Building a Better Big Rig: Simulations Improve Aerodynamics
Annually, millions of tractor-trailer and tanker combination trucks burn billions of gallons of fuel across the United States. To improve fuel-efficient transportation technology, the Department of Energy’s (DOE’s) Heavy Vehicle Aerodynamic Drag Consortium studies air resistance and fuel consumption and guides industry partners toward fuel efficiency improvements. Lawrence Livermore supports the consortium in an initiative known as SuperTruck 1, which focuses on the design and development of drag-reducing innovations (see “Rise of the Super Truck” in Science & Technology Review). Laboratory researchers have used high performance computing to simulate a range of solutions, and recently these heavy vehicle simulations came to life inside a wind tunnel.
Principal investigator Kambiz Salari, from the Center for Applied Scientific Computing’s Computational Physics Group, along with engineer Jason Ortega, leads the Livermore team participating in the consortium. Among their innovations is the Generic Speed Form (GSF) model, which combines different components of vehicle geometry to simulate airflow over, under, and around heavy vehicles. GSF’s versatile code enables researchers to quantify fuel economy performance of body modifications and add-on devices. Ultimately, GSF results inform new vehicle designs.
In the first phase of the project, Salari’s team designed the first generation of the GSF model (GSF1), improved the performance of several aerodynamic devices for use on heavy vehicles, and conducted the first series of wind tunnel tests. In the second phase, the team designed GSF2 using analysis of wind tunnel results, adapted attachable devices for better drag reduction, and investigated under-body airflow. In addition to design improvements and wind tunnel tests, these activities required close collaboration with the rest of the consortium to establish a virtual testing environment and conduct track and on-road demonstrations.
From the moment they get under way, big rigs expend energy trying to overcome drag and rolling resistance (friction caused by wheels on pavement). Nearly every part of the truck affects its aerodynamics in some way: hood, tires, wheel wells, mirrors, door handles, and so on. Moreover, because the cab must move independently from the trailer/tanker, the space between them causes complex flow patterns that contribute to drag.
Drag-reduction solutions take many forms. For instance, standard trucks can be retrofitted with aerodynamic devices along the bottom of the trailer (skirt), on the tail, and covering the gap between cab and trailer. Using single wide-base tires instead of doubling up can help lower rolling resistance. More comprehensively, the vehicle can be redesigned to minimize opportunities for drag.
Because there are too many potential designs to test physically, airflow simulations are crucial to narrowing down the possibilities. The most promising SuperTruck designs were tested as one-eighth-scale prototypes in a small wind tunnel (7x10 feet) at the National Aeronautics and Space Administration’s Ames Research Center in Mountain View, California. After adjustments based on prototype data, full-size models were tested in an 80x120-foot tunnel with 55 to 65 mile-per-hour winds to duplicate road conditions.
Figure 1. A truck’s shape and speed, as well as air pressure over, under, and around the vehicle, affect aerodynamic drag. Livermore’s Generic Speed Form (GSF) model simulates flow patterns for the SuperTruck vehicle. (Click to enlarge.)
The Power of Geometry
As a meshed hydrodynamics model, GSF simulates the geometry of separate, highly turbulent flow fields around a vehicle. The team optimizes the geometry of large and small flows, such as eddies near a vehicle’s surface, with Navier-Stokes partial differential equations. “In order to retrofit and replicate what’s on the road, we have to model in detail,” says Salari. “It takes a significant amount of computer resources to simulate these heavy trucks.” GSF simulations are run on Sequoia, Livermore’s most powerful supercomputer.
GSF uses a baseline geometry that measures drag and yaw—the crosswind—which present in a U-shaped curve. A yaw angle of 0 degrees indicates the lowest body-axis drag level (no crosswind), and drag increases as the vehicle pitches to the right or left. When GSF geometries are applied, however, the behavior is “flipped”: The maximum drag occurs at 0 degrees. “Beyond a certain point, we see a sailing effect where the forward force overcomes and exceeds body-axis drag,” explains Salari. The drag coefficient decreases, and eventually the drag trends negative. These results signify a breakthrough for the SuperTruck design. Salari says, “The GSF team is the first to create a situation in which there is no body-axis drag.”
Figure 2. Drag and yaw data for Generic Speed Form (GSF) simulations are markedly different from the geometry of the baseline U-shaped curve. (Click to enlarge.)
By the Numbers
An individual Class 8 tractor-trailer rig logs approximately 66,000 miles every year, achieving only 5.8 miles per gallon on average. With even worse gas mileage, tanker trucks represent another opportunity for fuel efficiency improvements. Class 8 trucks are long, tall, and constrained by size regulations, posing a major challenge for design improvements. According to Salari, vehicle shape can evolve only so much within these limitations, making it difficult for the team to improve aerodynamics and fuel economy. Nevertheless, the SuperTruck 1 wind tunnel tests demonstrate increased mile-per-gallon efficiency and reduced carbon dioxide emissions.
The team has entered the project’s third phase by beginning GSF3 designs, and the work could potentially extend to light-duty and other heavy-duty trucks. Additional tasks include continued tanker fleet improvements and investigations of truck “platooning” in which the lead vehicle’s aerodynamics affect airflow around the trailing vehicle(s). “To radically change fuel efficiency, we need a radical change in vehicle design,” notes Salari. “With our GSF capabilities, the nation can conserve billions of gallons of diesel fuel and tens of millions of tons of carbon dioxide emissions per year.”
Figure 3. Fuel efficiency estimates due to SuperTruck’s decreased aerodynamic drag. (Click to enlarge.)