One of the subject areas garnering the most attention at the SAE 2014 World Congress is aerodynamics because of its effect on vehicle efficiency through drag reduction and the potential for real-world fuel economy gains. Since last year, the number of technical sessions related to aerodynamics at the SAE World Congress has increased from seven to 10, and the number of technical papers on the subject has nearly doubled, according to Adrian Gaylard (Member, 2007), Activity Chair for Aerodynamics at the SAE 2014 World Congress and Technical Specialist–Aerodynamics & CFD at Jaguar Land Rover. SAE Magazines Assistant Editor Matthew Monaghan spoke with Gaylard prior to the World Congress to discuss the current state of the field and areas of further improvement.
Is the underbody the main area for aerodynamic improvement as automakers look to become more efficient?
There are things that you can improve on the underbody. Also the efficiency of cooling systems and vehicle cooling, generally. But there’s also still a lot of opportunity in the upper body. What tends to happen over the years is that styles change, and style language shifts, so you always have to go back and look at what opportunities that raises.
Is more attention being paid to aerodynamics with respect to cooling as a result of the move to more sophisticated powertrains?
Aerodynamic cooling flows have always been very important. Traditionally, you have grilles and inlets in the front of the car, so it is able there to be drawn through the radiators or the heat exchangers, and that whole process generates drag. So there’s always been an optimization exercise to make sure you get enough cooling flow to your heat exchangers but not inducing too much extra drag. Automakers are starting to do things like having grille shutters to make sure that cooling flow isn’t being taken into particular intakes when it’s not needed. As we move into different powertrains, EVs need cooling flowing different places for different reasons, so it’s been a long-term concern for automakers and will continue to be something that we look at very carefully.
What are the trends related to aeroacoustic improvement? Are side mirrors considered the most important factor?
Side mirrors are a big contributor, but by no means the only contributor. As powertrains have gotten quieter, road noise has decreased, and our products have become more refined, aeroacoustic sources that perhaps weren’t so intrusive in the past you can now hear them. Door mirrors are important as they flow into the A-pillar and the vortex is generated. But also noise sources are generated over the windscreen, windscreen wipers, as well as underbody noise is generated by the airflow. Those all become important topics and important concerns.
How big of an impact does aerodynamics have on handling and stability?
It’s certainly influential. Speaking from experience with our own products, we’ve not had particular issues, but it is something you have to keep an eye on. Certainly as cars become lighter, then the aerodynamic impact of crosswinds and wakes of other vehicles becomes more important. During the design process, you just have to keep an eye on the side force and yawing moments that your vehicles are experiencing to make sure that you keep those at acceptable limits. The trend for lightweighting will mean that we pay a bit more attention to that area in the future.
One of the papers you’re presenting at the World Congress is related to ground simulation. What are the challenges associated with replicating that?
Certainly in Europe, moving-ground wind tunnels have become the new standard. It’s different in the U.S. I think there’s only one current full-scale moving-ground wind tunnel in the U.S. (Windshear). There’s a growing realization that if we’re going to capture the aerodynamic resistance of a car traveling on the road, then we need to have realistic boundary conditions, so the relative motion of the vehicle on the road, and also wheel rotation, is important.
How do you see the mix between wind-tunnel, on-road testing, and CFD simulation for aerodynamic evaluation moving forward?
The balance is shifting. At Jaguar Land Rover we’re relatively forward-looking, so CFD is our principal design tool for aerodynamics, which we then support with wind-tunnel testing. Traditionally, wind-tunnel testing has been the lead technique, so what’s happened over the past 15 years is CFD has moved out of the research environment and into the development environment. That’s enabled us to work on our designs earlier and to exploit opportunities earlier in the process and do fewer late changes. I do foresee a strong role for wind tunnels in the future, particularly advanced moving-ground wind tunnels—maybe tunnels that can start to simulate more of the real-world on-road environmental impacts. But certainly CFD is a bigger and more important part of what we do.
What are the major challenges still to be overcome related to CFD for aerodynamics evaluation?
Probably the biggest challenge is that all practical CFD simulation has to account for turbulence using some sort of general model. We can’t simulate it directly. There’s no general theory of turbulence; it’s one of the last unsolved problems in classical physics. So we’re always having to make some approximations in effect to turbulence, and that does mean that there’s going to be some sort of difference in what we simulate from what we measure. Also, to do a good job with aerodynamic simulation, you need to run large models that are very computationally hungry, so the challenge is having the full speed of feedback that you want in your interactions with designers.
How does an event like the SAE World Congress help in your role as an aerodynamics specialist?
For me, it’s three days of immersing myself in a subject I love that I’ve been doing for the last 25 years. I get to see some of what my colleagues are thinking about in other parts of the industry, get to see some of the latest thinking from academia, and get to show some of my own work and have people ask me questions about it. So we share knowledge; I think it’s really very valuable for that. Also, for the informal contacts you make and the network you build up of people with similar interests, I think it is very important.Continue reading »