ANSYS Fluent Multiphase Modeling
#+CATEGORY: ansys
ANSYS Fluent Multiphase Modeling :: VOF, Mixture, Eulerian, and Discrete Phase Models
ANSYS Fluent provides four multiphase modeling approaches. Each targets different physical regimes. The range goes from sharp-interface free-surface flows to dense particle-laden suspensions. The key question matches OpenFOAM: what is the distance between phases relative to the mesh cell size? Use VOF for a "sharp interface within one or few cells." Use Eulerian for "closely interspersed phases." Use DPM for "sparsely dispersed particles."
VOF (Volume of Fluid) Model
The VOF model in Fluent matches the VOF in OpenFOAM's interFoam. Both solve shared momentum equations for multiple immiscible fluids. Both also solve a volume fraction equation for each fluid.
VOF Model Setup (ANSYS/Fluent)
The VOF model in Fluent is configured through: 1. Activate "Multiphase" → select "VOF" 2. Define the number of phases (up to 3 in VOF, typically 2) 3. Assign phase properties (density, viscosity, surface tension) 4. Select interface tracking method (GeoReconstruct recommended for sharp interfaces) 5. Surface tension: specify interphase surface tension coefficient
Fluent offers these interface reconstruction methods:
- GeoReconstruct: geometric reconstruction (sharp interface, most accurate for free-surface flows)
- Compressive: numerical compression of interface
- CICSAM: interface compression scheme (similar to HRIC in OpenFOAM)
For surface tension modeling:
- Continuum Surface Force (CSF) model (Brackbill et al., 1992) – same as OpenFOAM's interFoam
- Curvature is computed from the gradient of the volume fraction field
- Surface tension force acts as a body force on the momentum equation proportional to the interface normal and curvature
Mixture Model
The Mixture model treats multiphase flow as a single fluid with effective properties. The properties are volume-fraction-weighted averages of the phase properties. The code computes phase velocity relative to the mixture velocity through drift velocity corrections:
where $\mathbf{v}_{\text{drift},p}$ is the drift velocity of phase $p$, which accounts for relative velocity caused by: 1. Pressure gradient 2. Body forces (gravity, centrifugal) 3. Phase interaction (drag)
The Mixture model is:
- Less computationally expensive than the Eulerian model
- Suitable for dispersed flows where the dispersed phase volume fraction is small (< 10-15%)
- Applicable to slurry flows, bubbly flows, cyclones, hydrocyclones, and liquid-liquid separators
- Solves one set of momentum equations (mixture) + $N-1$ velocity correction equations (drift flux)
Eulerian Model
The Eulerian model treats each phase as a continuous fluid that interpenetrates the others. Each phase has its own velocity field, volume fraction, and a shared pressure field at the cell level. This is the most general and most expensive approach:
where:
- $\alpha_p` is the volume fraction of phase $p$ ($\sum\alpha_p = 1$ is enforced)
- $\mathbf{M}_p` is the interphase momentum transfer (drag, lift, virtual mass, turbulence dispersion forces)
- The pressure gradient is shared among all phases (single pressure)
Interphase momentum exchange models (same as OpenFOAM):
- Stokes drag: spherical particles, low-Re
- Schiller-Naumann: extended Stokes drag to higher Re
- Gidaspow: Ergun + Wen-Yu (for dense granular flows)
- Syamlal-O'Brien: gas-solid turbulent flows
The Eulerian model is used for:
- Fluidized beds
- Cyclones
- Sedimentation tanks
- Bubble columns (high gas hold-up)
- Gas-solid, gas-liquid, and liquid-liquid systems with multiple dispersed phases
Discrete Phase Model (DPM)
The DPM model tracks discrete particles, droplets, or bubbles through the continuous phase. It uses an Eulerian-Lagrangian approach. The continuous phase is Eulerian (Navier-Stokes). The dispersed phase is Lagrangian (particle trajectory integration).
DPM Model Setup (ANSYS/Fluent)
DPM in Fluent solves: 1. Continuous phase (Navier-Stokes, standard or multiphase) 2. Discrete phase (particle trajectory, heat transfer, mass transfer, combustion) 3. One-way coupling (particles affect continuous phase only through source terms) 4. Two-way coupling (bidirectional: continuous affects particles, particles affect continuous) 5. Four-way coupling (includes particle-particle interactions)
Particle forces acting on a discrete particle:
where $\mathbf{F}_D` is the drag force per unit mass, and $F_{\text{other}}$ includes:
- Saffman lift
- Magnus force (rotation)
- Pressure gradients
- Virtual mass force
- Gravitation/buoyancy
Particle tracking in Fluent supports:
- Constant density or temperature-dependent density
- Size distribution (Rosin-Rammler, log-normal)
- Multi-species fuel evaporation (single/distributed fuel species, wet fuel)
- Particle-particle collisions (Discrete Element Method/DEM coupling)
- Wall interaction (inertia, rebound, trap, escape, slide)
- Injection types: point, surface, sphere, cylinder, cone, fan
Comparison: Fluent Multiphase vs OpenFOAM Multiphase
| Model | ANSYS Fluent | OpenFOAM Equivalent |
| ------- | ------------- | ------------------- |
| VOF (sharp interface) | MRF → VOF model with GeoReconstruct | interFoam |
| VOF (compressive) | VOF with CICSAM or Compressive | interFoam with MULES |
| Mixture | Mixture model | interFoam / mixtureFoam (custom) |
| Eulerian-Eulerian | Eulerian model | twoPhaseEulerFoam |
| Discrete Phase (Eulerian-Lagrangian) | DPM model | DPMFoam |
| Sprays | DPM with evaporation + combustion | sprayFoam |
Summary (Multiphase)
1. VOF is the default for free-surface flows with sharp interfaces 2. Mixture model is the efficient choice for dilute dispersed phases 3. Eulerian model is the general-purpose model for dense multiphase flows 4. DPM (Lagrangian) is for sparse particle tracking 5. DPM with evaporation handles sprays and combustion 6. The choice is driven by the dispersed phase volume fraction and interphase distance
See Also