Modifying a Car for Better Aerodynamics

Abstract

Have you ever seen those cool wind tunnel pictures of cars with streams of smoke blowing over them? You do not need access to a wind tunnel to do a car aerodynamics science project! In this project you will use cheap, readily available materials—tape and yarn—to visualize airflow over a car. You will examine how you can affect the airflow over the car by making changes, like adding or removing accessories or making body modifications out of cardboard and duct tape. Maybe you can even reduce drag on the car and improve its fuel economy!

Summary

Areas of Science

Aerodynamics & Hydrodynamics

Difficulty

Method

Scientific Method

Time Required

Short (2-5 days)

Prerequisites

None

Material Availability

Readily available

Cost

Very Low (under $20)

Safety

Do not tape anything to the car that might fall off and present a hazard to other drivers.

All video recording should be done by a passenger in the vehicle, never the driver.

Wear seatbelts at all times when the vehicle is moving.

Credits

Ben Finio, PhD, Science Buddies

Science Buddies is committed to creating content authored by scientists and educators. Learn more about our process and how we use AI.

https://www.youtube.com/watch?v=YTnjxnA292E

Objective

Examine how changes to a car can affect airflow over the car and reduce drag.

Introduction

A piece of foam board extends out and downward from the back of the car's roof. It is taped to the car on the top and sides. Attached to the board are about 18 lengths of yarn, each about 4 to 6 inches long, evenly spaced. One end of each piece of yarn is taped to the board. The loose ends of the yarn will move as the car moves, indicating the direction of airflow.

Have you ever noticed that some cars look much more aerodynamic than others? For example, race cars and expensive sports cars typically have a sleek, streamlined design (Figure 1) that is quite different from big, boxy vans and trucks (Figure 2). A car's shape determines the amount of aerodynamic drag that it experiences. Drag, also called air resistance, is the force that results from air pushing back on a car as it drives forward. For cars that need to go really fast, minimizing drag is important.

Image Credit: Wikimedia Commons user SamH / Creative Commons Attribution 3.0 Unported license.
Figure 1. A Formula One race car with a very streamlined, aerodynamic design that helps minimize drag. (Image credit Wikimedia Commons User SamH, CC BY-SA 3.0)

Image Credit: Wikimedia Commons user Artaxerxes / Creative Commons Attribution 3.0 Unported license.
Figure 2. A box truck. (Image credit Wikimedia Commons user Artaxerxes, CC BY-SA 3.0)

However, aerodynamics is also important for everyday family cars like sedans and SUVs. Reducing drag can help improve the car's fuel economy since the engine does not have to work as hard to overcome drag. One way to reduce drag is to try to maintain attached flow on the car. This is airflow that closely follows the car's shape. Flow separation, or separated flow, occurs when the air flow "detaches" from the car's surface and no longer closely follows the shape of the car. In general, flow separation will result in increased aerodynamic drag. You can visualize airflow over a car, and therefore attached and separated flow, using a wind tunnel and streams of smoke, as shown in this video:

https://www.youtube.com/watch?v=E9ZSAX56m0E

However, most students do not have access to a wind tunnel for their science project! Instead, you can use a much simpler, cheaper method to visualize air flow over a car: attaching lengths of yarn to it with tape (Figure 3). You can take pictures or video as the car drives around and observe the movement of the yarn pieces to analyze the airflow over the car. Yarn pieces that are straight and point along the car's body indicate attached flow, whereas pieces that flap around a lot, curve or even point in the opposite direction indicate flow separation (and increased drag).

Image Credit: Ben Finio, Science Buddies / Science Buddies
Figure 3. Pieces of yarn attached to the window of a car with painter's tape.

For your science project, you can choose a variable to change and measure how it affects airflow over one or more regions of the car. The Procedure section of this project provides some suggestions, as do the many videos listed in the bibliography. Can you figure out how to best maintain attached flow and decrease drag on the car?

Terms and Concepts

• Drag
• Air resistance
• Fuel economy
• Attached flow
• Flow separation (Separated flow)

Questions

• What is the difference between attached and separated flow?
• Why can flow separation be bad?
• What are some ways to maintain or improve attached flow on a car?

Materials and Equipment

• Car
• Yarn (color should contrast with the car's paint color)
• Scissors
• Masking or painter's tape
• Driver to drive the car while you record video. The driver should not try to film while driving!
• Straight section of road where the car can drive at a constant speed, ideally not near other cars
• Something you can change or modify about the car. (See procedure for suggestions; materials will vary.)
• Lab notebook

This project follows the Scientific Method. Review the steps before you begin.

1. Before you do an experiment, you need to decide what the independent variable will be for your project. What will you change? The project overview video provides the following four suggestions, but you can find many more ideas in the bibliography or by doing your own research online:
   1. Driving with the windows up vs. windows down
   2. Adding or removing exterior accessories (hood ornament, roof rack/bike rack, off-road lights, flags or decorations, etc.)
   3. Modifying or extending the body using cardboard/foam board and duct tape
   4. Adding vortex generators
2. Based on what you decide to change, decide where to tape pieces of yarn on your car. You probably do not need to cover the entire car. For example, if you are putting a roof extension or vortex generators on the back of the roof, you do not need to put yarn on the hood of the car. If you are testing a hood ornament, then yarn on the hood of the car should be sufficient (Figure 4).

Image Credit: Ben Finio, Science Buddies / Science Buddies
Figure 4. Lengths of yarn taped to the hood of a car.

3. Decide how you will film the yarn while the car is moving. Do not attempt to drive while holding a camera and filming! Either get another person to drive or set up a tripod that will hold the camera for you.
4. Once you have everything set up, you are ready to conduct your control trial. Have the driver drive the car, ideally at a constant speed on a straight road. Take video of the lengths of yarn as the car moves.
5. Now you are ready to make changes and conduct another trial. The change you make will depend on what you decided in step  1.
6. Things like the weather (wind speed) and movement of nearby cars can affect the airflow over the car. If possible, do more trials in different conditions.
7. Make a data table like Table 1 to analyze your videos.

Swipe left to see more

Trial	Conditions	Total # of yarn strands	# of straight yarn strands	% attached flow

Table 1. Example data table.

7. One at a time, watch each video and fill out the data table. Watch the yarn closely. If the strand stays relatively straight and does not oscillate (flap back and forth) or bend, count it in the "straight" column. Then, divide the number of straight strands by the total number of strands to calculate the percentage of attached flow. (If every single strand in the video is straight, that would indicate 100% attached flow.)
8. Analyze your data. Did the changes you made to the car improve attached flow? Did they make it worse?
9. For your science project report or display board, you can create visualizations of your data using the tool below. The tool allows you to overlay transparent colored rectangles on the image. You can use different colors for areas of attached and separated flow.
   1. Export a single frame from one of your videos as an image.
   2. Click the "Choose File" (or "Browse" depending on your browser) button below and find the image on your computer.
   3. Click the "Add Rectangle" button below the image.
   4. Enter x and y coordinates for the upper left corner of the rectangle in pixels. The origin (0,0) of the coordinate system is in the image's upper left corner, with x increasing to the right, and y increasing downward.
   5. Enter the width and height of the rectangle in pixels.
   6. Select a color for the rectangle.
   7. Set the opacity for the rectangle.
   8. Click the "Draw" button to overlay the rectangle on the image.
   9. Instead of editing the individual text boxes, you can also edit the line of code to change the rectangle's appearance. The color is set by red, green, blue (RGB) values. Opacity is a number between 0 (totally transparent) and 1 (totally opaque).
   10. If you make any changes, click "Draw" again to redraw the rectangle.
   11. Click "Add rectangle" again to add another rectangle. Continue adding rectangles to your image and editing them until you are satisfied with an image representing the different regions of flow. Use the trash can icon if you need to delete a rectangle.
   12. When you are done, click "Download image" to download a copy of the image. You can then print it for your display board or include it in a report.
   13. Click the "Browse" button again to upload a new image and start over. Be careful, because if you leave or refresh the page, your work will be lost.

Download Image

Add Rectangle

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Global Goals

The United Nations Sustainable Development Goals (UNSDGs) are a blueprint to achieve a better and more sustainable future for all.

This project explores topics key to Industry, Innovation and Infrastructure: Build resilient infrastructure, promote sustainable industrialization and foster innovation.

This project explores topics key to Responsible Consumption and Production: Ensure sustainable consumption and production patterns.

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