Measure the interface area of a VOF free surface

In a volume-of-fluid simulation, the alpha.water = 0.5 iso-surface is a good proxy for the liquid–gas interface. Its area is a useful diagnostic: it grows during breakup, collapses on coalescence. pyOFTools’ iso_surface builder + Sum gives you the number in one line.

Open the case

import os
import subprocess

import pyOFTools.patch_pybfoam  # noqa: F401
from pyOFTools import clone_example

CASE = clone_example("damBreak")
subprocess.run(
    ["./Allrun"],
    cwd=CASE,
    check=True,
    env={**os.environ},
    capture_output=True,
    text=True,
)

from pybFoam import Time, fvMesh, volScalarField

time = Time(str(CASE.parent), CASE.name)
mesh = fvMesh(time)
volScalarField.read_field(mesh, "alpha.water")

<pybFoam.pybFoam_core.volScalarField object at 0x7f46cba33a90>

Iso-surface area

iso_surface returns a WorkFlow seeded with a SurfaceDataSet whose field slot is empty. Pipe into area() to populate the field with face-area magnitudes, then Sum to reduce.

from pyOFTools.aggregators import Sum
from pyOFTools.builders import area, iso_surface

result = (iso_surface(mesh, iso_field="alpha.water", iso_value=0.5) | area() | Sum()).compute()
interface_area = result.values[0].value
print(f"interface area at t=0: {interface_area:.6g} m^2")

interface area at t=0: 0.00687285 m^2

Over time, wire this into a @Table

In-situ, you register the computation once and the framework calls it each write step:

from pyOFTools.postprocessor import PostProcessorBase
postProcess = PostProcessorBase()

@postProcess.Table("interface_area.csv")
def interface_area(mesh):
    return iso_surface(mesh, "alpha.water", 0.5) | area() | Sum()

See Wire pyOFTools into system/controlDict for the system/controlDict wiring.