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From: Bert Harleman on 11 Dec 2009 19:08

> What do you imagine would happen if you used a hot soap bubble
> (which relies on surface tension) to hold up this basket?
> http://www.fantasyfliers.com/balloon001.jpg
> That's the same effect surface tension would have on a tsunami.
> I must be thinking you have no concept of scale.

I get your point. Thanks for the balloon

From another post:
> So, for a mixture of the two types of waves--which is what you
> get in the real world--there is a density effect.

I presume this density effect would be very smal
From: Darwin123 on 11 Dec 2009 22:22
On Dec 10, 6:25 pm, Bert Harleman <b...(a)bytegraag.nl> wrote:
> Suppose you measure the speed of waves at open sea (with a large depth)
> Then suppose you change the ocean water with a liquid that has twice the
> mass.
> Will the waves travel faster? Or slower?
I suppose that the wavelength of the wave is long enough so
surface tension doesn't matter. That describes most of the surface
waves we see on the ocean. If that is so, the dispersion relation for
these gravitational surface waves is:
Phase speed =sqrt(g*wavelength)
where g is the gravitational acceleration. Density is not part of this
formula. So surface waves under the conditions that you are thinking
of do no change speed with density.
>
> I found the next part of information:
>
> > Wave speed is also dependent on the density of the
> > medium.  The denser the medium (for the same phase of a medium,
> > in this case a liquid), the slower the speed of a wave
> > of a particular frequency.  Since sea water can vary in density
> > (density is determined by temperature, depth, and salinity),
> > waves in the ocean can vary in wave speed depending on the sea
> > water density.
I have a strong feeling that you copied from an article on
internal waves in the ocean. Deep down, there is a boundary where the
density of the water changes abruptly. At this boundary, there are
transverse waves called internal waves. They act precisely the way you
describe.

> Waves are formed by the wind. Moving denser liquid requires more energy,
> so I think these waves are less high. But what would the effect on the
> speed be?
Internal waves are not always caused by the wind. They are often
driven by tides. However, their speed does change with the ratio of
densities on each side of the boundary.
From: Darwin123 on 12 Dec 2009 11:27
On Dec 11, 6:17 pm, Bert Harleman <b...(a)bytegraag.nl> wrote:
> >  http://paws.kettering.edu/~drussell/Demos/waves-intro/waves-intro.html
>
> Thanks, you must be thinking I'm an idiot.
> I get a good idea of different waves types, how they transfer energy.
>
> What still puzzles me is if large (gravity) oceanwaves and capilary
> waves interact in a way that changes the velocity of the gravity waves.

Every surface wave is a combination of a gravity wave and a
capillary wave in this sense. There are formulas that express the
velocity of a surface wave that is affected by both surface tension
and gravity. The longer the wavelength, the larger the affect of
gravity. Hence waves with long wavelength follow a "gravity only"
formula, and waves with short wavelength follow a "surface tension
only formula." The exact equation for wave velocity is accurate at
intermediate wavelengths.
If you want to see a wave that has equal proportions of "gravity"
and "surface tension," throw a pebble into a pond. The ripples you see
will hav equal amounts of "gravity" and "surface tension" forces. The
reason is that at a certain wavelength, the phase velocity of the pure
"gravity wave" exactly equals the phase velocity of a pure "capillary
wave." Waves at this juncture are stable. They don't spread out as
fast as waves at other wavelengths. So the ripples that survive a
short distance are at this special wavelength.
From: Bert Harleman on 12 Dec 2009 12:01

>>I found the next part of information:
>>
>>
>>>Wave speed is also dependent on the density of the
>>>medium. The denser the medium (for the same phase of a medium,
>>>in this case a liquid), the slower the speed of a wave
>>>of a particular frequency. Since sea water can vary in density
>>>(density is determined by temperature, depth, and salinity),
>>>waves in the ocean can vary in wave speed depending on the sea
>>>water density.
>
> I have a strong feeling that you copied from an article on
> internal waves in the ocean. Deep down, there is a boundary where the
> density of the water changes abruptly. At this boundary, there are
> transverse waves called internal waves. They act precisely the way you
> describe.

I copied it from this article:
http://www.newton.dep.anl.gov/askasci/gen06/gen06205.htm

I understand the density-change in water (Because of scuba diving I'm
familiar with thermoclines; layers with different temperature in water,
which also have a density-change)

Now I see another part of the article and I think they are talking about
internal waves:
----
For a wave with a large amplitude (that would reach deep into the
ocean), the top of the wave (near the ocean surface)
could move faster than the bottom of the wave (which would be in
denser water lower down), resulting in the bottom of the wave
lagging the top of the wave. This can result in a tilted wave
after a while and possibly a reduced amplitude and longer wavelength.
-----


>>Waves are formed by the wind. Moving denser liquid requires more energy,
>>so I think these waves are less high. But what would the effect on the
>>speed be?
>
> Internal waves are not always caused by the wind. They are often
> driven by tides. However, their speed does change with the ratio of
> densities on each side of the boundary.
From: tadchem on 12 Dec 2009 13:48
On Dec 10, 6:25 pm, Bert Harleman <b...(a)bytegraag.nl> wrote:
> Suppose you measure the speed of waves at open sea (with a large depth)
> Then suppose you change the ocean water with a liquid that has twice the
> mass.
> Will the waves travel faster? Or slower?
>
> I found the next part of information:
>
> > Wave speed is also dependent on the density of the
> > medium.  The denser the medium (for the same phase of a medium,
> > in this case a liquid), the slower the speed of a wave
> > of a particular frequency.  Since sea water can vary in density
> > (density is determined by temperature, depth, and salinity),
> > waves in the ocean can vary in wave speed depending on the sea
> > water density.
>
> Waves are formed by the wind. Moving denser liquid requires more energy,
> so I think these waves are less high. But what would the effect on the
> speed be?

Sam gave you some good links, but this may be more apropos:
http://en.wikipedia.org/wiki/Airy_wave_theory

The article discusses the theory behind waves at a fluid-fluid
interface.

Neither the pahse velocity nor the group velocity depends on the
density, but the energy density does.

Note the expression for "mean wave-energy density per unit horizontal
area" in the Table of second-order wave properties.

Bottom line: increase the density of the fluid and you decrease the
wave *amplitude.*

Tom Davidson
Richmond, VA
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