|
[Sponsors] |
February 22, 2007, 01:05 |
CFX mass flow boundary condition
|
#1 |
Guest
Posts: n/a
|
Hello,
since the beginning of the year I am working with CFX (previously I used Fluent) and I have a question concerning the mass flow BC. I set up a problem with velocity components as inlet BC and mass flow at outlet. The solution converged quite well (RMS residual->1.e-05) but the mass imbalance was 12%! By setting the same BC in Fluent the static pressure at the outlet was itterated in order to meet the mass flow and the imbalance was ok. Has anybody experienced the same problem? The imbalance (in CFX) reduced to 0.01% by setting the static pressure at the outlet, however I was confused that the previously used BC gave that bad result with regard to mass imbalance. The problem is subsonic. I calculated the flow between turbine exit and combustion chamber inlet thus no rotating domains were included. |
|
February 22, 2007, 07:49 |
Re: CFX mass flow boundary condition
|
#2 |
Guest
Posts: n/a
|
Setting a velocity inlet and a mass flow outlet is an unbounded solution, and a poor choice for boundary conditions. You could have an infinite number of pressure/density fields that could result in the solution. You should use a pressure boundary somewhere, as you discovered.
|
|
February 22, 2007, 10:37 |
Re: CFX mass flow boundary condition
|
#3 |
Guest
Posts: n/a
|
Dear Michele,
In addition to Johnny's comments, the mass flow (numerical integral) at the inlet may not necessarily be the same mass flow at the outlet; therefore, there is no way the solver can enforce continuity.. Opaque. |
|
February 22, 2007, 16:52 |
Re: CFX mass flow boundary condition
|
#4 |
Guest
Posts: n/a
|
Hello Johnny, hello Opaque,
thank you very much for your answers. Today everthing is clearer. I thought the solver would set the "reference pressure" at the inlet and change the relative pressure at the outlet to meet the desired mass flow. I think I should read a bit more carefully what is written in the help manual. To be honest I feel a bit uncomfortable for having posed such a "stupid" question. However your comments were a big help. Thanx. Michele |
|
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
Boundary condition problem for open channel flow | Andy | CFX | 9 | June 11, 2016 08:20 |
ATTENTION! Reliability problems in CFX 5.7 | Joseph | CFX | 14 | April 20, 2010 16:45 |
How can apply mass flow rate boundary condition? | Sima | Phoenics | 1 | December 1, 2007 19:55 |
Mass flow boundary condition | Renato. | Main CFD Forum | 0 | July 21, 2006 23:07 |
Mass flow boundary condition | vivian | Main CFD Forum | 0 | April 11, 2006 00:32 |