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Drag Analysis of Class-8 Trucks Utilizing CFDSalman K. Rahmani Aerospace DepartmentSr4h@mtmail.mtsu.edu

Research Mentor: Dr. Nate M. Callender

AbstractAs a Class-8 Truck accelerates, it creates a rift within the

airstream that it is traveling within. As the air particles separate around the front of the vehicle and re-attach at the rear, it creates a region in which the air particles “tumble” and create a fundamental force known as drag. This drag, also known as parasitic drag, hampers the vehicle’s ability to heighten or maintain its desired velocity. This experiment observes the magnitude of the drag at the rear of the Class-8 Truck.

Once the drag on the “traditional L-shape” trailer has been observed, efforts will be made to then modify the edge of the trailer in an attempt to reduce the degree of the drag acting upon the rear of the vehicle. These modifications include rotating cylindrical geometries at the rear of the vehicle, as well as rounding (otherwise known as filleting) the sharp edge of the trailer. Some of these modifications have been under observation by Nick Myhre in a separate study that examines these modifications only on the trailer itself. This project will be a continuation of his efforts (headed by Dr. Callender as the Research supervisor), but in regards to drag analysis of the whole vehicle instead of just the trailer.

If satisfactory drag reduction is achieved, this project may possess applications beyond just Class-8 Trucks and instead, shed some light on how various factors all play a part in aerodynamic drag. The software utilized for these simulations will be ANSYS-Fluent and funding for these trials will be provided by URECA at MTSU. This study will also be carried out in the Aerospace Technology Laboratory

Background

The Future…

References

Visuals Results Results cont.

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 reduces 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).

Simulation Parameters

In the future I intend to run more tests by changing different variables to see if there are trends that may be examined.

For example, simulations will be carried out to see if various rotational speeds have an effect on the drag coefficient. Another example of a variable that could be tested is the radius of the cylinders (smaller radius leads to less surface area which equals less surface area drag?) Once testing all variables is completed at the current Reynolds number, I hope to carry out all the same examinations at different Reynolds numbers to try and observe a correlation between various variables and the outcome of the Cd.

If it is determined that no modifications to the cylinders (at any Reynolds number) cannot reduce the cd, experiments on modifying the trailer fins will begin. The idea would be that by modifying the surface of the fins (dimples, ridges, materials etc). further benefits may be extracted out of the same principle design but with slight modifications.

• 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:

• Madugundi, Dinesh and Anna Garrison (2013). Class 8 Truck External Aerodynamics. Choice of Numerical Methods, 9

• Commercial Vehicles | CD-adapco. (n.d.). Retrieved January 28, 2016, from http://www.cd-adapco.com/industries/ground-transportation/commercial-vehicles

• Vanhoenacker, M. (2013, April 3). What Are Those Odd Panels on the Backs of Trucks? Retrieved January 28, 2016, from http://www.slate.com/blogs/browbeat/2013/04/03/truck_panels_what_do_they_do_explained_photos.html

• After running simulations on all three of the the L-step geometries, some questions and hypotheses were established:

• Is there a correlation between the rotational speed of the cylinders and how they affect the drag coefficient?

• Will the same results hold for different Reynolds Numbers?

• Free-Stream Velocity: 70mph• Walls and Ground Condition: No slip• Fluid: Air• Temperature: 15ºC• Pressure: 1atm• Width of trailer: 8ft• Height of trailer: 13ft• Length of Trailer: 53ft• Density of fluid: 1.225 (kg/m3)• Viscosity of fluid: 1.7894x10-5 (kg/m∙s)• Rotational speed of cylinders: 140mph (2x speed of flow)

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Cd vs. Time (L-step)

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Figure 1. (Adaptco’s Depiction of drag following a Class-8 Truck)

Figure 2. (Isolated Cylinder geometry rendered in ANSYS Modeler)

Figure 3. (Snapshot of tetrahedral “mesh” around truck geometry)

Figure 4. (Truck with cylinders attached to the trailer)

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Cd vs. Time (Trailer with fin)

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Figure 5. (Representation of how trailer fins effectively streamline airflow)