[JulianP]

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,
Julian

[ghorrocks]

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]

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]

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]

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]

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]

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

[sasanghomi]

Dear All,

I am a bit doubtful about the Opening Boundary Condition for Temperature.
I suppose that this boundary condition uses Total Temperature if the flow is reversed and it uses Static Temperature when the flow is flowing out of the domain.

Please correct me if I am making any mistakes!!

Best Regards

[ghorrocks]

Your description is correct - if you use the default settings an opening uses a defined total temperature when the flow is going into the domain and static temperature when the flow is exiting the domain.

[sasanghomi]

Thank you Glenn Horrocks,

However, it seems a bit bizarre to me. Let me clarify it by giving an example...
Imagine you are simulating the fluid flow inside a pipe. At inlet boundary condition, Static temperature is specified and in the outlet, Opening boundary condition is chosen and Static temperature is specified by using Opening Temperature.
Don't you think that there is a problem in this example?
Both Static Temperatures are specified in both inlet and outlet boundary conditions!!!!

Best Regards

[ghorrocks]

I can see no problem with this. What problem are you suggesting occurs?

[sasanghomi]

Originally Posted by ghorrocks

I can see no problem with this. What problem are you suggesting occurs?

I think static temperature should rise because of friction. (I know that friction has a tiny role in this example)

What if we have some heat flux on the walls of the pipe? Can we use the same static temperatures for inlet and outlet!!?

[ghorrocks]

You won't get friction heating (commonly known as viscous heating) unless you turn the viscous heating option on.

You are getting confused between boundary condition definition and what you think should happen inside the domain. The boundary conditions are just that, just the conditions at the boundary. They don't define the physics of the simulation. They have to be consistent and physically possible, but if there is a temperature rise through the domain it is the domain physics which should capture this, not the boundary conditions.

Don't forget that the outlet temperature is just used when you have reverse flow. If you have forward flow then the outlet temperature is convected from the domain, which means it will be hotter than the inlet if there is heat added through the domain.

Finally - I think you are saying your outlet is actually an opening. The "outlet" boundary condition does not allow back flow and does not require a temperature to be set.