src-local/two-phase-tag.h

Two-phase Interfacial Flows with Interface Tagging

This file extends the standard two-phase.h setup for flows of two fluids separated by an interface. It is typically used in combination with a Navier-Stokes solver.

Overview

The interface between the fluids is tracked with a Volume-Of-Fluid method. The volume fraction in fluid 1 is \(f=1\) and \(f=0\) in fluid 2. The densities and dynamic viscosities for fluid 1 and 2 are rho1, mu1, rho2, mu2, respectively.

Interface Fields

This header declares two VOF fields:

f[]: The primary VOF field used for all physical calculations (density, viscosity, surface tension). This is the field that appears in the Navier-Stokes equations.
ftag[]: A secondary VOF field used only for tracking purposes. This field can be manipulated (e.g., filtered with tag.h to remove satellite droplets) without affecting the physical simulation.

Typical Usage

In coalescenceBubble-tag.c, the ftag field is used with connected component analysis (tag.h) to identify and track only the main bubble, filtering out small satellite droplets that may form during coalescence.

// In main():
f.sigma = 1.0;      // Physical surface tension
ftag.sigma = 0.0;   // No surface tension (tracking only)

// In logging event:
foreach() ftag[] = f[];           // Copy current interface
// ... apply tag() filtering to ftag ...
// ... compute diagnostics using ftag (filtered) ...

When to Use This vs Standard two-phase.h

Use two-phase.h for most simulations where you only need to track a single interface and don’t need satellite droplet filtering.
Use two-phase-tag.h when you need to:
- Filter out satellite droplets for cleaner diagnostics
- Track multiple disconnected regions separately
- Compute geometric measurements on only the main fluid body

VOF Field Declarations

Both fields are added to the interfaces list for proper advection and refinement handling by the VOF solver.

scalar f[], ftag[], * interfaces = {f, ftag};

#include "two-phase-generic.h"