## this is a question to do with fluid dynamics

A forum designed mainly for high school physics students in New South Wales, Australia.
mrcool91
Posts: 3
Joined: Tue Aug 05, 2008 2:19 pm
Location: australind, south of perth, WA

### this is a question to do with fluid dynamics

i am reserching airodynamic drag for a physics project and i keep finding references to reynolds number, the laws state that for reynolds number greater than 1000, use quadratic form and for reynolds number less than 1 use the liniar form. can anyone explane to me:

what is reynolds number?

how is it calculater?

what hapens for reynolds number between 1 and 1000?
joe
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Joined: Fri Aug 29, 2003 11:57 am
Location: Sydney
The Reynolds number NR is proportional to the ratio of inertial forces (those required to accelerate the medium) to those of viscous drag.

The first goes as v^2, because the work done in accelerating the medium goes as (1/2)rho.v^2. The viscous drag is proportional to v.

At low speed and in high viscosity media, viscous forces are large and the flow is laminar. At high speed and in low viscosity, the medium is accelerated substantially.

At high values of NR, one gets turbulence. At low values, laminar flow. The value at which turbulence starts depends on the geometry and even with given geometry is uncertain, as it depends on history. It is typically at values of order 1000.

There is an article on Wikipedia
http://en.wikipedia.org/wiki/Reynolds_number

Joe

Joe
mrcool91
Posts: 3
Joined: Tue Aug 05, 2008 2:19 pm
Location: australind, south of perth, WA
cheers joe, but what about when a object travles thrugh a medium at:

just below speed of sound of medium

above speed of sound in medium

at hypersonic speeds

i've done some reserch on wave drag and know that it forms at very high speeds, but what causes supersonic drag and how is it different?
joe
Posts: 755
Joined: Fri Aug 29, 2003 11:57 am
Location: Sydney
Above the speed of sound, the object produces a conical shockwave. Draw a set of spherical wavefronts and you'll see that they add to form a cone.

That all of the wavefronts add together produces very high pressures.

Joe
mrcool91
Posts: 3
Joined: Tue Aug 05, 2008 2:19 pm
Location: australind, south of perth, WA
ok, so as the circles get closer the force gets larger, thus the speed barior,

but what about objects which are considerably smaller for example electrons or single molicules?

dose small masses create the same effect at high speeds as large masses?
umm, what do i put here?
joe
Posts: 755
Joined: Fri Aug 29, 2003 11:57 am
Location: Sydney
Individual molecules are often travelling faster than sound -- in fact the RMS speed is a bit greater than that of sound.

If molecules or electrons are going so much faster (say a dozen times the speed of sound) that they have enough energy to ionise atoms, then spectacular effects happen.

Best
Joe