Turbulence and How to Avoid It

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Darleen Burke
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1 Turbulence and How to Avoid It Lesson Overview 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. Grade Level Grades 9-12 (secondary school) Time Required One class (indoors) for discussion of concepts; one class for data collection (if desired). Curriculum Connection (Province/Territory and course) Atlantic Provinces Education Foundation: General Curriculum Outcomes for Social Studies: 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. Interdependence 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. Link to the Canadian Atlas Online (CAOL) Additional Resources, Materials and Equipment Required Computers LCD projector Internet access Coloured ribbon (several metres) Canadian Wind Energy Association: Guided Tour of Wind Energy-Danish Wind Energy Association: Wind Energy Links:

2 Main Objective To recognize the effect of obstacles in generating turbulence from wind. Learning Outcomes By the end of the lesson, students will be able to recognize the effects of obstacles (buildings, groves of trees) in generating turbulence.

3 Teacher Activity The Lesson Student Activity Introduction 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 dropdown 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? 1. Gather background information from: as 2. Small group discussion/presentation of possible answers prior to extensive investigation. These suggestions will be looked at throughout the course of the lesson.

4 Lesson Development (See attached diagram) 2. 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. 3. Inquire: 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? 2. Discover through investigations: a) The height of the school. b) The envelope of turbulence that would be produced by the school 3. Respond to questions. (Turbulence at the leading edge produces lift; tapering reduces turbulent drag along the trailing edge).

5 Demonstrations & Measurements The wind plumb-bob is described in the Lesson Extension section 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.* 4. 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 Conclusion: Implications for Turbines 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 m above the ground surface. 5. After discussion, make a list of criteria identified through discussion and observation.

7 Wind Laminar Flow Zone 2 x building height Turbulent Flow Zone 2 x building height 20 x building height Turbulence generated by a Building Lesson Extension #1 Estimating wind speed without using an anemometer If you do not have an anemometer, the wind speed can be estimated by using a wind plumb-bob. This also allows you to measure gusting or inconsistent winds by taking measurements over minutes. Requires: Protractor, cm long (can be made by cutting a wooden semi-circle) Monofilament fishing line (30-50 cm long) Ping-pong ball 1. Attach one end of the fishing line to the centre (zero point) of the protractor. Attach the ping-pong ball to the other end. Ensure that the fishing line can move freely, and that the ping-pong ball is at least 15 cm below the protractor (the length of line required will thus depend on the size of the protractor). 2. Estimating wind speed requires two people. One person holds the protractor assembly at arm s length, ensuring that the protractor edge remains level. The second person estimates the angle made by the fishing line on the protractor (and should also help ensure that the protractor is level). Allow at least 30 seconds for the wind to move the ball. 3. Wind speed can be estimated as: Angle m/s km/h Beaufort Scale Description Calm; smoke rises vertically 85 ~ 1 ~ 4 1 Very light winds; paper airplanes affected 80 ~ 3 ~ 15 3 Leaves rustle; wind felt by people

8 70 ~ 5 ~ 20 4 Moderate breeze; papers and dust move 55 ~ 8 ~ 30 5 Fresh breeze; small deciduous trees sway 40 ~ 11 ~ 40 6 Strong winds; large branches moving, wires whistling 30 ~ 14 ~ 50 7 Whole trees moving; difficult to walk For safety, do not attempt to measure stronger winds! Lesson Extension #2 Conduct further investigations concerning the design of aircraft wings, to show how aeronautical engineers use turbulence to their advantage. Lesson Extension #3 See Construction of a homemade anemometer in the Lesson Plan Wind Speed and Height: Why do wind turbines have to be so tall? (Newfoundland and Labrador Grades 9-12) Assessment of Student Learning Poster presentations of diagrams and key elements. Link to Canadian National Standards for Geography Essential Element#3: Physical Systems Components of the Earth s physical system Global ocean and atmospheric systems Geographic Skills #2: Acquiring Geographic Information Systematically locate and gather geographic information from a variety of sources. Geographic Skills #5: Answering Geographic Questions Formulate valid generalizations from the results of various kinds of geographic inquiry

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