# CFX mass flow boundary condition

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

 February 22, 2007, 01:05 CFX mass flow boundary condition #1 Michele Cagna 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 Johnny 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 opaque 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 Michele Cagna 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

 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 Andy CFX 9 June 11, 2016 07:20 Joseph CFX 14 April 20, 2010 15:45 Sima Phoenics 1 December 1, 2007 19:55 Renato. Main CFD Forum 0 July 21, 2006 22:07 vivian Main CFD Forum 0 April 10, 2006 23:32

All times are GMT -4. The time now is 13:55.