< 1.7. dimensional analysis > physical parameters
TRANSCRIPT
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Aerodynamics 2015 fall - 1 -
Introduction to Aerodynamics
< 1.7. Dimensional analysis >
Physical parameters
Q: What physical quantities influence forces and moments?
V
l
A
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Aerodynamics 2015 fall - 2 -
Introduction to Aerodynamics
< 1.7. Dimensional analysis >
Physical parameters
Physical quantities to be considered
Generally, resultant aerodynamic force:
R = f(ρ∞, V∞, c, μ∞, a∞) (1)
Parameter Symbol units
Lift L' MLT-2
Angle of attack α -
Freestream velocity V∞ LT-1
Freestream density ρ∞ ML-3
Freestream viscosity μ∞ ML-1T-1
Freestream speed of sound a∞ LT-1
Size of body (e.g. chord) c L
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Aerodynamics 2015 fall - 3 -
Introduction to Aerodynamics
< 1.7. Dimensional analysis >
The Buckingham PI Theorem
The relation with N physical variables
f1 ( p1 , p2 , p3 , … , pN ) = 0
can be expressed as
f 2( P1 , P2 , ... , PN-K ) = 0
where K is the No. of fundamental dimensions
Then P1 =f3 ( p1 , p2 , … , pK , pK+1 )
P2 =f4 ( p1 , p2 , … , pK , pK+2 )
….
PN-K =f ( p1 , p2 , … , pK , pN )
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Aerodynamics 2015 fall - 4 -
< 1.7. Dimensional analysis >
The Buckingham PI Theorem
Example)
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Aerodynamics 2015 fall - 5 -
< 1.7. Dimensional analysis >
The Buckingham PI Theorem
Example)
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Aerodynamics 2015 fall - 6 -
< 1.7. Dimensional analysis >
The Buckingham PI Theorem
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Aerodynamics 2015 fall - 7 -
< 1.7. Dimensional analysis >
The Buckingham PI Theorem
Through similar procedure
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Aerodynamics 2015 fall - 8 -
Introduction to Aerodynamics
< 1.7. Dimensional analysis >
Dimensionless form
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Aerodynamics 2015 fall - 9 -
Introduction to Aerodynamics
< 1.8. Flow similarity >
Dynamic similarity
Two different flows are dynamically similar if
• Streamline patterns are similar
• Velocity, pressure, temperature distributions are same
• Force coefficients are same
Criteria
• Geometrically similar
• Similarity parameters (Re, M) are same
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Aerodynamics 2015 fall - 10 -
Introduction to Aerodynamics
< 1.8. Flow similarity >
Dynamic similarity
Airfoil flow example
• Consider two airfoils which have the same shape and angle of
attack, but have different sizes and are operating in two different
fluids.
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Aerodynamics 2015 fall - 11 -
Introduction to Aerodynamics
< 1.8. Flow similarity >
Dynamic similarity
Airfoil 1 (sea level) Airfoil 2 (cryogenic tunnel)
α1=5° α2=5°
V1=210 m/s V2=140 m/s
ρ1=1.2 kg/m3 ρ2=3.0 kg/m3
μ1=1.8e-5 kg/m·s μ2=1.5e-5 kg/m·s
a1=300 m/s a2=200 m/s
c1=1.0 m c2=0.5 m
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Aerodynamics 2015 fall - 12 -
Introduction to Aerodynamics
< 1.8. Flow similarity >
Dynamic similarity
The PI products evaluate to the following values.
Two airfoil flows are dynamically similar.
Airfoil 1 (sea level) Airfoil 2 (cryogenic tunnel)
α1=5° α2=5°
RE1=1.4e7 RE2=1.4e7
M1=0.7 M2=0.7
21
222111 ,Re,,Re,
ll cc
MgMg
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Aerodynamics 2015 fall - 13 -
Introduction to Aerodynamics
< 1.9. Fluid statics : Buoyancy force >
Fluid statics
The force on an element of fluid itself
• Pressure forces from the
surrounding fluid exerted on the
surface on the element
• Gravity force due to the weight of
the fluid inside the element
dzdxdydy
dpp
dzdxp
dxdy
dz
dzdydxg
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Aerodynamics 2015 fall - 14 -
Introduction to Aerodynamics
< 1.9. Fluid statics : Buoyancy force >
Fluid statics
Net pressure force & Gravity force
dzdxdydy
dpp
dzdxp
dxdy
dz
dzdydxg
dzdydxdy
dp
dzdxdydy
dppdzdxp
forcepressureNet
gdzdydxforceGravity
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Aerodynamics 2015 fall - 15 -
Introduction to Aerodynamics
< 1.9. Fluid statics : Buoyancy force >
Fluid statics
The fluid element is in equilibrium,
→ Hydrostatic equation
dzdxdydy
dpp
dzdxp
dxdy
dz
dzdydxg
dygdp
dzdydxgdzdydxdy
dp
0
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Aerodynamics 2015 fall - 16 -
Introduction to Aerodynamics
< 1.9. Fluid statics : Buoyancy force >
Fluid statics
Let the fluid be a liquid, for which we can assume ρ is
constant.
.
2211
1212
2
1
2
1
constghp
ghpghp
hhgpp
dygdph
h
p
p
11,hp
22 ,hp
21 hhh
dzdxdydy
dpp
dzdxp
dxdy
dz
dzdydxg
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Aerodynamics 2015 fall - 17 -
Introduction to Aerodynamics
< 1.9. Fluid statics : Buoyancy force >
Fluid statics
Applications : Walls of container
ap
11 ,hpp a
hp,
0, 22 hp
hhg
pp a
1
ap
1ghpa
hhgpp
ghpghpghp
a
a
1
111
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Aerodynamics 2015 fall - 18 -
Introduction to Aerodynamics
< 1.9. Fluid statics : Buoyancy force >
Fluid statics
Applications : U-tube manometer
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Aerodynamics 2015 fall - 19 -
Introduction to Aerodynamics
< 1.9. Fluid statics : Buoyancy force >
Buoyancy force
Archimedes principle
=Buoyancy force
on body
Weight of fluid
displaced by body