overPimpleFluid

Overview

overPimpleFluid solves the transient, incompressible Navier-Stokes equations for a Newtonian single-phase fluid on an overset (Chimera) mesh using the PIMPLE pressure-velocity coupling algorithm. It is the multiPhysicsFoam port of OpenFOAM’s overPimpleDyMFoam and shares its feature set with pimpleFluid: turbulence modelling (laminar, RANS, LES), MRF and fvOptions. In addition, it masks hole cells (cellMask), tags fringe cells (interpolatedCells) and can make the predicted fluxes conservative across the overset fringe (oversetAdjustPhi). The region mesh must be a dynamicOversetFvMesh, which makes it suitable for flows around bodies that undergo large rigid-body motions, e.g. rotors or moving objects.

Governing Equations

Continuity

Momentum

Overset coupling between the mesh components is not part of the equations above: values in fringe (interpolated) cells are supplied by the overset cell-cell stencil (oversetInterpolation in fvSchemes), and the overset patch type couples the matrices of the individual mesh zones.

Overset masks

Both masks are derived from the cell types of the cell-cell stencil (Stencil::New(mesh).cellTypes()):

The face mask $\chi_f$ is obtained by localMin interpolation of $\chi_c$, i.e. a face is masked if either neighbouring cell is a hole. The flux is multiplied by $\chi_f$ after each pressure correction and after mesh motion.

Stress tensor (for FSI coupling)

As in pimpleFluid, the Cauchy stress tensor is updated after each pressure corrector:

Numerical Algorithm

usesPIMPLE() returns true, so the framework drives the region through prePredictor → momentumPredictor → pressureCorrector for each outer PIMPLE iteration (nOuterCorrectors from the top-level system/fvSolution). solveRegion, postSolve, correct and setCoupledEqns are no-ops.

1. Pre-predictor (mesh motion and masking) — executed at most once per time step (latched on the time index) unless moveMeshOuterCorrectors is enabled; meshMotionCorrector() runs the same block. If the mesh changed:
   • update MRF zones;
   • recompute cellMask, the face mask and interpolatedCells from the new overset stencil;
   • correct Uf and phi on faces that changed from hole to active (faceMaskOld), set $\phi = \mathbf{S}_f\cdot\mathbf{U}_f$ and mask hole faces;
   • make $\phi$ relative to the mesh motion;
   • optionally print the mesh Courant number (checkMeshCourantNo).
2. Momentum predictor — assemble the momentum equation (cached for the pressure corrector), relax and constrain it, and, if momentumPredictor is on, solve it with the pressure gradient masked on hole cells: $A_U\mathbf{U} - \mathbf{H} = -\chi_c\nabla p_\text{kin}$.
3. Pressure corrector (PISO loop) — for each of nCorrectors:
   • $r_{AU} = 1/A_U$, $\hat{\mathbf{U}} = r_{AU}\mathbf{H}$ (via constrainHbyA), $\phi_{\hat{U}} = \hat{\mathbf{U}}_f\cdot\mathbf{S}_f$ stored in the persistent field phiHbyA; MRF makes it relative;
   • if oversetAdjustPhi is set, phiHbyA is made relative to the mesh motion, corrected by oversetAdjustPhi(phiHbyA, U, adjustZone) and made absolute again;
   • non-orthogonal loop solving $\nabla\cdot\left(r_{AU,f}\nabla p_\text{kin}\right) = \nabla\cdot\phi_{\hat{U}}$; on the final non-orthogonal iteration $\phi = \phi_{\hat{U}} - \text{flux}(p\text{Eqn})$ and $\mathbf{U} = \chi_c\left(\hat{\mathbf{U}} - r_{AU}\,\text{reconstruct}(\text{flux}/r_{AU,f})\right)$; with oversetPatchErrOutput the overset patch flux error is reported via oversetPatchPhiErr;
   • continuity errors are evaluated with fringe and hole cells excluded ($\chi_i\chi_c\nabla\cdot\phi$);
   • Uf is reset to $\overline{\mathbf{U}}_f$ with its normal component replaced by $\phi/|\mathbf{S}_f|$;
   • $\phi$ is made relative to the mesh motion and multiplied by $\chi_f$.
4. Turbulence correction — transport model and turbulence model are corrected once per outer iteration (turbCorr).
5. Post-processing fields — p = pKin·rho and sigma are updated.

Time-step control

getMinDeltaT() re-reads the DyM controls and returns $\Delta t_\text{max} = \text{maxCo}/\text{Co}\cdot\Delta t$, where the Courant number is computed from $|\phi|$ weighted by the localMin interpolation of $\chi_c + \chi_i$ (as in overPimpleDyMFoam). The maxDeltaT cap is applied by the solver’s setDeltaT.H.

Differences to pimpleFluid

Sub-Models

Required Fields

All patches of these fields on overset boundaries must be of type overset.

Additionally created at runtime:

Case Setup

Required dictionaries

constant/transportProperties    – nu and rho
constant/turbulenceProperties   – turbulence model selection
constant/dynamicMeshDict        – dynamicOversetFvMesh + motion solver
system/fvSolution               – PIMPLE sub-dict and linear solver settings
system/fvSchemes                – discretisation schemes + oversetInterpolation
system/controlDict              – libs (overset fvMotionSolvers)

multiRegionProperties entry

regions
(
    ( fluid (overPimpleFluid) )
);

controlDict (singleRegionMode)

libs                (overset fvMotionSolvers);

singleRegionMode    yes;
regionType          overPimpleFluid;

adjustTimeStep      true;
maxCo               1;          // default 1.0
maxDeltaT           1e-3;       // default GREAT

transportProperties

transportModel  Newtonian;
nu              1e-05;
rho             1;      // used for p = pKin*rho and sigma

dynamicMeshDict

dynamicFvMesh       dynamicOversetFvMesh;

solver          multiSolidBodyMotionSolver;

multiSolidBodyMotionSolverCoeffs
{
    movingZone1
    {
        solidBodyMotionFunction rotatingMotion;
        rotatingMotionCoeffs
        {
            origin      (0.005 0.005 0.005);
            axis        (0 0 1);
            omega       100.0;
        }
    }
}

Key fvSolution settings

PIMPLE
{
    momentumPredictor        false;
    nOuterCorrectors         1;
    nCorrectors              3;
    nNonOrthogonalCorrectors 0;

    // Overset controls
    oversetAdjustPhi         false;   // default false
    adjustZone               -1;      // default -1, zone used by oversetAdjustPhi
    oversetPatchErrOutput    false;   // default false

    // Moving mesh
    checkMeshCourantNo       no;      // default false
    moveMeshOuterCorrectors  no;      // default false

    // Pressure reference (pKin without fixed-value patch)
    pKinRefPoint             (0.0001 0.0001 0.001);  // or pKinRefCell (default 0)
    pKinRefValue             0.0;                    // default 0
}

correctPhi (default: mesh.dynamic()) is read but currently has no effect.

Key fvSchemes settings

oversetInterpolation
{
    method                  fastOversetCellStencil;   // or inverseDistance, cellVolumeWeight, ...
    weightMethod            correctedCellVolumeWeight; // fastOversetCellStencil only
    allowInterpolatedDonors false;
    excludedReachablePatches ( hole );
}

oversetInterpolationSuppressed
{
    grad(pKin);
    surfaceIntegrate(phiHbyA);
}

fluxRequired
{
    default         no;
    pKin            ;
}

Mesh preparation

The overset mesh components are combined into a single mesh and zoneID is set per component, e.g. (from twoSimpleRotors/Allrun.pre):

blockMesh
topoSet
subsetMesh box -patch hole -overwrite
topoSet
restore0Dir
setFields          # writes zoneID from cellSets

Coupling

No interface type in src/regionInterfaces refers to overPimpleFluid specifically. The region registers the same fields as pimpleFluid (U, pKin, p, phi, sigma), so interfaces and partitioned BCs that look these up by name (fsiInterface via regionCoupledTraction/sigma, heatTransferInterface/defaultInterface via additional region types such as transportTemperature) can in principle be attached to it.

None of these combinations is covered by a tutorial.

Known Limitations

• Overset mesh required — the constructor calls Stencil::New(mesh()), so the region mesh must be a dynamicOversetFvMesh (and the overset library must be loaded).
• No correctPhi / pcorr solve — after mesh motion the flux is only corrected on hole ↔ active faces via the face mask; the correctPhi keyword is read but not used.
• No ddtCorr — the fvc::ddtCorr flux correction is not added to phiHbyA.
• Monolithic coupling not implemented — setCoupledEqns is a no-op.
• solveRegion, postSolve, correct are no-ops — all work happens in the PIMPLE hooks.
• Incompressible, Newtonian, single phase, no energy equation — same restrictions as pimpleFluid; pair with a temperature region type for heat transfer.
• Multi-region use untested — only a single-region tutorial exists.

Tutorials

References

• Alkfri, A., Habes, C., Marschall, H., multiRegionFoam: A Unified Multiphysics Framework for Multi-Region Coupled Continuum-Physical Problems, Engineering with Computers, 2023. DOI: 10.1007/s00366-024-01974-4