Pressure outlet "presdist" instead of an area behind exit?

sstachu · May 8, 2020, 16:47

Hello everyone,
I'm new here and I have a question. I was wondering if you could help me.
I want to use a CONVERGE software to simulate a flow through some channel with a 5 bar difference of a pressure between inlet and outlet. Simply put, a flow from a container with high pressure, through a channel into the atmosphere. This is the simplest case, I plan on adding some other features to it later but this simple one is enough for my question. So usually I would need to simulate a big enough dump tank or an area around the outlet of the channel because of all the structures connected with the expansion of the gas while going into the atmosphere. However, I've found the 'presdist' parameter in CONVERGE. The CONVERGE Studio manual says about it:
"For steady-state simulations, you can improve convergence rates by entering a value for Presdist. This value is the distance (in m) from the boundary to the ambient pressure and dampens reflecting pressure waves in the domain."

My question is: Can I use it to save some computational power and do not model the dump tank/area behind the exit of the channel? And also, why shouldn't i use it for transient cases?

I hope I stated the problem clearly, if not please feel free to ask for specifics and many thanks in advance!

drowinsk · May 22, 2020, 08:08

Hello and thank you for your question. The pres_dist value may be used for transient or steady state simulations. However, depending on the application, the dampening effect could be much less than what you'd get from adding a plenum. Furthermore, pressure reflections may still occur at the outlet boundaries. I would suggest to try your application both ways to observe the effects on solution accuracy and computational cost. One strategy you can make use of in CONVERGE is to leave the plenum area with a relatively coarse grid so that there is a reduction of computational effort for this area. Feel free to let us know any time if you have any further questions.

Best regards,
David

sstachu · May 22, 2020, 11:18

David,
thank you for your response, I will do as you suggest and come back if I have any other questions. Thanks!

May 8, 2020, 16:47	Pressure outlet "presdist" instead of an area behind exit?	#1
sstachu New Member Stanislaw Join Date: May 2020 Posts: 2 Rep Power: 0	Hello everyone, I'm new here and I have a question. I was wondering if you could help me. I want to use a CONVERGE software to simulate a flow through some channel with a 5 bar difference of a pressure between inlet and outlet. Simply put, a flow from a container with high pressure, through a channel into the atmosphere. This is the simplest case, I plan on adding some other features to it later but this simple one is enough for my question. So usually I would need to simulate a big enough dump tank or an area around the outlet of the channel because of all the structures connected with the expansion of the gas while going into the atmosphere. However, I've found the 'presdist' parameter in CONVERGE. The CONVERGE Studio manual says about it: "For steady-state simulations, you can improve convergence rates by entering a value for Presdist. This value is the distance (in m) from the boundary to the ambient pressure and dampens reflecting pressure waves in the domain." My question is: Can I use it to save some computational power and do not model the dump tank/area behind the exit of the channel? And also, why shouldn't i use it for transient cases? I hope I stated the problem clearly, if not please feel free to ask for specifics and many thanks in advance!

May 22, 2020, 08:08		#2
drowinsk New Member David Rowinski Join Date: Dec 2015 Location: Madison, WI Posts: 13 Rep Power: 10	Hello and thank you for your question. The pres_dist value may be used for transient or steady state simulations. However, depending on the application, the dampening effect could be much less than what you'd get from adding a plenum. Furthermore, pressure reflections may still occur at the outlet boundaries. I would suggest to try your application both ways to observe the effects on solution accuracy and computational cost. One strategy you can make use of in CONVERGE is to leave the plenum area with a relatively coarse grid so that there is a reduction of computational effort for this area. Feel free to let us know any time if you have any further questions. Best regards, David __________________ David H. Rowinski, Ph.D. Principal Engineer Convergent Science, Inc. Madison, WI, USA david.rowinski@convergecfd.com www.convergecfd.com

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May 22, 2020, 11:18		#3
sstachu New Member Stanislaw Join Date: May 2020 Posts: 2 Rep Power: 0	David, thank you for your response, I will do as you suggest and come back if I have any other questions. Thanks!