Chapter-02-Basic Properties of Fluids

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d u v dy
momentum flux = (momentum diffusivity) x (momentum concentration gradient)
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3. Viscosities of gases and liquids
viscosity = f (temperature, molecular structure, pressure)
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3.3 RHEOLOGICAL PROPERTIES OF FLUIDS
3. Time-dependent flow
• Thixotropic（触变性） liquids break down under continued shear and on mixing give lower shear stress for a given shear rate; that is, their apparent viscosity decreases with time.
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• Liquid mixture: • For unassociated liquid mixture
lg m xi lg i
•xi : molar fraction of i component in the mixture
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Magnitude of viscosity •Liquid :
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• In this course, only Newtonian fluid will be discussed.
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3.4 VISCOSITY黏度
• • • • 1. Concept of viscosity 2. Viscosity and momentum flux 3. Viscosities of gases and liquids 4. Kinematic viscosity.

Definition of infinitesimal fluid element:
an infinitesimally small fluid element in the flow, with a differential volume.
It contains huge large amount of molecules Fixed and moving infinitesimal fluid element. Focus of our investigation for fluid flow.
Fixed control volume and moving control volume. Focus of our investigation for fluid flow.
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2.3.2 Infinitesimal fluid element approach
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2.3.3 Molecule approach
Definition of molecule approach:
The fluid properties are defined with the use of suitable statistical averaging in the microscope wherein the fundamental laws of nature are applied directly to atoms and molecules. In summary, although many variations on the theme can be found in different texts for the derivation of the general equations of the fluid flow, the flow model can be usually be categorized under one of the approach described above.

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Application ?
p0
pA = p0 + ρgh pG = ρgh
绝压 表压
h p
Absolute pressure （绝对压力） Gauge pressure（表压力）
Pressure source Connected water tube
Relative pressure（相对压力） Vacuum degree（真空度）
p = p0
ax z =− x g
Equipressure surface（等压面）
ax z = − x +C g
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Example:
A cup of coffee is 7cm deep at rest. 1. Will it spill out while ax=7m/s2? 3 2. Gage pressure of point A? (ρ =1010 kg m )
dp = ρ( Xdx +Ydy + Zdz)
Pressure increase in the direction of body force.
Equipressure surface（等压面）
Surfaces in fluid with same pressure, vertical to body force everywhere, in gravity field it is a horizontal plane.
∂p dx ∂p dx p− dydz − p + dydz + ρXdxdydz = 0 ∂x 2 ∂x 2
Force on left surface Force on right surface Body force in x direction

2021/8/11
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Experimental Projects (1)
In-class (Required) (Total hours: 6)
E1 Hydrostatics (1 hour) E2 Velocity measurement with Pitot tube (1 hour) E3 Flow measurement with Venturi tube (1 hour) E4 Verification of Bernoulli equation (1 hour) E5 Reynolds experiment (1 hour) E6 Measurement of friction loss coefficient for circular pipe
Water and air are researched mostly in fluid mechanics. Besides these, fluid also includes vapor as work medium in turbine, lube oil, water which mixing with mud and sand, blood, melting metal and the gas with complicated components coming from burning, plasmas under high temperature condition, etc.
B2 Boundary layer theory and flow past body (4 hours)
Part C (Total hours: 4) Extended contents
C1 Resaerch progress in Fluid Mechancis (1 hour) C2 Modern flow measurement technologies (1 hour) C3 Introduction to numerical simulation of turbulence flow