Learning Forum

[eliocpereira]

How could I make a report of the mass flow rate through a surface for some conventional normal direction?

It looks like that Fluent computes the mass flow rate using a pre-defined normal of the surface. A normal of the surface can be either be of one or the reverse direction. Hence, to know the direction of the flow it would be important to know which direction is Fluent regarding. Is there any way of defining this direction? I am not finding any option for that in the Surface Report Definition dialog box.

[Rob]

No, it's based on the normal, and you may find the normals aren't aligned on internal surfaces. There's a "proper" way to fix this, but turning the internal into a wall and then back to internal (interior) forces the normal alignment.

[eliocpereira]

So, the normal of a internal surface in Fluent may have not the correct orientation, that is, the normal may be misaligned as you mention? Will your suggestion allow me to define the signal of the normal (either positive or negative). Note that when I was talking about "direction" in the original post I was really referring to the signal. I already assumed that the normal had the correct orientation.

[Rob]

The interior boundary doesn't store a normal so the normal is created in Fluent - not all facets on the boundary necessarily point in the same direction. Hence the interior-wall-interior trick.
If you want the net mass flow then either align the normals or use a Custom Field Function of the sum of velocity component*cell area*density

[eliocpereira]

Thanks for the suggestion. The Custom Field Function way was what I then ended doing. Using the interior-wall-interior trick may make the normals in each facet to have the same direction but I guess that they could be either all positive or negative and we cannot decide which one, right?

[Amine Ben Hadj Ali]

Yes but the trick makes it consistent.

[eliocpereira]

I get it. But it is still important to know the normal of the surface. If we don't know the normal, we can't tell through the value of the mass flow rate if the flow is in the direct or reverse direction.

[Amine Ben Hadj Ali]

Area vector is not stored and not available for post processing on interior bcs. One can reconstruct the area vector per UDF. If you look into customization manual you will see that you can access only pressure and flux per UDF without any additional functions. We will make all this accessible directly in the future.

[eliocpereira]

Ok. Thanks for the clarification.