AIAA Technical Paper, Reduction of Semi-Truck Aerodynamic Drag

1
American Institute of Aeronautics and Astronautics
Reduction of Semi-Truck Aerodynamic Drag
Matthew A. Johnson1
, Gabriela Mendoza2
and Sifat Syed3
Viterbi School of Engineering, University of Southern California
3650 McClintock Ave, Los Angeles, CA 90089
The study focuses on a suspension system designed by Road Hoss Inc. which reduces
the frontal surface area of semi-trucks. The system replaces the traditional chassis by
lowering the freight container between the drive and rear axles, thereby reducing the height
and aerodynamic drag on the vehicle. To maximize this reduction, it is necessary to reduce
the adverse effect of an enlarged gap between a semi-truck’s drive axles and the trailing edge
of the cab which results from lowering the container. Wind tunnel testing was conducted on
a 1/34th scale model of a semi-truck at Re = 2.8 × 105
in order to determine the drag
reduction benefits of height reduction and the addition of fairings around the cab-container
gap of the truck. Computational fluid dynamics analysis was used to further analyze gap and
surface flow regions and model overall drag reduction to confirm similarity between the
wind tunnel model and a full-scale road condition. The drag reduction due to change in
height was 24% from wind tunnel testing and 27% from CFD results. The addition of gap
fairings created an additional benefit yielding an overall 38% reduction in drag.
Nomenclature
A = frontal area of the truck
CD = drag coefficient
D = drag force
G = gap size
!
"
= normalized gap
h = height
#
"
= normalized height
k = roughness parameter
L = length
Re = Reynolds number
U = free stream velocity
𝜌 = density of air
𝜈 = kinematic viscosity of air
I. Introduction
In recent history, standardized intermodal containers have dramatically improved cargo transportation
speeds on ships, trains and semi-trucks. However, the efficiency of commercial highway trucks has been nearly
stagnant since the 1980s and currently accounts for 20% of US greenhouse gas emissions.1
In the common road
configuration, a container sits on a flatbed above the vehicle’s axles and tires, about 50 inches off the road. An
emerging company, RoadHoss, is developing a system that suspends a container between front and rear axles,
moving the container closer to the road and reducing the frontal surface area of the tractor, as shown by the
streamlines in Fig. 1.
1,2,3
Undergraduate Student, Mechanical Engineering, Student Member

2
In order for a product like RoadHoss to maintain use of the global trucking infrastructure, the suspension system
would need to be compatible with current tractor designs. Due to the location of the rear axles on the cab, a
significant gap is created between the cab and container as shown in Fig. 2. Gap treatments have been well
researched to avoid the increase of pressure on the leading edge of the trailer caused by the flow being entrained into
the low-pressure gap.2
This study will measure the reduction in a tractor-trailer’s drag caused by the lowering of the
trailer and the addition of gap fairings. These combined improvements will be quantified through drag
measurements in a wind tunnel and with Computational Fluid Dynamics (CFD) using Star CCM+.
II. Methods
A. Wind tunnel testing
A moving ground plane is the largest difference between a truck travelling on the road and a stationary
wind tunnel model. There exist several techniques to simulate the moving ground phenomenon on a small scale.
This allows for effective wind tunnel testing when there is a relatively large boundary layer compared to the model
size. The results from these small scale experiments can therefore be used to quantify trends in drag coefficient with
changing height and gap size.3
Three common methods to represent the ground plane in wind tunnel testing are the image method, a
moving ground plane and a stationary ground plane. The stationary plane technique uses a thin plate the length of
the model and raises it above the boundary layer of the wind tunnel wall. This method causes drag effects that do not
occur in a real road condition due to the ground plane boundary layer interacting with the flow on the underside and
wheels of the truck model.4,5
This effect is shown on a cylindrical bluff body in Fig. 3.
Figure 1: Flow comparison of a standard semi-truck and the RoadHoss design 19
Figure 2: Truck model with varied quantities height (h) and gap size (G) as well as a diagram of wind
tunnel setup and dimensions.

3
It can also be seen from Fig. 3 that the image method produces a uniform flow field that closely parallels
the moving ground plane simulated using a moving belt that matches the speed of the oncoming flow. Due to the
complexity of a creating a high speed moving belt, the image method has been selected as the most accurate and
manageable simulation method.
In the image method, two models are fixed to each other where they would normally be in contact with a
moving ground plane. Due to the symmetry of the model and flow, no air particles should pass the plane of
symmetry.5
This better simulates a road condition than a model resting on the floor with a developing boundary
layer.6
The characteristic length used to determine the Reynolds number in the wind tunnel was calculated
according the square root of the frontal surface area of one of the mirrored truck models. This value was 𝐴'() =
0.306 𝑓𝑡 and the wind tunnel speed during testing was 𝑈 = 145 67
8
. This length parameter is used because the size of
the frontal area contributes to the dominant factor of pressure drag in bluff body aerodynamics.
𝑅𝑒 =
𝑈 𝐴
𝜈
(1)
A truck on the road experiences a Reynolds number near 5.3×10<
while the wind tunnel model will be
2.8×10?
, a factor of 19 lower, due to the limitations of the wind tunnel. Similar Reynolds numbers have been used
in past research on scaled models of bluff body heavy vehicles in order to determine drag. 7,8,9
It is common at
subcritical Reynolds numbers (𝑅𝑒 < 10<
for bluff bodies) for drag reduction benefits to be converted to a
percentage as ∆𝐶D/𝐶D.3
This is how the data will be analyzed so that in the case of CD being different than a full-
scale tractor-trailer, the relative drag contributions of varying height and gap distance will remain the same.
Boundary layer tripping was attempted to eliminate Reynolds number dependencies in the areas of interest,
especially where the gap width is being adjusted. Using previous research on tripping, the roughness Reynolds
number, 𝑅𝑒F = 62510
and characteristic length, 𝑥/𝐿 = 0.111
were chosen as parameters to extract roughness sizes
from an empirical relationship.12
Sandpaper strips with roughness heights of 𝑘JK(LJMK = 230 𝜇𝑚 and 𝑘JK(PQRK =
260 𝜇𝑚 were attached to the corresponding section of the model as this was the minimum requirement for inducing
a turbulent boundary layer.
Many variables contribute to the overall aerodynamic drag of a truck and it can be difficult to study
individual variables in isolation. For this study on the RoadHoss design, reducing pressure drag by lowering the
container requires closing the resulting gap with fairings which then create additional skin friction. When looking at
Figure 3: Constant velocity contours at 2.4 m downstream of the leading edge of a longitudinal cylinder.
Three ground plane simulation methods are shown. Reproduced from Diuzet.6

4
the results it is clear that the total drag coefficient is made up of an unchanging magnitude that is not under
consideration as well as the drag corresponding to the dependent variables in question.
𝐶DJMJ = 𝐶DS + ∆𝐶DU(K (2)
According to the breakdown of drag contributions from Machado13
, calculations were made for expected
reductions in drag from the proposed modifications. The lowering of the container reduced the frontal surface area
by 20% and the container then blocked 75% of the rear wheels. This combination gave a predicted 21% reduction
in drag. Additonally, it was predicted that when the tractor-trailer gap was reduced to the minimum practical
distance for a turning truck, the total drag could be reduced by around 33%.
Calculations for the magnitude of drag on the wind tunnel model were performed in order to determine
force balance resolution requirements in order to measure these changes. The parameters being used are 𝑈 = 145 𝑓𝑡
𝑠
for the free stream velocity, and 𝐴'() = 𝑤 ⋅ ℎ = 13.5 𝑖𝑛^
for the maximum frontal area.
𝐷R)`RLJRa =
1
2
𝐶D ⋅ 𝜌 ⋅ 𝑈^
⋅ 𝐴 = 6.5 𝑁
(3)
An estimate for the drag coefficient 𝐶D = 0.6 is based on previous research and is a result of the geometry
of an average tractor-trailer shape independent of scale.3,9,13
This number was found repeated in several studies
including one that used a 1/24th
scale model of a tractor-trailer configuration at a similar 𝑅𝑒 = 2.1×10?
.8
This 𝐶D
value was used as an estimate and was measured directly when drag data was obtained. Values for density of air
were considered at standard sea level conditions, 𝜌 = 0.0765 cde
fgh . The relative drag uncertainty due to the resolution
of the AMTI FS6-100 force balance was ∆i
ijkl
= m.n o
p.q o
= 3%, which was enough to measure the 21% change
expected.
Gap size was normalized according to !
"
as defined in Fig. 2. Without the use of more complicated spring-
loaded gap fairings, semi-trucks have a necessary minimum gap size of about 3.5 feet in order to complete turns
without the container damaging the back of the cab. This corresponds to a normalized gap size of !
"
= 0.3. The gap
size of our wind tunnel model was reduced by incremental gap fairings from !
"
= 0.67 to !
"
= 0.09, in order to get a
full picture of the contribution of the gap to the overall drag. Additionally, the height of the trailer was expressed as
a ratio #
"
and compared to the drag coefficient in testing.
B. Full scale CFD testing
Computational Fluid Dynamics was used to confirm these results and examine local flow conditions in
Figure 4: Distribution of drag coefficient (𝐶DJMJ = 0.6) across vehicle.

5
more detail. STAR CCM+ software was used to perform CFD on the model as an alternative option since many
operating conditions can be varied beyond the capabilities of the wind tunnel. The advantages of performing CFD is
that it provided detailed visual capabilities of the flow around the model and allows for data of simulated real road
conditions from a full-scale model to be obtained.7
Since the wind tunnel Reynolds number is lower than the full-scale, computational fluid dynamics analysis
is conducted to determine the differences occurring in these scenarios. With many parameters to vary in CFD
software, previous researchers have detailed parameters and setup they used for a high fidelity fluid dynamic
model.16
The numerical method selected to run this analysis is Reynolds-Averaged Navier-Stokes (RANS) equations
because it is most suitable for external flows with time dependencies at lower Reynolds numbers. RANS equations
provide information to simulate turbulence by calculating the time-averaged properties of the flow since high-speed
flow is generally turbulent, three-dimensional and time dependent.21
With RANS equations, these fluctuations can
be simplified for steady state situations by averaging the transient behavior.
In the boundary layer region, the model has a finer mesh to resolve the fluctuations due to the surface
roughness. The mesh gradually becomes more coarse in the region far from the wall where the flow becomes more
steady and laminar. For this reason, all y+ wall treatment was selected due to its capability of adapting to the
combination of coarse and fine meshes.15
The turbulence model selected for the simulation was the Shear Stress Transport 𝑘 − 𝜔 model (SST). This
option within STAR CCM+ is for transition models and offers a shear stress limited and automatic wall treatment. It
has also been shown to be an efficient and accurate selection for many vehicle applications.15
It has been previously
suggested that this is the best option for both steady-state and unsteady simulations.15,16
It utilizes transport
equations from the 𝑘 − 𝜀 model and transforms it into an omega transport equation by variable substitution. Both,
𝑘 − 𝜔 and 𝑘 − 𝜀 are combined to study the flow close to the walls and in the outer boundary layers.15
The meshing model selected is surface remesher with prism layers and a volume polyhedral mesh.
Polyhedral mesh was selected given that, compared to the other models available, it is more efficient, requires no
surface preparations, and produces less cells which reduces the simulation time for a complex model.15
Prism layers
are the best choice for resolving turbulent boundary layers and they improve accuracy on flow solutions due to their
gradual height growth normal to the wall.15,20
III. Design and measurements
A. Wind tunnel model design
The wind tunnel model was designed as a simplified 1/34th
scale of a full-size semi-truck. Small details like
mirrors and hood contours were omitted due to their negligible effects on overall drag within the scope of this study.
The height of the tractor and trailer were adjustable to allow for the reduction of the frontal surface by 20% and the
height between the trailer and the ground by 75%.
The model was hotwire cut out of high-density foam and coated with epoxy to minimize surface roughness.
As the trailer was lowered, the tractor height was reduced to an equal height, to eliminate unnecessary pressure drag.
Presented in Fig. 5 are the tractor tops, which were exchanged to allow the height of the tractor to equal the height
setting of the trailer. In order to utilize the image method to simulate a moving ground plane, two identical models
were fixed together with epoxy at the tractor and rear wheels, as shown in Fig. 2. The force balance mount was
attached below the center of mass of the assembled model, to reduce moments applied to the force balance.
Model fairings were made to simulate the gap fairings that are commonly used in the trucking industry to
ensure that the flow does not separate at the rear of the tractor and collide with the trailer, causing additional
pressure drag and leading to a two-body effect. Six fairing sizes were constructed so that the relationship between
gap distance and drag could be accurately quantified, as shown in Fig. 6.

6
In order to try to replicate the
turbulent boundary layer caused by road
conditions, strips of sandpaper were inserted
on the front surface of the tractor to trip the
flow, as shown in Fig. 2. However, no
differences were resolvable from the boundary
layer trips so they were omitted from the
model in most of the tests. Their minimal
effect combined with the result of a Reynolds
number sweep showed that the boundary layer
was likely already turbulent and represented a
larger scale road-condition well.
B. Wind tunnel testing procedure
Slots in the front and back of the
container allowed it to be raised to the desired
height for testing. Tractor caps were also used
in order to have a continuous height across the
vehicle and avoid abrupt flow changes as seen
in Fig. 5. To reduce vibrations during testing,
wooden wedges were inserted into the locking
mechanism that held the model together.
Upstream velocity measurements were taken
using a pitot-static tube and pressure
transducer. The average velocity was also
calculated and its uncertainty was determined
to be the precision, 0.2 m/s. The velocity was
varied from 40 to 10 m/s when testing each
tractor height to determine any Reynolds
number dependencies.
Force balance calibrations were
performed to ensure reliable results during
testing. The wind tunnel fan was set to the
desired velocity and was given several minutes
to ensure that the flow was steady. Twenty
data samples were taken from the force balance over the course of thirty seconds and were averaged to determine the
drag. The drag uncertainty resulted from the force balance resolution of 0.2 N.
C. Full scale CFD setup
The optimum mesh size was selected by analyzing previous CFD performed on semi-trucks and refining
the mesh in three stages; fine, medium and coarse mesh. The initial mesh base size was selected based on a previous
CFD study performed on Mesh Optimization for Ground Vehicle Aerodynamics. Analysis of CFD and wind tunnel
testing for a 1:3 scale model showed that the difference in drag coefficient results decreased from 5.5% to 0.9%
when the mesh size relative to the length of the vehicle reduced from 7.2% to 3.6%.20
Additionally, the difference in
drag coefficient remained under 5% when decreasing scale ratio. The prism layer thickness used in previous
research was 33% with 2 prism layers as a first attempt to obtain accurate estimation of velocity near the wall.20
For
the first simulation, the mesh size selected was 4.8% relative to the length of the truck. A refinement to 2.4% mesh
base size was found to change the drag coefficient by less than 1%; however, the run time doubled. Therefore, in the
remaining simulations, an absolute mesh size of 4.8% of the length of the model was used and the number of prism
layers was 9 with a 33% prism layer thickness. Increasing the number of prism layers while keeping the mesh size to
4.8% relative to the model’s length contributes to a larger total number of cells at the viscous sublayer. This created
Figure 5. CAD model of wind tunnel model with adjustable
tractor and trailer heights. Tractor top attachments with
heights A=0.25”, B=0.50”, C=0.75”, and D=1.00”.
Figure 6. Tractor with examples of gap fairing lengths that
ranged from 0.33” to 2.00”

7
thinner layers closer to the wall and thicker in the outer flow regime as shown in Fig. 7.
After the model was partitioned to create the inlet, outlet, ground, symmetry planes, and the surface of the
truck, the moving ground was set up as a translating frame to simulate the motion of the truck. The velocity for the
inlet and moving ground was set to a common road condition of 29.05 m/s (65 mph). The truck was analyzed by
splitting it in half longitudinally and utilizing symmetry to reduce computation time. Physic models were set to
coupled flow and coupled energy which solves conservation of mass, momentum and energy simultaneously for a
steady and turbulent flow.
IV. Results and discussion
In order to determine whether the wind tunnel can adequately describe the road condition despite Reynolds
number discrepancies, a Reynolds number sweep was performed to observe the changes.
In Fig 8., it can be seen that
among each data set taken, the drag
coefficient remains constant. Drag
coefficient uncertainties are large at low
Reynolds numbers due to the constant
force balance resolution and the
exponential relationship to velocity. In
order to confirm that the drag coefficient
remains constant over the wind tunnel
range, velocity was compared to the drag
measured by the force balance. With
frontal area and drag coefficient being
constant this should yield a linear
relationship between drag force and
velocity squared as seen in Eq. (3).
(a)

(b)
Figure 7: Representation of mesh on a) truck model and b) flow domain.
Figure 8: Variation of drag coefficient over wind tunnel and
full scale Reynolds number range.

8
A linear fit characterized the entire
range of data when the model was set to the
largest height. This confirms that the drag
coefficient was constant over the wind tunnel
Reynolds number range: 75 ×10v
to 275 ×10v
.
Additionally, the range of Reynolds numbers
tested using CFD showed a constant drag
coefficient near the road condition of
𝑅𝑒 = 6.8 × 10<
. An increase of 3% resulted
from dropping the CFD Reynolds number
down to 𝑅𝑒 = 1.0 ×10?
which is generally
considered the baseline for truck testing.9
The reason that the Reynolds
variations are not occurring in the wind tunnel
is likely due to a turbulent boundary layer
condition on the wind tunnel model. It was
found that the free stream turbulence intensity
ranged from 0.30% at 40 m/s to 0.44% at 20
m/s which would assist in locally energizing the boundary layer until it transitions to turbulence. Additionally, a
separation bubble occurred at the leading edge of the model that does not satisfy the condition of edge curvature for
a smooth transition.22
This separation was observed to reattach to the model quickly by using tufts for flow
visualization. After reattachment, the turbulent boundary layer modeled a road condition well with no Reynolds-
dependent variation in drag. Due to reattachment, variations in gap size were able to be measured which would not
be possible if a detached flow was dominant in this region because the arbitrary flow direction would not produce a
net force on the front of the trailer. The drag reduction measurements due to gap size were taken at the lowest height
configuration that the RoadHoss design would allow.
The slope of drag reduction with gap width correlates well to the sharp leading edge condition from
Allan.18
As Reynolds number decreases, a larger radius is required to minimize separation at a semi-truck’s leading
edges. Since the radii of the wind tunnel model was simply scaled in proportion to semi-truck dimensions, more
separation was expected. This separation reduces interaction with the gap immediately behind the trailer and leads to
a moderate drag reduction as the gap is sealed. The data in Figure 10 also suggests that if a normalized gap size
beyond 0.37 is required for a design configuration, a sharp leading edge can paradoxically reduce the overall drag.
Figure 10: Variations in drag with changing gap correlated to leading edge radius that
governs flow separation. Reference drag data offset to show variation with single variable.
Figure 9: Confirmation of steady state drag coefficient
according to Eq. (3).

9
As the effects of closing the gap
were analyzed using CFD, there was no
quantifiable drag reduction. However, the
velocity vectors in the gap region reveal
high-speed entrained fluid that would
physically indicate a momentum exchange
and additional drag forces applied to the
truck.
The velocity scene from STAR
CCM+ was created to analyze the behavior
of the flow by reducing the distance of
separation between the tractor and the
container. It is noted that the velocity of the
entrained fluid ranges from 33 to 60 ft/s
when the gap distance is 5.67 ft and
decreases to approximately 3 ft/s when gap
distance is 1.41 ft. Velocity fields from Fig.
11 also show that an additional reduction of
drag coefficient is achieved with the
implementation of gap fairings due to
reduction of entrained air and therefore
pressure drag.
Gap size was returned to the
largest configuration for the height
variation measurements in the CFD and the
wind tunnel models. The variation of drag coefficient with height was expected to occur linearly according to Eq.
(3) as the frontal area is a product of the constant width and variable height.
The drag reduction effect of lowering the trailer height is clear in Fig. 12. The y-intercept of these lines
represent the drag not associated with the trailer height as described by Eq. (2). These include skin friction, gap
effects and undercarriage drag. The variations in slope between the two tests show that the effect of frontal pressure
drag, which is proportional to the model height, was more prominent in the wind tunnel testing. This is due to the
relatively sharp leading edge as previously discussed. In order to measure the benefit of the RoadHoss design, the
Figure 12. Comparison of drag reduction from lowering of the truck’s overall height.

(a)

(b)

(c)

(d)
Figure 11: Velocity field vectors demonstrating fluid behavior
at gap distances: a) 1.73 m, b) 1.30 m, c) 0.87 m, d) 0.43 m.

10
result of maximum height reduction was compared to a normal road condition model that had a high configuration
and the minimum practical gap size for road vehicles.
V. Conclusion
Wind tunnel testing and CFD analysis have shown that there is a significant benefit in reducing the height
and therefore frontal area of a semi-truck. The initial approximation of a 21% benefit due to height reduction likely
did not take into account the magnitude of pressure drag on the exceptionally bluff model that was designed. This
effect led to an actual drag reduction of 24% and 27% for wind tunnel and CFD analysis respectively. Free stream
turbulence intensity and a turbulent boundary layer appeared to reduce Reynolds number effects on the 1/34th
scale
model in the wind tunnel. The adverse gap effect has been shown to be mitigated with gap fairings through flow
visualization and drag reduction measurements. The result of closing a large gap size that is necessary for the
RoadHoss suspension system led to an additional 14% reduction in drag.
A cost analysis was conducted based on the data from the wind tunnel and CFD results to determine the
savings that could be associated with a system like RoadHoss. The assumptions here were based on the FHWA
reporting combination heavy trucks using 29 million gallons of fuel in 2015, with diesel prices at $2.40/gallon.
Additionally, a highway condition was assumed, where 30% of a truck engine’s power is used to overcome
aerodynamic drag.23
The trucking industry had revenues of $726 billion in the US in 2015 and the private trucking companies
that dominate the industry generally have a 6% profit margin.24
This means that net profit of these companies lies
around $43.6 billion. Therefore, saving $7.9 billion would increase profits by more than 18%.
References
1
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for Medium-and Heavy-Duty Engines and Vehicles: Regulatory Impact Analysis." Washington, DC (2011).
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trucks. No. 2003-01-3377. SAE Technical Paper, 2003.
3
Van Raemdonck, G. M. R., and M. J. L. van Tooren. "Numerical and Wind Tunnel Analysis Together with Road Test
of Aerodynamic Add-Ons for Trailers." The Aerodynamics of Heavy Vehicles III. Springer International Publishing, 2016. 237-
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9
Hammache, M., and F. Browand. "On the aerodynamics of tractor-trailers." The Aerodynamics of Heavy Vehicles:
Table 1. Model and real road comparison of savings and drag reduction.
WT CFD
Configuration
Common
Truck
Reduced
Height
Reduced
Height & Gap
Common
Truck
Reduced
Height
CD 0.58 0.54 0.45 0.8 0.72
Drag Reduction - 24% 38% - 27%
Annual Savings - $5.2 billion $7.9 billion - $5.8 billion

11
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10
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roughness particles for boundary-layer transition at Mach numbers from 0 to 5. National Advisory Committee for Aeronautics,
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Information Engineering Design Symposium (SIEDS), 2014. IEEE, 2014.
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34th AIAA Fluid Dynamics Conference and Exhibit (2004): n. pag. Web.
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18
Allan, J. W. "Aerodynamic drag and pressure measurements on a simplified tractor-trailer model." Journal of Wind
Engineering and Industrial Aerodynamics 9.1-2 (1981): 125-136.
19
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ISSR Journals. Mechanical and Automotive Engineering Department, Faculty of Engineering and Computing, Coventry
University, UK, 15 Feb. 2010. Web. 1 Oct. 2016.
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McDonough, J. M. "Introductory Lectures on Turbulence." N.p., n.d. Web. 30 Sept. 2016.
22
Hammache, Mustapha, Mark Michaelian, and Fred Browand. Aerodynamic forces on truck models, including two
trucks in tandem. No. 2002-01-0530. SAE Technical Paper, 2002.
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III. Springer International Publishing, 2016. 195-210.
24
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https://www.sageworks.com/datareleases.aspx?article=202&title=U.S. [retrieved 25 February 2017].

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