How to simulate valve using pressure BC??

Luk_Fiz · August 16, 2010, 12:46

Hi,
I am trying to simulate the following situation during 1 second transient run:
The domain is a pipe with inlet and outlet:
a) between 0[s] and 0.5 [s] there is given mass flow in the inlet. In the same time the outlet is closed by the valve (no mass flow). In this time the domain is "pumped up", with growing gas pressure.
b) between 0.5[s] and 0.8[s] both inlet and outlet are closed.
c) between 0.8[s] and 1[s] the outlet valve is opened giving free outlet (zero pressure).
I want to use compressible air.

And now: inlet is simple: MassFlow with step function. But I have no idea what to give to the outlet. I was trying to set up pressure equal to inlet but anyway there is mass flow through the outlet (because inlet mass flow sets up velocity).

Any ideas?
Big thanks,
Luk

serezhkin · August 16, 2010, 15:01

Imho, you can split it into 2 runs:
1) 0-0.8s, wall BC on the outlet
2) 0.8-1s, pressure BC, using ouput file from run 1 as IC for run 2.

Luk_Fiz · August 16, 2010, 15:29

Well, I agree but You know...

))

serezhkin · August 16, 2010, 15:46

I guess another way of doing it is to set pressure BC side as outlet and specify a very high pressure for 0-0.8s. When ANSYS detects a backflow from the outlet, it puts a virtual wall over it to prevent it, and gives you a warning message. This way, you will have a virtual wall over the outlet for 0-0.8s.

Pretty sure this is not what ANSYS programmers had in mind when they implemented this, though

ghorrocks · August 16, 2010, 19:27

Your high pressure outlet idea might work, but it also might be dreadfully numerically unstable. As you say, it was never intended to do this so who knows what might happen. Splitting the run into two and changing the outlet BC seems like the best approach to me. Using the new multi-definition run thing (V12) I think you can do this in the same def file so it is easy to implement.

I assume these flow regimes come from opening and closing valves. Are you interested in the flow near the valves? In this case you should model the valve opening and closing and that will give you your boundaries and there is no need to restart the run.

August 16, 2010, 12:46	How to simulate valve using pressure BC??	#1
Luk_Fiz Member Lukasz Join Date: Mar 2009 Posts: 67 Rep Power: 17	Hi, I am trying to simulate the following situation during 1 second transient run: The domain is a pipe with inlet and outlet: a) between 0[s] and 0.5 [s] there is given mass flow in the inlet. In the same time the outlet is closed by the valve (no mass flow). In this time the domain is "pumped up", with growing gas pressure. b) between 0.5[s] and 0.8[s] both inlet and outlet are closed. c) between 0.8[s] and 1[s] the outlet valve is opened giving free outlet (zero pressure). I want to use compressible air. And now: inlet is simple: MassFlow with step function. But I have no idea what to give to the outlet. I was trying to set up pressure equal to inlet but anyway there is mass flow through the outlet (because inlet mass flow sets up velocity). Any ideas? Big thanks, Luk

August 16, 2010, 15:29		#3
Luk_Fiz Member Lukasz Join Date: Mar 2009 Posts: 67 Rep Power: 17	Well, I agree but You know...))

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August 16, 2010, 15:01		#2
serezhkin New Member Join Date: Jun 2010 Posts: 20 Rep Power: 16	Imho, you can split it into 2 runs: 1) 0-0.8s, wall BC on the outlet 2) 0.8-1s, pressure BC, using ouput file from run 1 as IC for run 2.

August 16, 2010, 15:46		#4
serezhkin New Member Join Date: Jun 2010 Posts: 20 Rep Power: 16	I guess another way of doing it is to set pressure BC side as outlet and specify a very high pressure for 0-0.8s. When ANSYS detects a backflow from the outlet, it puts a virtual wall over it to prevent it, and gives you a warning message. This way, you will have a virtual wall over the outlet for 0-0.8s. Pretty sure this is not what ANSYS programmers had in mind when they implemented this, though

August 16, 2010, 19:27		#5
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	Your high pressure outlet idea might work, but it also might be dreadfully numerically unstable. As you say, it was never intended to do this so who knows what might happen. Splitting the run into two and changing the outlet BC seems like the best approach to me. Using the new multi-definition run thing (V12) I think you can do this in the same def file so it is easy to implement. I assume these flow regimes come from opening and closing valves. Are you interested in the flow near the valves? In this case you should model the valve opening and closing and that will give you your boundaries and there is no need to restart the run.