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CE 150 Fluid Mechanics

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open-channel flow (CE) water treatment (CE) piping systems (CE, ... Textbook uses the International System (SI) and British Gravitational (BG) System of units ... – PowerPoint PPT presentation

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Title: CE 150 Fluid Mechanics


1
CE 150 Fluid Mechanics
  • G.A. Kallio
  • Dept. of Mechanical Engineering, Mechatronic
    Engineering Manufacturing Technology
  • California State University, Chico

2
Introduction
  • Reading Munson, et al., Chapter 1

3
Fluid Mechanics
  • Fluid mechanics is the study of fluids at rest
    (fluid statics) and in motion (fluid dynamics)
  • Applications
  • fluid forces on structures (CE)
  • open-channel flow (CE)
  • water treatment (CE)
  • piping systems (CE, ME)
  • porous flow (CE, ME)
  • air pollution control (CE, ME)
  • aerodynamics (ME)
  • turbomachines (ME)
  • rocket propulsion/supersonic flight (ME)

4
Fluid Characteristics
  • Solids
  • molecules are very dense
  • not easily deformed or compressed
  • Liquids
  • molecules are moderately dense
  • easily deformed but not compressed
  • Gases
  • molecules are relatively sparse
  • easily deformed and compressed
  • Fluids include liquids and gases
  • substance that deforms continuously when
    subjected to any shearing force

5
Fluid Characteristics
Break-up of a liquid jet
6
Dimensions Units
  • Primary dimensions length (L), time (T), mass
    (M), and temperature (?)
  • Secondary dimensions velocity (LT-1),
    acceleration (LT-2), force (MLT-2), etc.
  • Textbook uses the International System (SI) and
    British Gravitational (BG) System of units
  • The English Engineering (EE) System (e.g., lbm,
    lbf) is still used but not emphasized here

7
Dimensions Units
8
Dimensions Units
  • All theoretically-derived equations are
    dimensionally homogeneous - i.e., dimensions of
    LHS dimensions of RHS
  • Empirical equations are often not dimensionally
    homogeneous - i.e., they contain numerical
    constants that have dimensions and must be used
    with a specific system of units

9
Fluid Mechanics Problem Solving
  • Required format for HW problems
  • Given (brief)
  • Find (list items)
  • Sketch (if applicable)
  • Assumptions (list those not included in the
    problem statement)
  • Analysis (show eqns. in symbolic form, then plug
    in values box or highlight your answer always
    include units)
  • Comments (if requested)

10
Basic Fluid Properties
  • Pressure
  • Temperature
  • Density
  • Viscosity
  • (Bulk Modulus)
  • (Speed of Sound)
  • Vapor Pressure
  • Surface Tension

11
Pressure
  • Pressure (N/m2, lb/ft2)
  • Other units
  • 1 pascal (Pa) 1 N/m2
  • 1 kPa 103 N/m2
  • 1 bar 105 N/m2
  • 1 MPa 106 N/m2
  • 1 atm 101.325 kPa
  • 14.696 lb/in2 (psi)

12
Pressure
  • Absolute pressure - total pressure experienced by
    a fluid
  • Gage pressure or vacuum pressure- difference
    between absolute pressure and atmospheric
    pressure (usually indicated by a measuring
    device)
  • pgage pabs - patm
  • pvac patm - pabs

13
Temperature
  • Temperature (ºC or K, ºF or R)
  • measure of a bodys hotness or coldness
  • indicative of a bodys internal energy
  • more description in ME152, Thermodynamics
  • unit conversions
  • K ºC 273.15
  • R ºF 459.67
  • ºF 1.8 ºC 32

14
Density
  • Density (kg/m3, slugs/ft3)
  • pressure and temperature have strong influence on
    gas density, little effect on liquid density
  • in thermodynamics, specific volume (m3/kg ,
    ft3/slug) is more often used than density

15
Weight Measures
  • Specific Weight (N/m3, lb/ft3)
  • Specific Gravity (nondimensional)

16
Ideal Gas Law
  • An ideal gas is a superheated vapor that is at a
    relatively low p or high T (i.e., not approaching
    condensation or liquefaction)
  • Ideal gases obey the following equation of state,
    known as the ideal gas law
  • where R gas constant (Table 1.7, 1.8)
  • p absolute pressure
  • T absolute temperature

17
Viscosity
  • Fluids stick to solid boundaries, i.e., fluid
    velocity is equal to the solid velocity this is
    called the no-slip condition
  • In Figure 1.3, a fluid velocity gradient (du/dy)
    exists, accompanied by a shearing stress (?)

18
Viscosity
  • For Newtonian fluids,
  • ? absolute viscosity (N-s/m2)
  • ? shearing stress (N/m2)
  • du/dy rate of shearing strain, or
    velocity gradient (1/s)
  • Most common liquids and all gases are Newtonian
    non-Newtonian fluids are divided into
    shear-thinning fluids (e.g., latex paint) and
    shear-thickening fluids (e.g., sand-water
    mixture)

19
Viscosity
  • Viscosity is relatively insensitive to pressure,
    but can be very sensitive to temperature (see
    Figure 1.6 and eqns. 1.10, 1.11)
  • Kinematic viscosity is the ratio of absolute
    viscosity to density
  • Other units
  • poise 10-1 N-s/m2
  • stoke 10-4 m2/s

20
Viscosity
21
Vapor Pressure
  • Vapor pressure (pv) is the pressure that a vapor
    phase exerts on the liquid phase at equilibrium
  • In thermodynamics, the vapor pressure at
    equilibrium is known as the saturation pressure
    (psat)
  • Vapor pressure is a function of T
  • H2O at 20 ?C, pv 2.34 kPa
  • H2O at 100?C, pv 101.3 kPa (boiling)
  • If the pressure of a liquid is reduced to the
    vapor pressure, vapor bubbles will form, leading
    to cavitation

22
Surface Tension
  • Surface tension (?) is a force per unit length
    (N/m) that develops at a liquid-gas or
    liquid-liquid interface
  • The tension is due to an imbalance of molecular
    forces at the liquid surface
  • Surface tension is important at liquid surfaces
    with small radii of curvature
  • liquid droplets and gas bubbles
  • liquids in small tubes
  • liquid jets or sprays

23
Surface Tension
  • Liquid droplet

24
Surface Tension
  • Liquid in small tube
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