Ed Sirett wrote:
On Wed, 08 Oct 2008 13:22:05 -0700, The Real Doctor wrote:

On 8 Oct, 13:26, Martin Bonner wrote:

I *think* that the force exerted by drag goes as the /square/ of the
speed.
Only for a constant drag coefficient. Cd itself, however, depends on the
flow pattern around the car and that in turn depends on the speed - it's
often expressed in term of the Reynolds number. As speed increases the
flow becomes more turbulent and drag increases enormously. Conversely,
when the speed is low the flow is (largely) laminar and drag is low.

Have to diasagree the Reynolds numbers for object of size car, in air,
mean that even a few mph and you are into unambigously turbulent flow,
where the drag is proportional to speed squared.

The Cd can be affected by things like a half closed door/hatch. But is
largely unaffected by speed for Reynolds numbers of 10K+.




That is consistent with my understanding as well.

Airflow behind a car is fully stalled from a few mph onwards.