Compute a surface integral over a patch

SurfIntegrate weights by face-area magnitude instead of cell volume. On a boundary patch, that gives you per-patch totals (mass flux, force, heat flux) as long as the per-face quantity you hand it is the right integrand.

Here we compute the area of the lowerWall patch on the damBreak case by feeding area() into Sum — same idea, simpler integrand.

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

time = Time(str(CASE.parent), CASE.name)
mesh = fvMesh(time)

Patch area via create_patch_surface + area + Sum

create_patch_surface returns a sampledSurface spanning the given patches. Wrap it in a SurfaceDataSet yourself (there’s no builder for this yet), then area() writes face-area magnitudes onto the dataset and Sum reduces them.

from pyOFTools.aggregators import Sum
from pyOFTools.builders import area
from pyOFTools.datasets import SurfaceDataSet
from pyOFTools.geometry import SampledSurfaceAdapter
from pyOFTools.surfaces import create_patch_surface
from pyOFTools.workflow import WorkFlow

patch_surface = create_patch_surface(mesh, name="lower", patches=["lowerWall"])
patch_ds = SurfaceDataSet(
    name="lowerWall",
    geometry=SampledSurfaceAdapter(patch_surface),
)

patch_area = (WorkFlow(initial_dataset=patch_ds) | area() | Sum()).compute()
print(f"lowerWall patch area = {patch_area.values[0].value:.6g} m^2")

lowerWall patch area = 0.00992797 m^2

Surface-weighted integral of a sampled field

To compute a flux-like quantity, interpolate a volume field onto the patch, then use SurfIntegrate (same as sample | Sum but weighted by face area):

from pyOFTools.aggregators import SurfIntegrate
from pyOFTools.builders import sample

flux = (
    WorkFlow(initial_dataset=patch_ds)
    | sample(mesh, "p")
    | SurfIntegrate()
).compute()

The surface variant is in Sample a field on a cutting plane where we also visualise the sampled field.