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

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

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

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

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