Wild Winds: Turbulent Flow Around Structures
Have you ever seen a wind turbine? Wind turbines are large towers with blades on top that are spun by the wind. They are a great source of clean, renewable energy. Environmental engineers have to figure out where to put wind turbines so they get the best exposure to steady, consistent winds. In this project, you will play the role of an environmental engineer and figure out whether it is a good idea to put wind turbines near tall buildings.
Have you ever watched water flow through a stream or river? If there are no obstructions in the way, the water probably flows in a smooth, straight line. This is called laminar flow. However, if there is something in the water, like a rock or a branch, this causes the water to swirl and tumble as it flows by. This is called turbulent flow. The individual "swirls" are called vortices (the plural of vortex).
The same thing happens in air, when wind blows around a tall structure like a building or a tree. This is an important factor to consider when placing wind turbines, because they will generate the most power from steady winds that blow in the same direction. Inconsistent, turbulent flow can decrease power output or even damage the turbines. In this project, you will use cardboard boxes to represent "buildings" and see how they affect the flow of "wind" blowing from a fan.
Of course, turbulent flow in air is much harder for us to see! Sometimes scientists do controlled experiments using colored smoke to observe airflow. In this project you will use a streamer tied to the end of a stick to visualize the airflow.
- Stick or rod, a few feet long
- Streamer, a few inches long
- Cardboard box. The size of the box depends on the size of your fan – use a smaller box with a smaller fan, and a larger box with a larger fan.
- Optional: if the box is empty, you might need to put something inside it to weigh it down
- Set up the cardboard box a few feet away from the fan, with the fan pointed at the box.
- Tie the streamer to the end of the stick.
- Turn the fan on.
- Standing to the side so your body does not interfere with the air flow, use the stick to hold the streamer "upwind" of the box (on the side of the box facing the fan). Which direction does the streamer blow? Is its motion consistent? Do you think this shows turbulent or laminar flow?
- Now move the streamer to the "downwind" side of the box (on the side of the box facing away from the fan), but hold it fairly close to the box. What happens to the streamer now? Is the flow in this area turbulent or laminar?
- Gradually move the streamer farther and farther away from the box on the downwind side. What happens as you move the streamer farther away from the box? What does this mean about the air flow?
- What conclusions can you draw about the airflow near tall structures? How would this affect the placement of wind turbines?
Extra: see how your results change if you change the speed of the fan.
Extra: try the experiment with different size boxes. How do the results change?
Extra: try the experiment with different or irregularly shaped objects, like furniture. How do the results change?
Extra: try the experiment outside on a windy day. What happens if you hold the streamer near the corner of a building or behind a tree?
Observations and Results
When you held the streamer "upwind" of the box, you should have seen it blow consistently in one direction, away from the fan. This is because the flow upwind of the structure is laminar (or straight). However, when you held the streamer just "downwind" of the box, it might have fluttered around in all directions or even blown backwards. This is because the structure interrupts the smooth, laminar flow and causes turbulence. As you gradually moved the streamer away on the downwind side, its motion should have again become consistent, blowing away from the fan. This is because the flow gradually "recovers" and becomes laminar again after the turbulence is introduced.
This is important when placing wind turbines because they should not be too close to large buildings on the "downwind" side. The buildings will create turbulent flow, which will decrease the performance of the wind turbines.
Ask an Expert
- Science Activities for All Ages!, from Science Buddies
- Wild Winds: Detecting Turbulence Around Structures, a science project idea from Science Buddies