ureca final report (spring 2016)

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 [email protected]

mailto:[email protected]

URECA Final Report 2

Figure 1. (Isometric View of Traditional L-Step)

Figure 2 (Top View of Traditional L-Step)


Figure 3 (Isometric View of rendering with fins)

Figure 4 (Top View of trailer with fins)


Figure 5 (Isometric View of rendering with cylinders and plate)

Figure 6 (Top View of rendering with cylinders and plate)


Figure 7 (Isometric View of Mesh around truck)

Figure 8 (Comparison of all three tested geometries)


Figure 9 (Depiction of Magnus effect rendered in AutoCAD)

Figure 10 (Rendering of Rotating Cylinder offsetting the lift vector)


Introduction

Throughout the course of my research, headed by Dr. Nate Callender, I analyzed and

tested two different designs that could be utilized on Class-8 Trucks to attempt to reduce the drag

coefficient on the vehicle as it accelerates at 70mph (cruising speed for most Class-8 vehicles

while transporting goods). The first design I simulated was the traditional “L-Step” or box design

that most tractor trailer companies have been using for many years since the very origins of

transporting goods by trucks (See Figure 1). This was done so that I could get a reference value

of the Drag Coefficient. The first modification that I examined was that of trailer fairings, a

recent modification that some Class-8 Truck companies have started to incorporate onto the

trailing edge of the trailers to try and limit the drag forces acting upon their vehicles (pressure

drag in particular). The second geometry that I tested was that of rotating cylinders. The

cylinders pose a radius of two feet and are placed vertically at the trailing edge of the trailer with

the side edge of the trailer being tangent to the surface of cylinders (See Figure 2).

Background

Due to the magnitude that this drag issue poses, extensive research has been invested into

this subject to try and reduce the drag’s effects on Class-8 trucks. For example, STEMCO has

developed a set of fins that attach onto the rear of the trailer that attempts to reduce the drag by

trying to providing a more seamless transition for the fluid. Another example of CFD being

utilized for drag analysis of Class 8 vehicles is Dinesh Madgundi and Anna Garrison’s

discussion in regards to how mostly 50% of the drag encountered by Class 8 vehicles is due to

the trailer (Madugundi 2013). This indicates that there are significant gains to be made by

making the trailer more aerodynamic. This statement is reinforced by a study conducted by Altaf


Alamaan, Omar Ashraf, and Asrar Waqar which states that by testing various geometries of flaps

at the end of the trailer, they were able to reduce the drag by over 11% (Alamaan 2014).

Methods

Methods can be the difference between successful research, passing an exam or any

aspect of life for that matter. How someone approaches a given situation is critical to the

outcome of the task that is placed before them. With that being said, I spent a proficient amount

of time planning and analyzing the method I chose to incorporate into my project. The approach

I decided to take was to spend two weeks running simulations to make sure that the mesh (See

Figure 3 for example), input parameters or geometries didn’t trigger an error while executing.

After this was completed, I began simulations on the L-step, fin geometry and cylindrical

geometry all 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). I would repeat this process three times for each geometry

and then compute the average of the three to try and reduce any computational errors that

occurred within each specific simulation. After I computed the average for all three geometries

with four seconds of flow time, I started the same process over again but instead of computing

for four seconds of flow time, I had the computer simulate fifteen seconds of flow 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 Nick Myhre, another student researcher, 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. We

decided that every Monday, Wednesday and Saturday would be Nick’s time to research while

every Tuesday, Thursday and Sunday would be my time. This was the best case scenario since

each of my simulations took roughly a day to execute, they would be completed just in time for

Nick to perform his research.

Results

As expected, the traditional L-Step had a higher drag coefficient than that of the fin

geometry. This can be seen by examining Figure 8. Since the fins are set with a taper (Figure 4),

they decrease the “pocket” size where the air can get trapped behind the vehicle as it is moving

which leads to a drop in pressure drag. In addition, since the size of the “pocket” is smaller, that

means the air doesn’t have to move through as much area to re-attach. We can figure out this

correlation between area and drag by deriving it from the Drag Equation:

After examining the equation, we notice that if we hold all other variables constant, drag will

increase as area increases. Which leads us with the relationship: D ∝ A, which states that drag is

proportional to area. And since the fins help to streamline the fluid flow as well as reduce rear

area for the particles to move through, it is easy to understand why the fins possessed a better

outcome than the traditional L-Step.

Although the above description was accurately predicted, the result of the rotating

cylinders was not. Surprisingly, the cylinders had a more detrimental impact to the drag than

even the L-Step. The drag coefficient (Cd) for the cylindrical geometry wavered at roughly 0.5


while the L-step posed an average Cd of 0.4, and the fins at 0.2. The Magnus Effect shows that if

a fluid is moving past a rotating cylinder or ball, the cylinder will actually produce a lift vector

90 degrees to the fluid flow vector (See Figure 9).

One hypothesis for the cause of high Cd is that the cylinders are rotating too fast. The

idea is that since the cylinders are rotating, they are increasing the velocity of the fluid above it

whilst simultaneously dropping the pressure. This causes the lift vector to be greater than 90

degrees from the fluid flow vector. The difference between the angle of the normal 90 degrees

lift vector and the offsetted lift vector is what causes the increase in drag (Figure 10). In order to

correct this, we must find the optimal speed at which the cylinder can rotate and produce lift

without offsetting the vector too much from its 90 degree threshold.

Another hypothesis as to why the cause for Cd is so high for the rotating cylinder

geometry is that the space between the plate, cylinders, and rear face of the trailer is causing a

recirculation region in which the air particles cannot escape (Visible in Figure 6). This region is

not a problem for the fluid that is coming around the side of the vehicle due to the fact that it

transitions onto the cylinders. However, the fluid that is coming over the top and bottom of the

trailer is getting caught in that region and remains there for quite some time. A possible solution

to this is to also put either a plate to cover the space, or another set of rotating cylinders located

along the top and bottom edges of the trailer rotating about the lateral axis to try and produce the

same effect as the cylinders on the sides. These solutions will eliminate the recirculation region

and help in decreasing the overall drag upon the vehicle.

One benefit that was noticed within the trend of the cylindrical geometry was that the Cd

did not oscillate nearly as much as the L-Step and the fin designs. This could save

aerodynamicists who are researching this topic some time in the future because they won’t have


to delve into anomalies that cause drag data spikes which could be a very time consuming

process.

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

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 Nick Myhre, for being an incredible friend and research partner who taught

me all that I know about fluid flow.


References

Altaf, A., Omar, A.A., & Asrar, W. (2014). Passive Drag Reduction of Square Back Road

Vehicles. Journal of Wind Engineering & Industrial Aerodynamics. 134: 30-43:

Commercial Vehicles | CD-adapco. (n.d.). Retrieved January 28, 2016, from http://www.cd-

adapco.com/industries/ground-transportation/commercial-vehicles

Lift of a Rotating Cylinder. (2015, May 05). Retrieved May 06, 2016, from

https://www.grc.nasa.gov/www/k-12/airplane/cyl.html

Madugundi, Dinesh and Anna Garrison (2013). Class 8 Truck External Aerodynamics. Choice of

Numerical Methods, 9

The Drag Equation. (2015, May 05). Retrieved May 05, 2016, from

https://www.grc.nasa.gov/www/k-12/airplane/drageq.html

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