mrhardman

As might be expected from reading Appendix A of the stella paper (https://www.sciencedirect.com/science/article/pii/S002199911930066X ) this particular numerical instability can be stabilised for zed_upwind = 1.0, see the attached input file....

@DenSto @ZedThree Have there been any modifications recently that might affect nonlinear physics?

[collisionless_kbm_explicit.zip](https://github.com/stellaGK/stella/files/13810233/collisionless_kbm_explicit.zip) [collisionless_kbm_implicit.zip](https://github.com/stellaGK/stella/files/13810234/collisionless_kbm_implicit.zip) [collisionless_kbm_driftsimplicit.zip](https://github.com/stellaGK/stella/files/13810236/collisionless_kbm_driftsimplicit.zip) [collisionless_mtm_benchmark_explicit.zip](https://github.com/stellaGK/stella/files/13811880/collisionless_mtm_benchmark_explicit.zip) [collisionless_mtm_implicit.zip](https://github.com/stellaGK/stella/files/13811881/collisionless_mtm_implicit.zip) Example input files for a KBM benchmark with the various different algorithm options, including an attempt to find a collisionless MTM with only phi and...

Brief experimentation with the input file for the implicitly treated MTM above suggests that the apparent bug with drifts_implicit =.true. is in fact caused by a bug with the stream_implicit...

Today with @GeorgiaActon we experimented with the MTM benchmark above to determine what caused the numerical instability. We found that for ky = 0.5, the instability was present with magnetic...

Looking specifically at the low ky instability without drives or drifts with the master branch, commit 4cdc5fcd34a6bd9252873e2542fcd1c8c4636dff, reveals the same instability as observed in the development branch in this PR....

> > Brief experimentation with the input file for the implicitly treated MTM above suggests that the apparent bug with drifts_implicit =.true. is in fact caused by a bug with...

@mabarnes @GeorgiaActon The last commits add the bpar contribution to the particle and heat fluxes, but the contribution to the perpendicular part of the momentum flux is not included. To...

> ### Notes on numerical instability: A further note that all of these experiments were done with `zed_upwind = 1.0`, demonstrating that even maximum first-order upwinding does not guarantee a...

An additional note: Examining the collisionless_mtm_implicit.zip example above, using `drifts_implicit = .true.` makes the example numerically unstable. This might be consistent with the fact that the implicit algorithm cannot accept...