Physics interfaces

Adding physics

Physics come from the Model Wizard, or from Home  Add Physics at any time. Each interface appears under the component with its default features. For Heat Transfer in Solids these are Solid 1 (the domain equation), Initial Values 1 and Thermal Insulation 1 on exterior boundaries.

Most interfaces solve a fixed Elmer variable (Heat Transfer solves Temperature), so a component has one of each: to cover more domains, select them in the interface’s Domain Selection. Two interfaces may be added more than once, each instance with its own dependent variable:

  • Transport of Diluted Species: a second interface gets the species c2 (setting Species), solved as the Elmer variable Concentration_c2. Its results are c2, tds1.tflux, tds1.tfluxMag and tds1.ntflux (with the interface’s tag). Several species in one domain need streamline diffusion: AdvectionDiffusion keeps each species' keywords apart, ModelPDE does not.

  • Coefficient Form PDE: a second interface gets the dependent variable u2. ModelPDE reads the same coefficients for every instance, so two PDE interfaces must be on different domains.

When two interfaces would still overwrite each other in Elmer (the same variable, or one Elmer keyword set to different values in the same domain or boundary), building the study stops with an error that names both.

Add features with Physics  Domains and Physics  Boundaries, or by right-clicking the interface. The ribbon lists the features of the interface that is selected, or that the selected node belongs to.

Each feature’s Settings window has:

  • its Selection (Selections);

  • an Equation section with the equation it contributes;

  • its settings, with symbols and SI units. Every field is an expression.

The interfaces

Interface Features Elmer solver

Heat Transfer in Solids (ht)

Solid, Fluid (prescribed or Laminar-Flow velocity), Heat Source, Temperature, Heat Flux (general or convective), Surface-to-Ambient Radiation, Symmetry, Thermal Insulation

HeatSolver + FluxSolver

Solid Mechanics (solid)

Linear Elastic Material, Body Load, Gravity, Rotating Frame, Fixed Constraint, Prescribed Displacement, Roller, Symmetry, Boundary Load (force per area, or total force), Spring Foundation, Free; plane stress or plane strain in 2D

StressSolver (stationary, eigenfrequency)

Electrostatics (es)

Charge Conservation, Space Charge Density, Ground, Electric Potential, Surface Charge Density, Zero Charge

StatElecSolver

Electric Currents (ec)

Current Conservation, Ground, Electric Potential, Normal Current Density, Electric Insulation

StatCurrentSolver

Magnetic Fields (mf), 2D and 2D axisymmetric

Ampère’s Law, External Current Density, Magnetic Insulation, Magnetic Potential, Perfect Magnetic Conductor

MagnetoDynamics2D + CalcFields

Laminar Flow (spf)

Fluid Properties, Volume Force, Gravity, Wall (no slip or slip), Inlet (velocity or pressure), Outlet (optionally normal flow), Symmetry, Open Boundary; Stokes flow option

FlowSolve (stabilized P1-P1)

Pressure Acoustics, Frequency Domain (acpr)

Pressure Acoustics (domain), Sound Hard Boundary (Wall), Sound Soft Boundary, Pressure, Normal Acceleration, Impedance, Plane Wave Radiation, Spherical Wave Radiation (with incident plane wave), Exterior Field Calculation

HelmholtzSolver

Transport of Diluted Species (tds)

Transport Properties (diffusion, convection by a given or Laminar-Flow velocity), Reactions, Concentration, Flux (including mass transfer), Electrode Surface Coupling, No Flux, Symmetry, Outflow; migration in the electrolyte potential

AdvectionDiffusion with streamline diffusion (default), or ModelPDE

Secondary Current Distribution (cd)

Electrolyte, Current Source, Electrode Surface (primary, Butler–Volmer, linearized, Tafel, limiting current, user-defined kinetics), Electrolyte Potential, Electrolyte Current Density, Insulation, Symmetry

StatCurrentSolve, HeatSolve or ModelPDE, + FluxSolver. See Electrochemistry.

Coefficient Form PDE (pde)

c, a, f, da, β coefficients; Dirichlet, Flux/Source, Zero Flux

ModelPDE

Defaults and overriding

  • Default domain features cover all domains. Default boundary features cover the exterior boundaries, as in COMSOL.

  • A feature lower in the tree overrides earlier ones on the boundaries they share.

  • Default features can’t be deleted or disabled, but any feature can override them.

Interface settings

Select the interface node itself (for example Heat Transfer in Solids (ht)) for settings of the whole interface. The details depend on the interface:

  • Discretization: the element order, linear or quadratic. The mesh follows it by default.

  • model options, such as 2D plane stress or plane strain in Solid Mechanics, or primary or secondary current distribution;

  • the choice of Elmer solver, where there is one (see Electrochemistry);

  • Consistent Stabilization (Transport of Diluted Species): Streamline diffusion, on by default (Convection-dominated transport);

  • Additional Elmer solver keywords: lines appended verbatim to this interface’s Solver section in the .sif, for anything the GUI doesn’t expose.

Convection-dominated transport

When convection dominates diffusion over an element, a Galerkin discretization makes the solution oscillate from node to node along the stream. Convection dominates when the element Péclet number, Peh = \|u\|h/(2D), exceeds 1. A dissolved species in a liquid is the usual case: D ≈ 10-9 m²/s gives Peh = 2.5 at 1 mm/s on 5 µm elements. Transport of Diluted Species therefore uses streamline diffusion by default. This is streamline-upwind Petrov–Galerkin (SUPG) stabilization, in Elmer’s AdvectionDiffusion solver. It is consistent: it vanishes for the exact solution, so it doesn’t smear a converged result.

  • Untick Streamline diffusion on the interface node for the plain Galerkin form (Elmer ModelPDE). Models saved before this option existed open with it unticked, and their results stay as they were.

  • A Reactions feature with a first-order rate constant k needs the Galerkin form: the interface falls back to it, with a warning in the log.

Heat Transfer and Laminar Flow are always stabilized.

The flow itself matters as much. A thin boundary layer at an outlet needs a velocity field that stays parallel there. Tick Normal flow on the Laminar Flow Outlet to set the tangential velocity to zero. Without it, the pressure outlet on stretched near-wall elements distorts the flow over the last few element columns.

Multiphysics

Home  Multiphysics adds couplings between two interfaces in the model. They appear under Multiphysics:

Coupling Couples Effect

Electromagnetic Heating

Electric Currents → Heat Transfer

The Joule heat Q = J·E is a heat source.

Thermal Expansion

Heat Transfer → Solid Mechanics

Thermal strain α(T − Tref).

Nonisothermal Flow

Laminar Flow ↔ Heat Transfer

Convection of heat by the flow, and Boussinesq buoyancy.

The Model Wizard entries Joule Heating and Thermal Stress add both interfaces with their coupling. Coupled physics are solved together in one Elmer run, iterating between the solvers until the coupling converges (Studies › Coupled physics).

Nonlinear problems

A model becomes nonlinear when a material property or a boundary value depends on the solution (for example k(T), radiation, or electrode kinetics). Elmer Studio detects this from the expressions and makes the solver iterate. The iteration settings are in the study’s solver configuration (Studies › Solver configurations).

Every interface’s features and settings are in the Reference.