Transcript Thermodynamics I

Flow types
 Internal
 External
• Relative velocity between fluid &
object
• Auto moving through air
• Water moving against bridge
abutment
• Wind against building
Drag force
 Resistance to “forward” motion –
push back in direction of fluid flow
 Depends on
• Fluid/object velocities
• Fluid properties
• Geometry of object
• Surface roughness
Drag Forces
 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
 Theory: integrate pressure & shear
forces over object surface.
• Complex mathematics
• Empirical approach
Similitude
 Model simulates prototype
 Reliance on dimensionless
parameters
• Reynolds Number
• Relative roughness
• Drag coefficient - CD
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
Drag Coefficient
 Includes both pressure & friction
drags: one usually dominates
• Airfoil – friction; viscous shear drag
• Auto – pressure; form drag
Drag force
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Assume for experimentation
No adjacent surfaces
Free stream velocity uniform & steady
No free surface in fluid
Drag force
 Simplification: power to move vehicle
on level ground
• Rolling friction
• Drag force
Vehicles
 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
Fluid Mechanics Lab
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Simple shapes
Disk
Hemisphere
Sphere
Teardrop
Pressure drag
 Flat disk
• All pressure; no friction drag
• Streamline separation → wake; low
pressure region. Adverse pressure
gradient
• P front-to-back
Pressure drag
 Sphere
• Streamline separation
• Wake
Pressure drag
 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%
0
Design Process: EWT Models
 Photo’s of autos
 SolidWorks design
 CFD analysis of design: streamlines, CD
prediction
 3D printer for models using SolidWorks
design
 Preparation of models for EWT: surface
& mounting
 EWT testing: Lab CD vs predicted CD.
Agreement within 10%.
Assignment
 Chapter 17 up to Section 17.8