

Drag coefficient and Reynolds' Number.
Name: anthony
Status: N/A
Age: N/A
Location: N/A
Country: N/A
Date: Around 1993
Question:
I am dealing with a project which tested scale models of a series
of automobiles and I found the optimum air speed (air speed that gave lowest
drag coefficient). I also found the rate of change of the drag coefficient
with respects to the air velocity. How can I explain why this happened. For
example, my drag coefficient dipped at 28 m/s and then skyrocketed beyond that
speed. Someone told me that the Reynolds' number might help me to explain the
cause for the optimum air velocity to be a specific speed and nothing else. I
am not really familiar with the Reynolds' number. All I know about it is that
it can tell you if you have laminar or turbulent flow. I know that it is VERY
important in aeronautical research and engineering. Please explain its
significance if that is the solution to my problem.
Replies:
That someone may be right. Reynolds' discoveries probably apply
here. Osborne Reynolds found that there were two types of viscous flows in
pipes. At low velocities, the fluids acted with laminar flow properties, and
they matched the analytical predictions. At higher velocities, however, the
flow breaks up into fluctuating velocity patterns or eddies (turbulent flow)
in a form that has not yet been fully predicted. Quite a bit of study has
been done on these properties recently. Reynolds also established that the
transition from laminar to turbulent flow was a function of a single number,
called the Reynolds number. It is a product of the velocity, fluid density,
and pipe diameter, divided by the fluid viscosity. If that number is less
than 2100, the flow will always be laminar. When it is higher than 2100, it
will NORMALLY be turbulent. This can be controlled, however. Another factor
is boundary layer theory, which through the work of Prandtl, von Karman and
von Mises, have greatly simplified the NavierStokes equations and included
the effects of boundary layer and inviscid fluids outside the boundary layer.
dipper
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