Mass flow rate boundary condition with negative values

ashtonJ · November 25, 2014, 21:34

Dear all,
I am modelling flow in a pipe. My inlet boundary condition is mass flow rate which is initially positive and toward the mid flow cycle becomes negative and eventually becomes positive toward the end of flow cycle.
If I set the mass flow rate as the inlet boundary condition, will the flow enter the flow domain when the mass flow is negative? How is it possible to set a negative mass flow rate as inlet boundary condition in CFX-Pre?

Thanks,
AshtonJ

ghorrocks · November 26, 2014, 00:24

Have you tried it? Give it a go and see what happens.

I suspect an outflow mass flow boundary will fail as you need to define additional flow parameters (eg temperature, turbulence etc) when flow enters a domain and you cannot set this on an outlet.

But it might work for an inlet.

ashtonJ · November 26, 2014, 00:30

Actually the model is running now.
If CFX gives error, will switching the boundary condition to "opening" help?
If not, is there any other method to consider the negative mass flow rate in CFX. The negative mass flow is very common case in arteries ...

ghorrocks · November 26, 2014, 05:21

If you know the flow direction is going to reverse then yes, I would recommend using an opening.

November 25, 2014, 21:34	Mass flow rate boundary condition with negative values	#1
ashtonJ Senior Member Ashkan Javadzadegan Join Date: Sep 2010 Posts: 255 Rep Power: 15	Dear all, I am modelling flow in a pipe. My inlet boundary condition is mass flow rate which is initially positive and toward the mid flow cycle becomes negative and eventually becomes positive toward the end of flow cycle. If I set the mass flow rate as the inlet boundary condition, will the flow enter the flow domain when the mass flow is negative? How is it possible to set a negative mass flow rate as inlet boundary condition in CFX-Pre? Thanks, AshtonJ

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November 26, 2014, 00:24		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,372 Rep Power: 139	Have you tried it? Give it a go and see what happens. I suspect an outflow mass flow boundary will fail as you need to define additional flow parameters (eg temperature, turbulence etc) when flow enters a domain and you cannot set this on an outlet. But it might work for an inlet.

November 26, 2014, 00:30		#3
ashtonJ Senior Member Ashkan Javadzadegan Join Date: Sep 2010 Posts: 255 Rep Power: 15	Actually the model is running now. If CFX gives error, will switching the boundary condition to "opening" help? If not, is there any other method to consider the negative mass flow rate in CFX. The negative mass flow is very common case in arteries ...

November 26, 2014, 05:21		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,372 Rep Power: 139	If you know the flow direction is going to reverse then yes, I would recommend using an opening.