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# Automotive Aerodynamics: A Guide for Dummies (Part I)

By: Harshita Singh Chauhan

“According to the laws of aerodynamics, the bumblebee can’t fly either, but the bumblebee does not know anything about the laws of aerodynamics so it goes ahead and flies anyway.”

Igor Sikorsky

FUN-DAMENTAL CONCEPTS #iliketocringe

Although bumblebees are probably one of the best insects, if automobiles were similar to the bumblebee - completely devoid of aerodynamic features - they would be very inefficient (sorry Mr. Bumblebee). With an increasing need to reduce pollution and a dwindling supply of oil, automobiles are being designed to be aerodynamic so they can be more fuel-efficient. But do you know exactly which features help a car become more aerodynamic? Do you know what the underlying physics concepts are?

Before we start, however, it is important to understand the various types of drag, a force that opposes the motion of the car. It’s just a brief introduction, I’ll try not to drag it for too long!

Aerodynamic drag is a combination of five main types of drag. The three types of drag relevant to automotive aerodynamics are:

Friction Drag is correlated with the smoothness of the automobile’s surface . It can be reduced by either reducing the surface area of the car or increasing smoothness of its surface.

Form Drag is dependant on the longitudinal section of an object as well as the angle of attack. Therefore, it can be reduced by streamlining components, as seen in the image on the left.

Pressure Drag is the result of vortices generated behind a solid, where airflow is unable to rejoin. In order to reduce pressure drag, we need to minimize pressure differences between the leading and churning eddies in the wake through reducing boundary layer separation. For instance, the vehicle on the right has a large wake (low-pressure zone at the rear), and a high-pressure front. As you might have learned in your Physics lessons, air travels from a region of high pressure to that of lower pressure. This means that the air will move in the direction opposing the movement of the car, creating pressure drag.

Drag Coefficient

The most commonly used measurement to quantify the “aerodynamic-ness” of an object is the Drag Coefficient (cd). The drag coefficient is a dimensionless quantity and a good gauge of how aerodynamic an object is as it considers both the skin and form friction. A flat plate held at right angles to the airflow has a Cd of 1.25, whereas the most efficient production car shapes at the moment have a Cd of about 0.22. The drag coefficient is also directly proportional to the frontal area of the object, which explains why sports cars have much smaller frontal areas compared to regular cars!

AERODYNAMIC FEATURES

We have now covered the basics of aerodynamics, which is to recognize the types of drag and the factors that influence them. Now let’s dive into the features of a car that results in aerodynamic!

Apart from the features mentioned in the annotated diagram, the grooves on the car are designed to channel air to reduce boundary layer separation and the wake of the car. As you can see in the picture below, the flow lines are very close to the car body, an indication of less boundary layer separation and the wake at the rear of the car has also been minimized to reduce pressure drag, making it more aerodynmic.

The model also has diffusers in the underbody. Diffusers are used to reduce turbulence by guiding the air through their channels Although diffusers are more commonly used in race cars, their inclusion in the CA 180 contributed tremendously in drag reduction as the front and rear of the cars are major contributors to the drag coefficient of an object. On the brighter side, this means that there is tremendous scope for enhancing these areas.

Lastly, the CA180 also has gentle slope in the hood and rear of the car causing air to be directed upwards near the front and downwards at the rear, resulting in a reduction of the boundary layer separation.

So the next time you see a car on the road, make sure you appreciate its beauty and realize that every groove on the car is there for a reason, not just to make the car aesthetically appealing!

There are many more features in cars that have not been uncovered yet but we’ll have to drag that to Part II, where we will discuss the features in F1 Race Cars. So buckle up and stay tuned!

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Image Credits

{1]Warner,Sally. (23/03/2010). Form Drag and Tidal Flow Over Topography. Oregon State University. http://people.oregonstate.edu/~warnersa/research_phd.html. Retrieved: 17/12/2018.

[2] Sheppard, Scott. (15/03/2013). Updated Project Falcon Wind Tunnel Simulation on Autodesk Labs. It’s Alive in the Lab. https://labs.blogs.com/its_alive_in_the_lab/2012/09/updated-project-falcon-wind-tunnel-simulation-on-autodesk-labs.html. Retrieved: 17/12/2018.

[3] 2019 BMW Cars. Car and Driver. https://www.caranddriver.com/bmw Retrieved: 17/12/2018.

[4] 2018 Lamborghini Huracan Performante. Car and Driver. https://www.caranddriver.com/reviews/2018-lamborghini-huracan-performante-first-drive-review Retrieved: 17/12/2018.

References

[5] Karthik. Vehicular Aerodynamics. https://gr8autotech.wordpress.com/2013/06/16/vehicular-aerodynamics/. Retrieved: 18/12/2018.

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