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 - C D

Wind tunnels  Experimental drag determinations Buildings Ships Bridge supports/abutments Vehicles

Wind Tunnel  DC 3 & B 17: about 100 hours of testing  F 15: hours of testing

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

Drag force  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 C D ; no concern < 30mph  Higher speeds concerns increased  Advances in metal-forming techniques for improved body designs  Control C D Fuel costs Conserve non-renewable resources Pollution

Vehicles  Nose of auto  Trunk of auto  Surface finish  Discontinuities Mirrors Door handles Wheel wells Air intakes

Vehicles  Reduced drag vs other factors Visibility Passenger accommodation Aesthetics

Fluid Mechanics Lab  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 flowForm drag Skin friction 0%100% ~10%~90% ~90%~10% 100%0

Design Process: EWT Models  Photo’s of autos  SolidWorks design  CFD analysis of design: streamlines, C D prediction  3D printer for models using SolidWorks design  Preparation of models for EWT: surface & mounting  EWT testing: Lab C D vs predicted C D. Agreement within 10%.

Assignment  Chapter 17 up to Section 17.8