# Talk:Wake

WikiProject Physics / Fluid Dynamics  (Rated Start-class, Mid-importance)
This article is within the scope of WikiProject Physics, a collaborative effort to improve the coverage of Physics on Wikipedia. If you would like to participate, please visit the project page, where you can join the discussion and see a list of open tasks.
Start  This article has been rated as Start-Class on the project's quality scale.
Mid  This article has been rated as Mid-importance on the project's importance scale.

If you are offended by the note in the text about the offline link, feel free to move it here, but I did want to catch people's attention to the odd, archival link that a {dlw} template will leave. I'm not sure if the site will be back up, but the note seemed to indicate so. Pax. --KJPurscell 00:58, 20 July 2006 (UTC)

## Wake angle

• Someone inserted "The angle of the wake of a body moving steady in a deep fluid is 2arcsin(1/3) <ref>{{cite web|url=http://www.maths.cam.ac.uk/undergrad/|title=Undergraduate Mathematics at the University of Cambridge |date=2007-09-19}}</ref> (approximately 39 degrees).". This is wrong. The angle varies with speed. I have seen plenty of boat and ship and swimming waterbird wakes. Anthony Appleyard (talk) 10:46, 22 May 2008 (UTC)
You are in terrible, terrible, terrible error. Observe better, for slow boats and ducks that swim efficiently, i.e. they are not constantly climbing up their own wake, so, then, Froude number less than 1/2, as the text takes care to specify. The group wake which is visible, as per Kelvin's, and Lighthill's and Whitham's lovely arguments, does, indeed, have an opening angle in the chevron of 39 degrees, independently of the speed of the waves, the boat, the gravitational constant, etc... That is why, I propose below to insert the argument in a hidebox---to forestall terrible misconceptions such as this one! Cuzkatzimhut (talk) 12:10, 30 January 2016 (UTC)

• Isn't the wake a phenomenon quite similar to that of Čerenkov radiation, so I think the angle is given by:
${\displaystyle \cos \theta ={\frac {\text{Speed of waves}}{\text{Speed of boat}}}}$
These waves are the transversal waves at the surface of the water (such as these wake waves) and not the sound waves (which are longitudinal waves).
But there are other people that claim (apparently, in particular, Lord Kelvin) that the 39 degrees angle is in fact the only angle seen.
In the update section of this blog entry: her it claims that dispersion cancels certain wavelengths such that the wake angle is approximately 39 degrees.
There's a bit more info here but I'm getting a bit tired, so I'll try to look it up some other day. but my first guess is that one (if going slow enough) can generate any angle between 180 and 39 degrees, but no further. but that's just a guess JunCTionS 03:28, 17 September 2008 (UTC)

In deep water the angle of the steady gravity surface waves created will 2arccos(1/3). This is due to the relationship between group velocity and phase velocity. At small scale the waves are dominated by surface tension and this relation no longer holds. In shallow water, the relationship between phase velocity and group velocity changes and the angle of the cusp-line will change.

I have added a section on Kelvin wake pattern. Instead of following the original derivation by Lord Kelvin, which is very mathematical and not very intuitive, I chose to describe the phenomenon using a rather less rigorous but far more intuitive approach. This line of reasoning was first suggested by Frank Crawford of UC Berkeley in 1983 and enhanced by myself recently. It is not a new invention, of course, but merely a way to skip most of the math and still get a good grasp about the phenomenon. Those of you who want to complain about not seeing this treatment on the subject in the textbooks please remember that those textbooks are meant for graduate students in Fluid Mechanics. So, unless you have a better way to explain the phenomenon to undergrads and laymen, I suggest that you leave the section alone.
The most significant change I have made here in regard to the original Crawford approach is in the "Jacobian intensity maximum", corresponding to the waves with a would-be-shock-wave-angle of 54.7° which leads to the maximum actual wake angle of 19.5°. Crawford simply interprets the former as the the angle of the wake wavelets and the latter as the angle of the wake. I consider averaging over a band near the Jacobian intensity maximum a better approximation.
Duduong (talk) 23:32, 21 June 2009 (UTC)

## Distinction between Kelvin wave pattern and turbulent wake

There should be some discussion of the difference between the Kelvin wake (the surface pattern of gravity waves) and the turbulent wake as they are very different phenomena. Most of the illustrations given are of Kelvin wakes. A few show surface manifistations of the turbulent wake. Note that the turbulent wake is not purely a free-surface phenomenon. [DMH —Preceding unsigned comment added by 138.162.0.44 (talk) 13:41, 28 October 2008 (UTC)

Nice to see we've got a physicist or two around to offset the editing nonsense. The Kelvin wake analysis is a fairly well known analysis - which is referenced by the editor who origianlly added it. If one understands what to look for, the phenomenon is fairly easy to observe. This deletion removed one of the accurate statements in a technically weak article.
It would be great to have someone fix it...
Skål - Williamborg (Bill) 19:25, 8 November 2008 (UTC)
Wake most often refers to the turbulent wake, also in ship hydrodynamics wake is usually (but not always) associated with the turbulent zone produced by flow separation and the propellers. Perhaps through use of Wake (disambiguation) and splitting this article into Turbulent wake and Kelvin wave pattern (or Ship wave pattern) may clarify the confusion introduced at the moment. There is also an article Wake turbulence, focussing on the effects of the turbulence in wakes on aircraft. -- Crowsnest (talk) 07:59, 25 April 2009 (UTC)
Article is short enough it seems best to keep any discussion of turbulent water wakes here.
Can some physicists address wave decay? BoatWakes.org compares several authors on persistence of wakes over large distances, which is of major importance for erosion and "no wake" zones, but none of them gives a solid answer. Conservation of energy means the wake carries its energy until it dissipates in heat, but how far is that? Or does the circling motion of water molecules in each wave consume energy? When catamaran wakes "cancel" each other, as some of those sources suggest, where does the energy go? -- Numbersinstitute (talk) —Preceding undated comment added 17:02, 29 August 2009 (UTC).

## Wake pattern of a boat : scheme

This section is difficult to read and understand, mainly because it refers to a non existent scheme. The positions of letters A,B,C,D and source would be much clearer with this scheme. Can someone do that ? Johnnywayne2010 (talk) 10:20, 16 April 2012 (UTC)

## Please rewrite "Wake Patterns in Water"

I can hardly follow the line of argument, particularly in the second half of the section. First it says the phase velocity goes with the root of the wavelength, then later it talks about non-dispersive waves. There are many more gaps in the argument, and perhaps also some mistakes.

## Plea for graphing help regarding the Kelvin wave pattern

Addressing the classic simple, lovely, derivation of the Kelvin wake statement I inserted in section 2,

A concise geometric construction demonstrates that, strikingly, this group shock angle w.r.t. the path of the boat, 19.47°, for any and all of the above θ, is actually independent of v, c, and g; it merely relies on the fact that the group velocity is half of the phase velocity c. On any planet, slow-swimming objects have "effective Mach number" 3!

and the Figure that section had and disappeared by dint of obscurity, I wonder if anyone expert in figure drawing could contribute to Wikimedia the equivalent of Fig 12.2 (and perhaps 12.3) of the classic ref provided, G.B. Whitham (1974). Linear and Nonlinear Waves (John Wiley & Sons Inc., 1974) pp 409-410 Online scan.   Specifically, Figs12.2 & 12.3

They are ancient, so nobody in his right mind could claim copyright, etc... If one did draw this drawing, I would further recommend, indeed, beg, that he also draw a short circular arc of the circle centered at Q with radius QS, so the osculation of the tangent PS on S is evident, justifying the right angle discussed. With such a figure available, I could then easily adduce a Hidebox with the concise argument, detailing how the semi-circle with diameter PQ is the locus of all ψs, hence all ks, vs, etc... and so does not depend on their individual values, and thus stanch the terrible, terrible misconceptions demonstrated at the top of the page.... Cuzkatzimhut (talk) 12:36, 30 January 2016 (UTC)

So, basically you are asking for the diagrams 12.2 and 12.3 to be drawn and uploaded to WP. It should be no problem, diagrams can be adapted from books and uploaded to WP provided they are sourced (to where they were adapted from). I'll try later today. MŜc2ħεИτlk 09:23, 31 January 2016 (UTC)
Done, are these OK? Should have I included the wavevector k in them to show the direction of wave propagation? MŜc2ħεИτlk 10:19, 31 January 2016 (UTC)
Envelope of the disturbance emitted at successive times, fig 12.2 p.409 of G.B. Whitham (1974) Linear and Nonlinear Waves. Here ψ is the angle between the motion of the wave source and the direction of wave propagation (the wave vector k), and the circles represent wavefronts.
Envelope of the disturbance emitted at successive times, fig 12.3 p.410 of G.B. Whitham (1974) Linear and Nonlinear Waves. The circles represent wavefronts.

Wow! Grateful. Will do my part now. No, wavefront propagation from the center to the circumference is self-evident and simpler trumps busy in diagrams. A version of 12.3 is in Cherenkov radiation, but it's fraught with extra stuff, the direction of motion is reversed, etc... Again, most appreciative. Cuzkatzimhut (talk) 12:35, 31 January 2016 (UTC)

Thanks, and no problem ^_^ MŜc2ħεИτlk 12:44, 31 January 2016 (UTC)

Did my bit, thanks to your Figs. Let me know if there were something to be optimized. Cuzkatzimhut (talk) 15:19, 31 January 2016 (UTC)

## Disruptive editing on "Marangoni waves" asides

Singaporean IPs 155.69.125.175 , ‎155.69.192.107 , serially insert inappropriate references in an inappropriate location, despite repeated warnings to cease and desist: The section deals with surface object water-wave wakes, not submerged source ones as in the vanity references spatchcocked here. I tried to accommodate the hankering for vanity referencing involved, but the IP malefactor clearly believes serial reverts are too easy to resist. I propose requesting blockage of disruptive IPs Cuzkatzimhut (talk) 17:22, 6 February 2016 (UTC)

An admin would have to provide autoconfirm user protection, which stops IPs from editing but allows ordinary editors to edit. I geuss here is the place to do it. MŜc2ħεИτlk 17:26, 6 February 2016 (UTC)
Thanks! We might be dealing with simple ignorance of how transparent this Nanyang Technological University malefactor is. His latest edit was inoffensive, so I suppose we can watch his actions for a while, before an event is triggered. Cuzkatzimhut (talk) 17:34, 6 February 2016 (UTC)