The development of a virtual reality computational fluid dynamics (CFD) code for free surface flows would clearly be a useful research tool. In particular there is considerable industrial interest in a code that could accurately simulate injection moulding or cavity filling. With this in mind Tome and McKee (J. Comp. Phys. 1994, J. non-Newt. Fluid Mech. 1996) produced a two-dimensional version. This work is concerned with its extension to three dimensions and its interface with solid modelling technology so that a video sequence of the numerical simulation produces flow which is essentially indistinguishable from reality. GENSMAC3D approximates curved boundaries and free surfaces in such a way that the discrete Poisson equation, resulting from the velocity update and pressure correction, requires the solution of a symmetric positive definite system at each time step. These linear systems are generally of the order of 1 million variables and are solved by the conjugate gradient method both robustly and efficiently. Details of the methodology and the equations involved can be found in Tome et. al. 2001 (GENSMAC3D: A Numerical Method for Solving Three-dimensional Free Surface Flows). GENSMAC3D has been implemented under the name of FreeFlow3D and is composed of three distinct modular parts:

Modflow3D - An interactive system for the specification of fluid flow model which includes the definition of elements in the fluid domain such as containers, nozzles, fluid properties, etc

Simflow3D - This system is the central part of FreeFlow3D as it implements the discrete governing equations and boundary conditions. It uses de GENSMAC3D method which is a modification of the GENSMAC method employing marker particles on the free surface only

VisFlow3D - An interactive system for the visualization of the output of Simflow3D.

The three modules were written using the programming language C under the operating system UNIX (LINUX). The graphic interfaces of Modflow3D and VisFlow3D use the windowing system Xview under X-Windows. The data structure was designed to permit easy access and data interdependency in order to simplify software maintenance and extensions. The modules comunicate with each other through files.

The FreeFlow3D system has the following features:

• It solves the full Navier-Stokes equations;

• The numerical treatment of the free surface stress conditions is accurate;

• The Poisson equation is solved by the Conjugate Gradient method;

• Automatic time-step calculation;

• Solid modelling and computer graphics techniques are employed;

• The fluid is treated as a closed 3D-surface