# Air ideal gas wrong density

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

 April 23, 2013, 11:49 Air ideal gas wrong density #1 Member   Sebastian Join Date: Apr 2013 Posts: 31 Rep Power: 4 Hi guys. I have a little problem concerning the air ideal gas. The gas should have a density of 1.2 kg/m^3 (calculated by hand, pressure 1 bar, T = 348.4K) Somehow Ansys always gives me a value of 2 kg/m^3 Has anyone a clue why this could be happening? Its a pretty simple case but I cant fix this problem Regards

 April 23, 2013, 12:55 #2 Senior Member   Chris DeGroot Join Date: Nov 2011 Location: Canada Posts: 387 Rep Power: 6 I have never noticed this issue. Are you using CFX?

 April 23, 2013, 16:44 #3 Senior Member   Edmund Singer P.E. Join Date: Aug 2010 Location: Minneapolis, MN Posts: 500 Rep Power: 11 Check your relative pressure. I bet you set it to 1atm and you didnt account for that in your BC's.

 April 23, 2013, 17:18 #4 Member   Sebastian Join Date: Apr 2013 Posts: 31 Rep Power: 4 Hey i checked everything, i did in fact set it to 1 bar on both domains. my setup looks like this: - 1d problem - first 50 cell ideal gas with 1 bar and ~350K - Domain interface - second 50 cells 0.1 bar and ~280K so my reference pressure is 1 bar on both sides and my initialization pressure is 1/ 0.1 bar. I dont get how the "air ideal gas" actually works.

 April 23, 2013, 17:28 #5 Senior Member   Edmund Singer P.E. Join Date: Aug 2010 Location: Minneapolis, MN Posts: 500 Rep Power: 11 Your BCs (and initialization) is relative to the 1bar relative pressure you are specifying. So, in essence, you have a 2 bar region, and the denisty is correct for that pressure. Either alter your relative pressure to 0 bar (make sure you are really specifying BCs to 0bar relative pressure) or change all your BCs and intializations to account for the 1 bar relative pressure.

 April 23, 2013, 17:33 #6 Member   Sebastian Join Date: Apr 2013 Posts: 31 Rep Power: 4 ah alright thanks I see. I thought its really referring to the pressure not adding up on it. gonna try it out tomorrow thanks for the help edit: it works with a relative pressure of 0 bar. thanks for that Last edited by Badi; April 24, 2013 at 07:23.

 May 2, 2013, 12:49 #7 Member   Sebastian Join Date: Apr 2013 Posts: 31 Rep Power: 4 Hey im having a strange problem again. Im basically still having the same setup but now with water as fluid. On my left side I have 0 bar reference and 1 bar relative pressure. On the right side I have 0 Pa reference and 5000 Pa relative pressure. When i start my transient simulation ill get strange results for for my very first timestep (0). It shows 0 Pa on the left side and -9.5e4 The range is correct but the numbers are wrong. I dont really know where the problem could come from Last edited by Badi; May 2, 2013 at 13:09.

 May 2, 2013, 18:52 #8 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 10,959 Rep Power: 85 Models often need a small timestep to start with if you are modelling the starting transient. So you need a smaller timestep - at least to begin with.

 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 Mohsin FLUENT 7 June 7, 2015 22:23 Zigainer CFX 0 July 25, 2011 11:43 yong Main CFD Forum 0 January 7, 2006 00:58 Dan Moskal Main CFD Forum 0 October 24, 2002 22:02 rafat Main CFD Forum 5 June 13, 2001 21:36

All times are GMT -4. The time now is 22:42.