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L 15 Fluids [4]

L 15 Fluids [4]. > Fluid flow and Bernoulli’s principle > Airplanes and curveballs > viscosity > surface tension. v 1 , A 1. v 2 , A 2. Basic principles of fluid motion flow rate = v x A (m 3 /s). v. A. I. Continuity principle: v  A = constant  v 1 A 1 = v 2 A 2

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L 15 Fluids [4]

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  1. L 15 Fluids [4] > Fluid flow and Bernoulli’s principle > Airplanes and curveballs > viscosity > surface tension

  2. v1, A1 v2, A2 Basic principles of fluid motionflow rate = v x A (m3/s) v A I. Continuity principle:v  A = constant  v1A1 = v2A2 II. Bernoulli’s principle states thatas the speed of a moving fluid increases, the pressure in the fluid decreases.

  3. Blowing air over the top of the tube lowers the air pressure on that side allowing the fluid to rise

  4. Low pressure High pressure Flow on top No flow Bernoulli’s principle • fast flow  low pressure • slow flow  high pressure

  5. Why does a roof blow off in high winds ? WIND Low Pressure Normal Pressure

  6. Wind tunnel visualization of air flow streamlines AIR

  7. Streamlines and fluid flow • The black lines are the paths that the fluid takes as it flows. • Wider spacing indicates low speed flow, narrow spacing indicates high speed flow • Color indicates pressure High Low pressurepressure

  8. Bernoulli’s Principle • Fluid flow velocity = v • Fluid pressure = P  where v is high, P is low  where v is low, P is high

  9. From the perspective of the jet, the air moves relative to the jet The jet moves through the air Frames of reference

  10. wing Streamlines around a wing

  11. Flow over an airplane wing

  12. Control surfaces on a plane • By extending the slats, the wing area can be increased to generate more lift at low speeds for takeoff and landing

  13. How does a plane turn? Lift Force Level Flight Banked turn

  14. Airflow around a baseball that is NOT spinning • The ball is moving but from the ball’s perspective the air moves relative to the ball • The streamlines are bunched at the top and bottom indicating higher flow speed • The pressure forces are balanced

  15. Curveballs The ball is rotating clockwise. The layer of air adjacent to the ball is dragged along by the rotation, causing the flow speed to be higher on the top side. The higher pressure on the bottom causes the ball to curve upward. High speed, Low P the lift on a rotating body moving through a fluid is called the MAGNUS FORCE low speed, high P

  16. Curveballs & Screwballs the dimples on a golf ball are there to increase the air drag on the rotating ball (back spin) causing it to experience extra lift.

  17. Viscosity • so far we have considered only “ideal” liquids  liquids that can flow without any resistance to the flow • “real” liquids (like ketchup) have a property called viscosity which is a tendency for the liquid to resist flowing

  18. viscosity • for example – pancake syrup flows more slowly than water – we say that pancake syrup is more “viscous” than water. • Ketchup and molasses are also good examples • viscosity is sometimes referred to as the “thickness” of a liquid • viscosity is the most important property of motor oil

  19. Seeing the effects of viscosity Pancake syrup Substances with higher viscosity take longer to flow down the ramp. viscosity is a measure of the resistance that one layer of liquid experiences when flowing over another layer.

  20. Viscosities of various substances • water has a viscosity of about 1 unit • pancake syruphas a viscosity of 2500 • ketchuphas a viscosity of 98,000 • Lava- 100,000 • peanut butterhas a viscosity of 250,000 • glass is a liquid with a very high viscosity of 1017 it does flow, but very slowly! • viscosity depends on temperature warm syrup flows faster than cold syrup

  21. Pitch drop experiment at the University of Queensland in Brisbane, Australia • Pitch- used as a roofing material to prevent leaks • Must be heated to be applied • viscosity ~ 1011 water • Experiment began in 1927 • 8 drops have since fallen, one every decade or so

  22. Motor Oil • SAE – Society of American Engineers • the viscosity of oil tends to decrease as it heats up (oil breakdown) • what does 10W-30 mean? viscosity index- hot engine viscosity index- cold engine A higher viscosity index indicates the viscosity changes less with temperature than a lower viscosity index.

  23. Measuring viscosity ball bearings low viscosity Liquid (e. g. water) high viscosity Liquid (e.g. syrup)

  24. D P1 P2 L Flow through a pipe Viscosity slows the flow of a fluid through a pipe • (eta) is the viscosity of the fluid, k is a constant • A 10 % reduction in diameter reduces the flow by 34 % • If D  D/2, the flow is reduced by 94 %

  25. clogged arteries A pipe clogged With calcium deposits

  26. SURFACE TENSION a surface tension force causes a fluid surface to behave like an elastic sheet Molecules at the surface feel a net force insect on water surface

  27. Light objects suspended by surface tension This effect is NOT due to the buoyant force

  28. If a segment of the soap film ispunctured, surface tension pullsthe strings apart

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