OPEN Boundary Conditions in CFX

acoustica · June 7, 2014, 08:50

Hello,
I am simulating Turbofan fan to capture the acoustic effects in far field. If I use OPEN BC's at the fan exit will it be acoustically reflective ? I mean the acoustic waves at the exit boundary will reflect back to main flow field ?

Thanks and regards

ghorrocks · June 9, 2014, 19:09

No, this boundary is not representative. You will have to expand it out to a large volume.

acoustica · June 9, 2014, 19:12

What do you mean by representative here ? sorry I didn't get what exactly you mean ..

ghorrocks · June 10, 2014, 07:32

If you want realistic reflections/transmittion of the sound waves at the entrance to the fan you cannot put an opening there. It will not accurately model the acoustic behaviour. You will need to expand out to the surrounding atmosphere some distance.

acoustica · June 12, 2014, 12:00

I got your point but this is not entrance boundary, this is exit boundary.
My question in general is about open boundary conditions (near boundary) are pressure reflective or not ?

ghorrocks · June 12, 2014, 18:37

Yes, it will be reflective.

June 7, 2014, 08:50	OPEN Boundary Conditions in CFX	#1
acoustica New Member Tijery Join Date: Jan 2011 Posts: 25 Rep Power: 15	Hello, I am simulating Turbofan fan to capture the acoustic effects in far field. If I use OPEN BC's at the fan exit will it be acoustically reflective ? I mean the acoustic waves at the exit boundary will reflect back to main flow field ? Thanks and regards

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June 9, 2014, 19:09		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	No, this boundary is not representative. You will have to expand it out to a large volume.

June 9, 2014, 19:12		#3
acoustica New Member Tijery Join Date: Jan 2011 Posts: 25 Rep Power: 15	What do you mean by representative here ? sorry I didn't get what exactly you mean ..

June 10, 2014, 07:32		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	If you want realistic reflections/transmittion of the sound waves at the entrance to the fan you cannot put an opening there. It will not accurately model the acoustic behaviour. You will need to expand out to the surrounding atmosphere some distance.

June 12, 2014, 12:00		#5
acoustica New Member Tijery Join Date: Jan 2011 Posts: 25 Rep Power: 15	I got your point but this is not entrance boundary, this is exit boundary. My question in general is about open boundary conditions (near boundary) are pressure reflective or not ?

June 12, 2014, 18:37		#6
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	Yes, it will be reflective.