldn't it?. But i am getting it completely opposite. Any comments why this is happening?

December 11, 2013, 12:16	Total Pressure	#5
MDB New Member Manuel Díaz Brito Join Date: Jun 2013 Posts: 16 Rep Power: 12	Hi all, I know I am joining quite late, but think that I can still contribute a bit. You should check the total pressure (static + dynamic), and recall that the dynamic pressure is $\frac{1}{2}\,\rho\,\cdot\,{u_{\infty}}^2$ , hence whenever the mean velocity is high, the dynamic pressure is so. With mass-flow-inlet and pressure outlet, Fluent will work out the pressure at the inlet for your flow to be possible. Verify that the mass-flow out matches the flow rate you are imposing, and should the computed pressure at the inlet not match your known pressure at the inlet, update the imposed pressure at the outlet (static and gauge) and keep your simulation running. Hope it helps, MDB

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February 17, 2012, 17:16		#2
famerfamer Member Join Date: Apr 2011 Location: US Posts: 43 Rep Power: 15	static pressure is not the driven force for the flow but it is the dynamic pressure. If you set outlet pressure as 0, the inlet pressure can be negative because of the pgh, the hydraulic pressure. But it depends on the difference of the hydraulic pressure and the inlet dynamic pressure. Hopefully it helps.

February 18, 2012, 08:51		#3
delaneyluke Senior Member Join Date: Mar 2009 Location: Indiana, US Posts: 186 Rep Power: 17	Hi Nikhil What is it you are analyzing? Regards Luke

December 11, 2013, 12:30		#6
Zaktatir Senior Member Join Date: Jan 2010 Location: Germany Posts: 268 Rep Power: 17	Manuel has said already what i usually suggest to my folks: -Check the total pressure / Mass flow -Check Convergence -Check your reference pressure (mean for incomp/ zero or mean for compr)