Those of us in the curved steel business are sometimes asked to roll steel bars into a helical shape. One application is for these helixes to be affixed to smoke stacks and chimneys to minimize the vibrating effects of vortex shredding. Smokestacks are often wrapped with ¼ x 2 bars rolled into a helix the “hard way” (against the strong axis) to an inside radius matching the outside diameter of the cylinder. The pitch is usually 57.9 degrees. Sometimes the strakes are attached to the stacks as the stacks are being built; sometimes the strakes are retrofitted to existing installed stacks.
Once you become aware of these strakes on smoke stacks, you will become aware of this vibrating phenomenon in other everyday experiences. For example, have you seen car antennas manufactured in a manner similar to cylinders with attached helical strakes?
Have you ever strapped down your surfboard or similar item to the roof of your car by using nylon straps only to hear an annoying vibrating sound as you speed down the highway? If you would only twist the straps, you can eliminate much, if not all, of the sound.
Helical strakes have been retrofitted for the stacks which are part of a power generating station in Alaska. After the tsunami in Japan, the operators of the power plant wanted to be sure that their stacks could withstand such forces.
In another application, helical strakes were retrofitted to electrical transmission towers in Nevada, once again to protect them from vibration that might damage them structurally.
So . . . next time you see a thin, long object swinging back and forth in the wind, you will know you may be witnessing the effects of vortex shredding which could be lessened or eliminated with the use of helical strakes.
