# atmospheric and/or far field BC?

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 January 23, 2013, 11:29 atmospheric and/or far field BC? #1 Senior Member   Pei-Ying Hsieh Join Date: Mar 2009 Posts: 334 Rep Power: 18 Hi, I am wondering whether Autodesk Simulation CFD has atmospheric BC and/or far field BC? Thanks! Pei-Ying

 January 25, 2013, 12:53 #2 New Member   Matt Brown Join Date: Jan 2013 Posts: 17 Rep Power: 13 We typically define the outer faces of the air volume to have the freestream velocity condition with a zero pressure outlet face. I am not sure what your application is, but that has worked for us in the past.

 January 25, 2013, 14:14 #3 Senior Member   Pei-Ying Hsieh Join Date: Mar 2009 Posts: 334 Rep Power: 18 Hi, Matt, Thanks for the reply. I am quite new to Autodesk simulation CFD. May I ask how is free stream velocity defined? After selecting velocity, I do not see free stream that I can select. By free steam, do you mean 0 m/s, in the atmospherics case? Pei-Ying

 January 25, 2013, 14:16 #4 New Member   Mark Decker Join Date: Jan 2013 Location: Central Virginia Posts: 21 Rep Power: 13 As Matt indicated, the far field downstream condition is typically a P=0. The drawback is that you need to extend the downstream domain enough to where a P=0 condition would exist so as to not artificially influence the area of interest. I see that NX Flow 8.5 has added a new convective outflow condition where pressure does not have to be specified. This would allow the downstream domain to be much shorter ... their example shows it right behind a classic cylindrical vortex shedding simulation.

 January 25, 2013, 14:26 #5 New Member   Matt Brown Join Date: Jan 2013 Posts: 17 Rep Power: 13 The freestream condition really depends on what you are modeling. For example, say I wanted to model an airplane at 5 degrees angle of attack and 200 mph. To the forward face (inlet) and side faces of the air volume I would apply the following velocity BC's: Vx=200*cos(5deg) Vy=0 Vz=200*sin(5deg) (This assumes that X points down stream, y points out the right wing and z is 'up'. aka air plane body axis) I have also heard of people applying zero pressure not just to the rear face (outlet) but also to the side faces of the air volume. I have not personally tried this because I have had success setting these to the velocity BC's with only the outlet face as zero pressure.