CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > ANSYS > FLUENT

Mass flow inlet and pressure outlet issue

Register Blogs Members List Search Today's Posts Mark Forums Read

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   July 12, 2011, 13:49
Default Mass flow inlet and pressure outlet issue
  #1
New Member
 
Nik
Join Date: Mar 2010
Location: Michigan
Posts: 25
Rep Power: 16
nikhil is on a distinguished road
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?
nikhil is offline   Reply With Quote

Old   February 17, 2012, 17:16
Default
  #2
Member
 
Join Date: Apr 2011
Location: US
Posts: 43
Rep Power: 15
famerfamer is on a distinguished road
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.
famerfamer is offline   Reply With Quote

Old   February 18, 2012, 08:51
Default
  #3
Senior Member
 
Join Date: Mar 2009
Location: Indiana, US
Posts: 186
Rep Power: 17
delaneyluke is on a distinguished road
Hi Nikhil
What is it you are analyzing?
Regards
Luke
delaneyluke is offline   Reply With Quote

Old   March 16, 2012, 00:17
Default
  #4
Member
 
banty
Join Date: Mar 2012
Posts: 52
Rep Power: 14
banty is on a distinguished road
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 View Post
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?
banty is offline   Reply With Quote

Old   December 11, 2013, 12:16
Default Total Pressure
  #5
MDB
New Member
 
Manuel Díaz Brito
Join Date: Jun 2013
Posts: 16
Rep Power: 13
MDB is on a distinguished road
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
MDB is offline   Reply With Quote

Old   December 11, 2013, 12:30
Default
  #6
Senior Member
 
Join Date: Jan 2010
Location: Germany
Posts: 268
Rep Power: 17
Zaktatir is on a distinguished road
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)
Zaktatir is offline   Reply With Quote

Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
mass flow in is not equal to mass flow out saii CFX 12 March 19, 2018 05:21
Compressible flow, no data at the outlet mireis FLUENT 6 September 3, 2015 02:10
Pulsatile pressure inlet with pressure outlet a.lynchy FLUENT 3 March 23, 2012 13:45
Terrible Mistake In Fluid Dynamics History Abhi Main CFD Forum 12 July 8, 2002 09:11
what the result is negatif pressure at inlet chong chee nan FLUENT 0 December 29, 2001 05:13


All times are GMT -4. The time now is 17:03.