Turbulence and How to Avoid it
Wind turbines work best when they are exposed to consistent winds moving with constant speed and direction. Turbulence (“swirling winds”) causes problems. In this lesson, students will investigate turbulence generated by obstacles such as school buildings or trees. Simple analysis and measurements illustrate the concepts of turbulence, and indicate how it can be avoided in site selection for wind turbines.
Time Required: 1-2 classes
Province: Nova Scotia
People, Place and Environment:
· Students will be expected to demonstrate an understanding of the interactions among people, places and the environment.
· Students will use maps, globes, pictures, models and technologies to represent and describe physical and human systems.
· Students will use location, distance, scale, direction and size to describe where places are and how they are distributed.
· Students will be expected to demonstrate an understanding of the interdependent relationship among individuals, societies and the environment and the implications for a sustainable future.
· Identify and describe examples of positive and negative interactions among people, technology and the environment.
By the end of the lesson, students will be able to recognize the effects of obstacles (buildings, groves of trees) in generating turbulence.
1. For background preparation, direct students to the information on the following site:
After choosing appropriate language, go to the “Explore by themes” on the left side of the screen. Choose “Extremes of Weather” then, “Generating Power” on the top bar. Then choose “Wind power” from the drop-down menu.
Wind turbines work best when they are exposed to consistent winds moving with constant speed and direction. Turbulence (“swirling wind”) causes problems.
2. Speculate on the following questions:
“Has anyone experienced turbulence in an airplane flight?”
“Can you define turbulence?”
“What influences turbulence?” “How can it be recognized and assessed?”
“ What are the implications for choosing a site and tower height for turbines?”
3. Instruct students that turbulence is near ‘zero’ directly above the ground surface. It increases with height both to windward and leeward of an object (e.g. a building). Observations indicate that a building (or grove of trees) will generate a ‘turbulent envelope’ that extends along the surface to approximately twice the height of the obstacle to windward. To leeward, the turbulent zone extends upwards to twice the height of the obstacle. To leeward, the envelope extends along the surface approximately 20 times the height of the obstacle.
“How high is our school?”
“What is the extent of the ‘envelope of turbulence’ that would be produced by the school?”
“Why are aircraft wings thicker at the leading edge, tapering towards the trailing edge?”
4. Instruct that turbulence can be demonstrated by the following:
a) If your school has a flagpole: Attach coloured ribbons at 1 m intervals to the rope used to hoist the flag. Raise the flag and observe the ribbons. Ribbons which stream in a straight line are subject to laminar (non-turbulent) flow; those which swirl are experiencing turbulence (Greater swirling, more turbulence).
b) In a location where a kite can be flown safely, the same experiment can be done by attaching ribbons to the kite string. Ribbons should be spaced at greater intervals (e.g. 5 m) and must be light enough to allow the kite to fly. When the kite reaches a height greater than twice that of the adjacent obstacles (trees, buildings), it will generally be subject to laminar flow, and ribbons near the kite will stream out in straight lines.
c) If you have either a commercial or homemade anemometer, or a wind plumb-bob, you can measure wind speeds at various places around the school building. Alternatively, wind directions only can be measured.
Variations in either direction or speed indicate turbulence. Generally, the envelope of turbulent flow extends around a low building (such as a school) as indicated on the diagram.*
5. Brainstorm and record responses to the following:
“If you wanted to set up a mini-turbine at your school, how high would it have to be to be above the turbulent flow zone?”
“What would be the best location on school property for it?”
“How high are the tallest trees in your area?”
“What is the minimum clear space required around a wind turbine?”
Generate a list e.g.:
a.) A wind turbine must never be located in a zone of excessively turbulent airflow.
b.) Light turbulence will decrease performance, because a turbine cannot react to rapid changes in wind direction.
c.) Extreme turbulence puts stress on the rotors and blades, and can result in wind turbine failure.
d.) The height of the turbine should be at least twice that of any trees in the vicinity.
Ideally, there should be no large trees within a distance approximately 10 times the height of the turbine. Locating turbines in open areas avoids problems with turbulence.
Typically, commercial turbines are mounted 50-80 m above the ground surface.
Instructions for Students
1. Gather background information from: www.canadiangeographic.ca/atlas
2. Small group discussion/presentation of possible answers prior to extensive investigation.
These suggestions will be looked at throughout the course of the lesson.
3. Discover through investigations:
a) The height of the school.
b) The ‘envelope of turbulence’ that would be produced by the school
4. Respond to questions.
(Turbulence at the leading edge produces lift; tapering reduces turbulent drag along the trailing edge).
5. Assemble ribbon pieces which should be at least 1 m long and wide enough to be easily visible. Use different colours for different heights.
a) Record the heights of ribbons influenced by turbulence on the flagpole.
b) Record the heights of ribbons influenced by turbulence.
c) Record the wind speed and direction at each location. Plot the locations on a map of the school grounds. Are the wind speeds and directions identical everywhere? If not, then turbulence is occurring.
*This only applies to essentially vertical obstacles (groves of trees and small buildings), not to sloping topography or hills. It also does not apply to a single isolated tree or pole.
6. After discussion, make a list of criteria identified through discussion and observation.
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