Learning Forum

[Alex Chiella]

Hi all!
I just realized that the thermal boundary condition for mass flow inlets requires total temperature. I usually just typed in the design static temperature, and it would not change a whole lot having studied slow gaseous flows.

But when I'll be studying faster, compressible flows, one hand I have Fluent asking me this piece of information, on the other hand my hard input data might be just static temperature and mass flow rate; inferring what the stagnation temperature (hence velocity, hence density) might be is not trivial.
So, my question here is not really about CFD, more like plain fluid dynamics: how would you suggest to proceed?
Would you typically do some test runs to figure out the ballpark of the stagnation temperature contribution, or would you do some pen-and-paper estimation?
Or again, do you ever happen to have total temperature as actual hard design input data?

I want to stress again that this might not really be a question about Fluent per se, as much as about your experience as engineers plugging in boundary conditions.
Of course I'd be better off just using the static temperature, but I can very much guess that the solution needs the broader instruction brought by the total one.

[Rob]

Difficult to answer, and there may be maths and engineering to consider. 

For an internal flow we'd likely know the upstream temperature & pressure at near enough rest. For an external flow, we can usually assume the fluid is at rest, but the object is moving (OK, so we move the fluid and not the object but it's all relative). 

So, sketch out the system, including what's moving where. Is the difference in static & total temperature meaningful, and will that alter the solution enough to worry about? You may need to review compressible flow theory (it's been 30 years since I last had to worry about it overly much) and also run models to determine sensitivity. 

[Alex Chiella]

Wow Rob, once again a very telling reply from you!

That's what I guessed, I have colleagues working with external flows who were hardly aware of the matter.
But it's good to know that it's OK to ask for (and work with) more substantiated input data when compressibility comes in the picture.

I will hopefully manage to check out some theory about that, for the moment I'm just happy I did not bring up a non-issue.
Thank you again Rob, always relieved when you come to help!!

[Rob]

It is potentially a non-issue, but always better to ask, and now you can review and check.

Questions stemming from a lack of knowledge/understanding are good: questions coming from not bothering to think, read or learn may get a slightly more sarcastic answer! ;)