CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Main CFD Forum

Total Pressure

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

Reply
 
LinkBack Thread Tools Display Modes
Old   August 8, 2013, 06:25
Arrow Total Pressure
  #1
Member
 
James
Join Date: May 2013
Posts: 82
Rep Power: 4
Tensian is on a distinguished road
Hi everyone,

I have a very stupid question (sorry I am new in CFD). I have to specify a Total pressure BC on my inlet. I know that Gauge pressure is 0 Pa on inlet and -20 Pa on outlet, but I have no information about velocity (pressure driven flow), so how can I compute 1/2*rho*v^2 for dynamic pressure?

Thanks in advance
Tensian is offline   Reply With Quote

Old   August 9, 2013, 08:26
Default
  #2
Member
 
Marco
Join Date: Jul 2013
Location: Italy
Posts: 36
Rep Power: 4
bertozzi_marco is on a distinguished road
Plase, explain better what is your domain and your bonduary condition....
bertozzi_marco is offline   Reply With Quote

Old   August 9, 2013, 09:43
Default
  #3
Member
 
James
Join Date: May 2013
Posts: 82
Rep Power: 4
Tensian is on a distinguished road
Hi Marco,

First of all, thakns for your reply. My domain is a little complicated, but imagine a channel with two inlets and an outlet. I need to specify a TotalPressure BC at inlets and a static pressure at outlet. So my question is how can I compute dynamic pressure on inlet and outlet if I donīt have any information about velocitites (P_dynamic=0.5*rho*v^2)?
In other words, if I understanding well the theory:

P_absolute=P_atmospheric+P_gauge
P_total=P_static+P_dynamic

Then P_total=P_gauge, so at inlets is enough to specify 0 Pa, but what happens with outlet? I need to obtain from P_total=-20 the value of P_static and P_dynamic, but how if I have no idea about v values at outlet?

I hope I am explaining well (and I am understanding well too...)

Thanks
Tensian is offline   Reply With Quote

Old   August 10, 2013, 04:23
Default
  #4
Member
 
Marco
Join Date: Jul 2013
Location: Italy
Posts: 36
Rep Power: 4
bertozzi_marco is on a distinguished road
why do you have information only about the pressure ?
are you immage to obtain information only with a pressure gauge
installed on your channel ?

The total pressure keep the contribution of the dynamic term only
if you have the inlet of the gauge normal to the velocity vector

if you can post the wbpj file I try to understand better....
bertozzi_marco is offline   Reply With Quote

Old   August 10, 2013, 05:42
Default
  #5
Member
 
James
Join Date: May 2013
Posts: 82
Rep Power: 4
Tensian is on a distinguished road
Marco,

I have only info about pressure at boundaries because itīs a pressure driven flow, a typical problem in fluid dynamics.

Sometimes is physically more suitable to specify this than a flow rate or a velocity inlet (because for example velocity profile is not uniform at my inlet, so if I suppose there a velocity inlet condition I am changing the physics, and for example the computing of volumetric flow rate will be altered).

I am trying to do the analysis in OpenFOAM, but I have seen that some solvers specifies a TotalPressure BC at inlet (and a pIOV condition for velocity) and static pressure (fixedValue)at oultet (and a zeroGRadient condition for velocity).

So if I want to setup the problem correctly I should be able to compute static pressure at outlet, and I think I have:

P_Total=-20, P_Total=P_static+0.5*rho*v^2 then:
-20=P_static+0.5*1.225*v^2.

There are 2 unknowns, and about velocity the only info I have is du/dn=0. How can I obtain static pressure then?

Thanks
Tensian is offline   Reply With Quote

Old   August 12, 2013, 02:14
Default
  #6
Member
 
Marco
Join Date: Jul 2013
Location: Italy
Posts: 36
Rep Power: 4
bertozzi_marco is on a distinguished road
in these situations I use to fix some values of first attempt,
after the solution you can verify other bonduary condition you know:
for example fix the value of static pressure and control the value of
another variable in another poit of your bonduary.
In every case you need an additional information about the problem.
Sorry, but I haven't a better idea...
bertozzi_marco is offline   Reply With Quote

Old   August 12, 2013, 03:56
Default
  #7
Senior Member
 
Lefteris
Join Date: Oct 2011
Location: UK
Posts: 192
Rep Power: 5
Aeronautics El. K. is on a distinguished road
So, at inlet Ptotal=0 and at outlet Ptotal=-20?
__________________
Lefteris

Aeronautics El. K. is offline   Reply With Quote

Old   August 12, 2013, 06:40
Default
  #8
New Member
 
Liam
Join Date: Aug 2013
Posts: 26
Rep Power: 4
Many is on a distinguished road
Thanks Marco for your suggestions, I īll take in count.

Lefteris, the problem is about computing static pressure from total pressure when the velocity at that boundary is unknown. Of course if the solver only needs total pressure at boundaries, these values are given (as you said), but I have read something like OF needs one TotalPressure condition in a boundary and a static pressure in another.

Thanks!
Many is offline   Reply With Quote

Reply

Thread Tools
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 On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
"Pressure Inlet" Boundary Setup Wijaya FLUENT 14 August 26, 2015 06:02
Total pressure in rel frame and total pressure Salut CFX 8 December 2, 2014 04:38
Total pressure loss Shljuki CFX 9 June 13, 2013 07:41
total pressure in CFX Atit Koonsrisuk CFX 0 January 1, 2005 06:46
Hydrostatic pressure in 2-phase flow modeling (long) DS & HB Main CFD Forum 0 January 8, 2000 16:00


All times are GMT -4. The time now is 16:11.