General Physics I. Lecture 16: Fluid Mechanics. Prof. WAN, Xin ( 万歆 )

Transcription

1 General Physics I Lecture 16: Fluid Mechanics Prof. WAN, Xin ( 万歆 ) xinwan@zju.edu.cn

2 Motivations Newton s laws for fluid statics? Force pressure Mass density How to treat the flow of fluid? Difficulties: Continuous medium Change of the shape (still incompressible!) Frame of reference

3 Density Density (scalar): mass of a small element of material m divided by its volume V For infinitesimally small V: = m / V For homogeneous material: = m / V

4 Pressure Under static condition, force acts normal or perpendicular to the surface vector. Pressure: the magnitude of the normal force per unit surface area scalar. Δ F A Δ A Δ F = p Δ A p = Δ F Δ A

5 Common Units 1 pascal 1 N / m [SI] 2 1 atmosphere Pa 5 1 bar 10 Pa 1 atm 1 atm 760 mm of Hg 760 torr 1 atm 14.7 lb / in. 2

6 Origin of Pressure Microscopic origin: collisions of molecules of the fluid with the surface Action and reaction: Newton s third law liquid f reaction Wall Δ v

7 Pressure in a Fluid at Rest (p+dp)a A (dm)g y+dy y dp dy g pa ( dm) g ( Ady) g gady pa gady ( p dp) A 0

8 Pressure in a Fluid at Rest Assumption:, g independent of y dp dy g dp ' g dy p p g( y y ) p p g( y y ) p gh p p y y '

9 Mercury Barometer How to measure atmospheric pressure? E. Torricelli ( ) p=0 Vacuum! P gh h atm P atm g p atm h For mercury, h = 760 mm. How high will water rise?

10 U-Tube Manometer p gh p gh A 1 1 atm 2 2

11 Snorkeling Can I use a longer snorkel to look closer at the underwater life?

12 Atmospheric Pressure Gases are compressible. Thus, varies! p p Pa 0 sea level p pv nrt, for constant T p dp dy dx g 0 g p p 0

13 Atmospheric Pressure dp p p dp ' g g 0 0 dx dy p p ' p p y y dy ' g ln p ln p ( y y ) p p p e ( g / p ) h 0 0 h0 p p gh 0 0

14 A.P. an Improved Version pv nrt p p T T Temperature decreases 6 o C for each 1,000 meters of elevation T T ( y y ) 0 0 dp p T g g 0 0 dy p T ( y y ) 0 0 0

15 A.P. an Improved Version dp g T dy 0 0 p p T ( y y ) p p 0 p p dp ' g T y dy ' 0 0 p ' p T ( y ' y ) 0 0 h 1 T 0 y T 0 0g p 0 0 p e 0 0g h p 0

16 Temperature Corrections T0 50 km

17 Pascal s Principle Pressure applied to an p ext enclosed fluid is transmitted undimished to every portion of the fluid and to the walls of the containing vessel. d F p p p gd p p ext ext

18 Hydraulic Lever F i d o d A i A i o mg Fo F / A F / A, A d A d i i o o i i o o

19 Hydraulic Brake Fred Duesenberg originated hydraulic brakes on his 1914 racing cars and Duesenberg was the first automotive marque to use the technology on a passenger car in In 1918 Malcolm Lougheed (who later changed the spelling of his name to Lockheed) developed a hydraulic brake system.

20 Archimedes Principle A body wholly or partially immersed in a fluid is buoyed up by a force equal in magnitude to the weight of the fluid displaced by the body. F Vg m g B fluid fluid F B mg object Vg mg

21 Two Comments Buoyant force volume/density F Vg m g B fluid fluid mg object Vg Center of buoyancy (not fixed) Center of gravity

22 Question: Galileo s Scale Can you invent a scale that one can use to read the percentage of silver in the gold crown without explicit calculations?

23 Fluid Dynamics Aerodynamics (gases in motion) Hydrodynamics (liquids in motion) Blaise Pascal Daniel Bernoulli, Hydrodynamica (1738) Leonhard Euler Lagrange, d Alembert, Laplace, von Helmholtz Airplane, petroleum, weather, traffic

24 The Microscopic Approach N particles r i (t), v i (t); interaction V(r i -r j )

25 The Macroscopic Approach A fluid is regarded as a continuous medium. Any small volume element in the fluid is always supposed to be so large that it still contains a very large number of molecules. When we speak of the displacement of fluid at a point, we meant not the displacement of an individual molecule, but that of a volume element containing many molecules, though still regarded as a point.

26 Euler s Solution For fluid at a point at a time: Field ρ(x, y, z,t), v(x, y, z,t) State of the fluid: described by parameters p, T. However, laws of mechanics applied to particles, not to points in space.

27 Ideal Fluids Steady: velocity, density and pressure not change in time; no turbulence Incompressible: constant density Nonviscous: no internal friction between adjacent layers Irrotational: no particle rotation about the center of mass

28 Streamlines Paths of particles P v P Q v Q R v R P Q R v tangent to the streamline No crossing of streamlines

29 Mass Flux Tube of flow: bundle of streamlines Q P v 1 v 2 A 1 A 2 m m A v t mass flux 1 A v t1

30 Conservation of Mass A v IF: no sources and no sinks/drains A v A v A v constant constant, for incompressible fluid Narrower tube == larger speed, fast Wider tube == smaller speed, slow Example of equation of continuity. Also conservation of charge in E&M

31 What Accelerates the Fluid? Acceleration due to pressure difference. Bernoulli s Principle = Work-Energy Theorem

32 Work-Energy Theorem Steady, incompressible, nonviscous, irrotational

33 Bernoulli s Equation kinetic E, potential E, external work m A x A x p A x p A x mv mgy mv mgy p1 v1 gy1 p2 v2 gy p v gy constant

34 Comments Restrictions of the Bernoulli s equation: Points 1 and 2 lie on the same streamline. The fluid has constant density. The flow is steady. There is no friction. When streamlines are parallel the pressure is constant across them (if we ignore gravity and assume velocity is constant over the crosssection).

35 The Venturi Meter Speed changes as diameter changes. Can be used to measure the speed of the fluid flow. 1 1 p v p v v A v A ,

36 Bend it like Beckham Dynamic lift

37 Banana Free Kick Distance 25 m Initial v = 25 m/s Flight time 1s Spin at 10 rev/s Lift force ~ 4 N Ball mass ~ 400 g a = 10 m/s 2 A swing of 5 m! ~ 5m