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

Specify both velocity AND total pressure

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

Reply
 
LinkBack Thread Tools Display Modes
Old   May 27, 2010, 11:08
Default Specify both velocity AND total pressure
  #1
New Member
 
Ian Helms
Join Date: Apr 2010
Posts: 9
Rep Power: 8
OxIan is on a distinguished road
Hi all,

Does anyone know how to specify both the inlet velocity and the total pressure at the boundary inlet? None of the options seem possible. I am running a compressible flow simulation so velocity inlet BC is not appropriate. Thanks
OxIan is offline   Reply With Quote

Old   May 27, 2010, 14:43
Default
  #2
jpo
Member
 
Join Date: Apr 2009
Posts: 93
Rep Power: 9
jpo is on a distinguished road
I have the same problem. If anyone has an idea, please share ...
jpo is offline   Reply With Quote

Old   June 2, 2010, 05:24
Default
  #3
Senior Member
 
Max
Join Date: Mar 2009
Posts: 133
Rep Power: 9
coglione is on a distinguished road
Quote:
Originally Posted by OxIan View Post
Hi all,

Does anyone know how to specify both the inlet velocity and the total pressure at the boundary inlet? None of the options seem possible. I am running a compressible flow simulation so velocity inlet BC is not appropriate. Thanks
Defining a total pressure at an inlet implies that you "know" something about the velocity there (see attachment). Thus, if you give correct static pressure, Ma-Number and total-temperature at the inlet, the velocity obtained from the solution should match with reality.

cheers
[IMG]file:///tmp/moz-screenshot.jpg[/IMG][IMG]file:///tmp/moz-screenshot-1.jpg[/IMG]
Attached Images
File Type: gif img688.gif (1.2 KB, 32 views)
coglione is offline   Reply With Quote

Old   June 2, 2010, 06:21
Default
  #4
New Member
 
Ian Helms
Join Date: Apr 2010
Posts: 9
Rep Power: 8
OxIan is on a distinguished road
Hi Coglione,

Thank you for your reply, I actually realized what my problem was. The original geometry has some streamtube variation in the axial direction, and since I was initially trying to utilize a 2D simulation the absence of the area ratio was essentially creating my problem. Turns out if you properly define your problem you tend to get a better solution .

Jpo I'm sot sure what type of problem you are modeling, so I don't know if that information helps you at all, good luck.
OxIan is offline   Reply With Quote

Old   June 2, 2010, 10:01
Default
  #5
jpo
Member
 
Join Date: Apr 2009
Posts: 93
Rep Power: 9
jpo is on a distinguished road
I am trying to achieve supersonic flow in a pipe. Because the flow is compressible, I cannot set supersonic velocity at inlet. Why? Because the available boundary conditions for compressible flow (pressure-inlet, mass-flow-inlet) do not accept velocity values. They accept mass flux or pressure values.

I tried setting "large" total inlet pressure and "small" (skip the numbers here) static inlet pressure. This produces high velocity at inlet. Still, as soon as calculation starts to converge, the flow chokes and drops down to subsonic.

Tried pressure-far-field as inlet. Same story.
jpo is offline   Reply With Quote

Old   June 2, 2010, 10:10
Default
  #6
jpo
Member
 
Join Date: Apr 2009
Posts: 93
Rep Power: 9
jpo is on a distinguished road
coglione,

Thank you for suggesting the formula p_total / p_static
Same statement is made by the Fluent manual sec. 7.3.1:

"If you choose to initialize the solution based on the pressure-inlet conditions, the Supersonic/Initial Gauge Pressure will be used in conjunction with the specified stagnation pressure to compute initial values according to the isentropic relations (for compressible flow) or Bernoulli's equation (for incompressible flow)"

It seems that even if the flow is initialized as supesonic at inlet, it still reverts back to subsonic after a number of iterations. This is my problem - how to keep the flow supersonic in a pipe.

If anyone has ideas, please share ....
jpo 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
Boundary Conditions : Total Pressure or Velocity Gearb0x OpenFOAM Running, Solving & CFD 2 February 28, 2011 22:18
Variables Definition in CFX Solver 5.6 R P CFX 2 October 26, 2004 02:13
Total pressure and velocity inlet BC ff FLUENT 1 June 14, 2003 20:09
Total pressure and velocity inlet BC ff FLUENT 1 May 28, 2003 21:24
what the result is negatif pressure at inlet chong chee nan FLUENT 0 December 29, 2001 06:13


All times are GMT -4. The time now is 18:12.