Not only is traditional CFD software difficult to integrate into CAD environments, it requires such a lot of effort to generate a grid, even large companies may be forced to resort to consultants to get their problems sorted out. Worse yet, designers may need some prior knowledge of the flow that they are looking at to optimise the solution. And even if they do, they need to spend a lot of time tuning a system to model complex properties such as turbulence.
Most developers now agree that next generation of CFD products will be meshless, seamless with their interface to common CAD packages and able to handle large problems and complex geometries with ease.
Now, Exa, based in Lexington, MA, claims to have embraced all of these characteristics. It has developed a lattice gas method that simulates and analyses the flow of fluids in and around solid objects. Called Digital Physics, the technology itself was initially developed by the company’s chief technology officer Dr Kim Molvig, a professor at Massachusetts Institute of Technology.
The Digital Physics philosophy is embraced in a new two- and three-dimensional fluid modelling software package called PowerFlow. Its algorithms predict the flow of fluids based on an enhanced version of the Lattice Gas particle method. And unlike first generation CFD products, the Powerflow software provides the results of the Navier Stokes equations without the need to perform the numerical approximations to differential equations.
Totally unlike its predecessors, Powerflow has a gridless set-up and can be seamlessly integrated with CAD packages. At the time of writing, the software interfaces to Pro/Engineer, although the company will be embracing other CAD packages shortly.
In operation, the software divides a fluid volume around an object like a car into a regular lattice of discrete volume elements called voxels, and the surface into a series of discrete surface elements called surfels. The software eliminates the need to refine a grid manually.
Particle dynamics are restricted to the voxel grid, with special algorithms handling particle interaction with surfels. As particles move from voxel to voxel, their motion is represented by simple shift operations. Particles in the same voxel can collide with one another, with the collisions represented by simple logic functions; mass, momentum and energy are conserved.
Because the grids are generated automatically, PowerFlow eliminates the preparation time associated with other packages. It provides accurate representations of macroscope quantities like lift, drag and discharge coefficients. No tuning is required, there is no need for extra iterations, and there is no need to change the solver, even when geometries change substantially.
To demonstrate its functionality, Exa recently set the software the challenge of predicting separation for the Morel Body, a nasty problem that is used as a standard test of CFD packages in the automotive industry. It represents a significant test case because it exhibits physical properties that are common in vehicle aerodynamics, and allows users to study the effects of back slant angle on overall forces. Varying the angle of the rear window approximates different body types, fastbacks, sedans, and so on. As the angle increases, the flow-induced drag rises to a point and then suddenly drops at 30 degrees.
No conventional software had previously been able to predict vortex breakdown, separation and reattachment, or the formation of a large separation bubble for angles beyond 30 degrees. Exa’s new generation of software has apparently has done so.
The comparison between the measured and the predicted drag coefficient as a function of the back angle showed a good correlation. So too did a comparison between the measured and predicted lift coefficient as a function of the back angle. Indeed, computed drag and lift were within 10% of experimental data.
The software has been ported to Sun UltraSPARC workstations and Enterprise Servers. In doing so, the Exa software developers were able to take advantage of the new VIS (Visual Instruction Set) which Sun had adopted for the UltraSPARC chip. The Visual Instruction Set was primarily written to assist developers of multimedia software. However, the Exa developers recognised that the instruction set could also be used to assist with the acceleration of their software too. PowerFlow software is available from Random Computing in the UK. Initial annual workstation single licenses start at £11,000 per seat.