[Brewster99]

Hello,

I am a very new ANSYS CFX user. I have been trying to simulate a rotational viscometer to practice my skills. I chose this case because there is an analytical solution for the velocity profile and accordingly it should be straightforward to set up the problem. However, I have been unable to get any results. Within 10-15 iterations of my simulation, I get a overflow error and the solution stops. Since this error is happening so early, I am guessing the issue is I have somehow posed the problem incorrectly. Here is my setup:

This is the geometry:



This is the fluid domain:



These are my stationary no-slip walls (the bottom and the outside wall):



These are my rotating no-slip walls and their specifications:



This surface is free slip because in reality the rotational viscometer would have a free surface:




Here is a top-down view of my mesh:



Can anyone offer any input as to where I may have made a mistake in posing the problem (or another reason for why I cannot get a solution)?

[ghorrocks]

FAQ: https://www.cfd-online.com/Wiki/Ansy...do_about_it.3F

If you want us to help in your specific case please attach your output file.

[Brewster99]

Thank you for the reply. I will post my output file shortly.

[AtoHM]

Just out of interest, are you sure you can handle 13M elements? Thats a huge simulation ...

[evcelica]

Originally Posted by AtoHM

Just out of interest, are you sure you can handle 13M elements? Thats a huge simulation ...

Right, That's a pretty large mesh for a newbie to "practice their skills on"
Perhaps some of those unstructured mesh cells do not have normal surfaces perfectly normal to the rotation surface. Or you huge mesh size ratio is causing problems. Are you using double precision?

Start with a coarser mesh, and use a structured mesh when possible, which should be easy on a geometry this simple.
What is your time step size? This could be too large as well. CFX will auto-calculate this based on inlet velocity and model size. Normally it has to be reduced for buoyancy driven flows, or when there are no inlets or outlets. I'm not sure what it calculates when there are no inlets/outlets, but there are moving walls.

[Brewster99]

Hi everyone,

Thanks for the replies. I appreciate the willingness to help me out. It turns out my internship project has completely changed and it's unlikely that I will be using the CFD software any further. Unless my project changes again, I won't need further assistance on this problem.