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The streamlined
shape and the flat plate shown in the top diagram are scaled such
that their drag is the same. The drag coefficient for the streamline shape is
about CD = 0.04 whilst that for the disc is CD = 1.1.
The two lower
diagrams show the flow around these two bodies. The real flow around the streamline
shape has an initial laminar boundary layer that then becomes turbulent
towards the end of the body. The turbulent wake is small in area and the
pressure recovery across the shape is good. For the flat plate, flow separation
occurs at the sharp corners and there is no pressure recovery behind the
body. The large area wake has a considerable kinetic energy associated with
the turbulent motion and this energy has to be provided by the flow.
In
human-powered systems drag reduction by careful shape selection is an essential
component for success. |
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