Truck aerodynamics researchers with scale
model truck inside the wind tunnel at Ames Research Center,
National Aeronautics and Space Administration. Clockwise
from top left are James Ross of NASA Ames and Rose McCallen,
Kambiz Salari, and Jason Ortega of Livermore.
FOR every mile down the freeway, the
average-size American car has to push 5.5 metric tons of air out
of its way. That is, quite literally, a drag. Overcoming this aerodynamic
drag takes a lot of energy. In fact, at 70 miles (110 kilometers)
per hour, the typical highway speed, as much as 65 percent of the
fuel the car uses goes to overcoming air resistance.
The numbers are similar for
big-rig tractortrailers, despite their far greater weight,
and even higher for high-speed trains. Traveling as fast as 185
miles per hour, high-speed trains may use up to 80 percent of their
fuel to overcome drag.
For more than five years, Livermore has led a Department of Energy
project to examine possible ways to make heavy trucks more aerodynamic,
reducing air resistance and thus increasing fuel efficiency. Obviously,
air resistance cannot be eliminated entirely. But engineers estimate
that truck drag coefficients could be reduced by as much as 25 percent
over the next 20 years. In the future, such a reduction would save
billions of liters of diesel fuel annually, or 12 percent of the
Our nations dependence on oil is a national security
issue, says project leader Rose McCallen. Minimizing
vehicle aerodynamic drag can significantly reduce this countrys
dependence on foreign oil.
Current predictions are that at present rates of consumption, world
energy demand will begin to exceed energy available from all sources
by the year 2050. Reducing consumption is just one approach to meeting
this challenge head on.
After the first oil crisis in the early 1970s, trucking companies
adopted a shield that curves up over the top of the trailer to reduce
drag. But two major components of drag on a heavy truck remain:
the gap between the tractor and the trailer and low pressure in
the trailers wake. Friction lossesthe air shear resistance
on the trailertractor sidescontribute 10 to 20 percent
to the total vehicle drag.
Trucks are much more complicated than cars and planes, which
are integrated and streamlined, notes McCallen. Not
only do trucks come in two parts, with a gap in between, but the
trailer has to be a big unstreamlined box to maximize cargo space.
As if the aerodynamic situation werent complicated enough,
tractors and trailers are built by different companies. It
would be wonderful to be able to manufacture a single unit,
she adds, but it just isnt going to happen.
of the back end of a trailer with boattail plates. (a) The
full-scale truck and the scale model, (b) diagram of the
flow field of boattail plates, and (c) simulations of a truncated
vehicle (showing just the back end of the trailer) with and
without boattail plates. Red and blue indicate counterrotating
vortices. The flow is from left to right in the side view.
An Aerodynamic Partnership
Responding to the complex
physics problem of truck aerodynamics is a consortium of Livermore,
Sandia, and Argonne national laboratories,
University of Southern California (USC), California Institute of
Technology (Caltech), National Aeronautics and Space Administration
Ames Research Center (NASA Ames), and Georgia Tech Research Institute
(GTRI), under the leadership of McCallen in Livermores Energy
Technology and Security Program. The DOE Energy Efficiency and Renewable
Energy, Office of FreedomCAR & Vehicle Technologies (CAR stands
for Cooperative Automotive Research), is supporting the consortiums
effort. Four tractor manufacturers have joined the partnership
well: AB Volvo, which owns Mack Trucks, Inc.; International Truck
and Engine Corporation; Freightliner Limited; and Paccar Inc.,
manufactures both Kenworth and Peterbilt trucks.
The aerodynamic design of
heavy trucks is currently based on performance estimates derived
from wind tunnel, track, and road experiments.
Now, with the availability of powerful supercomputers, scientists
can begin to simulate complex tractortrailer air flows. The
trick is to make the simulations reliable and thus predictive.
simulations must also run efficiently, with quick turnaround times.
Only then will they be useful for designers of heavy tractors and
The simulations must be
able to accurately portray the complex interaction between a vehicles many different surfaces and the air striking
or moving past it. The air flow around the front end of the tractor
is complicated by the bumper, head lamps, hood, mirrors, and any
other trim that the driver has chosen to add. The contribution to
drag from the gap flow between the tractor and the trailer and behind
the trailer has already been noted. Air also flows along the underbody
of the truck and in the wheel wells. Computational fluid dynamics,
McCallens area of expertise, is the tool of choice on this
As computer simulations
of various parts of a trucks air flow
are being performed by Caltech and the Lawrence Livermore, Sandia,
and Argonne national laboratories, companion wind tunnel experiments
using models of tractortrailers are under way at NASA Ames,
USC, and GTRI. Livermore, USC, and GTRI are also developing devices
that can be attached to tractors or trailers that reduce aerodynamic
Air Flow in Action
Complex turbulent flowswhether
in the explosion of a nuclear weapon or in the wake of a heavy tractortrailerare
particularly challenging problems in fluid dynamics. Livermore has
been working to simulate turbulence for decades, first for weapons
design and more recently for stewardship of the nations nuclear
stockpile. For this project, Livermores focus is again on
turbulence, this time in the trailers wake. Engineer Kambiz
Salari is leading the Livermore effort.
Salaris team first
developed a simplified three-dimensional (3D) form representing
the typical tractortrailer combinations, a geometry that is
now being used by other researchers around the world. With this
form, Salari and his team are using large-eddy simulations (LES)
to model what happens at the back end of the trailer.
Some of their simulations
have included boattail plates attached to the back of the trailer
to reduce the wake. The figure above shows snapshots
of the flow field with short and long boattail plates and without
these plates. (The plates were designed by a private firm not involved
in the partnership.) The simulations with the plates indicate a
reduction in the trailer wake, which is consistent with wind tunnel
experiments at NASA Ames.
The consortium recently obtained
the first 3D particle image velocimetry (PIV) results in a large
production wind tunnel. Three-dimensional PIV techniques being developed
at NASA Ames use laser beams to measure the velocity and direction
of air flow in a series of planes. Two-dimensional PIV is far more
common, and experiments usually take place in small research wind
tunnels. But NASA Ames is home to a 2-meter by 3-meter wind tunnel,
where the first 3D PIV images were obtained using a one-eighth scale
model. More recently, experiments were run in a 3.5-meter pressurized
wind tunnel at NASA Ames.
using large-eddy simulation software show air flow in (a) the
tractor–trailer gap and (b) the trailer’s wake.
The pressurized tunnel is
capable of simulating the air flow at realistic highway speeds.
Because the wind tunnel is housed in a pressure vessel, the PIV
experiments in the vessel are controlled remotely to avoid blowdown
(pressure release) of the vessel when the laser or cameras are repositioned;
obviously, no one can be in the tunnel during an experiment.
Engineer Jason Ortega, a
member of Salaris team, has been developing new devices that
show promise for reducing aerodynamic drag. Detailed tests incorporating
these devices started this spring at one of the wind tunnels at
Sandia has been using steady
Reynolds-Averaged Navier Stokes (RANS) models to simulate air flow
around a heavy tractortrailer. But researchers are finding
that steady RANS models, which are routinely used to simulate fluid
dynamics and are computationally efficient, do not accurately predict
tractortrailer wake or low-pressure region. So Livermore is
developing hybrid techniques that combine LES and unsteady RANS
turbulence models, resulting in more accurate, predictive simulations.
Drag estimates must be correct before solutions can be found.
results for an unsteady Reynolds-Averaged Navier Stokes simulation.
The truck model used is the cab-over design. Air flow in the
gap between the tractor and the trailer and in the trailers
wake are visible. This simulation methodology more accurately
models wake and hence total drag than steady Reynolds-Averaged
Navier Stokes methods.
A World of Interest
Outside interest in the
partnerships work was evident in the
large turnout for a December 2002 conference in Monterey, California,
on The Aerodynamics of Heavy Vehicles: Trucks, Buses, and
Trains. DOE and the United Engineering Foundation sponsored
this conference that brought together researchers in aerodynamics
from national laboratories, universities, and corporations around
the world. Interest was so high that a similar conference is planned
for 2004. Saving energy is importantreducing drag on heavy
vehicles is one way to do it.
Key Words: aerodynamics, computational fluid dynamics, heavy trucks,
large-eddy simulations (LES), Reynolds-Averaged Navier Stokes (RANS)
For further information contact Rose McCallen (925) 423-0958 (firstname.lastname@example.org).
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