Learning Forum

[carlos.lange]

My student has been developing a model of a geobag in a water channel, trying to find the critical velocity when the bag starts to move. She is using Fluent 2020R2 with overset mesh and a couple of UDF functions, one for allowing the geobag in the overset to recognize the walls of the channel and another to calculate the drag and lift forces.

After extensive testing and verification, she finds that Fluent is calculating a drag force that is 50 times smaller than expected, when compared with an airfoil of similar shape and size. As a result, the bag does not move with the flow, even at speeds 10 times larger than the known critical flow speed from experiments. The force calculated in the UDF with the undocumented Compute_Force_And_Moment macro function has the same value as the force reported in the console, so it appears not to be a problem in the macro function. The UDF is needed, so that a friction force can be later estimated and added to the force balance.

How can we get the correct force in the UDF?

(The student needs to use version 2020 R2 of Fluent, since it is the latest one compatible with the Visual Studio compiler (2017), which is needed for the compilation of the UDF.)

[Rob]

I'll raise the force question on this side. 

If the student uses 2022R2 she can use the included Clang compiler, so no need to figure out how to install Visual Studio. 

[carlos.lange]

Thank you, Rob.

The drag force problem turned out to be a mistake in our setup, which we thought we had checked, but a miscommunication allowed the mistake to slip through. Please, disregard it.

The hint about the Clang compiler was really helpful. Now the student can use our Linux servers for her simulations and benefit from significantly increased performance.

[Rob]

You're welcome. Thanks for letting us know it's not our fault (for once!). 

Clang is for Windows. Do NOT use the included compiler on Linux, someone forgot to check the OS!