# Static Temperature / Opening Temperature

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March 18, 2013, 05:48
Static Temperature / Opening Temperature
#1
New Member

Join Date: Sep 2012
Posts: 11
Rep Power: 6
Hello everybody,

I want to make sure I understand this correctly. When using the "Opening" BC I have two options for setting the temperature: Static Temperature and Opening Temperature.

CFX Help says:
Quote:
 Specify the Heat Transfer as for an inlet boundary condition. This value is only used for the inflow portion of the opening. You can specify either: Static Temperature Opening Temperature The opening temperature is similar to the Total Temperature, but the dynamic component is based only on the normal component of velocity.
My first concern: This means that the temperature value I set for the boundary at which I expect the fluid to flow out of the domain doesn't have any influence because it "is only used for the inflow portion of the opening"?

My second concern: I have imposed a value for Opening Pressure on the boundary at which I expect the fluid to flow into the domain. If I use the "Opening Temperature" option for this boundary - then is it practically the same as if I had used "Static Temperature" with the same value? Because I haven't set any velocity value as BC ther, so I suppose the Total Temperature is the same as the Static Temperature?

With kind regards,
Julian

 March 18, 2013, 06:16 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 12,718 Rep Power: 99 Concern 1: Yes, exactly. When the flow is outward the temperature at the boundary is convected from inside the domain. So no need to specify a boundary condition temperature. Concern 2: As the doco says, the opening pressure uses a total temperature approach, using the normal compnent of velocity. So if the normal velocity is small then this will be just about the same as the static temperature. If the normal velocity is high there will be a large difference. jthiakz likes this.

 March 18, 2013, 06:41 #3 New Member   Join Date: Sep 2012 Posts: 11 Rep Power: 6 Hello Glenn, thank you for your response. So if I don't know about the flow velocity on my Openings, it is safer to use the Static Pressure Option and let the solver determine the velocities. Am I right?

 March 18, 2013, 06:57 #4 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 12,718 Rep Power: 99 Not at all. You choose the option which best describes what you know about the boundary. That could be static T or opening depending on what is upstream.

 March 18, 2013, 07:02 #5 New Member   Join Date: Sep 2012 Posts: 11 Rep Power: 6 OK thank you, probably I didn't express myself good enough because that's what I wanted to say. I only know about the pressure and the temperature but nothing about the velocity. So it's the best to use Static Pressure and Static Temperature for my opening?

 March 19, 2013, 06:10 #6 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 12,718 Rep Power: 99 No, it is not as simple as that. If the velocity is low then the static and total pressures/temperatures are just about the same and it makes little difference. Then the choice will not matter. But if the velocity is high it will make a big difference. You are going to have to determine somehow whether static or total pressure/temperature is more appropriate. Also read the documentation on well posed boundary conditions. The choice of boundary condition also affects the numerical stability of simulations.

 March 23, 2013, 17:03 #7 Member   Thiagu Join Date: Oct 2012 Location: India Posts: 59 Rep Power: 6 Can you tell us, in reality how the flow is for your case.

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