Learning Forum

[KhyphenH]

Hi everyone,

 I am working on a Fluent simulation involving a gas phase and a liquid metal phase (laser additive manufacturing). I need to impose a heat source at the interface between the two phases, where the expression for the heat source involves the gradient of the phase volume fraction (grad(f), with f as the volume fraction of liquid metal). The heat source is added as a volumetric heat source acting on fuild bodies using the expression language in Fluent.

I have been using the "smoothedVOFgradient" variable for this purpose. However, I noticed that the value provided by "smoothedVOFgradient" in Fluent is dimensionless, while the gradient of volume fraction should have units of [1/m]. I plotted both variables in one of my models; while they indicate the interface between the two phases, the numerical values differ.

From a previous post (https://innovationspace.ansys.com/forum/forums/topic/smoothed-vof-gradient/), I understand that the difference in numerical values may be due to the smoothing process. However, I'm unclear why there is also a difference in units. I attempted to use the gradient of volume fraction directly in the heat source expression, but this caused issues during the initialization step.

To summarize:

1. Is using "smoothedVOFgradient" appropriate for computing the gradient of a phase's volume fraction?
2. Why is there a discrepancy in units?
3. Can I simply multiply "smoothedVOFgradient" by 1 [m^-1] to correct the unit issue?

Thank you for taking the time to read my problem. Any insight would be extremely appreciated!

[Rob]

You need to decide how you're applying heat to the system. Gradient or isovalue are sensible, but check you don't have any other regions meeting those criterion too. 

Not sure about the units, it could be an omission in the documentation or code notes. If adding those units allows you to use the value then good, and double check it's giving you what you expect. Expressions are fairly new, and features/functions/values are being added with nearly every update. 

[KhyphenH]

Thank you for your advice! I'll make sure that everything is checked. I am also wondering if there is any resource other than Fluent theory guide and user guide that may have the documentation of how the variables are coded?

[Rob]

There is a course covering UDFs and Expressions but that's in the subscription side of the Learning system. That's likely to show how to use a function rather than the theory. Nothing else I can share though. 

[KhyphenH]

Thanks for this information. Do I need to subscribe something to get that course? Even if that course is not directly related to my current question, I am also very interested in learning how to better use expression language.

[Rob]

It is/will be. I'm not quite sure of it's status as I was only involved in part of the discussions around the UDF course. I'm also wary of recommending buying the sub for just the one course in case it's not what you want/need.