Using a wind tunnel for the purpose of product development is certainly nothing new in the aircraft industry. The coefficient of friction and other aerodynamic aspects in aircraft are more important than simply miles per gallon (i.e., flight is about more than fuel efficiency). And although auto manufacturers have long been using high-velocity blown air as a means by which they can check out the aerodynamic attributes of body designs (often using such things as smoke that streaks along the surface, thereby providing visual cues as to how smooth things are—or aren't), arguably, the plane manufacturers have an experiential advantage in this wind tunnel arena.
One of the best places to learn about how to do something better is somewhere else. That is, instead of looking at how your competitors are doing something (which is, in effect, the same place where you are, but with a different address), it can be advantageous to view the practices in other industries. And this is what the people at Ford's Scientific Research Laboratory did. When it came to wind tunnel tests, they looked to the aircraft industry.*
Drilling & Plumbing.
One of the ways by which the researchers at the Ford Dearborn Proving Grounds measure pressure build-up on surfaces is through the use of what are called "pressure ports," which involves precise hole drilling on a surface. As Dr. Patsy Coleman, senior technical specialist at the Scientific Research Laboratory puts it, "If you measure pressure on a passenger side window, for example, you'd first remove the glass, replace it with a metal or plastic plate, and then carefully drill the holes and plumb each to pressure ports, or taps." This drilling and plumbing for the side window (which involves 120 holes), she says, may require a person to spend a week doing the setup.
The information obtained from these pressure ports is good. But there are a few drawbacks. For one thing, there is the time involved for setup. For another, it is an intrusive process (there may be some surfaces that you can't replace with a plate and you might not want to have something with holes in it when you are done). And the data obtained relates to the points, and there may be things happening between the points, as in:
·_ _ _ _·_ _^^·_^^^·_ _ _ _·
One might not catch those upswings, as the point-to-point information seems right.
A Period Piece.
So, back to the aerospace approach. One of the means by which they obtain pressure data is through the use of an optical imaging technique that's made possible through the deployment of a ruthenium-based paint that's sourced from Boeing. (For those of you who have forgotten your periodic table, that element, Ru, has an atomic weight of 44; it is a member of the platinum group—which explains why, when asked what the paint costs, Mark Walter, a product manager for Sverdrup Technology, which provides asset management of the climatic wind tunnels and cold rooms at the Dearborn Proving Ground for Ford, simply states that it is a whole lot more expensive than the house paint that you might buy at K-mart. A word about Sverdrup Technology. It was established in 1950 in order to support the development and operation of the U.S. Air Force Arnold Engineering Development Center—which happens to be the largest aeronautical testing complex in the U.S. Among Sverdrup's clients are NASA, Lockheed Martin, and Boeing. Clearly, these folks know more than a little about wind tunnel testing.)
One of the differences between aircraft testing and automotive testing is that whereas the aircraft personnel are interested at what goes on at speeds of 600 mph or so, Ford is more interested in, say, wind noise at 70 mph. So there had to be modifications made to the chemistry of the paint that is used in aerospace in order to obtain the results that they're looking for at Ford. Coleman, by the way, is a chemist.
A bit of chemistry. If you had a surface painted with the pressure sensitive paint and placed it in a nitrogen atmosphere, it would glow brightly (assuming, of course, that you were observing this with the appropriate optical setup). Oxygen quenches the luminescence. But cars don't drive in pure nitrogen or pure oxygen atmospheres. But the luminescent and quenching aspects still hold.
A Quicker Process.
Now for how the vehicle aerodynamics are measured in the Ford wind tunnel with the paint. If the measurements are to be done on a real or concept vehicle, technicians first apply a thin sheet of adhesive plastic to the area of interest, such as the driver's side window. If this is going to be done on a scale model, then this step is ignored. (The plastic peels off of the vehicle and the underlying surface is cleaned up with glass cleaner; the paint would be tough to remove—and when talking about full-sized products, chances are, some other people within the organization want the vehicle for other purposes, purposes that probably are better served without a sickly yellow painted surface.) A white primer is applied. Then, using a touch-up spray gun, the area is painted. Coleman says that the paint must be smooth and even. How is this achieved? Through good hand-eye coordination of the person with the spray gun. Once the surface is painted, black locator points, or targets, are affixed to the surface because two separate images are taken of the surface (one at ambient temperature and pressure; one at tunnel velocity) and these points are used to coordinate the images.
The surface is illuminated with light that passes through blue filters. Setting up the lights so there is even illumination takes the greatest amount of time—although the time involved is on the order of a couple of hours, not days. The CCD camera views the surface through a red filter. The camera has a 1,024 x 1,024-pixel field. The camera is linked to a computer that acquires and processes the information, and displays the results in (near) real-time. If you look at the surface, you wouldn't be able to see the colors that show up on the screen. There, areas of the greatest air pressure show up as red. These areas are the ones where there is likely to be the greatest wind noise. By being able to get this information comparatively quickly (say, in 15 minutes), designers and engineers are able to make modifications, then run other tests.
As mentioned, the conventional pressure tap approach provides good information. But Mark Walter points out that while there are 120 points collected on a window from the pressure taps, the paint is more comprehensive: they're generally viewing an area of that size with 300 x 700 pixels, which translates to 210,000 points. This digital data lends itself to comparisons with the results of computer-based tests run with computational fluid dynamics (CFD) software, which can help lead to more digital-based and less physical-based testing—the sort of thing that the aircraft industry is doing more and more of.
*In this case, the looking to another industry was rather simple. It seems that a person who had been working at McDonnell-Douglas, which is now part of Boeing, who is familiar with the paint took a job at Ford.
A Big Wind
Wind tunnel #5 at the Dearborn Proving Grounds where pressure sensitive paint testing is done has a test section that measures 28 x 14 x 30 ft. The room features temperature control (from 75o to 140o F); the temperature at which the testing is done isn't as important as having a consistent temperature. The air speed is generated by a fan with a 10-ft. diameter blade that's powered by a 1,250-hp motor.