# Static Temperature / Opening Temperature

 User Name Remember Me Password
 Register Blogs Members List Search Today's Posts Mark Forums Read

 LinkBack Thread Tools Search this Thread Display Modes
March 18, 2013, 05:48
Static Temperature / Opening Temperature
#1
New Member

Join Date: Sep 2012
Posts: 11
Rep Power: 12
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: 17,265 Rep Power: 136 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 and pooya_1375 like this.

 March 18, 2013, 06:41 #3 New Member   Join Date: Sep 2012 Posts: 11 Rep Power: 12 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: 17,265 Rep Power: 136 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: 12 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: 17,265 Rep Power: 136 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: 60 Rep Power: 12 Can you tell us, in reality how the flow is for your case.

 April 6, 2019, 10:17 Opening Temperature #8 Senior Member   Sasan Ghomi Join Date: Sep 2012 Location: Denmark Posts: 289 Rep Power: 13 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

 April 7, 2019, 00:38 #9 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,265 Rep Power: 136 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. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

 April 10, 2019, 01:17 #10 Senior Member   Sasan Ghomi Join Date: Sep 2012 Location: Denmark Posts: 289 Rep Power: 13 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

 April 10, 2019, 07:22 #11 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,265 Rep Power: 136 I can see no problem with this. What problem are you suggesting occurs? __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

April 10, 2019, 12:59
#12
Senior Member

Sasan Ghomi
Join Date: Sep 2012
Location: Denmark
Posts: 289
Rep Power: 13
Quote:
 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!!?

 April 10, 2019, 19:00 #13 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,265 Rep Power: 136 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. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

 Thread Tools Search this Thread Search this Thread: Advanced Search Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are Off Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post MASOUD Fluent UDF and Scheme Programming 14 December 6, 2012 14:39 tccruise Fluent UDF and Scheme Programming 2 September 14, 2012 07:08 NPU_conanxie Main CFD Forum 0 April 6, 2011 22:30 jak FLUENT 6 April 18, 2007 01:55 Zurayk CFX 4 April 5, 2007 12:39

All times are GMT -4. The time now is 03:13.

 Contact Us - CFD Online - Privacy Statement - Top