Chapter 3 — Turbulence Models and Cavitation

Figure 5.1 — Turbulence Models: k-ε, k-ω SST, Smagorinsky LES

CFDrs models three coupled physical regimes: turbulence (RANS / LES closures), multiphase flows (phases with distinct equations of state), and cavitation (liquid–vapour phase transition at low pressure). This chapter documents the integration contracts used to combine them.

Turbulence Closures

cfd-3d integrates the canonical closures through a single trait:

pub trait TurbulenceModel<T: NumericElement> {
    fn turbulent_viscosity(
        &self,
        flow: &FlowField<T>,
    ) -> Vec<KinematicViscosity<T>>;
    fn turbulent_kinetic_energy(
        &self,
        flow: &FlowField<T>,
    ) -> Vec<SpecificEnergy<T>>;
}

The public results retain their physical dimensions through Aequitas: turbulent viscosity is m²/s and turbulent kinetic energy is J/kg. Canonical closures store their dense solver state in the existing scalar arrays, then wrap values at the public metric boundary. Formula kernels and dense-field assertions explicitly extract the base scalar. This preserves Eunomia’s real-valued turbulence contract; a complex value is appropriate for a genuine phasor or Fourier field, not for a real turbulence metric, so no imaginary physical unit is introduced.

CFDrs distributes three models:

The closure returns a turbulent viscosity ν_t per cell that the momentum solver applies as an additional diffusion term.

Multiphase Coupling

For multiphase (e.g. liquid + gas), CFDrs uses a volume-of-fluid style coupling: each cell carries a phase-volume fraction α ∈ [0, 1], and mixture properties are computed by a weighted sum:

let rho_mix = rho_l * (1.0 - alpha) + rho_g * alpha;
let mu_mix  = mu_l  * (1.0 - alpha) + mu_g  * alpha;

The MomentumCoupling enum selects between co-located and staggered schemes; SSE-style momentum exchange is supported through [cfd_3d::multiphase::exchange].

Cavitation Physics

For details on phase transition models, Rayleigh–Plesset closures, and damage integration, see Cavitation: Liquid-Vapour Phase Transition.

Integration Contract

All three regimes compose through the same Integrate trait:

pub trait Integrate {
    fn step<F: FloatElement>(&mut self) -> Result<(), CfdError>;
}

The implementation multiplexes the closure choice at the cavity-call site. This makes regime switching a configuration-change, not a code rewrite.

Examples Referenced by This Chapter

Part III opens with seven example chapters:

• turbulence_models_demo — three closures compared on a turbulent channel.
• turbulence_validation_demo — closure against DNS reference data.
• turbulence_momentum_integration_demo — closure-integrated momentum solve.
• venturi_cavitation — venturi cavitation inception and collapse.
• simple_cavitation — single-bubble Rayleigh–Plesset in a constrained channel.
• cavitation_damage_simulation — long-time damage integration under turbulent inflow.
• 1d_venturi_cavitation — 1-D reduced cavitation model.

Further Reading

• cfd-3d turbulence module
• cfd-3d multiphase module
• cfd-3d cavitation module
• Geometry, Meshing, and CSG — CSG primitives feeding the cavitation geometry.