[Nek5000-users] Significant computational time increase from 2D to 3D runs

[nek5000-users at lists.mcs.anl.gov]

Hello,

I am running a DNS cross-flow, two density fluid, turbulent duct simulation and I see a significant increase in computation time going from 2D to 3D that cannot be attributed to a simple increase in grid points. I believe I am at the small end of simulation size for Nek5000, as I am only running on average 4-500k grid points and running on 1-32 cores. Keeping all other things the same (i.e., parameters, physical set-up, etc), I’ve run a 2D simulation of 100x4 elements and a 3D simulation with 100x4x4 elements, both with 9 grid points per element and on 4 cores, and found a increase in computational time of more than 100 fold. I’ve also run 2D simulations of 200x16 elements on 4 cores (again everything else the same) and found it still ran ~100 times faster than the 3D simulations that had half the amount of grid points. This doesn’t make sense to me and implies that this issue would not be simply resolved by following the “Performance and Memory Considerations” advice in the Nek5000 Docs as it is not just about an increase in grid points. It seems to be linked to going from 2D to 3D, rather than due to an increase in grid points. 

I feel that I am missing something fundamental here. Is there something one should do when switching from 2D to 3D in Nek5000 that I am not aware of? I have searched through old email threads but could not find anything that mentions something like this.

Thanks!
- Kat