Frequently Asked Question

Begin with focused tests: a stationary interface for surface-tension balance, a single-particle settling case for drag, an advection test for volume fraction, and a simple phase-change balance. Each test isolates one mechanism before combining them.

Report interface thickness, boundedness, mass conservation, spurious currents, and sensitivity to mesh and time step.

Phase interaction

Multiphase flow requires conservation of mass and momentum for each phase together with interfacial exchange. The important scales include volume fraction, interface or particle size, relative velocity, density and viscosity ratios, surface tension, residence time, and phase-change rate. The representation must match the desired output: a resolved free surface, an averaged mixture, or dispersed particle and bubble statistics.

α is volume fraction, We is the Weber number, ρ is density, U is relative speed, L is a characteristic size, and σ is surface tension.

Worked example

With ρ = 1000 kg/m³, U = 1 m/s, L = 0.005 m, and σ = 0.072 N/m, We = 69.4. Inertia is therefore significant relative to surface tension, so interface deformation should be investigated rather than assuming a spherical phase.

Check: assess phase conservation separately, resolution, exchange time scale, coalescence or breakup assumptions, and inlet phase-fraction sensitivity.

Engineering check

For Multiphase Verification and Validation, maintain traceability from requirement to risk, design output, evidence, and approval. Record the configuration, acceptance criterion, test or analysis conditions, open actions, and residual risk. A method is not complete when the document is filled in; it is complete when the evidence supports the decision and affected controls are updated.