Microbursts

In article om, wrote:


wrote:

Sailplaners will have a good understanding of natural air flow.
This text seems to suggest that you can take an unenclosed 'parcel'
of air, and move it through the surounding air, like you can throw
a solid object through the air.
== Chris Glur.

Chris,
What is happening is that the parcel has momentum, which is conserved.

All momentum is conserved.
But the molecules escape from the 'parcel' since it's not sealed.
So the combined system conservs momentum.

The momentum vector remains intact in the air until it is dissipated by
friction. Think of it as billiard balls passing along the momentum.

This is valid for each molecule.
The macro model is more complex.

Jim Ketcham

For the physicists: what's the AVERAGE velocity of the nitrogen
molucules at 25 degree C & atmospheric pressure ?

Now I know what they mean by "herding cats".

== Chris Glur.

wrote:

snip

For the physicists: what's the AVERAGE velocity of the nitrogen
molucules at 25 degree C & atmospheric pressure ?

From kinetic molecular theory of gases:

The *most probable* velocity (peak of the distribution)
c(p) = SQRT(2*k*T/m)

The average scalar velocity
c-bar = SQRT(8*k*T/pi*m)

The velocity of the molecule with average kinetic energy
SQRT(c^2-bar) = SQRT(3*k*T/m)

where k = Boltzmann's constant, T is absolute temperature, and m is
molecular mass.

Note that pressure is not a significant system variable in these
results - only temperature. Pressure *does* contribute to other
quantities such as collision frequencies and mean free path.

FWIW, c-bar for a nitrogen molecule at 25° C is about 1.31*10^5
cm/sec.

Tom Davidson
Richmond, VA

tadchem wrote:
wrote:

snip

For the physicists: what's the AVERAGE velocity of the nitrogen
molucules at 25 degree C & atmospheric pressure ?

From kinetic molecular theory of gases:

The *most probable* velocity (peak of the distribution)
c(p) = SQRT(2*k*T/m)

The average scalar velocity
c-bar = SQRT(8*k*T/pi*m)

The velocity of the molecule with average kinetic energy
SQRT(c^2-bar) = SQRT(3*k*T/m)

where k = Boltzmann's constant, T is absolute temperature, and m is
molecular mass.

Note that pressure is not a significant system variable in these
results - only temperature. Pressure *does* contribute to other
quantities such as collision frequencies and mean free path.

FWIW, c-bar for a nitrogen molecule at 25° C is about 1.31*10^5
cm/sec.

Oops! Make that 1.50*10^5 cm/sec


Tom Davidson
Richmond, VA

"tadchem" wrote in message
oups.com...

tadchem wrote:
wrote:

snip

For the physicists: what's the AVERAGE velocity of the nitrogen
molucules at 25 degree C & atmospheric pressure ?

From kinetic molecular theory of gases:

The *most probable* velocity (peak of the distribution)
c(p) = SQRT(2*k*T/m)

The average scalar velocity
c-bar = SQRT(8*k*T/pi*m)

The velocity of the molecule with average kinetic energy
SQRT(c^2-bar) = SQRT(3*k*T/m)

where k = Boltzmann's constant, T is absolute temperature, and m is
molecular mass.
...

Is this the point to jump in and argue that velocity is a vector quantity
and, therefore, the AVERAGE velocity is nearly zero? I would think that with
a little effort, it should be possible to spin this thread off into a
urinating contest about units or something like that.

:-)

--
Geoff
The Sea Hawk at Wow Way d0t Com
remove spaces and make the obvious substitutions to reply by mail
When immigration is outlawed, only outlaws will immigrate.

wrote:
Remarkably valuable material is available these days on wikipedia.

But I've got problems with this
http://en.wikipedia.org/wiki/Microburst
text.

Sailplaners will have a good understanding of natural air flow.
This text seems to suggest that you can take an unenclosed 'parcel'
of air, and move it through the surounding air, like you can throw
a solid object through the air.

I can't find good explanations of why the text is 'wrong'.

Under certain conditions, a toroidal vortex can & does move an
'enclosed` volume of fluid through the surrrounding fluid.
A smoke ring is a good example in air.
MadDog