Honors Thesis Proposal
Clarkson University
The Study of the Effect of a Gap Seal on the Aerodynamic Drag of
Heavy Vehicles
Timothy Deschenes
Advisor: Dr. Kenneth Visser
Motivation
It is estimated that a tractor-trailer moving at highway speeds of 70 mph uses 65%1 of the total
engine power to overcome the drag of the vehicle. If one were able to reduce the aerodynamic
drag of the vehicle, less power would be required to sustain the same speeds, which would in
turn require less fuel usage. The United States Department of Transportation reports that a total
of 2,245,0852 combination trucks, which include nine-axle tractor trailers, were used in 2003
traveling over 1382 billion miles, resulting in 26.92 billion gallons of fuel consumed by these vehicles
over the year.
These figures indicate an obvious economic motivation to reduce the fuel consumption of
tractor-trailers on the road today. Even a 10 percent reduction of fuel consumption would save
billions of dollars for the transportation business. In addition to the economic benefit, increased
fuel efficiency will have positive environmental impacts as well. It has been estimated that
tractor-trailers emit an average of 6.1 grams3 of hydrocarbons per mile and 61.07 grams3 of
carbon monoxide per mile. Considering the total mileage of all tractor-trailers in 2003, the total
emissions for that year were 8.42×108 kg3 of hydrocarbons and 8.43×109 kg3 of carbon
monoxide2. Because carbon monoxide is a product of engine exhaust, and the quantity of engine
exhaust is directly related to the amount of fuel consumed, it is assumed that a reduction of fuel
use would reduce the quantity of pollutants emitted into the atmosphere. With an increasing
push for federal regulations of vehicular and other emissions, there may become an increasing
legal incentive to reduce fuel consumption as well.
Before the oil crisis of the 1970’s, little was done to reduce the aerodynamic drag on all vehicles
including combination trailer vehicles. During the 1970’s and 1980’s, research on the reduction
of the drag was performed and some modifications of the tractor-trailer geometry were
implemented4. The design of the front the truck was heavily modified in order to reduce the
overall aerodynamic drag of the tractor-trailer combination. Other devices were briefly studied
as well, including the effect of sealing the gap between the cab and the trailer, which is the
intended study of this proposal. At the time, it was determined that the incremental reduction of
drag would not be large enough to offset the economic cost of constructing and installing such a
device. Since then, the price of fuel has steadily increased from an average value of $1.28 per
gallon in 19985 to $1.92 per gallon in 20045. Because of this rise in fuel prices, it is believed that
a device that is designed based on the optimum geometry found through experimental testing
will result in a positive impact both economically and environmentally.
This study will look into the effectiveness of different possible geometries that could be used to
fill the gap between the cab of the truck and the trailer on a tractor-trailer combination vehicle. It
is believed that a device that completely seals the gap will be the most beneficial, but the number
of plates needed to achieve this seal is still unknown. After the effectiveness of each device is
calculated, a full-scale prototype based on the best geometry will be constructed. The full-scale
prototype will then be installed on a tractor-trailer and fuel usage will be collected and compared
to the fuel usage on a tractor-trailer with no gap-sealing device.
Background
Before one can begin to test the effects of different geometries to fill the gap in a wind tunnel,
the environmental conditions presented to a full-scale tractor-trailer must be studied. As a truck
travels down the road the direction of the velocity vector that the truck experiences is almost
always offset by an angle, called the yaw angle (ψ), to the centerline. Before testing different
devices, the probability of a yaw angle occurring should be determined to ensure that the device
is not designed for yaw angles that almost never exist. As one can see in Figure 1, as the
traveling speed is increased, the range of yaw angles is decreased. Since tractor-trailers are used
mainly for travel in highway conditions traveling at speeds greater than 55 mi/h, one can test the
yaw angles of less than 12º and produce the yaw angles that are experienced in the environment
over 95% of the time.
Figure 1: Probability of Exceeding a given yaw angle4
In general, the drag force on any object is expressed in terms of Equation 1 where D is the drag
force on the object, ρ is the air density, V is the velocity, A is the frontal area, and CD is the drag
coefficient. The air density and velocity cannot be altered in an attempt to reduce the drag since
they are based on the environment and the speed with which the vehicle is moving. In addition,
for this study, the frontal area of the truck is not a parameter that will vary. Therefore, the only
parameter that can be altered to reduce the overall drag on the tractor-trailer is the coefficient of
drag.
D
1
V 2 AC D
2
(1)
Because of this, the percent drag savings can be computed as the percent reduction of the
coefficient of drag. The percent reduction of the coefficient of drag can be computed as the
change in drag of the modified device with respect to the baseline case divided by the baseline
case as seen in Equation 2 where C D ,Test is the drag coefficient of the model with a test device
installed and C D , Base is the drag coefficient of the baseline model.
% DragSavings 
C D , Base  C D ,Test
C D , Base
* 100%
(2)
Experimental Methodology
Apparatus
The testing will take place in Clarkson University’s subsonic wind tunnel. The wind tunnel test
section has cross sectional dimensions of 36 in by 48 in and is 65 in long. A simulated road bed
elevated about 12 in will be installed so as to avoid the effect of the boundary layer buildup that
occurs on the test section floor which does not exist for tractor-trailers traveling down the road.
The road bed is held up by 8 threaded rods and is constructed out of a .5 in plywood. The bed
will be stationary throughout all tests. The operating speed of the wind tunnel will be about 50
mph.
The base model to be used in the wind tunnel is a 1:15.25 scale of a 1990’s generation Peterbuilt
model 379 tractor-trailer combination with a 48 foot trailer. This model was the same model that
was previously used to test different aft-end devices on the trailer. It has a frontal area of 61.7
in2 and a side cross-sectional area of 372.6 in2. These areas do not include mirrors, handles, and
many of the undercarriage protrusions except axles and 18 rubber wheels which are included.
The total length of the model is about 51.35 inches with the gap between the cab and trailer
being 2.5 in which is 38.125 inches at full scale.
The cab is constructed out of foam and covered with a layer of plastic filler and multiple coats of
paint to reduce to porosity. A steel plate is placed on the back of the cab in order to easily
remove and attach different test sections between the cab and trailer. The trailer is constructed
out of plywood and connects to the cab with a length of standard 2” by 4”. An example of the
tractor-trailer with one of the test devices installed is shown in Figure 2.
Figure 2: Tractor-Trailer Model with Possible Drag
Saving Device Installed
The model is installed in the center of the wind-tunnel test section and is connected to a biaxial
force balance with a .75 in diameter pipe that is rigidly installed to the approximate center of
gravity of the model. The exposed part of the pipe between the test section floor and the
simulated road bed will be covered by a 1.25 in diameter PVC in order to avoid an increase in
the drag due to the exposed area of the pipe. The wind tunnel set-up and model position can be
seen in Figure 3.
Figure 3: Wind Tunnel Set-up and Model Position
Drag and side force measurements will be collected using a custom-built biaxial force balance as
shown in Figure 4. This device is installed below the wind tunnel and the .75 in pipe connects
into a housing that has two degrees of freedom. For each degree of freedom, drag and side, a
transducer is installed and transmits a voltage that is linear with respect to the force being applied
to it. Both the drag and side transducer are Precision Transducers model PT1000. Each one
requires an excitation voltage of 10V and has a precision of 2 mV/V. The drag load cell has a
range of 0-29.4 N and the side load cell has a range of 0-49 N. This is acceptable since the
expected loads range from 5-15 N in the drag direction and from 0-25N in the side force
direction. A LabView script that was developed by Coon and Visser6 will be used to collect the
voltage measurements.
Figure 4: Close up of Drag and Side Transducers
In order to determine the test section velocity a differential pressure transducer is used.
Assuming incompressible flow, the velocity can be found using Eqution 3.
V
2 RT P
AC PTotal
(3)
In Equation 3, V is the velocity of the test section, R is the universal gas constant for air, T is the
absolute pressure, ΔP is the pressure differential, PTotal is the total atmospheric pressure and AC is
the area contraction ratio. In order to find the velocity, these values must be calculated or
collected using various instrumentation.
The area contraction ratio, AC, is found using Equation 4 where A1 is the cross sectional area at
the inlet of the wind tunnel and A2 is the cross sectional aread at the test section of the wind
tunnel.
A
AC  1   2
 A1



2
(4)
The absolute temperature T, is collected using a digital thermometer. The pressure differential,
ΔP, is collected using a differential pressure transducer made by Modus Instruments model
number DT105E010L and the total pressure, PTotal, is collected using a Setra barometric sensor
model 276.
Figure 5: National Instruments Data Acquisition Board
used to collect Load Cell Data
One National Instruments data acquisition board model SC-2043 SG as shown in Figure 5 will
be used to collect the voltages from the load cells. A National Instruments acquisition board
model BNC-2120 as shown in Figure 6 will be used to collect the voltage excitations from the
differential pressure transducer and the total pressure transducer. Both data acquisition boards
are connected to a Dell Dimension XPS model T800r through National Instruments PCI cards
model 6024E. The temperatures are read off a digital thermometer and entered manually into the
LabView script.
Figure 6: National Instruments BNC Board used to
collect Total and Differential Pressure Readings
Testing will occur over the yaw angles of 12º, 9º, 6º, 3º, 0º, and -3º. Both 3º and -3º
measurements will be collected in order to verify that the data collected is symmetric as it should
be. Yaw angles are achieved by rotating to model on the axis that the .75 in pipe connects
through. Yaw angles greater than 12º are not tested because they rarely occur over the speeds
that tractor-trailers are commonly operated at. Multiple test geometries will be study in order to
determine the best method of filling the gap and are summarized in Table 1. The dimensions
given are full-scale dimensions. An example of some of the test devices is shown in Figure 7.
Table 1: Test devices geometry
Geometric Parameter
Types
Number of Plates
3 (top and two sides), 2 (two sides), 1 (center fill)
Yaw Angle
-3º, 0º, 3º, 6º, 9º, 12º
Plate Length
0, 12.7, 25.4, 38.125
Plate Thickness
.9 in
Figure 7: Example of some of the possible drag reducing
devices
Wind Tunnel Procedure
In order to collect accurate measurements, the load cells will be calibrated with a known set of
weights. This is done with a pulley system as shown in Figure 8 that can add known masses in
both the drag and side force directions. Three to four different masses are used and the voltage
data is collected using the LabView script.
Figure 8: Pulley system used to calibrate the load cells.
Since the relationship between force and voltage is linear, a simple calibration curve can be done
in Excel of Force versus Voltage. An example of one calibration curve is shown in Figure 9. A
linear trend line is then added to the plot and the slope and y-intercept are inputted into the
LabView script so all the later measurements are in terms of a force, Newtons, rather than a
voltage.
25
Force (N)
Drag
Lift
20
Lift Calibration 15
y = 3354.1x + 8.0201
10
R2 = 0.9954
Drag Calibration
y = 1836.7x + 7.3339
5
R2 = 0.9989
0
-0.006 -0.004 -0.002
0
0.002
0.004
0.006
0.008
-5
Load Cell Voltage (V)
Figure 9: Example of a load cell calibration curve.
Each data set will be collected over 5 seconds at a sampling frequency of 1000 Hz. These 5000
samples are averaged to produce one single point. This procedure is repeated 5 times and the
mean of the 5 procedures is used to determine the mean drag force on the model. This drag force
will be used in conjunction with the velocity, air density, and frontal area of the model to
calculate the coefficient of drag.
Baseline data and wind-off tests will be periodically done in order to check the reliability of the
calibration curve previously entered. If the force measurements deviate far from the original
values, recalibration or repositioning of the model will be done. In addition, the corresponding
wind-off values will be subtracted from the test values to account for the shift in the voltage
curves that occurs with the pressure differential.
After drag forces are collected, the coefficient of drag can be calculated by using Equation 3 and
the percent drag savings can be found using Equation 4. Using this data, the optimal geometry
will be used as the basis for the design and construction of a full-scale prototype.
Preliminary Results
Zero yaw data for 2- and 3- plate devices was already collected and corresponding drag
coefficients were calculated. As seen in both Figures 10 and 11, overall drag of the model was
reduced for gap lengths of both .33 and full gap closure. In addition, at .66 gap fill, for both two
and three sided, there was an adverse effect from the device. All data was collected at velocities
between 45 and 55 mph.
Three-Sided Devices
0.03
0.02
Change in Coefficient of Drag
0.01
0
-0.01
-0.02
-0.03
-0.04
-0.2
0
0.2
0.4
0.6
0.8
1
1.2
Gap Fill
Figure 10: Change in CD vs. Gap Fill for ThreeSided Devices at Zero Yaw
-3
6
Two-Sided Devices
x 10
Change in Coefficient of Drag
4
2
0
-2
-4
-6
-8
-0.2
0
0.2
0.4
0.6
0.8
1
1.2
Gap Fill
Figure 11: Change in CD vs. Gap Fill for Two-Sided
Devices at Zero Yaw
Among the both the two sided and three sided devices, the three sided devices outperformed the
two sided devices with the three-sided device with entire gap fill produced the best results. This
three-sided design resulted in a three percent reduction in overall drag reduction.
Initial results seem promising; however, repeat testing is required to find the optimum
configuration. A full-scale prototype will be built based on the geometry of the optimum
configuration. This prototype will then be installed on a tractor-trailer and the fuel consumption
of a tractor-trailer with the optimum geometry will be compared to the fuel consumption of a
standard tractor-trailer configuration.
Plan of Study
Date
March 11, 2005
Mid April 2005
Summer 2005
Fall 2005
February 10, 2006
March 2006
Early April 2006
Goals
 Finish Initial Data Collection
 Finish Analyzing Data
 Repeat Tunnel Data
 Design and Build a Full-Scale Prototype
 Test Full Scale Concept
 Begin Writing Thesis
 Finish First Draft of Thesis
 Submit Thesis
 Thesis Presentation
References
1
Storms, B., Satran, D, Heineck, J., Walker, S. A Study of Reynolds Number Effects and Drag-Reduction Concepts
on a Generic Tractor-Trailer. American Institute of Aeronautics and Astronautics. 2004.
2
National Transportation Statistics 2004. www.bts.gov. Bureau of Transportation Statistics. 2004. Accessed 22
Feb 2005.
3
Maragno, D and Visser, K. The Effects of Aft Cavity Inset and Boat Tail Angle on Drag Reduction of Tractor
Trailers. MAE Report 365, Clarkson University, May 2003.
4
Cooper, K. R. Truck Aerodynamics Reborn – Lessons from the Past. SAE Paper 2003-01-3376. November 1012, 2003.
5
“Monthly Energy Report.” www.eia.doe.gov. Department of Energy. 27 Jan 2005. Accessed 22 Feb. 2005
6
Coon, J. and Visser, K. The Effects of Non-Ventilated Plate Cavity Devices on Drag Reduction of Tractor
Trailers. MAE Report 361, Clarkson University, June 2002.