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.
  • 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.