Thermodynamics I

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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

• 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 lt 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

• Photos 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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Assignment

• Chapter 17 up to Section 17.8