Gyrofluid Codes
Gyrofluid codes take velocity-space moments of the gyrokinetic equation and add closures that mimic kinetic effects, trading accuracy for speed.
Between fluid and kinetic
Full gyrokinetics is expensive because it resolves velocity space. Gyrofluid models reduce the cost by taking a few velocity-space moments, density, parallel velocity, parallel and perpendicular temperature, and evolving those fields instead of the full distribution. The catch is that the moment hierarchy is not closed: each moment equation contains the next-higher moment.
Gyrofluid closures approximate the unclosed terms with models that reproduce key kinetic effects, especially Landau damping and finite-Larmor-radius corrections, using carefully fitted coefficients. Done well, a gyrofluid model captures much of the linear and nonlinear behavior of gyrokinetics at a fraction of the cost.
Landau closure
The signature achievement of gyrofluid theory is the Landau-fluid closure, which represents collisionless kinetic damping through a term that mimics the phase mixing responsible for it in the true kinetic system. The coefficients are chosen to match the kinetic response function at the moments retained.
Uses and limits
Gyrofluid codes are attractive for fast parameter scans and for building intuition, and historically seeded reduced transport models. Their weakness is that closures are approximate and can fail when the true distribution is far from the assumed form, so results near marginal stability or with strong energetic populations are treated cautiously.
Design relevance
For rapid scoping of turbulence trends in the Hyperion breeder design, gyrofluid tools give quick answers that are then confirmed with full gyrokinetics where it matters. They are one tier in a hierarchy of models used in simulation before construction.
- Evolves velocity moments, not full distribution
- Closures mimic Landau damping and FLR effects
- Much faster than gyrokinetics
- Approximate; verified against kinetic runs