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JulianP March 18, 2013 05:48

Static Temperature / Opening Temperature
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:

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,

ghorrocks March 18, 2013 06:16

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.

JulianP March 18, 2013 06:41

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?

ghorrocks March 18, 2013 06:57

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.

JulianP March 18, 2013 07:02

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?

ghorrocks March 19, 2013 06:10

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.

jthiakz March 23, 2013 17:03

Can you tell us, in reality how the flow is for your case.

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