Applying named expressions correctly?

UchihaMadara · June 4, 2021, 15:13

Hello all,

I wanted to ask about named expressions, as I have been told in the ansys forum that using named expressions would be better if I have to use DEFINE_PROFILE and F_PROFILE macros.

My question is

Lets say I have a circular inlet patch of a tube and I want to calculate reynolds number through this section of inlet. I have the following in my named expression

Code:

Density*VelocityMagnitude*Dia/DynamicViscosity

, where Dia is constant.

And then I use this in a friction factor formula, which goes into pressure calculation formula, that gets applied to this inlet patch.

my question is, is this applied correctly?
or it should be

Code:

Average(Density,['inlet'], weight='Area') * Average(VelocityMagnitude,['inlet'], weight='Area') * Dia  / Average(DynamicViscosity,['inlet'], weight='Area')

???

pakk · June 5, 2021, 03:06

Both expressions are Reynolds numbers. There are many ways to define a Reynolds number, and only for the simplest geometries there is an "obvious" choice.

In your case, one is a local, the other one is a global. Which one do you need? The one that was used in the friction factor calculation.

Unfortunately, this is not always specified. The best thing you can do is ask the author. Second best thing: choose what is easiest, and write in your report/article/presentation which one you used.

The good thing: it should not matter that much in the results.

June 4, 2021, 15:13	Applying named expressions correctly?	#1
UchihaMadara Member Join Date: Jan 2018 Posts: 31 Rep Power: 8	Hello all, I wanted to ask about named expressions, as I have been told in the ansys forum that using named expressions would be better if I have to use DEFINE_PROFILE and F_PROFILE macros. My question is Lets say I have a circular inlet patch of a tube and I want to calculate reynolds number through this section of inlet. I have the following in my named expression Code: DensityVelocityMagnitudeDia/DynamicViscosity , where Dia is constant. And then I use this in a friction factor formula, which goes into pressure calculation formula, that gets applied to this inlet patch. my question is, is this applied correctly? or it should be Code: Average(Density,['inlet'], weight='Area') * Average(VelocityMagnitude,['inlet'], weight='Area') * Dia / Average(DynamicViscosity,['inlet'], weight='Area') ???

June 5, 2021, 03:06		#2
pakk Senior Member Join Date: Nov 2013 Posts: 1,965 Rep Power: 26	Both expressions are Reynolds numbers. There are many ways to define a Reynolds number, and only for the simplest geometries there is an "obvious" choice. In your case, one is a local, the other one is a global. Which one do you need? The one that was used in the friction factor calculation. Unfortunately, this is not always specified. The best thing you can do is ask the author. Second best thing: choose what is easiest, and write in your report/article/presentation which one you used. The good thing: it should not matter that much in the results. __________________ "The UDF library you are trying to load (libudf) is not compiled for parallel use on the current platform" is NOT the error after compiling. It is the error after loading. To see compiler errors, look at your screen after you click "build".

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