atmospheric and/or far field BC?

phsieh2005 · January 23, 2013, 11:29

Hi,

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

Thanks!

Pei-Ying

matt.brown.ae · January 25, 2013, 12:53

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.

phsieh2005 · January 25, 2013, 14:14

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

marktm250xc · January 25, 2013, 14:16

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.

matt.brown.ae · January 25, 2013, 14:26

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.

January 23, 2013, 11:29	atmospheric and/or far field BC?	#1
phsieh2005 Senior Member Pei-Ying Hsieh Join Date: Mar 2009 Posts: 249 Rep Power: 7	Hi, I am wondering whether Autodesk Simulation CFD has atmospheric BC and/or far field BC? Thanks! Pei-Ying

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January 25, 2013, 12:53		#2
matt.brown.ae New Member Matt Brown Join Date: Jan 2013 Posts: 17 Rep Power: 2	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
phsieh2005 Senior Member Pei-Ying Hsieh Join Date: Mar 2009 Posts: 249 Rep Power: 7	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
marktm250xc New Member Mark Decker Join Date: Jan 2013 Location: Central Virginia Posts: 20 Rep Power: 2	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
matt.brown.ae New Member Matt Brown Join Date: Jan 2013 Posts: 17 Rep Power: 2	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=200cos(5deg) Vy=0 Vz=200sin(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.