Science of Submarines

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Science of Submarines

• CSS Hunley

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CSS Hunley

• CSS Hunley background

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Module Overview

• Learn about the submarine Hunley, its history and
  its service during the Civil War.
• Learn why the Hunley was a leap in tech-nology
  for the period.
• Learn about form drag, a retarding force to the
  forward motion of a submarine due to its shape.

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The Hunley a leap in technology

• Adversary ship had gotten larger, no longer
  an-chored in small harbors for long periods like
  the ships the Turtle faced. Steam power had been
  introduced for capital vessels and larger vessels
  had deeper drafts.
• Distances were much larger, and a submarine had
  to go several miles under her own power.
• The power selected was human power a crew of 8,
  turning a screw.
• What was required was a submarine shape that was
  more efficient moving through water under power.

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Forces on a Submarine
Lift
Drag
Thrust
Weight

• There are four forces that affect the
  movement of a submarine lift, weight, thrust,
  and drag. We exa-mine one of these, drag, and
  one form of drag, form drag.

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Form Drag

• Form drag, or pressure drag, is directly related
  to the shape of submarine body a smooth,
  streamlined shape will generate less form drag
  than a blunted or flat body.
• Form drag is easy to demonstrate using a hand out
  the window of a moving car. If the hand is held
  flat, like a wing, its a streamlined object. The
  person feels only a small tug or drag. If the
  hand is turned so that the palm is facing
  forward, the drag force is greatly increased, and
  the hand is pulled backwards! Its no longer
  streamlined.

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Scientific Principle

• An equation commonly used to represent the drag
  force on an object as it moves through a fluid is
• F 0.5Cdrv2S
• where
• Cd is the coefficient for drag
• r is the density of the fluid
• v is the velocity of the object
• S is the projected area of the object
  perpendicular to the plane of the flow

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A Virtual Experiment

• A graduated cylinder is filled with water. Four
  different shaped objects (cube, tear drop,
  dimpled golf ball, smooth sphere) are dropped in
  the cylinder of water and the time for each to
  reach the bottom of the cylinder recorded.
• What is the relationship between the times to
  reach the bottom of the cylinder and the shape
  of the objects?
• Object drop experiment

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Experimental Results
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Experimental Conclusions

• Our virtual experiment suggests that the
  resistance of the water to the fall of the
  objects depends on the shape of the objects.
• The more streamlined objects fell to the bottom
  of the tank more rapidly than the less
  streamlined objects.

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More Examples of Drag

• Swimmer on the left has more drag than the
  swimmer on the right

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More Examples of Drag

• On the left, flow visualization of smoke blown
  over a tennis ball. Air flow is moving left to
  right. The large gap behind the ball is known as
  the "wake". Compare this to a smooth bowling ball
  on the right. A smaller gap infers less drag.

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References
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Glossary

• Viscosity is a measure of much a fluid will
  resist flowing. Units of viscosity, defined in
  terms of mass, are kilograms per meter second
  (kg/m s).
• Density is a measure of how much mass (number of
  molecules) an object contains. Units of density
  are mass per unit volume (kg/m3).

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Computational Problems