Fluid mechanics ppt by sanj2681

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									           Fluid Mechanics
• Liquids and gases have the ability to flow
• They are called fluids
• There are a variety of “LAWS” that fluids
• Need some definitions
• Regardless of form (solid, liquid, gas) we
  can define how much mass is squeezed into
  a particular space

         density 
• A measure of the amount of force exerted
  on a surface area

     pressure 
          Pressure in a Fluid
• The pressure is just the weight of all the
  fluid above you
• Atmospheric pressure is just the weight of
  all the air above on area on the surface of
  the earth
• In a swimming pool the pressure on your
  body surface is just the weight of the water
  above you (plus the air pressure above the
          Pressure in a Fluid
• So, the only thing that counts in fluid
  pressure is the gravitational force acting on
  the mass ABOVE you
• The deeper you go, the more weight above
  you and the more pressure
• Go to a mountaintop and the air pressure is
Pressure in a Fluid
                Pressure acts
                to the surface
                and increases
                at greater
Pressure in a Fluid
           Net upward
           force is
           called the
           Easier to
           lift a rock
           in water!!
Displacement of Water
             The amount of
             water displaced is
             equal to the
             volume of the
        Archimedes’ Principle
• An immersed body is buoyed up by a force
  equal to the weight of the fluid it displaces.
• If the buoyant force on an object is greater
  than the force of gravity acting on the
  object, the object will float
• The apparent weight of an object in a liquid
  is gravitational force (weight) minus the
  buoyant force
• A floating object displaces a weight of fluid
  equal to its own weight.
• The primary difference between a liquid
  and a gas is the distance between the
• In a gas, the molecules are so widely
  separated, that there is little interaction
  between the individual moledules
• Independent of what the molecules are
Boyle’s Law
              Boyle’s Law
• Pressure depends on density of the gas
• Pressure is just the force per unit area
  exerted by the molecules as they collide
  with the walls of the container
• Double the density, double the number of
  collisions with the wall and this doubles the
Boyle’s Law
         Density is mass
         divided by
         Halve the
         volume and you
         double the
         density and thus
         the pressure.
              Boyle’s Law
• At a given temperature for a given quantity
  of gas, the product of the pressure and the
  volume is a constant

       P V1  P2 V2
        Atmospheric Pressure
• Just the weight of the air above you
• Unlike water, the density of the air
  decreases with altitude since air is
  compressible and liquids are only very
  slightly compressible
• Air pressure at sea level is about 105
          Buoyancy in a Gas
• An object surrounded by air is buoyed up by
  a force equal to the weight of the air
• Exactly the same concept as buoyancy in
  water. Just substitute air for water in the
• If the buoyant force is greater than the
  weight of the object, it will rise in the air
Buoyancy in a Gas

          Since air gets less
          dense with altitude,
          the buoyant force
          decreases with
          altitude. So helium
          balloons don’t rise
Bernoulli’s Principle
        Bernoulli’s Principle
• Flow is faster when the pipe is narrower
• Put your thumb over the end of a garden
• Energy conservation requires that the
  pressure be lower in a gas that is moving
• Has to do with the work necessary to
  compress a gas (PV is energy, more later)
         Bernoulli’s Principle
• When the speed of a fluid increases,
  internal pressure in the fluid decreases.
Bernoulli’s Principle
      Bernoulli’s Principle

Why the streamlines are compressed is
quite complicated and relates to the air
boundary layer, friction and turbulence.
Bernoulli’s Principle

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