Watching the wind blow. (Design).

Computational fluid dynamics programs are often cumbersome and erratic, which limits their usefulness. Exa Corp. claims its powerflow aerodynamics software eliminates these problems, and creates a simulation tool that not only replicates wind tunnel testing, but accurately models airflow.

The science of aerodynamics is all about the flow of air around an object and the forces it exerts on that item. Yet air is an invisible gas. Visualizing the flow around a vehicle, for example, often has meant measuring oil flows, doing pressure taps, or using a smoke wand to create a picture of what the wind is doing as it dances across the surface. The common factor in all these methods is a session in the wind tunnel, an expensive and time consuming process that limits design experimentation and results in a reliance on tested vehicle silhouettes.

"You don't need to do 100 iterations of a vehicle," says Stephen Remondi, president, CEO, and co-founder of Exa Corp. (Lexington, MA), an aerodynamic visualization software company. He suggests as few as 10 iterations of a design may be enough to optimize a vehicle's aerodynamics -- if you know what to look for. Which is where his company's PowerFLOW software comes in.

"It's a real 'What if?' tool," says Remondi. "Since you can change your model quickly, cost effectively and have confidence in the simulation, you're able to try out more aggressive, more creative ideas in the time available." While it's presently in the domain of engineers and aerodynamicists, they're working on a digital interface for designers so that 3D models will be, in effect, "sculpted" as if they were in clay.

Correlation

PowerFLOW places external airflow models in a "digital wind tunnel," though custom-made tunnels can be created to correlate with those the customer currently uses. This allows test results to be compared, and gives users greater confidence in the results. In conjunction with the European Aerodynamic Data Exchange (EADE), Exa has modeled Ford's two wind tunnel facilities in Cologne, Germany, Audi's and BMW's wind tunnels, as well as Pininfarina's test center in Italy. For the tests, vehicles were tested in each tunnel, then digitized and tested in virtual models of the same wind tunnels. (Not all vehicles were tested in each tunnel.) The results correlated well.

optimization

An aerodynamically optimized shape depends on more than a nice silhouette. Countless arguments have taken place over the size of grille openings, sunroofs that "boom" when the windows are closed, deck lid height, and other details, and each is related to the lack of aerodynamic testing. "The traditional process in the auto industry is to design, build, then test," says Remondi. "What we want to do is take aerodynamic simulation and completely integrate it into the design process so that you can move to a design, build, confirm model."

For example, underhood flow testing must typically wait for the creation of physical prototypes, and changes here are often costly. "There's usually a fight between the designers and engineers over the size of the grille opening and the amount of surface area needed," Remondi says, "because it's not known ahead of time how much will be needed to meet the targets. So one group asks for as much as it can get, while another asks for as little as possible. It's a never-ending battle."

Adding the thermal management component to the aerodynamic model can reduce this tension by mapping the mass airflow through the radiator and around the engine compartment. Heat transfer can be tracked around critical components like computer modules and the battery, and updated as the design is changed. This ability extends to the climate control system as well.

Exa says its software also is being used in simulations of powertrains, air filters, exhaust systems, disc brakes, intake and exhaust manifolds, and valves. The next version will add movement through the addition of dynamic geometries, and allow users to calculate the dynamics of these systems in use.

Race Ready

This dynamic functionality will be in addition to the current system's ability to do more than static airflow testing. Right now, the system can simulate a moving ground plane and rotating wheels. Not surprisingly, this has piqued the interest of a number of race teams, from NASCAR to the current Le Mans 24-Hour champions. Says Remondi, "They've looked at everything from how their cars respond when running in a pack, to how to get the greatest amount of downforce with the lowest amount of drag from the underbody." Formula One, where thousands of man-years can be spent optimizing a vehicle during the season, is another series taking an interest in PowerFLOW.

Yet the primary target for the company is the auto industry at both the supplier and OEM levels. As the number of prototypes drop, the need for robust simulation tools rises in order to make certain the vehicle is right the first time. "We think the market for CFD is bigger than the structural and crash analysis markets combined," declares Remondi, "and the application space is broader in terms of long-term potential." One supplier told Remondi it will go from art to part in five years, and automakers like BMW are working to eliminate scale-model testing altogether.

"We're convinced that talking to the design community is the best way to meet these needs," he states, "because this is where the process begins." Already there are successes. "One automaker has seen the conflict between the aerodynamicists and designers decrease because the designers can now see that the changes the aero guys want are necessary. As a result, more of the aerodynamicists' suggestions are accepted, and the designs are closer to the target much earlier in the process. "If you listen closely, you can almost hear the cash register ringing.

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Traditional computational fluid dynamics [CFD] techniques start with Navier-Stokes equations, which mathematically describe a fluid at its continuum level. The equations are highly non-linear, and the models analyzed are bult from a grid in which there can be no distortion in any of the grid elements if results are to be accurate. "The more the grid is contorted," says Exa Corp.'s Stephen Remondl, "the more likely it is that you won't get an answer.

Remandi claims his company's PowerFLOW software package eliminates this problem by modeling the fluid traveling around the vehicle, not the universe in which the vehicle travels. "Power-FLOW uses macro-molecules, several times larger than actual air molecuies, that possess the properties (heat, mamentum, elasticity, etc.) of the real thing. These particles interact, and we ascribe to them certain properties that are based on physical first principles." The result, he says, is a navier-Stakes fluid without the hassle of solving a traditional Navier-Stokes equation.

Detail is generated by importing CAD geomentry [from Alias, NASTRAN, SDRC, Unigraphics, CATIA, etc.], customizing regions of higher resolution and measurement, and specifying characteristic properties and flow parameters. The software creates the underlying grid by placing a Cartesian lattice over the CAD model. This causes the grid to intersect with the model, thus creating a surface representation the software understands. "With this method, the distribution of particles exists at discrete locations in space and move in discrete directions, speeds, and intervals of time," says Remandi. "This provides full recovery of the Navier-Stake continuum fluid equations." Some thing traditional CFD approaches apparently can't do.

The software also is being used to model airflow across computer circuit boards, wind loads on and HVAC flow within buildings, wind load studies on offshore oil platforms, aerodynamic lift on highspeed disc drive heads, nuclear reactor cooling flow, and maritime hydrodynamics. All this from a company that opened its doors in 1992 with some MIT-based algorithms regarding how to solve fluid dynamics on a computer.