review final – fluid propertiesbolster/diogo_bolster/fluids_files... · 2019. 12. 5. · review...
TRANSCRIPT
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C E 3 0 4 6 0 - F L U I D M E C H A N I C S D I O G O B O L S T E R
REVIEW FINAL – FLUID PROPERTIES
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IMPORTANT EQUATIONS CHAPTER 1
• Specific Weight γ=ρg
• Ideal Gas Law p=ρRT
• Newtonian Fluid Shear Stress τ=µ du/dy
• Bulk Modulus
• Speed of Sound
• Capillary Rise in a Tube
€
γπR2h = 2πRσ cos(θ )€
c =dpdρ
=Ev
ρ€
EV =−ΔpΔV /V
=−ΔpΔρ /ρ
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IMPORTANT EQUATIONS CHAPTER 2
• (pressure gradient in a stationary fluid)
• (hydrostatic force on a plane surface)
• (Location of hydrostatic force on a plane)
• Fb=γVdisp (buoyant force)
• Manometer Equation
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USEFUL EQUATIONS – CHAPTER 3
• Streamwise Acceleration
• Bernoulli Equation
• Free Jet
• Flow Meter
• Sluice Gate
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CH5 - CONSERVATION OF MASS
• Often called ‘The Continuity Equation’
• What is velocity not uniform
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CH5 - CONSERVATION OF LINEAR MOMENTUM
• F=ma
• Steady State
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MOMENTUM - RECIPE APPROACH
• Draw Control Volume • Identify all inlets and outlets • Identify all forces acting and their specific direction of action
(break into x and y components) • For each inlet and outlet calculate mass flow rate (ρAv) • For each inlet outlet identify velocity vector (v) • Break velocity vector v into x and y components • Combine last three steps to identify x and y components of
momentum flux, i.e. v|xρAv and v|yρAv • Substitute into x and y momentum equations
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CH5 - MOMENT OF MOMENTUM
Torque
Power
Work per Unit Mass
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CH5 - APPLICATION (FROM TEXTBOOK)
U – velocity of head due to angular rotation (velocity of nozzle as measured relative to a fixed surface) - ωR W – velocity of nozzle exit flow as viewed from the nozzle – Q/A V – fluid velocity as measured relative to a fixed control surface (typically what you need for formulas on previous slide)
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CH5 - CONSERVATION OF ENERGY
• For steady, incompressible flow with shaft work
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CH6 - VORTICITY (ROTATION)
• Counterclockwise rotation is positive (z component is component out of the page). Others, in x-y also exist
• vorticity (zero => irrotational)
• Related, rate of shearing strain
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CH6 - FORMS OF CONTINUITY EQUATION
• General Form
• Steady
• Incompressible
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CH6 - CYLINDRICAL POLAR COORDINATES
• General Form
• Steady
• Incompressible
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CH6 - STREAMFUNCTION
• For incompressible, plane two dimensional flow we can define a streamfunction psi, such that
• Quantifies the flow rate between two streamlines (lines of constant psi)
• Cylindrical:
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THE NAVIER STOKES EQUATIONS
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IN CYLINDRICAL COORDINATES
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OUR ASSUMPTIONS
• 2d (x-y or r-z or r-θ) • Incompressible • Steady State (d/dt=0) • Fully Developed (d/dx=0 or d/dz=0 – except
pressure)
• With these we are typically left with a 2nd order ordinary differential equation that we can readily solve.
• We then must apply boundary conditions for unknown constants • No flow (velocity normal to a boundary is zero) • No slip (fluid velocity tangential to a boundary is the same
as the velocity of the boundary)
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CH7 – DIMENSIONAL ANALYSIS
Buckingham Pi Theorem
• I an equation involving k variables is dimensionally homogeneous, it can be reduced to a relationship among k-r independent dimensionless products where r is the minimum number of reference dimensions required to describe the variables • Mathematically
can be reduced to
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DETERMINATION OF PI TERMS
• List all variables that are involved in the problem • Express each on in terms of primary dimensions (MLTθ)
• Determine required number of Pi terms (each independent) – i.e. k-r
• Select a number of repeating variables (equal to number of dimensions)
• Form pi terms by multiplying one of the nonrepeating variables by the product of repeating variables
• Repeat last step for all nonrepeating variables • Check that all resulting Pi terms are dimensionless • Express in final form
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DATA
• Problems with One Pi Term :If only one term exists then according to the Pi Theorem we can say:
Π1=C
• For two terms Π1=f(Π2)
Π1=f(Π2)
• Means when you have data you should plot Π1 against Π2 to deduce relationships or at least make best fits that can be used predicatively.
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TYPES OF SIMILARITY
• Geometric (ratio of length scales the same) • Kinematic (velocity structures are the same) • Dynamic (ratio forces the same) • The best situation is: Get all dimensionless
variables (Pi groups) the same between model and prototype. Then all similarities are preserved….. • If N-1 dimensionless groups between model
and prototype are the same then the last one must be also.
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CH8 - PIPEFLOW
Re<2000
Re>4000
Re=UDρ/µ
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FULLY DEVELOPED FLOW
Entrance length:
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ENERGY EQUATION
Recall Energy Equation
Therefore we can say that (part of) the loss in a pipe is
€
hL,min = KLV2
2g
€
hL,major =lDV2
2g f Re,εD⎛
⎝ ⎜
⎞
⎠ ⎟
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MOODY DIAGRAM (FRICTION FACTOR)
For non-laminar flow approximately true that
€
1f = −1.8log
εD3.7
⎛
⎝
⎜ ⎜
⎞
⎠
⎟ ⎟
1.11
+6.9Re
⎡
⎣
⎢ ⎢
⎤
⎦
⎥ ⎥
For laminar
€
f =64Re
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PIPE FLOW MEASUREMENT
€
Q = CQideal = CA02 p1− p2( )ρ 1− β4( )
C is a constant that depends on geometry