Ask A Scientist© Engineering Archive

Index Key:   ENG024
Author:      anthony
Subject:     Drag coefficient and Reynolds' Number.
Text:        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.

Response #:  1 of 1
Author:      dipper
Text:        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 Navier-Stokes equations and included 
the effects of boundary layer and inviscid fluids outside the boundary layer.