Force received from wind 2 (Effect of shape)

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What type of experiment is this?

Experimental procedure and explanation:

• What happens to the force received from the wind that acts on an object when we change its shape?
• Let us compare a square bar and a round bar. Examine the magnitude of the force caused by the wind from the elongation of the rubber string when exposed to the wind.
• With the square bar, the rubber stretches about 32 cm. With the round bar, it extends up to about 22 cm. Thus, the force due to the wind (air resistance) is smaller for the round bar compared to the square bar.
• With the round bar, the wind turns slightly behind the bar and the vortex that forms at the back becomes smaller (the separation will become smaller). The force by the wind (air resistance) became smaller as the vortex became smaller.

[Development]	Drag FD (such as air resistance, water resistance) can be expressed by the following formula. For a cylinder (round bar) and a square pillar (square bar), the cylinder has a smaller drag coefficient CDvalue and a smaller drag FD. (Drag FD) ＝ (Drag coefficient CD) × (Fluid density ρ) × (Speed U )2 × (Frontal projected area S) ÷ ２ 　Here, the drag coefficient CD is determined by the shape of the object. The drag coefficient of the cylinder is about CD = 0.7–1.2, and the drag coefficient of the square pillar is about CD= 1.1–2.0 (each varies depending on the length, etc.).
[Keywords]	Air resistance, Drag
[Related items]	Function of wind and rubber, Force received from wind 1 (Effect of size), Jump with buoyancy 1
[Reference]	“The Wonders of Flow”, Japan Society of Mechanical Engineering, Kodansha Blue Backs, P148-155. “Illustrated Fluid Dynamics Trivia,” by Ryozo Ishiwata, Natsume Publishing, P54-55 and P58-63.

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