Transcript FLUID PROPERTIES - Southern Polytechnic State University

Flow types
Internal
 External
• Relative velocity between fluid & object
• Auto moving through air
• Water moving against bridge abutment
• Wind against building
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Drag force
Resistance to “forward” motion – push in
direction of flow
 Depends on
• Fluid/object velocities
• Fluid properties
• Geometry of object
• Surface roughness
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Friction drag
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Two types
Friction drag: viscous shear effects as flow
moves over object surface. Acts parallel
to surface
Form drag: affected by geometry of object.
Acts perpendicular to object
Drag force
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Theory: integrate pressure & shear forces
over object surface.
Complex mathematics
FEA or CFD?
Empirical approach
Similitude
Model simulates prototype
 Reliance on dimensionless parameters
• Reynolds Number
• Relative roughness
• Drag coefficient - CD
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Wind tunnels
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Experimental drag determinations
Buildings
Ships
Bridge supports/abutments
Vehicles
Wind Tunnel
DC 3 & B 17: about 100 hours of testing
 F 15: 20 000 hours of testing
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Drag Coefficient
FD = CD A ρ (V2/2)
 V – free stream velocity
 Characteristic area –e.g. frontal for auto
 Air density
 CD – drag coefficient characteristic of
geometry
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Drag Coefficient
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Includes both pressure & friction drags:
one usually dominates
Airfoil – friction; viscous shear drag
Auto – pressure; form drag
Drag coefficient
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Affected by geometry & Reynolds Number
Drag force
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Assume for experimentation
No adjacent surfaces
Free stream velocity uniform & steady
No free surface in fluid
Drag force
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Simplification: power to move vehicle on
level ground
Rolling friction
Drag force
Vehicles – Drag factors
Proximity to other surfaces & objects
 Wind speed & direction
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Vehicles
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Early autos – high CD; no concern < 30mph
Higher speeds concerns increased
Advances in metal-forming techniques for
improved body designs
Control CD
Fuel costs
Conserve non-renewable resources
Pollution
Vehicles
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Nose of auto
Trunk of auto
Surface finish
Discontinuities
Mirrors
Door handles
Wheel wells
Air intakes
Vehicles
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Reduced drag vs other factors
Visibility
Passenger accommodation
Aesthetics
Drag Factor
Trucks
• 70% front
• 20% rear
• 10% sides
 Fairing
• Without – 0.75 to 0.95
• With – 0.7
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Wind tunnel tests
Drag forces
 Pressure distribution
• High pressure: nose & windscreen base
• Low pressure regions: Effect on
friction/power & traction/handling
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Fluid Mechanics Lab
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Simple shapes
Disk
Hemisphere
Sphere
Teardrop
Pressure drag
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Flat disk
All pressure; no friction drag
Streamline separation → wake; low
pressure region. Adverse pressure
gradient
P front-to-back
Pressure drag
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Sphere
Streamline separation
Wake
Surface roughness
Affects boundary layer
 Point of separation
 Golf ball
 Knuckle ball
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Pressure drag
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Tear drop – streamline
Reduce separation – farther along surface
yields smaller wake
Increase in friction drag; optimum
streamline design
Shape and flow Form Skin
drag friction
0%
~10%
~90%
100%
~90%
~10%
100%
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Compressible fluid
Mach Number = v/c
 Ma < 0.3 to 0.5; flow behaves as
incompressible
 Critical velocity ~ 370mph (544fps)
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