Go with the Flow: Why are Lighthouses Round?

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Updated on Aug 24, 2011

Lighthouses must be built along the coast and they must be tall, but that subjects these structures to fierce winds. Builders have learned to make the shape of lighthouses round, causing air to flow around them with less resistance, and allowing them to withstand strong winds.

Sea coasts are beautiful, but it's not unusual for them to experience violent storms with furious winds. Through the years, builders have had to take this environment into account when they design lighthouses. These unique buildings that have aided storm-driven sailors for centuries must be constructed to withstand hard winds and weather. Lighthouses are also used for identification by those at sea to help them get their bearings as to where they are in relation to the coast, a shoal, or a safe harbor.

A good defense against the wind is to offer as little resistance as possible and to deflect the moving air past the structure, so it flows smoothly around it. Have you ever held a large sheet of plywood and tried walking with it on a windy day? Think about a sail on a sailboat; it presents a lot of resistance to the wind and uses the wind's force to propel the boat.

A building with the shape of a cylinder guides the air flow around it and allows the air to continue behind it. Such a structure can withstand higher winds, as it has less force than on a similar structure that catches the wind. Therefore, you may have noticed from seeing pictures or visiting lighthouses that most of them are cylindrical in shape. Now you know why!

Hypothesize that moving air flows more easily around a cylindrically shaped object than one with a flat surface facing the wind and, therefore, offers less resistance to wind.

  • Thirty-three (33) long straight pins
  • Spool of thread
  • Piece of plywood 1 foot square
  • Piece of balsa wood 1 foot square (or several smaller pieces that can be laid side by side to cover a 1-foot-square area)
  • Glue
  • Ruler
  • Pencil
  • A cylindrically shaped object between 3 and 3½ inches in diameter (a glass jar or a can of fruit—we recommend a cardboard container for bread crumbs)
  • Two pieces of 2×4 lumber, each about 5 or 6 inches long
  • Hair dryer
  • Pair of scissors
  • Possible adult supervision needed

The constant in this project is the velocity of the approaching air. The variable is the shape of the object around which the air must flow.

For us to see the pattern of air flow around an object, we must first construct a device that visually shows us the presence and direction of air flow (an "air flow table"). Obtain a piece of plywood that is at least 12 inches square. Glue a 12"×12" sheet of balsa wood on top, or attach it by using several very small screws or nails. If you cannot get a single sheet of balsa wood that big, use several smaller pieces, lay them side by side, and carefully cut them with a utility knife to form a 1-foot-square area. Use extreme caution when you work with a utility knife.

Using a ruler, mark a grid pattern of lines at 1½ inch increments, both horizontally and vertically, on the balsa wood. At the point where each line intersects, carefully push a long straight pin into the balsa wood with your thumb. As shown in the illustration, do not put pins near the front of the board in the locations covered by the shaded circle. This is where the objects under test will be placed.

Tie thread onto each pin, and position it near the pin's head. Using scissors, trim the thread to a length of 1½ inches. You can use a small drop of glue on the pin to hold the thread securely in place. This is helpful if you plan to move the project from home to a classroom or a science fair.

Secure two pieces of 2×4 wood together, each about 5 inches tall (use glue, string, screws, or nails). This makes a structure that is almost square on four sides. Stand it upright in the empty space on the balsa board.

Hold a hair dryer in front of the balsa board and aim it directly at the 2×4 wood block. Place the hair dryer on a setting that blows the most air. If the hair dryer has a cool setting, use it, because heat is not needed. Observe the pattern of the threads. Do the threads directly behind the block move?

Remove the wood block and replace it with a cylindrically shaped object that is about 3½ inches in diameter. A large cardboard bread-crumb container works well.

With the hair dryer in the same position and at the same setting, do the directions of the threads look different? Do the threads directly behind the cylinder now indicate a strong air flow?

Write down the results of your experiment. Document all observations and data collected.

Come to a conclusion as to whether or not your hypothesis was correct.

Something More
  1. Experiment with structures of different shapes (try a pyramid, for example). Observe the flow of air around them.
  2. Construct a stream table, a device through which water can flow and objects can be inserted to study how shape affects the flow of water. Food coloring dye is dropped into that water to better visualize the pattern of water flow. Prove that the behavior of the flow of water and the behavior of the flow of air around an object are similar because they are both "fluids."