1. Streamlined body
- flow follows contours of body :
Bluff body
- flow separates :
vortices formed by rolling up of shear layers - may re-attach

2. Basic bluff-body aerodynamics
Flat plates and walls normal to flow
Smooth flow
CD = 1.1
SQUARE PLATE
Turbulent flow
CD = 1.2
Shear layer
Turbulence decreases (more negative) leeward side or ‘base’ pressure by increasing entrainment of flow from wake by ‘shear’ layers

3. Basic bluff-body aerodynamics
Flat plates and walls normal to flow
Smooth flow
TWO-DIMENSIONAL PLATE
CD = 1.9
No flow path around the sides (out of screen) - strong vortex generation and shedding - lower base pressure - higher drag

4. Basic bluff-body aerodynamics
Flat plates and walls normal to flow
TWO-DIMENSIONAL PLATE
splitter plate
CD = 1.4
Splitter plate induces re-attachment of flow - weaker, smaller vortices - lower drag

5. Basic bluff-body aerodynamics
rectangular prism (two dimensional)
Effect of turbulence
Higher drag
Lower drag
Decreased radius of curvature and hence lower pressure due to increased rate of entrainment of wake fluid into the more turbulent shear layer.
d/b = 0.1 b d
Low turbulence
High turbulence
Partial reattachment lower drag
Higher drag
d/b  0.5