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Flow types

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

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Flow types

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  1. Flow types • Internal • External • Relative velocity between fluid & object • Auto moving through air • Water moving against bridge abutment • Wind against building

  2. 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

  3. 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

  4. Drag force • Theory: integrate pressure & shear forces over object surface. • Complex mathematics • Empirical approach

  5. Similitude • Model simulates prototype • Reliance on dimensionless parameters • Reynolds Number • Relative roughness • Drag coefficient - CD

  6. Wind tunnels • Experimental dragdeterminations • Buildings • Ships • Bridge supports/abutments • Vehicles

  7. Wind Tunnel • DC 3 & B 17: about 100 hours of testing • F 15: 20 000 hours of testing

  8. 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

  9. Drag Coefficient • Includes both pressure & friction drags: one usually dominates • Airfoil – friction; viscous shear drag • Auto – pressure; form drag

  10. Drag force • Assume for experimentation • No adjacent surfaces • Free stream velocity uniform & steady • No free surface in fluid

  11. Drag force • Simplification: power to move vehicle on level ground • Rolling friction • Drag force

  12. 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

  13. Vehicles • Nose of auto • Trunk of auto • Surface finish • Discontinuities • Mirrors • Door handles • Wheel wells • Air intakes

  14. Vehicles • Reduced drag vs other factors • Visibility • Passenger accommodation • Aesthetics

  15. Fluid Mechanics Lab • Simple shapes • Disk • Hemisphere • Sphere • Teardrop

  16. Pressure drag • Flat disk • All pressure; no friction drag • Streamline separation →wake; low pressure region. Adverse pressure gradient • P front-to-back

  17. Pressure drag • Sphere • Streamline separation • Wake

  18. Pressure drag • Tear drop – streamline • Reduce separation – farther along surface yields smaller wake • Increase in friction drag; optimum streamline design

  19. 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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