Flow types Internal External Relative velocity between fluid & object Auto moving through air Water...
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Transcript of Flow types Internal External Relative velocity between fluid & object Auto moving through air Water...
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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 back in direction of fluid flow
Depends on• Fluid/object velocities• Fluid properties• Geometry of object• Surface roughness
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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
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Drag force
Theory: integrate pressure & shear forces over object surface.
• Complex mathematics• Empirical approach
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Similitude
Model simulates prototype Reliance on dimensionless parameters• Reynolds Number• Relative roughness• Drag coefficient - CD
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Wind tunnels
Experimental drag determinations
• Buildings• Ships• Bridge supports/abutments• Vehicles
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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
Includes both pressure & friction drags: one usually dominates
• Airfoil – friction; viscous shear drag• Auto – pressure; form drag
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Drag force
Assume for experimentation• No adjacent surfaces• Free stream velocity uniform & steady• No free surface in fluid
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Drag force
Simplification: power to move vehicle on level ground
• Rolling friction• Drag force
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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
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Vehicles
Nose of auto Trunk of auto Surface finish Discontinuities• Mirrors• Door handles• Wheel wells• Air intakes
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Vehicles
Reduced drag vs other factors• Visibility• Passenger accommodation• Aesthetics
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Fluid Mechanics Lab
Simple shapes• Disk• Hemisphere• Sphere• Teardrop
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Pressure drag
Flat disk• All pressure; no friction drag• Streamline separation → wake; low
pressure region. Adverse pressure gradient
P front-to-back
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Pressure drag
Sphere• Streamline separation• Wake
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Pressure drag
Tear drop – streamline• Reduce separation – farther along surface
yields smaller wake• Increase in friction drag; optimum
streamline design
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Shape and flow Formdrag
Skinfriction
0% 100%
~10% ~90%
~90% ~10%
100% 0
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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%.
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