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PHYSICA and SMARTFIRE: From solidification of metals to the spread of fire

Collaboration between Centre staff has enabled CFD technology originally developed for the metals processing industry to be adapted for the simulation of fire spread in buildings, aircraft and ships.

Two software products produced by the CENTRE are the multi-physics code PHYSICA and the fire simulation code SMARTFIRE. Both codes are based on Computational Fluid Dynamics (CFD) but both codes have very different application areas. The PHYSICA code was primarily intended to address complex multi-physics problems in the metals industry involving solidification, phase change, shrinkage, fluid-structure interaction, the interaction of charged flows with magnetic fields etc. The SMARTFIRE software addresses how fire spreads within structures, and so is concerned with the generation of thermal radiation, turbulent flows, toxic gases, combustion, smoke generation, etc.

However, both codes have the same common heritage: the unstructured mesh code CWNN, developed by the CENTRE in the early 1990s. CWNN was developed as a computational engine, written in FORTRAN, to solve the Navier-Stokes equations on an unstructured mesh. The concepts developed for this code would be adopted and modified by various CENTRE teams for eventual application within the PHYSICA and SMARTFIRE codes.

Project Optimisation Diagram

SMARTFIRE:
SMARTFIRE is an open architecture CFD environment written in C++ that is comprised of four major components: CFD numerical engine, Graphical User Interfaces, Automated meshing tool, and the Intelligent Control System. The code uses the SIMPLE algorithm and can solve turbulent or laminar flow problems under transient or steady state conditions. As part of its development, the software has undergone considerable validation.

The CFD engine used in the software can trace its ancestry back to the FORTRAN code CWNN. For use in SMARTFIRE, CWNN was completely re-engineered and re-written in C++ using object orientated techniques. In addition, a number of additional physics features were included in the re-written code that are required for fire modelling. These include a six-flux radiation model, a multiple ray radiation model, provision for heat transfer through walls, a volumetric heat release model or gaseous combustion model (using the eddy dissipation model) to represent fires, smoke modelling and turbulence (using a two equation K-Epsilon closure with buoyancy modifications).

A key feature of SMARTFIRE is its ease of use, making this powerful fire analysis technique accessible to non-CFD experts such as fire fighters, architects and safety engineers. This ease of use has been achieved by developing a Case Specification Environment, an automated meshing system and an interactive CFD Engine. By embedding expert knowledge into the CFD software and the meshing system, SMARTFIRE removes the black-art associated with the CFD analysis. Critical tasks such as the selection of solvers, relaxation parameters, convergence criteria, time steps, meshing and boundary condition specification are all supported by automated tools and expertise within the SMARTFIRE system. The user is given the option of overriding these decisions, thus retaining ultimate control. In this way, the safety professional need simply specify a scenario in terms of known geometry and fire characteristics, using familiar terms of reference, and the SMARTFIRE system will use embedded expertise to configure the simulation. SMARTFIRE can also run in parallel, utilising multiple PC computers on a standard PC-network, greatly reducing the run-time incurred by very large simulations.

Another important feature of the software its ability to utilise three-dimensional unstructured meshes, enabling complex irregular geometries to be meshed without the recourse of cruder methods such as stepped regular meshes or body fitted meshes.

PHYSICA:
PHYSICA is an open architecture multi-physics environment written in FORTRAN in an object oriented manner. The original version of PHYSICA can be traced back to the CFD code CWNN. Since then the code has been re-engineered with extra physics being added. PHYSICA has a user interface routine (known as INFORM) which allows the user to define the physics that they are interested in solving, plus other user-defined variables such as material properties, boundary conditions and solver parameters. PHYSICA comes with the pre and post processor FEMSYS but can also interface to a host of other state-of-the-art pre and post processing software.

A key feature of PHYSICA is its ability to model complex applications that are governed by interacting physics such as fluid flow, heat transfer, electro-magnetics, chemical reactions, and mechanical stress. One industrial application, that was the main early driver in the development of PHYSICA, is the processing and casting of metals. This requires the modelling of metal flow, with free surface dynamics, into a mould cavity (i.e. the shape of an aircraft engine), heat transfer, solidification (i.e. liquid metal becomes solid) and mechanical stress. As well as the processing and casting of materials PHYSICA is now being used as the basis for simulating aero-acoustics, magneto-hydrodynamics, dynamic fluid-structure interaction, and high speed impacts.

PHYSICA has the ability to use fully unstructured 3D meshes to solve the above physics over complex geometries. It contains a range of high and low Reynolds number turbulence models and can predict flow of both Newtonian and Non-Newtonian fluids. The enthalpy technique is used to predict how liquid materials solidify. The code also has the ability to model the deformation of elastic and non-elastic (i.e. visco-elastic and visco-plastic) solid materials.

Another Key feature of PHYSICA is its ability to exploit parallel computing hardware infrastructure using both distributed and shared memory machines. The software has been parallelised using both MPI and OpenMP which provides the ability to run very large simulations across many computer processors using dynamic load balancing techniques.

Today, both the PHSYICA and SMARTFIRE codes are used throughout the world to solve some of the most difficult and challenging engineering problems in the metals processing industry, e.g. Hismelt® a new environmentally friendly technology to smelt iron ore, and in fire engineering, e.g. official investigation into the Swiss Air MD11 fire and crash.