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Fluid Mechanics

Fluid Mechanics. Today…. Understand the ways in which flow affects fluid forces Explore buoyancy and its variables Define lift and drag and their effect on movement. Introduction . Why do golf balls have dimples? Why can some people float and other cannot?

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Fluid Mechanics

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  1. Fluid Mechanics

  2. Today…. • Understand the ways in which flow affects fluid forces • Explore buoyancy and its variables • Define lift and drag and their effect on movement

  3. Introduction • Why do golf balls have dimples? • Why can some people float and other cannot? • Why do some athletes try to streamline their bodies during competition?

  4. A little golf ball history • Original golf balls were smooth • Nicks and cuts from use • “turbulators • Created turbulent boundary layer • Reduces drag

  5. Flow • Caused by object moving through fluid • Laminar flow: characterized by smooth parallel layers of fluid • Turbulent flow: layers of fluid near the surface of the object mix • If turbulence is present… flow is non-laminar • **speed related properties • Effect of surface area

  6. The nature of fluids • Are all fluids liquids? • Fluids: all liquids and gases. • Water is a fluid and a liquid. Air is a fluid and a gas. • Liquid tends always to occupy the same volume • Gas changes volume by expanding or contracting to fill or fit into any container • substance that tends to flow & deform when acted on by a shear force… • Air and water act on the body

  7. Relative motion • Influence of fluid depends on velocity of body & velocity of fluid • Relative velocity: velocity of body w/respect to the velocity of the surrounding fluid

  8. Relative velocity • If motion is opposite fluid flow the magnitude of the velocity of the body is the sum of the speeds of the body and the fluid • If motion is in the same direction as the fluid, the magnitude of the body’s velocity is the difference in the speeds

  9. Relative velocity Cyclist velocity 15 m/s Velocity of cyclist Relative to wind 20 m/s Cyclist velocity 15 m/s Velocity of cyclist Relative to wind 10 m/s Wind velocity  5 m/s

  10. Other fluid properties • Density = mass/volume • Specific weight = weight/volume • Viscosity: resistance to flow • All affected by atmospheric pressure & temperature

  11. Buoyancy • Archimedes’ principle: buoyant force is equal to the weight of the fluid displaced by the body Fb = Vd

  12. Buoyancy • Example: water polo ball with a volume of 0.2m3 is completely submerged in 20°C water, the buoyant force acting on the ball is equal to the ball’s volume X the specific weight of water at that temp Fb = (0.2m3)(9790N/m3) Fb = 1958 N

  13. Buoyancy • >density of fluid, > magnitude of buoyant force • Buoyant force acts on center of volume

  14. Flotation • Related to body density • Two people w/identical volume • One weighs more (higher density) • Volume must be large enough to create Fb> body weight

  15. Flotation • Relationship between center of volume and center of gravity

  16. A look at airplanes

  17. …airplanes…. • Thrust = Drag • Lift = Weight • If drag > thrust, the plane will slow down • If thrust and is > drag, the plane will speed up. • if lift drops < the weight of the airplane, the plane will descend.

  18. Thrust • force that must be created to overcome the drag • thrust propellers, jet engines, kicking

  19. Drag • force that resists the motion of an object moving through a fluid • Hand our car window example • Factors affecting drag • drag increases velocity • Skiing example • Landing gear

  20. Drag • Skin friction, surface drag, viscous drag ..derived from friction between adjacent layers of fluid near a body moving through the fluid.. • Factors: • Relative velocity of fluid flow • Surface area of the body • Roughness of surface • Viscosity of fluid

  21. Drag • Form drag, profile drag, pressure drag …resistance created by a pressure differential between the lead and rear sides of a body moving through a fluid.. • Factors • Velocity of body • Pressure gradient between front & rear ends of body • Size of surface area perpendicular to flow

  22. Drag • Wave drag ..resistance created by the generation of waves at the interface between two different fluids..

  23. Wonder about 747’s?

  24. Lift • force that holds airplane in the air • created primarily by wings • exist in presence of moving fluid • relative difference in speeds between the object and the fluid • Object moving/fluid stationary • Object still/fluid moving

  25. How is lift created? • force on a wing in a moving fluid • acts perpendicular to the flow of the fluid • Drag = same thing, but parallel to the direction of the fluid flow

  26. How is lift created? A = compressed into the air above it as it moves upward B = slowed, compressed and redirected in a downward path C =lift component D = net force E = drag

  27. Lift force • Factors • Velocity of foil relative to fluid • Fluid density • Surface area of the flat side • Angle of attack

  28. Magnus effect/force • spinning object traveling through liquid is forced to move sideways • Curve ball example • spinning action on release • spinning causes air to flow differently • top of the ball vs under the ball • top of the ball is spinning directly into air and the bottom of the ball is spinning with the air flow • air under the ball is flowing faster than air on top • creates less pressure, forces the ball to move down or curve.

  29. Magnus force • Spinning objects generate lift • Effect: trajectory deviation toward the direction of spin

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