EPX is a simulation software dedicated to the analysis of fast transient phenomena involving structures and fluids in interaction. The program is co-owned by the Commissariat à l’Energie Atomique et aux Energies Alternatives (CEA) and the Joint Research Centre of the European Commission (EC/JRC). Its development is carried out through a Consortium involving the co-owners and so-called major partners who are granted a complete access to the source code and development tools.

EPX is based on a space discretization by means of Finite Elements, SPH Particles (Smooth Particle Hydrodynamics) or Discrete Elements for structures, or by means of Finite Elements, Finite Volumes or SPH Particles for fluids. Time integration is achieved through a conditionally stable explicit scheme.

The solving algorithm is completely non-linear, at both geometric level (large displacements; large rotations) and material level (constitutive laws implementing plasticity or damage for example).

The programme provides a large number of kinematic links between entities, for instance for boundary conditions, contact between structures or fluid-structure interaction. EPX is characterized by its minimal use of non-physical parameters to enforce these links, such as penalty coefficients. It relies on direct methods to compute the link forces whenever it is possible and otherwise, the links are dualized by means of Lagrange Multipliers, the unknown forces being then deduced from the resolution of an additional linear system.

For CEA and JRC, the main development teams are hosted by :


Nuclear Energy Division

Saclay Nuclear Activity Division

Systems and Structures Modeling Department

Mechanical and Thermal Studies Service

Directorate for Space, Security and Migration 

Safety and Security of Buildings

EPX implements advanced fluid models and Finite Volume schemes for multi-component flow, reactive flows or multi-phasic flows. Below are some results for illustrative tests for which a reference solution is available :

  • shock-bubble interaction simulations,
  • water-gas Richtmeyer-Meshkov instabilities,
  • Woodward-Colella wind tunnel with a step,
  • validation of Hydrogen detonation model against experiments (FZK 12 meters shock tube).


EPX implements specific models able to analyze various mechanical situations, such as shocks, impacts, explosions, wave propagations and their consequences on structures…

Spatial discretization for structures is mainly achieved through finite elements, but some meshless models, such as SPH particles, or discrete models are also available.

EPX implements kinematic constraints of many kinds to provide boundary conditions and couplings between entities, such as unilateral contact between different pieces of structures, fluid-structure interaction or coupling between different formulation (pipe SPH-Finite Elements, Discrete Elements-Finite Elements…).

Arbitrary numerical parameters are avoided in the way these constraints are enforced, mainly by means of a dual approach through Lagrange Multipliers.

Below are given two demonstrative examples of the capabilities of EPX for strongly coupled systems.

2024. EUROPLEXUS Software - Joomla! is Free Software released under the GNU General Public License.