Wave Sets..

Don Carnage

Hi.

As it can be seen at the ocean waves arrange in sets. Typical a set of 5 to 7 waves
where the middle ones are the biggest. What I don't understand is how this can happen.
Is it really possible for waves to exchange energy with each other..?? I though that
linear plane waves could "interact" in forms of constructive/destructive interference
but not that wave in the middle of a wave set could somehow suck energy from the others..
Maybe its because I am comparing EM waves with mechanical longitudinal waves or what..?

jdg812

Even linear waves can be arranged in such sets. Because they are not 'monochromatic'. Combination of two waves with approximately the same frequencies produces beating of waves, which you saw.

If wave is nonlinear, then there are many other mechanisms of sets formation, for example modulational instability.

Don Carnage

But if the waves are not-monochromatic (of different wavelengths) but linear they must travel with different speeds because their wave speed is v = omega / k, where k is a function of lambda (the wavelength)..
and if they have different speeds sets wont exist.. but they do !?

Don Carnage

Furthermore waves sets suck energy from its surroundings which can be seen when a set hits the shore because there will be a moment of totally calm right after....

jdg812

A wave in sea consists of many crests, from shore to horison
Another wave consists of many crests as well.
They have slightly different speeds, but they overlap all the time, so sets exists.

In popular English WAVE means ONE crest.
In physics, wave means a whole thing, from Florida to Africa.

Don Carnage

hehe i still dont get it :P

Don Carnage

or well.. I understand what you are telling me but how this makes them arrange in sets..

Don Carnage

When waves arrange in sets is it then an intrinsic sort of property contained in the waves or is it due to external forces.?

Don Carnage

I mean if it was some kind of intrinsic property i must be rather simple to write a program which show how the arrange in sets :D

jdg812

If you just make an animation of the couple of linear waves

A*sin (W1*t -K1*x) + B*sin (W2*t -K2*x)

(use K2 = 1.05*K1, and t as parameter of the animation, K1*x changes from 0 to 100, not from 0 to 2*Pi only)

you would see not only your sets, but change of the amplitude of the largest crest. Then next crest would become the largest one.

In this example everything is linear and intrinsic

Don Carnage

Ok -I will give it a look tomorrow.. thx a lot..

Peter

Don Carnage

Ok now I have written a program that generates 50 *.dat files in gFortran.
I can open them in gnuplot one at the time but it simply doesn't illustrate it.
Does anybody know an animation tool for Linux that can read datefiles..?

jdg812

A_1
A_2

Don Carnage

very nice.. are these just *.gif animations. What program has been used to make the plots?
Doesn't look like Gnu plot.. maybe Matlab..?

Don Carnage

So will a superpositioning of two linear waves with different ang. frq.'s and wavevector's always result in these sets.. damn I have to get my animation working.. its just so exiting and non-intuitive.. wonder what happens when I superposition a lot of waves.

Well on the other hand I know from quantum theory that the most located quantum particles are the ones of huge Fourier Sums there trough containing A LOT of superpositioned waves. One thing is to read it - another is to actually see it :P

jdg812

I prefer Maple or Mathematica

If frqs are about equal, yes
If not, still yes, but beating too quick and not easy to observe...

Actually you dont need animation or numerical simulations...
Sum of a couple of trig functions is product of trig with arguments
(a + b)/2 and (a - b)/2
The second one is beating term...

Don Carnage

I don't understand what you mean..