poroMechanicalLaw

This page documents the linear-geometry wrapper that obtains effective stress from another run-time selectable mechanical law and subtracts a scaled pore pressure. The runtime type is:

poroMechanicalLaw

User Guide

What it computes

The law asks the nested mechanical law to update the effective stress and then forms the total stress as

sigma = sigmaEff - b*(p + p0)*I

where b is the Biot coefficient. For a face-centred stress field, the current pressure and initial pressure are interpolated to the faces:

pf    = interpolate(p)
p0f   = interpolate(p0)
sigma = sigmaEfff - b*(pf + p0f)*I

On the first call, the effective-stress field is initialised by adding the pore term to the supplied total stress. Subsequent calls retain that field and pass it to the nested law.

The law implements correct(volSymmTensorField&) and correct(surfaceSymmTensorField&). The point-centred overload is inherited from mechanicalLaw and aborts with notImplemented. The CompactListList quadrature-point overload is also inherited and aborts with notImplemented; that overload is compiled only when FOAMEXTEND is not defined. The nested effective-stress law must itself implement whichever of the cell- or face-centred overloads the selected solid model calls.

Model options

The nested dictionary must contain its own type and all entries required by that law. Density belongs to the outer dictionary because rho() reads it from the wrapper's dictionary.

Pressure lookup first checks pressureFieldRegion. If that registry does not exist, the law falls back to the solid registry. It does not fall back when the selected registry exists but does not contain pressureFieldName.

The outer solvePressureEqn and pressureSmoothingScaleFactor entries are read by the base constructor, but this wrapper never calls updateSigmaHyd(), so they do not alter its pore-pressure term. A nested law may read and use its own copies of those entries.

Recommended dictionary setup

The following setup represents a saturated soil with an isotropic linear elastic effective-stress response:

planeStress     no;

mechanical
(
    saturatedSoil
    {
        type            poroMechanicalLaw;
        rho             rho [1 -3 0 0 0 0 0] 2000;

        effectiveStressMechanicalLaw
        {
            type        linearElastic;
            E           E [1 -1 -2 0 0 0 0] 20e6;
            nu          nu [0 0 0 0 0 0 0] 0.3;
        }

        // Optional
        // biotCoeff          biotCoeff [0 0 0 0 0 0 0] 1.0;
        // pressureFieldName  p;
        // pressureFieldRegion region0;
        // p0                 p0 [1 -1 -2 0 0 0 0] 0;
    }
);

Field glossary

• sigmaEff: cell-centred effective stress, created on the first cell-centred update and written automatically.
• sigmaEfff: face-centred effective stress, created on the first face-centred update and written automatically.
• p: externally maintained cell-centred pressure field; its actual name is selected by pressureFieldName.
• p0: uniform cell-centred initial pore pressure, held by the law and neither read nor written as a field.
• p0f: demand-created face interpolation of p0.
• biotCoeff: temporary uniform cell field returned by biotCoeff() and not written.

Developer Notes

Class role

poroMechanicalLaw derives directly from mechanicalLaw and is registered in the linGeomMechLaw selection table. It owns the nested law through an autoPtr<mechanicalLaw>, keeps demand-created cell and face effective-stress fields, and stores the Biot coefficient, pressure field name, pressure region, initial pressure field and a pointer to its face interpolation.

The class contains no FOAMEXTEND preprocessor guards. Its source appears in both Make/files.openfoam and Make/files.foamextend. The inherited quadrature-point interface is absent from foam-extend because that interface is guarded by #ifndef FOAMEXTEND in mechanicalLaw.

Construction

The mechanicalLaw constructor first reads solvePressureEqn and pressureSmoothingScaleFactor, then uses regionName or detects solid or region0. It emits a fatal error if no solid region can be found.

The wrapper then requires the effectiveStressMechanicalLaw sub-dictionary, reads its type, and constructs that linear-geometry law. It next reads biotCoeff, pressureFieldName, pressureFieldRegion and p0, in that order, using the defaults listed above. A non-zero p0 produces an informational message. The wrapper itself emits no construction warning. Missing required dictionary entries cause dictionary lookup errors, and the nested law can emit its own validation errors and warnings.

At stress correction, lookup aborts if the pressure field cannot be found in the configured region or in solid. The demand-driven p0f constructor also contains a defensive fatal error if its pointer is already set.

Key methods

• impK() delegates directly to the nested effective-stress law. It is the diffusivity of the solid model's implicit Laplacian term and affects the outer-iteration convergence rate rather than the converged answer.
• correct(volSymmTensorField&) looks up p, creates sigmaEff if needed, updates it with the nested law, and subtracts the cell-centred pore term.
• correct(surfaceSymmTensorField&) interpolates p, creates sigmaEfff if needed, updates it with the nested law, and subtracts the face-centred pore term.
• biotCoeff() returns a uniform cell-centred field containing b.
• materialTangent() is not overridden or delegated. The inherited implementation aborts with notImplemented.

Extension points

A related linear-geometry wrapper can copy this class, replace the added stress contribution, and retain delegation of impK() and stress correction to a nested law. Any additional dictionary entries should be read in the constructor. Support for point or quadrature-point solid models requires explicit overloads that also delegate to compatible nested-law interfaces. The new class must be registered in the appropriate mechanical-law selection table and added to the OpenFOAM and foam-extend build lists where supported.

The source is at poroMechanicalLaw.C.

Tutorials

• solids/poroelasticity/stripFooting
• solids/poroelasticity/suctionCaission
• solids/poroelasticity/rodAndSeabed