# Mass flow inlet and pressure outlet issue

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

 July 12, 2011, 13:49 Mass flow inlet and pressure outlet issue #1 New Member   Nik Join Date: Mar 2010 Location: Michigan Posts: 25 Rep Power: 7 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?

 February 17, 2012, 18:16 #2 Member   Join Date: Apr 2011 Location: US Posts: 43 Rep Power: 6 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, 09:51 #3 Senior Member   Join Date: Mar 2009 Location: Indiana, US Posts: 185 Rep Power: 8 Hi Nikhil What is it you are analyzing? Regards Luke

March 16, 2012, 01:17
#4
Member

banty
Join Date: Mar 2012
Posts: 52
Rep Power: 5
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?

 December 11, 2013, 13:16 Total Pressure #5 New Member   Manuel Díaz Brito Join Date: Jun 2013 Posts: 16 Rep Power: 4 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 , 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

 December 11, 2013, 13:30 #6 Senior Member   Join Date: Jan 2010 Location: Germany Posts: 268 Rep Power: 8 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)

 Thread Tools Display Modes Linear Mode

 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 OffTrackbacks are On Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post a.lynchy FLUENT 3 March 23, 2012 14:45 mireis FLUENT 1 July 28, 2010 05:22 saii CFX 2 September 18, 2009 08:07 Abhi Main CFD Forum 12 July 8, 2002 09:11 chong chee nan FLUENT 0 December 29, 2001 06:13

All times are GMT -4. The time now is 19:20.