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Fluid Mechanics

Fluid Mechanics. Chapter 8. Fluid State. Flow and have no definite shape Liquids and gasses are fluids Pressure is the force on a surface divided by the area of the surface P = F/A The force (F) on the surface is assumed to be perpendicular to the surface area (A).

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Fluid Mechanics

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  1. Fluid Mechanics Chapter 8

  2. Fluid State • Flow and have no definite shape • Liquids and gasses are fluids • Pressure is the force on a surface divided by the area of the surface • P = F/A • The force (F) on the surface is assumed to be perpendicular to the surface area (A)

  3. When you stand on ice, the ice exerts on your body an upward normal force that has the same magnitude as your weight • The upward force is spread over the area of your body that touches the ice, which is the soles of your feet. • If you lay on the ice there is a larger area for the force to be spread out over

  4. Pressure • Pressure (P) is a scalar quantity • Unit is the pascal (Pa) = to 1N/m2 • Atmospheric pressure- On every square centimeter of the Earth’s surface at sea level, the atmosphere exerts a force of approximately 10 N. • Balanced on your body, notice when ears pop

  5. Fluids at rest • Ideal fluid- has no internal friction among its particles • Used as a model for fluids • Examples of Fluids • Water, honey, oil, tar, and air

  6. Pascal’s Principle • Blaise Pascal (1623-1662), a French physician, noted that the shape of a container has no effect on the pressure of the fluid it contains at any given depth • Pascal’s principle- any change in pressure applied at any point on a confined fluid is transmitted undiminished throughout the fluid • Squeeze a tube of toothpaste • Pressure exerted at bottom is transmitted through the tube, forcing the paste out the top

  7. Hydraulic pressure • When fluids are used in machines (hydraulic lifts) to multiply forces, Pascal’s principle is being applied • In a hydraulic system a fluid is confined to two connecting chambers with a piston that is free to move. • Force exerted by a hydraulic lift • F2 = F1A2 / A1

  8. Gas Pressure • Gases are made up of small particles, widely separated, in constant, random motion at high speeds, making elastic collisions with each other and surfaces. • The forces exerted by these collisions result in gas pressure on the surface

  9. Pressure of Water on a Body • P = Fg/A = pAhg/A = Dhg • The pressure of the water on a body depends on the density of the fluid, its depth, and g.

  10. Buoyancy • The increase of pressure with depth creates an upward force on all objects • Called the buoyant force • Buoyant force is Fbuoyant = pVg • The volume of the object is equal to the volume of the fluid that was displaced

  11. Archimedes’ Principle • The relationship that the buoyant force (pVg) is equal to the weight of the fluid displaced was discovered by Archimedes in 212 B.C. • All objects in a liquid have an apparent weight that is less than when the object is in air • Fapparent = Fg - Fbuoyant

  12. Uses of Archimedes’ principle • As a result of the buoyant force ships can be made of steel and still float as long as the hull is hollow and large enough so that the density of the ship is less than the density of water • Submarines take advantage of Archimedes’ principle as water is pumped into or out of chambers to change the net vertical force

  13. Fluids in Motion • The relationship between the velocity and pressure exerted by a moving fluid is described by Bernoulli’s principle • As the velocity of a fluid increases, the pressure exerted by that fluid decreases

  14. Uses of Bernoulli’s Principle • Most aircraft get part of their lift from this principle • As the wings travels through the air, the air moving over the top surface travels farther, and therefore must go faster than the air moving past the bottom surface

  15. Forces within Liquids • In real liquids particles exert electromagnetic forces of attraction on each other – called cohesive forces • Surface tension- a result of the cohesive forces among the particles of a liquid. It is the tendency of the surface of a liquid to contract to the smallest possible area

  16. Capillary Action • Adhesion is the attraction force that acts between particles of different substances • If a piece of glass tubing with a small inner diameter is placed in water, the water rises inside the tube. • The adhesive force between the glass and water is stronger than the cohesive force of the water molecules- called the capillary action

  17. Examples of capillary action • Molten wax rises the wick of a candle • Paint moves up through the bristles of a brush • Water moving through the soil and up the roots of a plant

  18. Evaporation & Condensation • The escape of particles from a liquid is evaporation • Cooling effect • Evaporated particles can return to the liquid phase if the Ke or temp decreases. Called condensation • fog

  19. Solid State • Close fitting particles • More dense than liquids • Water is an exception- solid water is less dense than liquid water

  20. Thermal expansion • When heated all forms of matter become less dense and expand to fill more space • Hot air expands and rises – Cold air contracts, becomes more dense, and sinks • Coefficient of Linear Expansion • α = ΔL /L1ΔT

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