Vehicle Aerodynamics of the cwru sae baja Team Car

One of the big problems with CFDs, for our application, is that they can be expensive and/or hard to obtain. CFDs are usually designed by automobile and aerospace companies in house or require a relatively large company to acquire licenses from a software company. However our CAD Program, Solidworks, is already equipped with a CFD which means we do not have to spend extra money on a CFD program.

Wind Tunnels

Wind tunnels are devices that generate airflow that flow over a physical body. In these wind tunnels scales and pressure sensors are used to determine how the air effects the forces and pressures exerted on the body. Some visual guide such as smoke or fluorescent tufts are used as aides to physically see how the airflow interacts with the body. Wind Tunnels can be large enough to fit planes with wingspans over 100 feet [7] but can be built smaller to house scale models [2]. Wind tunnels are built using several sections including a compressor, diffuser, exhaust and a fan.

Wind Tunnel Schematic [6]

The first step will be to learn the Solidworks CFD program. I am really new to the program and need to learn how to properly use it. I will use the previous year vehicle designs for learning during this stage. When I figure out how to use the program I can also use the previous year’s design to get a head start on determining what areas of the car has the biggest effect on the aerodynamic performance. Since the team will still be in the theoretical development of the car there will not be any CAD files for several months which is why I have this step taking two months. The next step will be taking the new designs and running them through the CFD program. Since the designs will be frequently changed from either new devices being added, availability of new materials, and functionality changes this step will take a while. Since this phase is also when most of the designing of the car will happen and changing a design after this point could result in a massive time set back this is when I will be making aero dynamic design changes to the frame and other components to the car. I will also design the aero-package that will be put on to the car during this time. At this point in time the CWRU Baja leadership and I will have to decide if the aerodynamic approach is worthwhile. If we determine that we want to proceed with a specifically designed aero-package I will proceed with designing a wind tunnel and/or a sensor package for field testing.

Due to inexperience with wind tunnel design and building along with other considerations such as floor space and enclosing the wind tunnel. I have also never designed or built a sensor package complete with sensors and data acquisition device. Both of these tools will require help from faculty and other students. I expect a minimum of two months to design and another two months to three months to build both the wind tunnel and the sensor package. During this time team members and I will be building the aerodynamic pieces for the car. Due to limited machining time and other components being more important to build first I expect this period to take around three months.

Once the wind tunnel and sensor package are built I will need to test them. Assuming these components were built right there should not be very many bugs to work out and should take around a month. Once the wind tunnel is built we can finally test the car and tune the aero-package up until the May competitions.

Designing a wind tunnel will be an expensive endeavor. Floor space somewhere on campus will be needed, an enclosure will need to be built, and electronic controllers to control the fan speed will be needed. The estimated cost is expected to be around $7000 as seen in the following budget (this budget also includes the sensor cost assuming a student does not design and build the package in-house).

For the sensors either a student can design and build a sensor package or we can use several pressure sensor systems to measure the pressures on the car. These sensors include a built in data acquisition device and a USB port to plug in to a computer. The student method would be cheaper since most of the parts needed are on hand in the circuits lab and the basic components can be obtained cheaply. This method will take a long period of time however. The solution involving the USB sensors will be more expensive and cost around $1000. However these sensor will be available in a quick time frame and are relatively user friendly. Either solution is fine with me but if the CWRU Baja team decides to go with the student design option a student will probably need to be found now to give them time to complete the sensor package.

In conclusion I will be using a CFD program to determine if a dedicated aero-package is worth it by seeing how certain vehicle designs and changes affect the drag, downforce, and sideforce of the car. If the results of CFD testing are promising I plan on obtaining funds to either build a wind tunnel and/or obtain a pressure sensor array to measure what the pressures are on the car and use these tools to tune the aero-package.

Sources

[1]J. Wendt and J. Anderson, Computational fluid dynamics. Berlin: Springer, 2009.

[2]J. Anderson, Fundamentals of aerodynamics. Boston: McGraw-Hill, 2001.

[3]S. Turns, Thermal-fluid sciences. Cambridge: Cambridge University Press, 2006.

[4]S. McBeath, Competition car aerodynamics. Sparkford, Yeovil, Somerset, UK: Haynes Pub., 2006.

[5]J. Katz, Race car aerodynamics. Cambridge, MA, USA: R. Bentley, 1995.

[6]G. Ritchison, 'Bird Flight II', People.eku.edu, 2015. [Online]. Available: http://people.eku.edu/ritchisong/554notes3.html. [Accessed: 01- Apr- 2015].

[7] Nasa.gov, 'NASA - NASA's Wind Tunnels fact sheet', 1992. [Online]. Available: http://www.nasa.gov/centers/langley/news/factsheets/WindTunnel.html. [Accessed: 01- Apr- 2015].

[8] Lotusenthusiast.net, 'Lotus Enthusiast » Hennessey Venom GT CFD renderings', 2009. [Online]. Available: http://lotusenthusiast.net/2009/09/hennessey-venom-gt-cfd-renderings.html. [Accessed: 01- Apr- 2015].

[9] Grc.nasa.gov, 'Wind Tunnel Design'. [Online]. Available: http://www.grc.nasa.gov/WWW/k-12/airplane/tunnozd.html. [Accessed: 01- Apr- 2015].