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Fluid Mechanics

Fluid Mechanics. Why is the electricity produced at the bottom of dams?. When you catch a deep-sea fish, why does its eyes pop-out?. Why do your ears pop on an airplane or up in the mountains?. The study of how fluids flow and how forces and energy are transferred through fluids

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Fluid Mechanics

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  1. Fluid Mechanics Why is the electricity produced at the bottom of dams? When you catch a deep-sea fish, why does its eyes pop-out? Why do your ears pop on an airplane or up in the mountains?

  2. The study of how fluids flow and how forces and energy are transferred through fluids Liquids and gases have the ability to flow They are called fluids There are a variety of “LAWS” that fluids obey Need some definitions Fluid Mechanics

  3. Defining a Fluid • A fluid is a nonsolid state of matter in which the atoms or molecules are free to move past each other, as in a gas or a liquid. • Both liquids and gases are considered fluids because they can flow and change shape. • Liquids have a definite volume; gases do not.

  4. Regardless of form (solid, liquid, gas) we can define how much mass is squeezed into a particular space Density

  5. A measure of the amount of force exerted on a surface area Pressure is measured in pascals (pa) Pressure

  6. Pressure • Pressure = Force/surface area • Pressure = Newtons (Kg x m/s2) meters2 • Units are in N/m² or Pascals (Pa)

  7. 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 water) Pressure in a Fluid

  8. 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 lower Pressure in a Fluid

  9. Pressure in a Fluid Pressure acts perpendicular to the surface and increases at greater depth.

  10. Pressure in a Fluid • Remember that when we are considering the pressure of a fluid: • At any given point the pressure exerted by the fluid is equal in all directions • In other words a fluid will exert the same pressure in every direction relative to the force acting on the fluid (that is the weight above the fluid)

  11. Pressure in a Fluid

  12. Buoyancy Net upward force is called the buoyant force!!! Easier to lift a rock in water!!

  13. Factors Affecting Fluid Pressure • Fluid density • Depth • Weight • NOT: • Shape or volume of the container

  14. Hydrostatic Pressure • Water pressure due to depth alone

  15. Instruments Used to Measure Pressure • Barometer • Mercury • Aneroid • Gauges • Gauge Pressure • Measures system pressure relative to the atmospheric pressure, 0 gauge pressure indicates the system is at atmospheric pressure • Absolute Pressure • Measures pressure relative to a perfect vacuum

  16. Specific Gravity • (s.g.)= The comparison of the density of a substance to the density of water • s.g.= density of substance density of water

  17. Displacement of Water The amount of water displaced is equal to the volume of the object displacing the water.

  18. 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 Archimedes’ Principle

  19. Archimedes’ principle: • Buoyant Force on an object immersed in a liquid equals the weight of the liquid displaced and the weight of the object if it floats.

  20. A floating object displaces a weight of fluid equal to its own weight. Flotation

  21. Flotation

  22. Pascal's Principle • When a force is applied to a confined fluid, the increase in pressure is exerted equally to all parts of the fluid.

  23. Transmitting Pressure in a Fluid • When force is applied to a confined fluid, the change in pressure is transmitted equally to all parts of the fluid.

  24. Hydraulic Machines • In a hydraulic machine, a force applied to one piston increases the fluid pressure equally throughout the fluid.

  25. Hydraulic Devices • By changing the size of the pistons, the force can be multiplied.

  26. Flowing Fluids: Liquid • A fluid flows because a fluid will exert an equal force in all directions. • When a fluid is contained the fluid remains within its boundaries (Newton’s 1st law) • When an opening is made the fluid will flow through that opening (Newton’s 3rd law) (a substance will flow where there is least resistance)

  27. Flowing Fluids: Gases • “Gases flow because of difference in pressure as well, but their behavior is also influenced much more by the way they diffuse and are compressed” –pg 184 • Gases are more susceptible to compressibility and diffuse more readily than liquids • Think of how a storm forms: • When low pressure winds are surrounded by high pressure winds the winds form a circular effect. • Low pressure is formed from the warm air rising from the ground, they reach the cooler air (high pressure) in the atmosphere. When these warm and cool airs combine we get a storm

  28. The whole system is a low pressure, but it dramatically decreases towards the eye of the hurricane. Very Low pressure Pressure always flows from high to low, which creates the high velocity winds. Higher Pressure

  29. Barometric Pressure • The barometer is used to forecast weather. • Decreasing barometer means stormy weather and an increasing barometer means warmer weather.

  30. 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 newtons/meter2 Atmospheric Pressure

  31. Barometers

  32. Bernoulli's Principle • The pressure exerted by a moving stream of fluid is less than its surrounding fluid. • Therefore, as the speed of the fluid increases its pressure decreases. • The sum of the water’s pressure, kinetic energy and potential energy, is equal at all points

  33. Flow is faster when the pipe is narrower Put your thumb over the end of a garden hose Energy conservation requires that the pressure be lower in a gas that is moving faster Has to do with the work necessary to compress a gas Bernoulli’s Principle

  34. Bernoulli’s Principle

  35. When the speed of a fluid increases, internal pressure in the fluid decreases. Bernoulli’s Principle

  36. Bernoulli’s and Baseball A non-spinning baseball or a stationary baseball in an airstream exhibits symmetric flow. A baseball which is thrown with spin will curve because one side of the ball will experience a reduced pressure. This is commonly interpreted as an application of the Bernoulli principle. The roughness of the ball's surface and the laces on the ball are important! With a perfectly smooth ball you would not get enough interaction with the air.

  37. Bernoulli’s and Air Foil Bernoulli's Principle says that increased air velocity produces decreased pressure. Lift is produced by an airfoil through a combination of decreased pressure above the airfoil and increased pressure beneath it. The principle states that "the pressure of a fluid [liquid or gas] decreases as the speed of the fluid increases." Within the same fluid (air in the example of aircraft moving through air), high-speed flow is associated with low pressure, and low-speed flow is associated with high pressure.

  38. The Condă Effect • Condă effect is the phenomena in which a fluid (mainly jet flow) attaches itself to a nearby surface and remains attached even when the surface curves away from the initial fluid/air (jet) direction.

  39. The primary difference between a liquid and a gas is the distance between the molecules In a gas, the molecules are so widely separated, that there is little interaction between the individual moledules IDEAL GAS Independent of what the molecules are Gases

  40. Kinetic theory of gases and … • Compressibility of Gases • Boyle’s Law • Pa collision rate with wall • Collision rate a number density • Number density a 1/V • Pa 1/V • Charles’ Law • Pa collision rate with wall • Collision rate a average kinetic energy of gas molecules • Average kinetic energy aT • PaT

  41. Physical Characteristics of Gases • Gases assume the volume and shape of their containers. • Gases are the most compressible state of matter. • Gases will mix evenly and completely when confined to the same container. • Gases have much lower densities than liquids and solids.

  42. An object surrounded by air is buoyed up by a force equal to the weight of the air displace. Exactly the same concept as buoyancy in water. Just substitute air for water in the statement If the buoyant force is greater than the weight of the object, it will rise in the air Buoyancy in a Gas

  43. Buoyancy in a Gas Since air gets less dense with altitude, the buoyant force decreases with altitude. So helium balloons don’t rise forever!!!

  44. Force Area Barometer Pressure = Units of Pressure 1 pascal (Pa) = 1 N/m2 Atmospheric Pressure at Sea Level: 1 atm = 760 mmHg = 760 torr 1 atm = 101,325 Pa

  45. Gas Laws • Boyle’s Law- states that volume and pressure of a confined gas are inversely related • As the temperature remains constant the pressure will decrease when the volume increases. • Charles’ Law- states that the volume of a fixed amount of gas is directly proportional to its absolute temperature when the pressure is constant. • As the pressure remains constant the volume will increase as the temperature increases

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