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Re: Question on currents and waves
From: John Huth
Date: 2009 Dec 19, 15:54 -0500
From: John Huth
Date: 2009 Dec 19, 15:54 -0500
Yes, I read the post. Maybe I wasn't clear - what I'm seeking to understand is how this might affect the steepness of the waves on one side - that is to say, they seem to be steep on the side facing the current, and not on the side away and I don't quite understand what causes that.
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The convergence in the presence of a gradient I can understand, and also the patchiness. One of my colleagues did some work on freak waves and has a fairly simple model that does a better job of predicting the frequency of these than some computer models. It's based on what you're describing about currents - so yes, I understand that part.
It's the steepness on the side "impacting" the current that I don't quite understand. That seems to be the common refrain. I'll ask my colleague, he may be able to give me an explanation on the steepness issue.
If I missed that in the post, my apologies.
Best,
JH
On Sat, Dec 19, 2009 at 2:52 PM, George Huxtable <george@hux.me.uk> wrote:
John Huth wrote-
If John returns to my posting [11219], he will find a suggested explanation
"I think already the current gradient effect of focusing the waves that
Frank mentioned is one distinct culprit, but I don't quite understand how
this would steepen them, although I could be convinced. I'll let you know
what I find out."
which seems at least plausible, although I can not claim it to be certainly
correct. I wrote-
energy adds; others where they diverge and the water becomes smooth. It is,
"But when the wavefronts meet these varying currents, they get refracted
accordingly, as the net speed of surface propagation varies. So
unpredictable patches occur, where wavefronts converge and their local
of course, the adding bitsIt's somewhat similar to the way that sunlight shines through one of those
that matter, and create the danger to small craft.
In just the same way, wave patterns, generated away from the locality of the
Gulf Stream, can get refracted by those powerful local currents when they
meet it, and give rise to characteristic and dangerous wave-patterns, as
they get refracted toward local focus-points.
But all these effects are, in my view, related to local surface velocity
gradients."
dimply bathroom windows, and falls on a surface behind it. The varying
slopes of the glass cause the light to refract in different ways, so that at
some points it adds, and concentrates, creating a dappling of the light
pattern.
The disordered surface velocities of the current-flow cause varying
refraction of the waves, giving rise to transient concentrations of energy,
varying with position and time. For a given wavelength of a surface wave,
there is only a limited amount of energy that it can carry before it spills,
and the menace to small-craft is of those spilling seas tumbling aboard.
On top of that, waves generated elsewhere, where surface current is low,
will find that they have to carry an increased energy-density when meeting
an adverse current, which also adds to the tendency to break.
And waves generated elsewhere, which meet a more organised surface gradient
at an angle, such as at the edge of the Gulf Stream, could well be subject
to rather more geometrical bending and focussing, compared with the more
chaotic turbulence that I've been discussing above. That could result in
larger, and more predictable, danger-patches.
If John, or anyone else, comes across a decent explanation of such effects,
on the web or in print, I would be very interested to learn about it.
Somehow, I doubt if that will be a work on theoretical fluid dynamics,
though. But you never know...
George.
contact George Huxtable, at george@hux.me.uk
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
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