That's because engineers at General Motors are using Computational Fluid Dynamics (CFD) to reduce the whooshing and whistling sound of wind noise that wipers create, keeping these foul-weather friends out of mind until the next rainfall.
Why wipers are such a challenge
It might seem that a small automotive part like wipers shouldn’t command so much attention from engineers, but today's automotive manufacturers are giving wipers more attention than ever. Wipers are essentially the victims of a variety of engineering successes that have reduced noise from other areas of the car, suddenly leaving wipers seeming rather noisy.
“Because tire and engine noise has been reduced with various kinds of new technology, wind noise becomes a greater overall contributor to what can be heard inside the automobile,” says Ken Karbon, a staff engineer at General Motors who specializes in aerodynamics, “and wipers are potentially a significant source of wind noise.”
When you consider that wind noise consistently ranks highly among drivers’ concerns in consumer surveys, wipers are suddenly no small matter. Furthermore, where there’s noise, there's generally aerodynamic drag, which auto manufacturers seek to minimize for improved fuel efficiency.
The most obvious way to eliminate wiper wind noise would be tucking idle wipers out of sight—and thus out of the airflow—as was done on many cars years ago. This idea, however, often runs head-on into engineering difficulties and aesthetic issues. First, as automobiles have become more compact, there's less room under the hood to stow anything that doesn't have to be there, including wipers, the wiper motor, and any necessary linkages. Second, the moment wipers emerged from beneath the hood's edge to sit in their idle position on the windshield, they immediately gained the status of styling elements. Now, stylists skillfully use wipers and hood features to add character to automobiles, making them more attractive to potential buyers.
Balancing style with performance
When concerns of styling and engineering overlap, traditionally a small game of tug-of-war develops. Stylists are constantly striving for a new and innovative look, while engineers are chiefly focused on performance, which puts the two design teams at odds by their very nature. After all, the best looking car is generally not the most aerodynamic, and vice versa, so compromises have to be made to achieve the fine balance that creates a winning design.
Cross-section of the aerodynamic model showing velocity vectors around the wipers.
Traditionally, one of the important tests that helped determine whether the right balance had been struck was the wind tunnel. This testing compared a vehicle's noise-level and aerodynamics with the company’s accepted standards.
In the past, there was a lot on the line during wind tunnel testing. One of the major reasons for this was that at any auto manufacturer, acoustic vehicle prototypes aren't generally ready until fairly late in the design process. With little time left, any surprises in the wind tunnel could jeopardize the entire production schedule. This situation left little room for error, and fewer options to fix wind-noise problems when they arose.
"By traditional methods, there was some minor tweaking we could do around the wipers, but in some cases we had no choice but to make changes that weren't ideal, like using thicker windshield glass—which adds mass—to prevent the transmission of the noise to the inside of the car, rather than tackling the source of the noise," notes Karbon. "Per federal safety standards, cleaning performance must also be preserved with any change to the wiper system."
The added cost—as well as the frustration—of having to sometimes make these kinds of late changes prompted Ken Karbon to develop a new process to optimize wiper design early in the schedule.
Virtual simulations streamline the process
Engineers are now able to greatly reduce the element of surprise in wind tunnel testing, thanks to virtual airflow simulations. As a matter of routine, an aerodynamics CFD model is made by GM analysts for every new car design. Ken Karbon decided to take advantage of this existing work and now uses these CFD models in a new process that ensures that windshield wipers won't affect a vehicle's timely completion.
Surface-restrict particle traces simulate oil streaks and identify near-wall flow behavior.
Karbon further analyzes the proposed wiper geometry with the CFD model. When the data is viewed in EnSight, a visualization software program from Computational Engineering International (CEI) in Apex, NC, the resulting graphics make it clear whether the proposed design will meet wind noise standards. And when it's obvious that improvements are needed, the graphics clearly show where improvements can be made.
"Now it's a matter of hours to check the wind noise created by the wipers. And these wiper results are practically free since we utilize existing aerodynamics models," points out Karbon. "Using one CFD simulation to produce a variety of useful results shows the power of computer-aided engineering."
Thanks to the new method, just a couple of hours of work give GM valuable wind-noise data six to eight months before the new vehicle prototype ever arrives in the wind tunnel. In other words, the team has the luxury of these months to fine-tune and adjust designs to reduce wiper wind noise to acceptable levels.
One of the reasons the process has also been so effective is because EnSight has made it incredibly easy to explain and share results with engineering colleagues, or with automotive stylists and other non-technical members of the team.
"The visual results have been a very effective communications tool—particularly with wiper designers. Graphics that show why the noise requirements are being violated tell the whole story. It's no longer just an engineer's opinion," explains Karbon.
In addition, because EnSight allows changes to be made on the fly, engineers can test various wiper positions and configurations, and then supply designers with specific boundaries to work within, such as a particular zone on the windshield where wipers can rest while idle. These guidelines allow room for creativity, while still meeting wind-noise standards.
"None of the techniques I've developed are possible with our other CFD tools. The post-processing features of EnSight are just so much more powerful than any other software we use. It lets us create detailed images and manipulate the data in a way that really fits our approach."
General Motors is now to some extent a model in this particular area of computer-assisted engineering. Not only has the company implemented effective measures for reducing wiper wind noise, but it has also created a better design environment. It has successfully reduced the industry's traditional "drag" and "friction" between styling and engineering colleagues, which means better teamwork. The company is now looking at other areas, such as the designing of side rear-view mirrors, to continue building on its success with CFD and EnSight visualizations.