An aerodynamics engineer conducts a wind-tunnel test on a 25-scale model at SOLUS Solutions and Technologies LLC.
RICHARD WOOD isn't on a mission. He's on two of them.
The 59-year-old president of SOLUS Solutions and Technologies LLC wants to continue his exhaustive research to find better ways to make trucks and trailers more aerodynamic, and use that research to make the products that will save fuel and make trucks and trailers more efficient.
But that's not enough. There's also his role as an educator.
Wood — who chairs the Society of Automotive Engineers' (SAE) Truck and Bus Aerodynamic and Fuel Economy Committee and is a member of the American Trucking Association's (ATA) Engineering and Technology Policy Committee and the Technology and Maintenance Council (TMC) Future Truck Committee — believes mounds of misinformation and factually incorrect statements are severely damaging the move to make aerodynamics more acceptable and understood.
Surprisingly, Wood says some of it is coming from the Environmental Protection Agency's (EPA) SmartWay effort and the Department of Transportation's (DOT) effort on fuel economy.
“When I hear an inaccurate statement, I ask, ‘Where did you get that information?’” he says. “They'll say, ‘Well, I talked to someone.’ So their only source is that someone told them. That's fine, but can you show me a published paper that supports the claim? Because five people say that Joe told them that does not mean Joe is right. It simply means that's what Joe says. So when the government is starting to use statements without supporting evidence, it becomes a concern, because guidelines and policies must be based on facts.
“The reason I'm interested in this topic is to reduce the number of false aerodynamic assumptions or statements being made that imply that one size fits all and there's no complexity to the function of aerodynamic technology. As a result, some fleets say they see huge benefits which are impossible to achieve because physics won't allow it. Or some fleets get no benefit because they are utilizing a skirt incorrectly on or they run in conditions where the slider is in a rearward position. In the latter case, the skirt is going to provide minimal savings.
“People need to understand that skirt fuel savings will vary with trailer setup, and it is likely to be lower than the EPA SmartWay test result. They also need to understand that depending on where they operate in the country, the effect of weather is dramatically different. In the Southeast, the average wind effect is less than half of the average wind in the Northwest. That difference in wind means a dramatic change in the fuel economy of your vehicle. People need to know how the device is going to work where they drive.”
Over the past six months, Wood has been tracking factually incorrect statements in articles he reads about aerodynamics.
“A typical claim is, ‘If you install a specific aerodynamic device, you're changing the air turbulence,’” he says. “Well, no you're not. Even if you did, you wouldn't change the aerodynamic force. So statements are made and printed. And the source is referenced as an ‘expert.’ They're entrepreneurs selling products or people who accept the title of ‘expert’ based on their use of buzz phrases.”
Wood started getting annoyed because these statements could be interpreted as facts and misused. He could envision how they'd factor into sales material. He thought, enough is enough.
He started creating a list of facts to replace the factoids that were spreading through the industry.
Fact: Smoothing the trailer surface will not create laminar flow or reduce friction drag.
Wood's explanation: Friction drag reduction can only occur by changing the boundary layer from turbulent to laminar. However, at operational speeds a laminar boundary layer is only possible in the first 10% of the trailer length. As a result, 90% of the trailer will have a turbulent boundary layer with a constant friction drag level.
“My motivation drew out of the frustration from repeatedly hearing the same misleading aerodynamic claims,” he says. “I'm not saying that it has been intentional, but they could have heard it, and pretty soon it becomes a factoid — something that everybody believes is true but is not really true because there's no basis for it or proof for it. It's just a phrase people use. You hear those phrases get cycled back and people start explaining the physics of their products based upon statements, which do not truthfully describe the way the aerodynamics are working. I said, ‘This is getting out of control. The fleets are going to start using these statements, and as a result, aerodynamics is going to go by the wayside. Instead of people embracing aerodynamics, it's going to decay.’”
Wood believes statements should be supported by published literature authored by recognized experts. To be accepted and published by professional societies, a work has to be supported by numerous published papers that support your findings.
“Opinions aren't accepted in the engineering community — only facts are accepted and only if they are published,” he says. “That's the gold standard. It means it's been peer-reviewed and accepted.”
For the past few months, Wood has been developing and writing two papers to present at the SAE Commercial Vehicle Engineering Congress (ComVec) October 2-3 in Chicago, where he chairs the sessions on heavy-truck aerodynamics and fuel economy, technology, and testing.
One is on the sensitivity of crosswinds and yaw effects on vehicles and how that changes with different devices, because different devices — such as a skirt or boat tail — all behave differently if the wind is not blowing versus when wind is blowing and coming across the vehicle.
The other is on the Reynolds number, an engineering parameter that guides when a test is valid or not, whether a researcher is getting good data out of a test. The Reynolds number is critical for wind-tunnel testing and also influences coast-down testing, which is required by the EPA and DOT as part of heavy-truck testing.
Loving what he does
Vic Suski, an industry veteran who retired last year as a senior engineer for ATA and TMC, first met Wood in 2000. At that time, Wood was working at NASA's Langley Research Center as a senior research aerodynamic engineer specializing in aerodynamic drag reduction, and was involved in a concept called passive porosity — porous panels attached to the rear of trailers to reduce aerodynamic drag. NASA and Southeastern Freight ran road tests, some results of which were reported in Simulation of Flows With Passive Porosity at the 2002 International Council of the Aeronautical Sciences (ICAS) Congress.
Suski says that at the time, Wood was one of the few people in NASA interested in truck drag reduction. Over the years, Suski and Wood collaborated on a number of presentations to TMC.
When NASA started de-emphasizing aerodynamic research, Wood started looking at other areas, and became fascinated with helping the truck and trailer industry deal with energy concerns.
“When you talk about airplane aerodynamics and go to an aircraft company, even small companies would have hundreds of aerodynamicists and mechanical and electrical engineers studying problems,” he says. “There's a whole body of knowledge out there that's relied upon.
“Trailer manufacturers have outstanding engineers and designers, but they're trying to reduce weight and produce high-quality products. They typically don't have aerodynamicists. Tractor manufacturers, when I first started, were lucky if they had more than a handful of experienced aerodynamicists. Now they have considerably more resources focused on the application of established aerodynamic principles to improve vehicle performance.
“So as I see problem areas that could impact fleets, and the acceptance of aerodynamics, I'm just trying to add some education to the conversation so that people can make the right decision. Whatever that decision is, at least they're basing it on facts and knowledge of what's happening on their vehicle.”
Testing aerodynamic devices
The other part of his mission is producing the devices that can deal with the forces that trailers and trucks encounter. Wood's company, SOLUS Solutions and Technologies LLC, develops advanced truck and trailer aerodynamic drag reduction technology. The company's current portfolio contains 12 patented or patent-pending innovations. Last October, 18 products became SmartWay-verified.
Trucks and trailers generate aerodynamic drag in four main areas: front of the tractor, gap between tractor and trailer, underside of the trailer, and rear of the trailer. Here are a few of the ways engineers have targeted these areas, along with Wood’s rough estimate of the percentage each idea is said to reduce drag: Top row: wake boards, 7%; vortex strake, 7%, high-momentum mudflaps, 3%; fairing, 3%. Bottom row: passive porosity, 4%; boattail, 5%; side skirts, 2%; and vortex traps, 7%.
Southeastern Freight's Foster says his company started using the wheel-cavity covers in March, putting them on a tandem-axle Volvo VN based out of Columbia, South Carolina, on a dedicated run to Jacksonville just about every day. Foster is gathering and analyzing the data and expects to come to some conclusions this fall.
“We were hoping to get 1% (fuel reduction),” he says. “If we could get that, I'd be tickled to death. The challenge is measuring it. One of the challenges we have with aerodynamics is the cost in ROI in our operation. If you're not running aerodynamics at a 40-45 mph road speed, what effect do they have? Pretty much nil.
“We have two major classes of vehicles: line haul and LTL. With line-haul trucks, because we do such a good job of utilization on the vehicles, we're running line haul at night and using the in LTL during the day. If we're not careful, it's hard to segregate data and figure out what, if anything, something's doing for you. From a trailer standpoint, we have trailers used in both operations as well, so if you put a lot of aerodynamics on a trailer, it's sometimes hard to keep that unit in a dedicated run to where it gets the full benefit of that expense and have an ROI on the product itself. We're a little bit different than a truckload guy because we're LTL.”
Charlie Fetz, Great Dane's vice-president of design and development, says he's surprised that Wood's ideas were not embraced more quickly.
“I think aerodynamically he's a whiz,” Fetz says. “His wheel cover with a hole will probably get some traction.”
For these types of products to succeed, they will have to overcome market skepticism.
“I've seen some of these aerodynamic devices in the past that I thought would probably work that didn't test out,” Fetz says. “They probably could have gotten somewhere but they just didn't pay off in terms of aerodynamic benefits: things on the front of the trailer to cut down on air flow through the gap and things on the rear of the trailer to help you with boundary layer and separation — strakes.
“I've seen some articles out there that are gratifying to me that say skirts really do work. If I was an owner-operator, I would have a whole bunch of aerodynamic devices on my trailer because fuel would be coming out of my pocket. But I understand why only certain devices fit into fleets' methodology. Only certain things will work for them.”
Wood says it's just as damaging for fleets to think there is a benefit to an aerodynamic device that is physically impossible as it is for them to hear someone say there is no benefit at all.
“I think if fleets are reluctant, it's because the information that's out there is piecemeal. There's not a consistent message. For example, the fundamental behavior of a skirt is related to the size of the skirt and its proximity to the rear wheels. If you overlay all skirts on the market, you will see they vary in size by less than 10%, but their claimed benefits vary by 30%. Well, the physics don't support the claims. In addition, tests data for a 4% skirt could show 7% fuel savings if it's tested under certain conditions and data for a 7% skirt could show 4% fuel savings.
“The advertised fuel savings don't tell the story of what that fleet is going to get when they use an aerodynamic device. Some fleets get a 6% skirt and say, ‘I'm getting 8.’ Well, they can't get 8. The physics aren't there. For example, a fleet may purchase a skirt that is EPA-verified at 5% fuel savings but the fleet claims they are measuring an 8% fuel savings. The problem is that a 5% skirt cannot provide an 8% fuel savings when hauling a load.”
Various non-aerodynamic factors can influence fuel savings. A fleet, for example, may see the additional 3% fuel savings by reducing vehicle weight.
“When you do an EPA test, you add a fixed load weight inside the trailer to make sure all rolling-resistance values are the same. But if someone changed the weight from a 35,000-pound load for a Type II (J1321 Fuel Consumption Test Procedure) to an empty trailer, then the rolling resistance is cut by 50% and fuel savings by 25%.”
He says sometimes fleets claim they put on a skirt and their mileage got worse. This can occur if the fleet started collecting data for the baseline vehicle in the summer and tested the skirt in the fall.
“The change in weather on aerodynamics going from summer into fall has a great effect on the performance of the skirt or any aerodynamic device (in terms of percent reduction in fuel usage),” he says. “As it gets colder, aerodynamic forces change. The skirt could be helping 5%, but compared to summer fuel-economy values, the vehicle could be losing more than 5% due to temperature change.
“There are all kinds of funny things going on out there. I don't want to assume any action is intentional, but if fleets are not aware of the facts, there's no way for them to evaluate product claims and do a valid in-service assessment.”
And so Wood continues his mission to educate.
What do you know about how air flows?
Consultant Richard Wood has spent a career studying aerodynamics. Here are a few of the things he has concluded over the years:
- The trailer freight box is still the best aero shape for hauling goods on US roads.
Trailers are built to haul freight. Because of state and federal laws, that shape is a box. Even so, some minor changes can be made to the box to significantly affect aerodynamic drag. Trailer manufacturers should focus on edge shaping and not reshaping the box. Edge shaping can recover lost volume and reduce drag on the front, rear, and undercarriage. The top and bottom side edges can be modified to generate controlled vortices to manage undercarriage, tire, and wake flows. Shape changes to undercarriage structures can reduce drag in crosswinds, and new wheel devices can control wheel drag.
- The radius front corner of a trailer reduces vehicle drag up to 5%.
The trailer front-side-edge radius reduces drag by accelerating the air flow around the edge, resulting in lower edge pressures. While the edge radius reduces flow separation on the trailer sides, the presence-flow separation does not impact trailer drag.
- Aerodynamic drag is composed of pressure drag and friction drag. However, trailer drag is more than 90% pressure drag.
Pressure drag results from air pressure pushing on forward and rearward facing surfaces while friction drag results from air molecules attaching to and scrubbing the surfaces. At low speeds, these two drag forces may be equal. However, at operational speeds, pressure drag dominates due to the increase in air pressure with vehicle speed while friction drag remains unchanged.
- The sides and top of a trailer do not generate pressure drag, not even in crosswinds.
Because the trailer sides and top are parallel to the vehicle direction, these surfaces can only generate friction drag, and cannot generate pressure drag. The increase in total drag with crosswind results from the air pressure pushing on the front surface, rear surface, undercarriage, wheels, and tires, and not the side of the trailer.
- A roll-up door trailer has lower drag than a swing door trailer.
A roll-up door is constructed with a small recessed cavity at the back of the trailer, which is not present for a swing-door trailer. This recess separates the rear surface of the trailer from the wake flow, creating a buffer and an increase in air pressure pushing on the trailer rear.
- Reynolds number is the most important criteria in aerodynamic design and testing.
Reynolds number varies with vehicle speed and vehicle/device size. The minimum representative Reynolds number is the Reynolds number value above which aerodynamic effects do not change. The minimum Reynolds number for a trailer occurs at ~30mph. Due to their small size, aerodynamic devices typically have a minimum Reynolds number different from the trailer. To maximize drag reduction the device and trailer, minimum Reynolds numbers should be similar.
- Skirts lose effectiveness with increased skirt-to-wheel gap.
To be effective, a skirt must cover most of the rectangular area between the landing gear and tandem, and between the trailer lower surface and road. The vertical dimension is fixed and is easily satisfied. However, the horizontal distance varies with bogie position. Skirts that fail to adjust for a varying skirt-to-wheel gap will allow air to impact the bogie and lose effectiveness.
- Gap fairings lose effectiveness with reduced truck-to-trailer gap.
To be effective, a gap fairing requires a large gap for air to enter the gap and interact with the device. For gaps less than 36 inches, the trailer OE side-edge radius maximizes the drag reduction benefit. Advanced edge-radius designs can be used to increase the drag reduction for all gaps.
- Boat tails lose effectiveness with increased crosswind.
A boat-tail design has a narrow performance band dictated by the boat tail angle and length, which are a function of vehicle speed, length, height, and width. Boat-tail performance also requires the trailer boundary layer to remain attached as it flows onto and along the device. In a crosswind, both the trailer and boat-tail flow change dramatically, lowering the boat-tail surface pressure, which results in a performance loss.
- A rolling road wind tunnel is not superior to any other wind tunnel.
Reynolds number is the most critical wind tunnel test parameter. Simulating the road surface (ie, tunnel floor) is just one of many lower order criteria that must be considered in the test process. The most critical feature in road-surface simulation is the tunnel-floor boundary layer and not the movement of the floor. There are numerous industry-established techniques to properly simulate and manage the tunnel-floor boundary layer.
Wood can be contacted at firstname.lastname@example.org.
Glossary of terms
Boundary layer: The layer of low-velocity air immediately adjacent to the surfaces of the vehicle. All boundary layers start as laminar and then transition to turbulent.
Laminar flow: A boundary layer in which the air is steady and flows parallel to the surface.
Turbulent flow: A boundary layer in which the air is unsteady and does not flow parallel to the surface.
Reynolds number: Aerodynamic parameter used to determine the boundary layer and aerodynamic similarity between a vehicle design of different sizes and/or operating at different speeds. Example: a ¼-scale model of a truck would need to be tested at four times the speed of a full-scale truck to have the same aerodynamic drag.