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atmospheric and/or far field BC?

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Old   January 23, 2013, 12:29
Default atmospheric and/or far field BC?
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Pei-Ying Hsieh
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Hi,

I am wondering whether Autodesk Simulation CFD has atmospheric BC and/or far field BC?

Thanks!

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Old   January 25, 2013, 13:53
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Matt Brown
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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.
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Old   January 25, 2013, 15:14
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Pei-Ying Hsieh
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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?

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Old   January 25, 2013, 15:16
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Mark Decker
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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.
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Old   January 25, 2013, 15:26
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Matt Brown
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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.
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