Abstract

Objective

To detect and measure the turbulent airflow around test structures.

Introduction

When you get a new piece of furniture, you have to figure out where to put it. You want your room to look nice, but at the same time, you want to make sure traffic can flow easily in and out of the room, and that the room functions as it was intended. For example, if the room is a play space, there must be enough room to spread out toys and games. If the room is a living space, seating must be arranged so people can easily talk to one another.

Environmental engineers—the people who decide where to place or site a wind generator—have a similar task. They want to avoid blocking views or creating noise pollution with the wind generators, and at the same time, they want the generators to function as well as they possibly can for the given location. They want this machine that changes the free energy found in wind forces into electrical power, to do the best job it can of making power, which requires consideration of factors like:

• Cost
• Aesthetics (beauty)
• Zoning laws
• Noise pollution
• Wind speeds
• Wind turbulence
• Surrounding structures
• Topography
• Installation
• Transportation
• Wind generator type (horizontal-axis or vertical-axis)
• Environmental conditions (extreme cold or heat)
• Access to a power grid, and
• Mechanical stresses.

For a wind generator to work effectively, it needs a location that has average winds of at least 10-13 miles per hour, or better, for over a year. A good location is also free from "bad winds" that can damage the wind generator, such as turbulent or chaotic winds that change quickly in force, speed, and direction.

The power the wind generator is able to produce increases by the cube of the wind speed. Cube means multiplying the wind speed by itself three times. Therefore, a small increase in wind speed results in a large increase in the power that the wind generator can produce. If you raise the wind generator up to as high as is allowed by building rules, you can take advantage of higher wind velocities. The trade-offs, though, are possible obstruction of views, difficulty in installation and transport, and mechanical stresses and fatigue from high winds, which can cause the wind generator to fail.

Click here to watch a video of this investigation, produced by DragonflyTV and presented by pbskidsgo.org.

In this science fair project, you'll focus on two of the factors used in siting a wind generator: wind turbulence and surrounding structures. You might have heard the word turbulence before if you've flown on an airplane close to a storm. The pilot might have come on the intercom system and cautioned the passengers to remain seated and buckled because the plane was expected to experience some "rough air" or turbulence.

Turbulence is chaotic, random changes in the flow of a fluid or gas. An example is the behavior of smoke rising from a birthday candle, or the motion of frothy, white waves around the hull of a speedboat. Turbulence creates eddies (in the case of liquids, like water) or vortices (in the case of gases, like air). In eddies or vortices (which is the plural of vortex), the flow of the liquid or gas is reversed. An ideal place to spot an eddy in a river is on the downstream side of a rock or other obstacle in the river. If you want to see eddies on the downstream side of boulders, visit the DragonflyTV video on the right and join Rasheed, Kohner, Scotty, and JB as they go whitewater rafting.

Just as eddies occur on the downstream side of a rock in a river, vortices occur on the downwind side of a building or obstacle. This is why airplanes cannot take off too close together. Wake turbulence develops behind a plane as it takes off, which churns up the air downwind, making wind conditions too dangerous for the next plane, so air traffic controllers wait 2-3 minutes, depending on the size of the planes, before allowing the next one to take off. This gives time for the turbulence to disappear.

When siting wind generators, environmental engineers try to avoid placing generators downwind of structures or obstacles because of wind turbulence on the downwind side of the structure. Turbulent wind is not an efficient way to generate power and the violent, changing winds can also damage the wind generator. In this science fair project, you'll detect wind turbulence behind a structure by observing the motion of a streamer. You'll also determine how far away you need to be from the structure for the turbulence to die down.

Terms, Concepts, and Questions to Start Background Research

• Environmental engineer
• Site
• Wind generator
• Wind turbulence
• Topography
• Chaos
• Cube
• Velocity
• Random
• Eddy
• Vortex
• Downwind
• Wake turbulence

Questions

• Why is it important to avoid turbulence?
• Where can you find turbulence?
• How can you avoid turbulence?

Bibliography

• TPT. (2006). Whitewater Rafting by Rasheed, Kohner, Scotty, and JB. DragonflyTV, Twin Cities Public Television. Retrieved July 17, 2008, from http://pbskids.org/dragonflytv/show/whitewaterrafting.html

This source discusses how to find the best location for placing small wind generators, as well as how structures and terrain influence winds:

• Southwest Windpower. (2007). Siting Wind Generators. Retrieved July 18, 2008, from http://windenergy.com/documents/guides/0372_Siting_guide.pdf

This source describes and shows examples of vortices:

• Wikipedia Contributors. (2008, July 15). Vortex. Wikipedia: The Free Encyclopedia. Retrieved July 18, 2008, from http://en.wikipedia.org/w/index.php?title=Vortex&oldid=225877466

For help creating graphs, try this website:

• National Center for Education Statistics (n.d.). Create a Graph. Retrieved May 23, 2008, from http://nces.ed.gov/nceskids/CreateAGraph/default.aspx

Materials and Equipment

• Streamer, such as party streamer, a piece of old cassette tape, or a strand of plastic tape from a pom-pom; streamer should be several inches long
• Slender stick or pole, at least 2 feet long
• Fan
• Boxes (3 of equal size), approximately 6.5 inches x 5 inches x 2.5 inches; examples of this size are boxes of pasta or salt. The boxes don't have to be empty, but first ask your parents if it is OK to use the boxes if they are full.
• Rocks, a few handfuls to use as weights if the boxes are empty
• Measuring tape
• Room that is free from drafts
• Lab notebook

Experimental Procedure

1. Clear your room of as many obstacles as possible so you have a path that is free from furniture or other items, and that is several feet long and several feet wide.
2. Tie the streamer onto the end of your stick.
3. Place your fan at one end of the room, facing the path.
4. Place one of the boxes upright, approximately 1-2 feet in front of the fan. If the box is empty, place rocks or other weights inside the box to keep it from blowing over.
5. Turn the fan on high.
6. Stand off to the side of the fan and box and dangle the streamer between the fan and the box. Look at the motion of the streamer. In which direction is it moving? Write down the streamer's behavior in your lab notebook.
7. Again, stand off to the side of the fan and box so that your body does not affect the airflow. Now dangle the streamer on the downwind side of the box (meaning the side of the box not facing the fan). Look at the motion of the streamer. Which direction is it moving? Write down the streamer's behavior in your lab notebook.
8. Stand off to the side of the fan and box again. Hold the streamer just below the top of the box on the downwind side, and slowly move the streamer away from the box and the fan observing the motion of the streamer.
1. As shown below, turbulent wind will make the streamer flow backwards toward the box and the fan, or it will make the streamer move around chaotically in all directions.
2. When the streamer changes direction and begins to flow only forward, away from the box and fan again, then stop moving away from the box and lower the streamer to the ground so you can take a measurement.
9. Measure the distance from the downwind edge of the box to the streamer with the measuring tape, as shown in the figure below, and record your measurement in a data table in your lab notebook, like the one below.

Figure 1. This drawing shows how to measure the turbulent flow distance.

10. Repeat steps 8-9 two more times so that you have data from a total of three trials.
11. Stack another box on top of the first box and repeat steps 5-10.
12. Stack another box on top of the first two boxes and repeat steps 5-10.

Turbulent Flow Distance Data Chart

13. Calculate the average distance at which the turbulent flow ended for each of the structure heights and enter it into the data table.
14. Plot the Height of the Structure on the x-axis and the Average Turbulent Flow Distance on the y-axis. You can make the line graph by hand or use a website like Create a Graph to make the graph on the computer and print it.
15. As the height of the structure increased, did you have to get farther away from the box and fan before the turbulence died down? Is your plot linear (does it form a line)? Using your graph, can you predict how far you would need to be away from the structure to see the turbulence die down for a structure higher than three boxes, or a structure shorter than one box?

Variations

• Evaluate how high above the structure you need to be to avoid turbulence.
• Investigate how air flows around more than one structure.
• Investigate how turbulence changes as you move away from the center of a structure.

• For more science project ideas in this area of science, see Environmental Engineering Project Ideas.

Credits

Kristin Strong, Science Buddies

This science fair project was inspired by this resource: TPT. (2006). Whitewater Rafting by Rasheed, Kohner, Scotty, and JB. DragonflyTV, Twin Cities Public Television. Retrieved July 17, 2008, from http://pbskids.org/dragonflytv/show/whitewaterrafting.html

Last edit date: 2008-11-14 10:33:00