How far to extend boundary

MachZero · June 8, 2013, 09:11

Hello All,

I am simulating a centrifugal fan to try and predict a fan curve. An experiment was done, so I have built my geometry to try and match the experiment. At the exit of the fan, a 2" tube is connected as the exhaust. This tube exhausts into atmosphere. Since my flow is still incompressible, I ended the domain at the end of this tube and called it a pressure outlet at 0Pa.

One thing that I always struggle to figure out is, should I extend this outlet further? Should I model the room that it is in? If so, how far do I need to extend. If I extend the domain and call all the extended walls pressure outlets, then I get reversed flow continuously, which slows the simulation down tremendously. How are these types of things handled by the pros out there?

Any information or examples would be awesome.

sina.sh · June 8, 2013, 23:49

Dear MachZero
you should extend your region until the effects of up wind boundary condition on down wind BC be negligible. its Try and Error!

Regards

MachZero · June 9, 2013, 09:16

Thank you for your response. Do you have any recommendations on how to check for this? What kind of approach to take? For example, should I extend it until there is almost no pressure gradient at the exit? Should the extension be just an extended tube or should it be a large open section representing a room. I look forward to hearing your thoughts. Thanks!

sina.sh · June 9, 2013, 23:46

Yes. For best solution, it should be larger than tube for example: large box. plz plot a variable (for example Uvelocity) in your main domain and each time -when you change extended domain- check deviation of variable from last one. if no change had been seen so size of extended domain is ok.

June 8, 2013, 09:11	How far to extend boundary	#1
MachZero Member Join Date: Aug 2011 Posts: 89 Rep Power: 14	Hello All, I am simulating a centrifugal fan to try and predict a fan curve. An experiment was done, so I have built my geometry to try and match the experiment. At the exit of the fan, a 2" tube is connected as the exhaust. This tube exhausts into atmosphere. Since my flow is still incompressible, I ended the domain at the end of this tube and called it a pressure outlet at 0Pa. One thing that I always struggle to figure out is, should I extend this outlet further? Should I model the room that it is in? If so, how far do I need to extend. If I extend the domain and call all the extended walls pressure outlets, then I get reversed flow continuously, which slows the simulation down tremendously. How are these types of things handled by the pros out there? Any information or examples would be awesome.

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June 8, 2013, 23:49		#2
sina.sh New Member sina Join Date: Sep 2011 Posts: 28 Rep Power: 14	Dear MachZero you should extend your region until the effects of up wind boundary condition on down wind BC be negligible. its Try and Error! Regards

June 9, 2013, 09:16		#3
MachZero Member Join Date: Aug 2011 Posts: 89 Rep Power: 14	Thank you for your response. Do you have any recommendations on how to check for this? What kind of approach to take? For example, should I extend it until there is almost no pressure gradient at the exit? Should the extension be just an extended tube or should it be a large open section representing a room. I look forward to hearing your thoughts. Thanks!

June 9, 2013, 23:46		#4
sina.sh New Member sina Join Date: Sep 2011 Posts: 28 Rep Power: 14	Yes. For best solution, it should be larger than tube for example: large box. plz plot a variable (for example Uvelocity) in your main domain and each time -when you change extended domain- check deviation of variable from last one. if no change had been seen so size of extended domain is ok.