SCIENCE CONCEPT:
- One would think that a boat could only move in the
direction that the wind was blowing - that is, downwind. But a
triangular sail allows a boat to move toward the wind (windward). A boat
is moved in a windward direction by using forces that are created on each
side of the sail. This total force is a combination of a positive
(pushing) force on the windward side and a negative (pulling) force on
the leeward side, both acting in the same direction. Though you wouldn't
think so, the pulling force is actually the stronger of the two.
STUDENT OBJECTIVE:
- The student will demonstrate on paper how the wind
affects the movement of the sailboat through the water. The student will
do this by labeling the various sections of the sailboat with a brief
description under the label of how the wind affects the parts of the
sailboat, highlighting the principles of Bernoulli.
OVERVIEW:
- The student will draw a picture of a sailboat and illustrate
how the wind blowing on the triangular sails affects the movement of the
sailboat. Also, the student will write an essay describing Bernoulli's
Principle in relationship to the windward side of the sail and to the
leeward side.
TEACHER TEXT:
- In 1738 the scientist Daniel Bernoulli discovered that an
increase in air flow velocity in relation to the surroundings free air
stream causes a decrease in pressure where the faster flow occurs. This
is what happens on the leeward side of the sail - the air speeds up and
creates a low pressure area behind the sail. Why does the air speed up?
Air, like water, is a fluid. When the wind meets and is divided by the
sail, some of it sticks to the convex (leeward) side and hitches a ride.
In order for the 'unstuck' air just above it to move past the sail, it
has to bend outward toward the flow of air unaffected by the sail. But
this free air stream tends to maintain its straight flow and acts as a
kind of barrier. The combination of the free air stream and the curve of
the sail creates a narrow channel through which the initial volume of air
has to travel. Since it can't compress itself, this air has to speed up
to squeeze through the channel. This is why the velocity of flow
increases on the convex side of the sail.
Once this happens, Bernoulli's theory takes effect. The
increased air flow in the narrow channel is faster than the surrounding
air,
and the pressure decreases in this faster flowing area. This creates a
chain reaction. As new air approaches the leading edge of the sail
splits
and more of it flows to the leeward side - air flow is attracted to low
pressure areas and repulsed by high pressure areas. Now an even larger
mass of air must travel faster to squeeze through the channel caused by
the convex sail and the free air flow, causing an even lower air
pressure. This continues to build until the maximum speed is achieved
for the existing wind condition, and a maximum low pressure area is
created on the leeside. Note that the air flow increases only until it
reaches the deepest point of the curved shape (the chord depth). Up to
this point the air is converging and speeding up. Beyond this point the
air diverges and slows down until it is again the speed of the
surrounding air.
In the meantime, just the opposite is happening on the
windward side of the sail. As more air travels to the leeward side there
is less air on the windward side to travel through the expanded space
between the concave side of the sail and the free air stream. As this
air flow spreads out it slows down to a speed less than the surrounding
air, creating an increase in pressure.
One part of this relationship between sail and wind is
called the angle of attack. The angle of attack must be very precise.
If the angel remains too close to the wind the front of the sail 'luffs'
or flaps. It it's angled too wide the flow lines along the curve of the
sail detach and rejoin the surrounding air.
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PREPARATION TIME:
- 20 minutes.
LESSON TIME:
- 45 - 60 minutes.
TEACHER PREP:
- Gather materials
WORDS TO KNOW:
- windward
- leeward
- convex
- chain reaction
- leading edge
- concave
- angle of attack
- luffs
- stall zone
- leech
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