Proper Boundary Conditions

shreesha87 · March 27, 2014, 21:38

Hello Everyone

I am trying to simulate a problem wherein I have a duct (lets assume cylinder for the moment). The inlet of the duct is being supplied air at 50 psi (static pressure) and the inlet flow rate is 30 lpm. The outlet is open to the atmosphere.

In CFX, I am aware that I can either specify static pressure or mass flow rate at the inlet but not both. In simulation run 1, I used the mass flow inlet condition (converted 30 lpm to kg/sec) and ambient pressure at the outlet. But when I measure the static pressure at the inlet, it is not 50 psi (understandable)

Similarly, in simulation run 2, I used the static pressure as the inlet boundary condition and entered 50 psi and the outlet was ambient as usual. Now, I got a ridiculously high velocity at the inlet since ANSYS calculated a very high mass flow rate at the inlet.

So, is there any way to tell ANSYS to interpret the inlet condition so that both the 50 psi and 30 lpm vol flow is considered?

Thanks
Shree

ghorrocks · March 28, 2014, 00:55

No. It is over specifying the flow and mathematically impossible.

The issue is that your simulation is not matching the measurements. If we assume that your measurement is accurate then your simulation is inaccurate - you are not getting a 50psi drop over the duct. Now the question is an FAQ - why are my results inaccurate: http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F

shreesha87 · March 28, 2014, 12:50

Can you please explain how is it mathematically impossible and overdefining?

Shree

ghorrocks · March 29, 2014, 05:03

Let us say CFX reckons the pressure drop at a certain flow is 40psi. If you say the inlet pressure is 50psi, and the inlet flow rate is that certain value then there is no way the outlet can be 0psi (giving a 50psi pressure drop). The only way to do this is to define the inlet to have the specified mass flow rate, the outlet to have 0 psi and then CFX will solve the inlet to have 40psi.

March 27, 2014, 21:38	Proper Boundary Conditions	#1
shreesha87 New Member Join Date: Oct 2013 Posts: 13 Rep Power: 12	Hello Everyone I am trying to simulate a problem wherein I have a duct (lets assume cylinder for the moment). The inlet of the duct is being supplied air at 50 psi (static pressure) and the inlet flow rate is 30 lpm. The outlet is open to the atmosphere. In CFX, I am aware that I can either specify static pressure or mass flow rate at the inlet but not both. In simulation run 1, I used the mass flow inlet condition (converted 30 lpm to kg/sec) and ambient pressure at the outlet. But when I measure the static pressure at the inlet, it is not 50 psi (understandable) Similarly, in simulation run 2, I used the static pressure as the inlet boundary condition and entered 50 psi and the outlet was ambient as usual. Now, I got a ridiculously high velocity at the inlet since ANSYS calculated a very high mass flow rate at the inlet. So, is there any way to tell ANSYS to interpret the inlet condition so that both the 50 psi and 30 lpm vol flow is considered? Thanks Shree

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March 28, 2014, 00:55		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,697 Rep Power: 143	No. It is over specifying the flow and mathematically impossible. The issue is that your simulation is not matching the measurements. If we assume that your measurement is accurate then your simulation is inaccurate - you are not getting a 50psi drop over the duct. Now the question is an FAQ - why are my results inaccurate: http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F

March 28, 2014, 12:50		#3
shreesha87 New Member Join Date: Oct 2013 Posts: 13 Rep Power: 12	Can you please explain how is it mathematically impossible and overdefining? Shree

March 29, 2014, 05:03		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,697 Rep Power: 143	Let us say CFX reckons the pressure drop at a certain flow is 40psi. If you say the inlet pressure is 50psi, and the inlet flow rate is that certain value then there is no way the outlet can be 0psi (giving a 50psi pressure drop). The only way to do this is to define the inlet to have the specified mass flow rate, the outlet to have 0 psi and then CFX will solve the inlet to have 40psi.