[Phys-L] Re: water outflow (was earthquake)

[jsd at AV8N.COM (John Denker)]

Chuck Britton wrote:

> Let's try viscosity as a major factor.

Did you try it?

If you haven't yet tried it, here are a couple of suggestions:

1a) Put a gallon or so of water in a gallon-sized pail.  Reach in
with your hand, grasp a handful of water, and pull.  Tell us
whether viscosity was effective enough to let you lift the
entire sample of water (or a large fraction thereof), or whether
gravitational and inertial forces grossly overwhelmed the
viscous forces.

1b) Tell us whether the ratio of viscous forces to inertial forces
will get larger or smaller if we progress to samples larger than
one gallon, such as a tsunami.  Hint:
   http://www-groups.dcs.st-and.ac.uk/~history/Mathematicians/Reynolds.html

2) If you don't want to get your hand wet, you can calculate the ratio
(inertial force / viscous force) from scratch, using the tabulated
properties of water.  An order-of-magnitude dimensional-analysis
estimate will suffice.  You can take 750 km/h as a typical propagation
speed for a tsunami.

> My simple model has the energy/momentum traveling as a giant vortex,
> with horizontal axis.
> The rotation is such that the leading edge of the 'rolling' water is
> traveling downward.
>
> There is a boundary layer between the vortex and the 'still' ocean water.
>
> As the rolling water encounters depths that are comparable to the
> diameter of the vortex, it must 'climb' up the sloping submerged
> beach.
>
> The boundary layer is pulling the beach water under the vortex,
> producing the outflow.

Is there any evidence, experimental or theoretical, to suggest that
this model has anything to do with reality?

> Anything having to do with vortices, relates to turbulence and is
> therefore beyond human understanding  ;-)

The humans on my planet are quite capable of discussing waves
without getting tangled up in vortices ... and discussing
vorticity without getting shaken up by turbulence.  This has been
well understood for well over 100 years.  Indeed it is one of
the standard simplifications in fluid dynamics and aeronautical
engineering, to describe the flow field as a superposition of
vortices.  Turbulence may be essentially absent ("laminar flow
wings") or if present it is ordinarily confined to a rather
thin boundary layer, and does not prevent the vortex-field
method from giving remarkably precise, convenient answers.
Indeed before the advent of supercomputers, this was about the
only tractable method for analyzing the D=3 flow pattern near
a wing.

See any good aero-engineering book, e.g. von Mises.

In addition to setting up quantitative calculations, the vortex
picture provides valuable qualitative insights.  For example:
If you imagine the wave as a giant smoke ring just below the
surface, you can easily predict that bits of driftwood on the
surface will be thrown forward, and the largest velocity they
can attain is at least twice the propagation velocity of the
wave ... which is in rather dramatic conflict with the
observed behavior of real driftwood.

A good place to start would be Feynman volume I chapter 51
(waves on the surface of water) which can be contrasted with
volume II chapter 40 (vortices).