Fluid-solid coupling schemes: fluidSolidInterfaces

Fluid-solid coupling schemes are implemented in solids4foam via the fluidSolidInterface base class and derived runtime-selectable fluid-solid interface coupling schemes.

The base class is implemented in fluidSolidInterface/fluidSolidInterface.{H,C}. It manages the fluid and solid models, interface mapping, residual evaluation, outer-correction controls, and shared coupling data.

Available Schemes

The following schemes are available through the fluidSolidInterface entry in constant/fsiProperties.

weakCoupling

Weak Dirichlet-Neumann coupling without outer FSI correctors in each time step. This is the cheapest option, but it is also the least robust for strongly coupled problems.

Implementation: weakCouplingInterface/weakCouplingInterface.{H,C}

fixedRelaxation

Strong Dirichlet-Neumann coupling with a fixed under-relaxation factor. This is a simple strong-coupling scheme and is often used as a baseline or for mildly coupled problems.

Implementation: fixedRelaxationCouplingInterface/fixedRelaxationCouplingInterface.{H,C}

Aitken

Strong Dirichlet-Neumann coupling with Aitken dynamic under-relaxation. This improves on fixed relaxation by adapting the relaxation factor during the outer iterations.

Implementation: AitkenCouplingInterface/AitkenCouplingInterface.{H,C}

IQNILS

Strong Dirichlet-Neumann coupling accelerated with the interface quasi-Newton inverse least-squares (IQN-ILS) method. This scheme reuses secant information from previous coupling iterations and time steps to improve convergence for challenging FSI problems. The implementation includes filtering of repeated or near-linearly-dependent secant modes before the least-squares solve to improve numerical robustness.

Common IQNILS controls in constant/fsiProperties include:

  • relaxationFactor: startup relaxation used before enough secant information is available.
  • couplingReuse: number of previous time steps whose secant information is retained.
  • minSignificant: absolute filtering tolerance for repeated or near-dependent modes.
  • relMinSignificant: optional relative QR2-style filtering tolerance based on the fraction of new information in each older secant mode.
  • normalizeCouplingColumns: optionally normalize each V/W secant column pair before the QR solve to improve the conditioning of the least-squares system.
  • residualSumPreconditioning: optionally scale each V column by the inverse of the current residual magnitude before the QR solve, mimicking preCICE's residual-sum approach.
  • residualPreconditioningEpsilon: regularization added to the residual magnitude when computing the above scaling weight.
  • reusePreviousStepModes: optionally reuse the previous time step's cached secants only in the first IQN-ILS solve of the new time step. If the immediately previous time step converged in one iteration and generated no new secants, the implementation falls back to the latest non-empty cached time step instead, which is closer to preCICE's backup LS-system behavior.
  • maxReuseUpdateNormRatio: optional safeguard that caps the norm of that first reused IQN-ILS correction relative to the current interface residual.
  • combinedCouplingSystem: optionally assemble one aligned least-squares system across all coupled interfaces instead of solving one interface at a time. On single-interface cases this should reproduce the per-interface result; it is mainly a structural step toward a more preCICE-like assembly. This can also be combined with preciceStyleCouplingQR.
  • requireAllResidualMeasures: optionally require all normalized residual measures in the shared FSI convergence check to satisfy the tolerance. Legacy behavior uses the minimum of the available residual measures, which can stop earlier when only one measure has decayed.
  • qrSolveTolerance: relative cutoff used to ignore nearly singular directions during the triangular back-substitution.
  • reorthogonalizeCouplingColumns: optionally apply a second Gram-Schmidt-style correction while assembling the QR system.
  • predictSolid: optionally solve the solid once before the outer FSI loop.

Implementation: IQNILSCouplingInterface/IQNILSCouplingInterface.{H,C}

oneWayCoupling

Pseudo-coupling scheme for one-way FSI. The fluid solution is assumed to be known already, and the fluid fields are read from a pre-run fluid case and applied to the solid.

Implementation: oneWayCouplingInterface/oneWayCouplingInterface.{H,C}

thermal

Strong thermal coupling interface with fixed under-relaxation. This extends the mechanical coupling infrastructure to temperature and heat-flux transfer and can optionally include mechanical coupling as well.

Implementation: thermalCouplingInterface/thermalCouplingInterface.{H,C}

Notes

  • The partitioned schemes share the common fluidSolidInterface machinery for transferring tractions and displacements across the interface.
  • Different schemes have different stability and cost trade-offs. In general, weakCoupling is the cheapest, fixedRelaxation and Aitken are simple strong-coupling options, and IQNILS is the most sophisticated partitioned acceleration scheme in this directory.
  • For IQNILS, increasing couplingReuse can improve convergence, but it is not guaranteed to make a case monotonically more robust. The best reuse level is case dependent and interacts with time-step size and the quality of the inner fluid and solid solves.