Fluid Mechanics
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Transcript of Fluid Mechanics
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MET 306Fluid Mechanics
Lecture # 1
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Objective
Determine Unit and DimensionsIdentify the key fluid properties used in the
analysis of fluid behaviorCalculate common fluid properties given
appropriate informationExplain effects of fluid compressibilityUse the concept of viscosity and surface
tension
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Unit and Dimensions
A dimensions is used to measured the physical quantity with out numerical value.
A unit is the way to assign a number to dimension
L= length is dimensionm= unit used for length
Two system of the fundamental dimension1. FLT2. MLT
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Unit and Dimensions
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Unit and Dimensions
Example:
Most of the fluid problem required the basic fundamental dimension witch (MLT)
1: LTT
L
s
mVelocity
2: MLTmaFForce
The equation of the velocity for uniformly acceleration body
atVV o
Where:Vo is the initial velocity a is the acceleration t is the time
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Unit and Dimensions
Dimensionally homogeneousAll theoretically derived equation are dimensionally homogeneous that is the dimensions of the lift side of the equation must be the same as those on the right side.
11
111
211
2
LTLT
LTLTLT
TLTLTLT
atVV o
The equation of the velocity for uniformly acceleration body is dimensionally homogeneous and the 2 is a constant number
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Unit and Dimensions
Dimensionally homogeneousCheck whether this equation is dimensionally homogeneous or NOT
1-Equation of a freely falling body
21.16 td Where d= distancet= time
2-Volume rate flow equation
ghAQ 261.0Where Q= Flow rateA= Areag= gravity h= height
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Systems of Units
International System (SI)British gravitational system(BG)
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Measures of Fluid Mass and Weight
DensityThe density of the fluid designated by Greek symbol (ρ)Defined as a mass per unit volumeUsed to characterize the mass of fluid BG= slugs/ ft3 and SI= kg/m3
Specific WeightThe specific weight of the fluid designated by Greek symbol (γ)Defined as weight per unit volumeUsed to characterize the weight of the systemBG= lb/ ft3 and SI= N/m3
Specific GravityThe specific gravity of the fluid designated by SGDefined as a ratio of the density of the fluid to the density of the waterThe density of the water @ 4oC is BG=1.94 slugs/ft3 and SI=1000 kg/m3
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Measures of Fluid Mass and Weight
Specific Gravity (Example)Calculate the density of the mercury in two system
BG and SI by knowing the SGmercury @ 4oC is 13.55
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Ideal Gas Law
Gas are highly compressible in comparison to liquid, So from the Ideal gas law change in the temperature or the pressure of the gases can directly change the density.
Where p is the absolute pressure, ρ is the density, T is the absolute temperature and R is gas constant
R=R/m R is universal gas constant 8314.3J/kg mole K m is the molar mass
TRp
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Ideal Gas Law
Example
The absolute pressure and temperature of a gas in large chamber are found to be 500 kPa and 60oC respectively. Find the density if the air has m =28.97
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Viscosity Dynamic viscosity
dy
du BG and SI
Slug/ft s Kg/ms
See Figure 1.1
Kinematic Viscosity
vSI
m2/s
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Viscosity
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Viscosity
Example
Determine the value of the Reynolds number using SI system for fluid with viscosity of 0.38 N.s/m2 and specific gravity of 0.91 flow into pipe with diameter of 25 mm with velocity of 2.6 m/s.
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Surface Tension
At interface between a liquid and the gas forces develop in the liquid surface which case the surface to behave as “skin” or “membrane”
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Surface Tension(σ)
The pressure inside the drop can be calculated using free body diagram .The force developed around the edge due to the surface tension is 2πRσ this force must be balance by the pressure difference
22 RpR
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Surface Tension(σ)
ϴR2
hR2 h
cos22 RhR
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Surface Tension(σ)
Example What diameter of clean glass tubing is required to
rise water at 20oC in tube to 1mm. Where σ of water is 0.0728 N/m and the θ =0.