Flow analysis from a pressurised chamber to a larger chamber

WT4 · November 4, 2015, 02:21

A pressurised chamber is connected to a large chamber through a membrane disc valve. The large chamber is initially at the same pressure as the small chamber. The air in the large chamber is then evacuated. When the pressure differential between the small chamber and the large chamber reaches a certain value the membrane disc ruptures allowing flow from the small chamber to the large chamber. I need to determine the pressure drop rate in the small chamber.

In CFX I create two fluid domains. I set the reference pressure to 0 atm. Then in the initial tab, I set the relative pressure to 0.3 atm for the small chamber and 0 atm for the large chamber. However I get floating point error whenever I try to run it. It only runs when I only have a very small pressure differential.
Am I specifying the pressure differential parameters correctly. I could not find any way of specifying different pressure values for the two domains other than in the initial tab.

fresty · November 4, 2015, 10:18

You may need to post some images of your settings.. did you define any boundary conditions? Like an outlet/ opening from one chamber to the other?

(Post your query in CFX forum to get a precise response)

WT4 · November 4, 2015, 16:03

I managed to solve the problem by reducing the time step by a factor of 10.
I did not realise that there was a separate forum for each of the CFD software. Next time I will post in the CFX forum.

Many thanks for your response to my query.

fresty · November 4, 2015, 22:19

Alright, good to know. Ideally one has to keep decreasing the time step till the solution is independent of further decrements in time step or if the percentage difference in results is small enough for your intended purpose. Cheers.

November 4, 2015, 02:21	Flow analysis from a pressurised chamber to a larger chamber	#1
WT4 New Member William Join Date: Nov 2015 Posts: 2 Rep Power: 0	A pressurised chamber is connected to a large chamber through a membrane disc valve. The large chamber is initially at the same pressure as the small chamber. The air in the large chamber is then evacuated. When the pressure differential between the small chamber and the large chamber reaches a certain value the membrane disc ruptures allowing flow from the small chamber to the large chamber. I need to determine the pressure drop rate in the small chamber. In CFX I create two fluid domains. I set the reference pressure to 0 atm. Then in the initial tab, I set the relative pressure to 0.3 atm for the small chamber and 0 atm for the large chamber. However I get floating point error whenever I try to run it. It only runs when I only have a very small pressure differential. Am I specifying the pressure differential parameters correctly. I could not find any way of specifying different pressure values for the two domains other than in the initial tab.

November 4, 2015, 10:18		#2
fresty Senior Member Join Date: Aug 2014 Location: UK Posts: 213 Rep Power: 12	You may need to post some images of your settings.. did you define any boundary conditions? Like an outlet/ opening from one chamber to the other? (Post your query in CFX forum to get a precise response) Last edited by fresty; November 4, 2015 at 22:16.

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November 4, 2015, 16:03		#3
WT4 New Member William Join Date: Nov 2015 Posts: 2 Rep Power: 0	I managed to solve the problem by reducing the time step by a factor of 10. I did not realise that there was a separate forum for each of the CFD software. Next time I will post in the CFX forum. Many thanks for your response to my query.

November 4, 2015, 22:19		#4
fresty Senior Member Join Date: Aug 2014 Location: UK Posts: 213 Rep Power: 12	Alright, good to know. Ideally one has to keep decreasing the time step till the solution is independent of further decrements in time step or if the percentage difference in results is small enough for your intended purpose. Cheers.