Multi-physics coupling

cad-sim ships 38 physics domains; Wave 67 added the partitioned coupling scheduler that lets two of them exchange a shared interface field iteratively. This recipe walks through choosing a coupling scheme + relaxation mode for your stiffness regime before you wire real solvers in.

See the IQN-ILS algorithm page for the math + paper citations.

When you need this

When two physics domains touch at an interface and the response of one feeds back into the input of the other:

  • Conjugate heat transfer — solid wall heated by adjacent fluid.
  • Fluid-structural interaction — flexible airfoil in transonic flow.
  • Electromagnetic-thermal — induction-heated piece.
  • Piezoelectric-mechanical — sensor strain feeding voltage.

Picking a scheme

Coupling stiffness is roughly the product of the two solvers' sensitivities to each other's interface output. simulate_coupled exposes a two-parameter linear toy α × β so you can test your stiffness regime against each relaxation scheme.

{
  "method": "tools/call",
  "params": {
    "name": "simulate_coupled",
    "arguments": {
      "coupling_alpha": 0.9,
      "coupling_beta": 0.95,
      "scheme": "implicit",
      "relaxation": "iqn_ils",
      "iqn_history_max": 10,
      "max_iterations": 200,
      "tolerance": 1e-8
    }
  }
}

Returns Converged in N iterations, final residual … plus the analytic fixed point so you can verify behaviour.

Which relaxation when

RegimeRecommendedWhy
αβ < 0.3 (weak)explicitOne solver pass per side per step. Cheapest, stable.
αβ ∈ [0.3, 0.7]constant ω = 0.5Sub-Newton, robust.
αβ > 0.7 (stiff FSI, CHT)aitken or iqn_ilsSub-linear vs. linear convergence rate.
Tight FSI, added-mass dominatediqn_ils history 10-20preCICE's workhorse for the hardest cases.

The Wave 67.B test shows IQN-ILS finishes in ≤ Aitken iterations on a stiff α=0.9, β=0.95 case — the assertion the substrate verifies on every build.

Wiring real solvers (Wave 67.C preview)

The current MCP tool runs the canonical linear toy so you can characterise convergence behaviour. Wave 67.C will accept real cad-sim domain handles — thermal, structural, cfd — and loop them through the same scheme + relaxation choice you picked on the toy. Same convergence properties; just plug different solvers in.