This document is a final report summarizing research on using computational fluid dynamics to analyze reducing drag on class-8 trucks through the use of rotating cylinders. The report finds that rotating cylinders unexpectedly increased drag rather than reducing it. Further investigation is recommended to understand why drag increased at certain cylinder rotation speeds and to test different cylinder surface patterns that may improve drag reduction. The report was submitted by Salman K. Rahmani for their undergraduate research experience and concludes by thanking those who supported the research.

1. Running Head: Drag Analysis of Class-8 Trucks 1
URECA Final Report: Drag Analysis of Class-8 Trucks utilizing Computational Fluid Dynamics
Salman K. Rahmani
Middle Tennessee State University
Author’s Note:
If any questions or concerns arise relating to this article, please contact Salman K. Rahmani at 615-351-
1114 or Salmanr96@gmail.com

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Drag Analysis of Class-8 Trucks utilizing Computational Fluid Dynamics
Introduction
Throughout the course of my research, headed by Dr. Nate Callender, I analyzed how
rotating cylinders could be utilized on Class-8 Trucks to reduce the drag coefficient (Cd) on the
vehicle as it propels at 70mph (cruising speed for most Class-8 vehicles while transporting goods).
The values of angular velocity, also known as alpha, were used relative to the flow velocity and
ranged from zero to two in increments of .5. I used data that was obtained from my previous years
of research on traditional tractor trailers (Seen in Figure 1.) to see what the standard Cd value was
to get a basis for the desired drag reduction value. As seen in Figure 2, the cylinders were mounted
vertically with their surfaces being tangential to the side of the trailer. The cylinders pose a radius
of two feet and a height of eight feet.
Background
As a result of drag being such an issue, billions of dollars have been invested by companies
to try and solve this issue. In addition to company efforts, a multitude of different independent
studies have been conducted in an attempt to try and address the issue of aerodynamics with class-
8 vehicles. One such study by the National Research Council of Canada shows that 35-55% percent
of engine power for Class-8 Tractor Trailers are consumed by Aerodynamic Losses (Patten, 2012,
p. 84). Another study, conducted by Dinesh Madgundi and Anna Garrison (2013), displayed that
approximately 50% of aerodynamic drag experienced by the vehicle was caused near the trailing
edge of the trailer (p. 9). This shows that by increasing aerodynamic efficiency at the trailing end
of the vehicle, the issue of harmful emissions may also be addressed by reducing energy
consumption within the engine.

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Three studies depict that drag reduction on box-shaped objects are possible using various
geometries. The first examination that shows this is Nicodemus Myhre’s investigation into Drag
Reduction Methods for a Rearward Facing Step. Mr. Myhre showed that by modifying the trailing
edge of a 2-Dimensional rectangular shaped geometry, drag reduction is possible by up to 20%.
The various modifications he tested were trailing edge flaps and filleting of the trailing edge the
trailing edge (2016, p. 15). Altaf Alamaan, Omar Ashraf, and Asrar Waqar’s examination into
Passive Drag Reduction of Square Back Road Vehicles state that by testing various flap geometries
at the end of a 3-Dimensional trailer, they were able to achieve a maximum drag reduction of 11%
(2014, p. 1).
Dr. Nate Callender’s research into Optimized Lifting Line Theory Utilizing Rotating
Biquadratic Bodies of Revolution provides additional insight into the theoretical aspects of how
this issue may be addressed utilizing 3-Dimensional moving bodies. Dr. Callender displayed that
at various rotational speeds about a cylinder’s longitudinal axis (also known as alpha), a fluid’s
boundary layer is modified to the point where the turbulent region is decreased substantially
(2013, p. 69). The major breakthrough with this idea is that instead of utilizing elongated, solid-
body boundaries such as flaps to streamline a 3-Dimensional airflow, we are now able to
accomplish the same results by having rotating cylinders create the same effects. This result is
crucial in the sense that it provides information as to how fluid flow relates to rotating 3-
Dimensional bodies, a relationship that will be further investigated throughout this project.
Methods
The approach I decided to take was to spend two weeks running simulations to make sure
that the mesh (See Figure 3), input parameters, and geometries didn’t trigger an error while
executing. After this was completed, I began to harness the data from my previous research to find

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the optimal value of drag reduction. Next, I set up the simulations and ran each case of alpha with
four seconds of flow time to try and see how the drag would act upon the vehicle within a very
short range of time. After each simulation, I would pull the data file from where ANSYS (the
simulation software) saved it, upload it to an excel spreadsheet and create a graph of time vs. drag
coefficient (Cd). Once the data was analyzed to make sure that no computational-based anomalies
occurred, I would initiate the same process but instead of computing for four seconds of flow time,
I had the computer simulate fifteen seconds of flow time. As a result, I would be able to see how
the drag coefficient behaved over a longer period of time.
Although the actual researching method that is carried out during the duration of the project
is one of the most critical aspects, I feel as if the time-management methodology also carries with
it a significant amount of importance. With this being said I feel that discussing this is essential.
Since Dalal Jondoul and Marco Hanna, other student researchers, also required the services of the
ANSYS-Fluent software, we had to generate a time budget that would not only fit our class
schedules, but also optimize our time researching without conflicting with one another’s work. We
decided that Tuesday, Thursday and Saturday would be my time to research while every other day
would be Marco and Dalal’s turn.
Results
Surprisingly, the act of rotating cylinders resulted in a detrimental impact to the drag. A visual
depiction of the results of all the simulation with fifteen seconds of flow time can be seen in Figure
4. After close inspection of the graph, it is noticeable that as the angular velocity of the cylinders
increase, so does the drag coefficient.

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One hypothesis for the cause of Cd is that when the air particles reach the cylinders, they
are already in a turbulent state due to the act of flowing past the front of the truck. Because of the
air being in a non-laminar state, the air particles meet the cylinders with an increased non co-linear
component of velocity. This causes the air particles to impact and recoil off the cylinders instead
of transitioning smoothly onto them. As a result of the air particles not sitting on the surface of the
cylinders due to the impact, the cylinders are not able to inject any kinetic energy into the particles
through the use of the no slip boundary condition.
Upon closer inspection of Figure 4, it is possible to notice that there is an anomaly which
is that the angular velocity of 1.5x the flow speed caused a higher Cd value than any other
rotational speed. As of now, it is unclear what the fundamental cause of the singularity within the
trend is. However, it is important to note that the range of the singularity occurred between angular
velocities values of one times the flow speed and twice the flow speed.
Conclusion
In conclusion, I believe that the anomaly that occurred at 1.5x the flow speed should be
further investigated. The follow-up study should be conducted within ANSYS-Fluent coupled with
the same simulation parameters that were used for this examination. The reasoning behind this is
that the as many variables should be kept identical to the original study to try and prevent
inconsistencies within the results.
The optimal method for the follow-up project would be to increment the angular velocity
of the cylinders by .1 (starting at 1x the flow speed) to try and identify the region where the Cd
value starts diverging from the trend expressed in Figure 4. Once the divergence of the Cd is

6. URECA Final Report 6
detected, the angular velocity of the cylinders should be incremented in values of .05 to capture
the divergence region as accurately as possible.
In addition to testing where the Cd trend is disobeyed, another supplemental examination
that would be quite useful would be to try and observe what type of surface patterns would best
dampen oblique impacts of air particles on a microscopic scale. This would force the particles to
stay attached longer which would mean that the cylinders have a higher chance of performing their
intended role of injecting kinetic energy into the system. For example, golf balls are rendered with
three-hundred and thirty six dimples because they cause the air to stay attached to the surface
longer so the ball won’t drift in flight. With this in mind, I believe that an aerodynamic analysis of
different skin designs would be beneficial.
The method for the analysis of the skin patterns should be conducted computationally as
well as experimentally to obtain results that are as accurate as possible. The two patterns that I
believe should be tested are golf ball dimples, and the “zig-zag” pattern that can be seen in Figure
5. Once testing of the surface patterns has been completed on a sole cylinder, the patterned cylinder
should then be incorporated onto the truck geometry to see if it has a positive effect on total drag
reduction.
With the conclusion of both of the skin-surface examination as well as the Cd divergence
study, I feel that more insight will be gained on the topic of why the rotating cylinders returned
negative results as well as why there was a disturbance in the overall trend of Figure 4.
Thank You
Although some individuals might not consider a thank you section within a final report to
be professional, I believe the exact opposite; that giving thanks to those who put so much faith

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within you is of the utmost professionalism. I would like to begin by thanking the URECA
committee who granted me the opportunity to perform this research and help prepare me for my
future. I also would like to thank Dr. Callender for being an exemplary mentor throughout this
project and guiding me as I pursue my dream of being an Aerospace Engineer. And last of all, I
would like to thank my family, who continues to support me throughout my brightest days and my
darkest nights.

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Figures
Figure 1. (Isometric View of Traditional L-Step)
Figure 2 (Isometric view of cylindrical modifications)

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Figure 3 (Isometric View of mesh)
Figure 4 (Graph depicting simulation results)
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 2 4 6 8 10 12 14
Cd
Time(s)
1.5x FS .5x FS 0x FS 1x FS 2x FS

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Figure 5 (Experimental skin pattern for follow-up examination)

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References
Ahanotu, D. N. (1999, July).Heavy-Duty Vehicle Weight and Horsepower Distribution;
Measurement of Class-Specific Temporal and Spatial Variability. Georgia Institute of
Technology, 1-275. Retrieved August 31, 2016, from
http://transaq.ce.gatech.edu/guensler/publications/theses/ahanotu dissertation.pdf
Altaf, A., Omar, A.A., & Asrar, W. (2014, Nov). Passive Drag Reduction of Square Back Road
Vehicles [Abstract]. Journal of Wind Engineering & Industrial Aerodynamics. 134, 30-
43. Retrieved August 30, 2016 from
http://www.sciencedirect.com/science/article/pii/S0167610514001640?np=y#ppvPlaceH
older
Callender, M. N. (2013, Dec). A Viscous Flow Analog to Prandtl's Optimized Lifting Line
Theory Utilizing Rotating Biquadratic Bodies of Revolution. Trace: Tennessee Research
and Creative Exchange, 1-92. Retrieved August 29, 2016, from
http://trace.tennessee.edu/cgi/viewcontent.cgi?article=2909&context=utk_graddiss
Madugundi, D and Garrison, A (2013, Mar). Class 8 Truck External Aerodynamics. Choice of
Numerical Methods, 1-19. Retrieved August 28, 2016
http://mdx2.plm.automation.siemens.com/sites/default/files/Presentation/3_Daimler_DM.
pdf
Myhre, N. (2016, May). Computational Analysis of Drag Reduction Methods for a Rearward
Facing Step. 1-27. Retrieved August 31, 2016, from
http://jewlscholar.mtsu.edu/bitstream/handle/mtsu/4856/Myhre-Nick
Thesis.pdf?sequence=1&isAllowed=y

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Patten, J. P., McAuliffe, B., Mayda, W. P., & Tanguay, B. (2012, May). Review of Aerodynamic
Drag Reduction Devices for Heavy Trucks and Buses. National Research Council
Canada, 1-100. Retrieved August 29, 2016, from
https://www.tc.gc.ca/media/documents/programs/AERODYNAMICS_REPORT-
MAY_2012.pdf.