6 Life in a Fluid Medium

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6 Life in a Fluid Medium
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• How is seawater different than Air?
• More viscous
• More dense (density increases linearly with
  salinity)
• Lower levels of O2 in water

• How is living in seawater different than living
  in air??
• O2 can be obtained from solution
• More supportive medium than air (no need for
  skeletons)
• Movement is much more difficult (viscous)

NOTE freezing temp of seawater is 1.9 C
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Streamline

• CONSIDER FLUID MOVING IN STREAMLINES
• Water flow can be visualized as
  streamlines.Particles entrained in flow move with
  
• streamlines and do not cross.
• Inertial and viscous forces compete.

Cylinder (in cross section)
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Reynolds Number, Re measure of relative
importance of viscous and inertial forces in fluid
Note that we are always working with seawater, so
we Consider no variation in ? (density) or ?
(viscosity)??Therefore we conclude that Re
increases with velocity (V) and size of object
(l).
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We can make a calculation of Re if an object is
moving in water or stationary, with the water
moving past the object.
l
V
l
V
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Reynolds numbers for a range of
swimming organisms and sperm
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Reynolds number implications

• Re gt 1000 inertial forces predominate
• Re lt 1 viscous forces predominate
• World of very small size and velocity is a
  viscous world takes continuous work to move an
  object. Particles will stop moving when no work
  exerted (e.g., ciliate can stop instantaneously
  and reverse direction by simply stopping waving
  of external cilia).
• World of large size and high velocity is an
  inertial world if work is done, object will tend
  to continue to move in fluid (e.g., supertanker
  at full speed will continue to move several km
  after propulsive power shut off).

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Drag

• Water moving past an object creates drag.
• At high Reynolds number, the pressure difference
  up and downstream explains the pressure drag.
  Streamlining and placing the long axis of a
  structure parallel to the flow will both reduce
  pressure drag.
• At low Reynolds number, the interaction of the
  surface with the flow creates skin friction.

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Drag and fish form. The left hand fish is
streamlined and creates relatively little
pressure drag while swimming. the right hand fish
is more disk shaped and vortices are created
behind the fish, which creates a pressure
difference and, therefore, increased pressure
drag. This disk shape, however, allows the fish
to rapidly turn.