Mass accumulation in transient Simulation
 

I work on a transient simulation in CFX. It is a Taylor-Couette flow in a narrow gap between two concentric cylinders, with the inner cylinder rotating. I get the expected flow phenomena (wavy taylor vortices).
However, the total pressure at the monitoring points constantly rises. The domain imbalance for mass ist ok (0.002%) also the other imbalances. Residuals after 4 iterations per timestep are ok (Max.Residual 10e-4..10-5), only H-Energy doesn't converge (Max. Res.= 10e-2). As boundary conditions at the cylinder ends I set first translational periodicity, I also tried with symmetry or free slip walls. There is no inlet or outlet. It is a laminar flow, however, I also tried with SST turbulence once.
Whatever I tried out, total pressure /mass was rising. Can anyone tell me what is wrong?

Hi

What is the time-step you are using. Generally you should choose 5/RPM ass the timestep. And also check if you have residuals in one or two cells only. Have you checked grid dependency?


Thanks
CCTech Pune

Hi CCTech Pune

Thanks for you ideas.
The grid should be ok, I did a study about this before starting the transient simulation.
My timesteps were much smaller, I used Courant-Nr ~= 1. An increase of timestep by a factor of 5 didn't change anything to my problem. I will set up a new simulation with ~5 dt/rpm and see what happens. Why do you say 5/rpm? Is there a special theory about that?
I think, this will solve the problem. In the meanwhile I finished my simulation of turbulent Taylor vortex flow. there is still some mass accumulation, but total pressure in monitor points didn't rise. There I used much higher timesteps (~1.5/rpm).

Unfortunately, increasing the timestep didn't help. Total Pressure / mass still rose (despite the closed system). When I wanted to increase it even more, the simulation broke up with errors only few timesteps after the adaption.

ERROR #001100279 has occurred in subroutine ErrAction. |
| Message: |
| c_fpx_handler: Floating point exception: Overflow |

+--------------------------------------------------------------------+

+--------------------------------------------------------------------+
| ERROR #001100279 has occurred in subroutine ErrAction. |
| Message: |
| Stopped in routine c_fpx_handler: |


Are there any other suggestions, what I could try?

Hi

Can you please send me the complete out file to consultancy@cctech.co.in

I could not figure out what could be reason for crash.

Thanks
CCTech Pune