IRIS

IRIS Finite Element Code — Main Features

IRIS is a general-purpose finite element program designed for computational solid, fluid, and structural mechanics. It emphasizes clarity, extensibility, and research flexibility while maintaining good performance for medium to large problems.

🔑 Key Characteristics

  • Handles linear and nonlinear problems.
  • Supports quasistatic and transient analyses.
  • Provides a flexible input language for defining models and analyses.
  • Built in C++, using external high-performance libraries for linear algebra.

🧩 Element Library

IRIS offers a wide range of elements for different physics:

  • Solid mechanics: small strain and finite strain solids, displacement-based, mixed, and enhanced formulations.
  • Fluid mechanics: Stokes and Navier–Stokes elements with stabilization (SUPG, GLS, VMS, OSS).
  • Thermal analysis: elements for (nonlinear) Poisson and diffusion equations.
  • Special analyses: topology optimization, eigenvalue/mode computation, dispersion diagrams.

🧪 Material Models

Implemented through the MUESLI library:

  • Elastic (small strain isotropic).
  • Elastoplastic (von Mises, Tresca, Drucker-Prager with isotropic/kinematic hardening).
  • Thermal conductors (Fourier-type heat conduction).
  • Extensions possible by adding custom models.

⚙️ Solvers and Integration

Nonlinear Step Solvers

  • Newton–Raphson
  • Quasi-Newton
  • Nonlinear Conjugate Gradient (NLCG)
  • Adaptive Dynamic Relaxation
  • Explicit and Fractional step methods

Linear Solvers

Interfaces with multiple sparse direct and iterative solvers:

  • SuperLU, LDL, Pardiso, WSMP, HSL, PETSc wrappers, and PCG.

Time Integration

  • Quasistatic solver
  • Implicit: Backward Euler, Newmark, Hilber–Hughes–Taylor (HHT), Midpoint
  • Explicit: Central Differences, Verlet
  • CFL-based or adaptive time stepping available.

📊 Analysis Types

  • Controlled load/displacement analysis
  • Eigenvalue and spectral analysis
  • Harmonic analysis
  • Dispersion analysis
  • Inf-sup analysis for mixed formulations
  • Stationary (quasistatic) and transient dynamics
  • Topology optimization
  • Staggered multi-physics analysis

🛠️ Input and Preprocessing

  • Plain text .iris input files with modular commands.
  • Built-in primitives: brick, sphere, cylinder, torus, etc.
  • Imports meshes from Gmsh or Abaqus.
  • Supports node sets, element sets, boundary conditions, and time-dependent loads via scaling functions.

📤 Output and Postprocessing

  • Log files with solver and iteration details.
  • Result files (stresses, strains, displacements, energies, etc.).
  • Reaction logs for boundary condition verification.
  • Compatible with external visualization tools (e.g., Gmsh).

📚 External Libraries

  • BLAS/LAPACK — basic linear algebra.
  • Intel TBB — multithreading.
  • SuperLU, WSMP, Pardiso, HSL, LDL — sparse solvers.
  • PETSc — linear and nonlinear solvers, preconditioners, ODE integrators.
  • MUESLI — material models.
  • Gmsh — mesh generation and postprocessing.

🎯 Philosophy

  • Focus on clarity and extensibility over raw speed.
  • Uses external high-performance libraries for computational bottlenecks.
  • Designed as a research and teaching platform, enabling the addition of new solvers, elements, or material models without breaking existing functionality.

📘 User manual

A user manual can be freely accessed.