User talk:Crowsnest/Archive 1

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Re:Thanks
Thank you, Kraaiennest. I'm really glad you liked it.--Mbz1 (talk) 22:13, 14 January 2008 (UTC)

list of wave topics
I've just added Boussinesq approximation (water waves) and Stokes drift to the list of wave topics. If you know of other articles that should be there and are not, could you add those? Thanks. Michael Hardy (talk) 05:01, 25 January 2008 (UTC)
 * Thank you! I just put dispersion (water waves) there and I will keep it in mind when browsing Wikipedia. -- Kraaiennest (talk) 10:19, 25 January 2008 (UTC)

Fluid dynamics
It's good that you have removed the section "Other fundamental engineering topics" from Fluid dynamics page.

I have some points about the defintion of hydrodynamics. If you see the history of fluid dynamics you can see that the hydrodynamics is a branch fluid dynamics mainly developed by mathematicians. The hydraulics is the other branch developed by the practicing engineers. The two branches were practically unconnected before the introduction of Boundary layer theory.

Hydrodynamics assumes that the flow is potential. Liquid flows cannot be treated as potential flows except for the special cases where viscous effects can be neglected. Milne-Thompson's book deals with potential flows and associated conformal mapping techniques. Although Lamb's books deals with effects of viscosity he does not attempt solve the full Navier-Stokes equation.

What I want to say is that hydrodynamics is essentially the study of ideal fluid flows becuse ideal fluid flows are generally potentail flows. Salih (talk) 17:27, 17 February 2008 (UTC)
 * For instance, see the MSN Encarta definition for hydrodynamics: "study of liquids in fluid dynamics: the area of fluid dynamics that is concerned with the study of liquids". The books of Lamb and Milne-Thompson are excellent, but old (1895 and 1938). Before Stokes, in the mid-nineteenth century, almost all the knowledge and interest was on inviscid flows. And then mainly of irrotational inviscid flows since these could be solved using potential theory. That is one of the main reasons why there is not much so focus on viscous flow in these books. If you look at modern hydrodynamics books, you see they deal to a large extend with viscous flow, e.g. Criminale, Jackson and Joslin (2003) "Theory and Computation of Hydrodynamic Stability", Cambridge University Press. Also, before the availability of high-speed computers, it was almost impossible to solve the rotational Euler equations or viscous Navier-Stokes equations. So, hydrodynamics is the sub-branch of fluid dynamics that studies moving liquids. Period. With as special interests, as compared to other branches of fluid dynamics: the presence of free surfaces, floating bodies and confined flows by a bottom and/or lateral boundaries. Kraaiennest (talk) 23:08, 17 February 2008 (UTC)
 * Thank you for your response. Of course the books by Lamb and Milne-thompson are old, but classics. As far a my understanding goes, hydro (meaning water) is a misnomer in the word hydrodynamics. The hydrodynamic stability is another topic and it does assume that the fluid is liquid or gas. Hydrodynamic stability deals with inviscid stability as well as viscous stability (for example, stability of laminar parallel flows). The words hydrodynamics and hydrodynamic stability are used in different contexts.


 * Also, I appreciate the fact that potential theory can be used for determination of frequency of oscillation of sloshing liquids etc. But inviscid assumption is used in the derivation of frequency. So, whether the fluid is liquid or gas invariably the inviscid and incompressible assumptions are used in the hydrodynamics. Salih (talk) 13:06, 18 February 2008 (UTC)
 * Well, in these sloshing motions the inviscid approximation is used because most often the viscous effects are limited to thin oscillatory boundary layers near the surfaces of the flow, and the dissipation is often relatively small and has only a small effect on the frequency of oscillation. Hydrodynamic stability is a sub-branch of hydrodynamics, as are coastal, marine, ship hydrodynamics etc. All these incorporate viscous and turbulent flows when appropiate. And use inviscid and potential flow if applicable. Also compressibility is part of hydrodynamics, e.g. cavitation for ship propellors and in case of e.g. "green water" or wave slamming. The field of hydrodynamics develops, and incorporate more complex fluid phenomena when needed and if possible. Science is dynamic, as is hydrodynamics. Kraaiennest (talk) 17:48, 18 February 2008 (UTC)

Well, I have done some research to find out the definition of hydrodynamics from modern fluid mechanics books which are referred by academics throughout the world. The information I obtained is given below:

Undergraduate level books:

1. R. W. Fox and A. T. McDonald, "Introduction to Fluid Mechanics", 6th ed., John Wiley & Sons, New York (2004).

This enormously popular book does not have the word hydrodynamics in it. It has a chapter on "Incompressible inviscid flows" (chapter-6). The Reference books given at the end of this chapter include the books by Lamb (1945), Milne-Thompsom (1960), Robertson (1965), and Vallentine (1959). All the books have the word "hydrodynamics" in their title.

2. F. M. White, "Viscous Fluid Flow", 3rd ed., McGraw-Hill, New York (2006).

No mention of the word hydrodynamics. It has a chapter on "Inviscid incompressible flows" (chapter-8). The Reference books given at the end of this chapter include the books by Milne-Thompsom - Robertson - Vallentine.

3. B. S. Massey and J. Ward-Smith, Mechanics of Fluids, 7th ed., Nelson Thornes, UK (1998).

Again no mention of the word hydrodynamics. It has a chapter on "The flow of an ideal fluid" (chapter-9). The Reference books given at the end of this chapter include the books by Lamb - Milne-Thompsom - Robertson - Vallentine.

4. P. K. Kundu and I. M. Cohen, Fluid Mechanics, 3rd ed., Academic Press (2004).

Again no mention of the word hydrodynamics. It has a chapter on "Irrotational flows" (chapter-6). The Reference books given at the end of this chapter include the books by Milne-Thompsom - Vallentine.

5. B. R. Munson, D. F. Young, and T. H. Okiishi, "Fundamentals of Fluid Mechanics", 4th ed., Wiley, New York (2002).

In this book the word hydrodynamics appears in one single page (No.31). It says "Hydrodynamics was the term associated with the theoretical or mathematical study of idealized, frictionless fluid behavior, with the term hydraulics being used to describe the applied or experimental aspects or real fluid behavior, particularly the behavior of water".

Graduate level books:

1. G. K. Batchelor, "An Introduction to Fluid Dynamics", Cambridge Univ. Press, Cambridge, UK (1967). No mention of the word hydrodynamics.

2. F. M. White, "Viscous Fluid Flow", 3rd ed., McGraw-Hill, New York (2006). No mention of the word hydrodynamics.

3. R. L. Panton, "Incompressible Flow", 3rd ed., John Wiley & Sons, New York (2005). No mention of the word hydrodynamics. It has a chapter on "Ideal flows in a plane" (chapter-8). But reference books are given only at the end of the book.

4. H. Schlichting, "Boundary Layer Theory", 7th ed., McGraw-Hill, New York (1979). No mention of the word hydrodynamics.

The above discussion shows that hydrodynamics is an old term used to describe the behavior of ideal fluids. Modern fluid dynamics books does not use this term. After the publication of books by Lamb - Milne-Thompsom - Robertson - Vallentine, is there any popular book titled hydrodynamics appeared? To my knowledge the answer is NO.

Hydrodynamic stability You have mentioned that hydrodynamic stability is a subset of hydrodynamics (study of liquid in motion). Now, consider this: Raleigh-Taylor instability, Rayleigh-Couette instability, (sometimes Benard instability) are topics discussed under hydrodynamic instability. But these three instabilities are not pertaining to liquids alone. Gases can also undergo these kind of instabilities (see books of Chandrashekhar and Drazin & Reid). So books on hydrodynamics stability and its numerical simulation will continues to appear in future.

PS: I must also mentioned that I have found something which supports your arguments. In the book

Y. A. Cengel and J. M. Cimbala, "Fluid Mechanics", McGraw-Hill, New York (2005), page no.2, the authors states "The study of the motion of fluids that are practically incompressible (such a liquids, especially water, and gases at low speeds) is usually referred to as hydrodynamics. A subcategory of hydrodynamics is hydraulics, which deals with liquid flows in pipes and open channels".

Note that Cengel and Cimbala book is relatively new and it is only the first edition. Regards, Salih (talk) 11:24, 19 February 2008 (UTC)
 * Well, you did quite a thorough search on finding book references to hydrodynamics. How many of these books mention gas dynamics or aerodynamics? Hydrodynamics is just another name for fluid dynamics applied to liquids, and the use of this term depends on the subject of study, the preferences of the author and what is common in the field. So hydrodynamic stability is the common term and also applied for gases. It has nothing to do with hydrodynamics being an old term and only applicable to ideal fluids. Just search http://scholar.google.com for combinations of hydrodynamics and compressible or viscous. Kraaiennest (talk) 12:05, 19 February 2008 (UTC)
 * By the way, I proposed to merge hydrodynamics into fluid dynamics, since there are no typical hydrodynamic topics mentioned in that article. Kraaiennest (talk) 12:08, 19 February 2008 (UTC)
 * Please don't merge fluid dynamics and hydrodynamics for the time being. Let us get a third opinion about this controversy. Salih (talk) 14:50, 19 February 2008 (UTC)


 * Also, I feel the description of aerodynamicsas given in the opening para of fluid dynamics is also not proper. Aerodynamics deals with the study lift, drag and other fluid dynamics phenomena connected to aircrafts, racing cars, etc. Further, the study of compressible flow of an inviscid fluid is usually refreered to as gasdynamics. Compressible flow of viscous fluid is called just compressible fluid flow. That way gasdynamics can be considered as a subset of compressible flow. Salih (talk) 15:10, 19 February 2008 (UTC)
 * Usually the dynamics of gasses is called gas dynamics, the dynamics of air called airodynamics, the dynamics of liquids (water) called hydrodynamics. Without further restrictions on being inviscid, compressible, etc. Or restrictions on the phenomena which are studied. All real fluids are compressible, gasses more than liquids. Under some conditions, e.g. low Mach number for aircraft, the description of the flow may be simplified by assuming it is incompressible. (In)compressibility is a statement about the kinematics of the flow, not the dynamics. And kinematics is a branch of dynamics. So, in my view, it is not logical to consider gas dynamics as a subset of compressible flow. Kraaiennest (talk) 18:42, 19 February 2008 (UTC)

I think you have completely wrong notion about gasdynamics. Do you agree that we can get to know what "gasdynamics" is by collecting all the popular books on gasdynamics (Benedict(1983), Cambel & Jennings(1958), Chapman & Walker(1971), John & Keith (2007), Liepmann & Roshko(1957), Oswatitch(1956), Owczarek(1964), Rotty(1962), Turrell(1997), Vincenti & Kruger(1965), Zucker and Biblarz(2002), Zucrow(1985), Zucrow and Ho�man(1976)) and going through the contents in them?

You are going to see that all the above mentioned books discuss compressible flows of frictionless fluids! Of course there is a Fanno flow in which wall friction is taken into account. But, not the fluid viscosity.

Similar is the case with hydrodynamics and and aerodynamics. We should collect all the books in these field and see the contents in it. Regards, Salih (talk) 07:34, 20 February 2008 (UTC)
 * Well, that seems an impossible task to me, and I will not start it or work on it. Besides, it does not solve the question, because science is evolving, common usage of these terms changes, and many textbooks are reprints of outstanding old ones. People tend to focus in writing new books on where the action is nowadays, and that is in the specialisations of these fields, or multi-disciplinary interactions. And in all these sub-fields, e.g. coastal hydrodynamics, ship hydrodynamics, marine hydrodynamics, offshore hydrodynamics, etc., you will see that viscid and compressible flows are also studied. Just   stick to the definition of what these terms say, and the problem is solved: aerodynamics is the dynamics of air, etc. and all these cumbersome exercises are not needed. Moreover, this agrees with how the majority of the scientists and engineers use these terms, as you can easily find out by searching the WWW. Kraaiennest (talk) 13:07, 20 February 2008 (UTC)
 * Don't worry I have most of the gasdynamics and hydrodynamics books with me (got these books as complementary copies over a period of seventeen years). It does't take much time to see what topics are covered in a book.


 * I agree with you that coastal hydrodynamics, ship hydrodynamics, marine hydrodynamics, offshore hydrodynamics, etc., are very specialized fields and their name perfectly indicate what topic is under consideration. But the word "hydrodynamics", standing alone, does not explicitly indicate what is under consideration. I also agree with you that meaning of certain words change with time. Thanks. Salih (talk) 16:42, 20 February 2008 (UTC)

Spelling
Hi Crowsnest. On 28 February 2008 you carried out some useful editing of Bernoulli's principle. I notice that you converted all the English spelling to the equivalent American spelling: Please be aware that the Wikipedia Manual of Style endorses the principle of retaining the existing variety, including retaining the first substantive variety of English spelling. See national varieties of English. In particular, the Manual of Style does not advocate exclusive use of American English spelling. See retaining the existing variety.
 * aerofoil to airfoil
 * summarise to summarize
 * recognise to recognize

Happy editing! Dolphin51 (talk) 03:46, 7 March 2008 (UTC)
 * Hi Dolphin51. Thanks. I did not know about these English/American spelling conventions on Wikipedia. I just saw some red underling in the Wiki editor, so I changed it. Thank you for pointing this out. Crowsnest (talk) 11:44, 7 March 2008 (UTC)

Undo on gewp
May we discuss why you undid my referencing to www.gewp.org, the 100 terawatt proposal, without discussion? Do you have corporate or government affiliations in energy, or do you find the site to present ideas that are simply wrong? I'm guessing here ... I would never be so bold as to make such a deletion on another editor without a talk, first. Reason, please. Even having held a Chair in Energy Studies at MIT, 20 years ago, I would not be so bold as to make such an undo. Nukeh (talk) 01:17, 8 March 2008 (UTC)
 * Please, see my response at Talk:Wave power on March 9. Crowsnest (talk) 08:43, 11 March 2008 (UTC)

Tide
You've reverted edits to Tide recently. I don't think User:86.4.26.89 is trying to vandalize the article, although he/she seems unable to create the simplest Edit Summary. I've read elsewhere that "high water" (etc.) is the preferred term over "high tide". Spiel496 (talk) 01:42, 11 March 2008 (UTC)
 * Thank you. I am not an expert on English grammar, I just saw in the dictionary that "high tide" and "low tide" are correct expressions, and I hear them often. Crowsnest (talk) 08:36, 11 March 2008 (UTC)

That user was me. Thank you for your acknowledgements and withdrawal of your earlier allegations of vandalism. The reason that high and low water are preferred is that to seafarers the term 'tide' properly refers to the current. So if 'high tide' means anything, it means 'strong tidal current'. It's a bit like confusing volts and amps. Tide flows (or ebbs and floods), the water level goes up and down.

This is why tide tables almost always show the abbreviation 'HW' for high water, rather than 'HT'. Indeed, if you look at the article, you will see several references to high water, including in the graphs near the top with 'higher high water' and 'lower high water'. It would be nice if the same terminology was used throughout the whole article.

Before editing the article I checked US usage (I am British), just to make sure that they did not deprecate 'high water' and prefer 'high tide'. A good place to start seemed to be http://tidesandcurrents.noaa.gov/publications/glossary2.pdf. You'll notice that 'high water' appears many times throughout the document, and high tide only once - to explain that it is the same as high water. To me, that suggests that 'high water' is the preferred term in the States too, though they are not as strict as I am in abhorring the term 'high tide'.

I hope that you'll now consider reversing your previous edits of the article, removing references to 'high tide' as I tried to do. Cheers. —Preceding unsigned comment added by 86.4.26.89 (talk) 19:12, 13 March 2008 (UTC)


 * I reverted Tide to your previous version made on March 10. No (unreverted) changes had occurred since then, so it was not difficult to do. Best regards, Crowsnest (talk) 09:18, 14 March 2008 (UTC)

Thanks for your adjustment of layout for the calculated tide plots. I wasn't happy with the results I had obtained, but lacked the patience to pursue the matter when I am supposedly working for the Electricity Commission. I have been considering adding some plots on tidal currents around NZ as further fallout from my assessment of tidal power generation, but patience is low, and the information I have is not very good: the tidal coefficients are held secret (even though prepared from public research) so that the holding organisations may profit from publications. The available information is from nautical charts, and schedules of calculated high and low water, both sources being annoyingly quantised (to a tenth of a knot, or a tenth of a metre) and I noticed a number of obvious mistakes in the nautical chart data as well. I have sent a grumble communique (would you like a copy?) to the masters of the nautical charts (a putsch by the land mappers took over control of nautical charts from the NZ navy), but so far no response either to the errors, or to reveal finer data. NickyMcLean (talk) 21:29, 6 July 2008 (UTC)

Wave harvesting energetics
The wave power article has many references that lead to many places, and I have been looking through them. If you have done that too, can you answer this question: Has wave force (mgh) based on displacement (or buoyancy) integrated over time into energy been neglected for large vessels as a possible source of energy? Also, in your earlier comment, do you believe there is special physics required to do calculations on ocean wave energy, for example, fluid dynamics? If you are thinking in terms of a large energy vessel getting down current from an anchor, I believe that is still treatable, by first order approximation, by ordianary vectors. Also, on citing work or energy per wave, I believe that is model dependent. For example, if a wave hits a stationary wall, no useful work is harvested. I don't see any of the links or references headed into references that are valid physics or thermodynamics (open or closed).

Another question: If I run orthonormalization of pseudocolor (www.youvan.com) methods on figures to make a new color scheme and associated color look-up map, do you think that is original research? The figure I am thinking about changing is a color-coded map of the US costs in kWH on a per state basis. It's in the sandbox link on www.gewp.org. For some strange reason, high cost states are coded blue and low cost states are colored into the red. That is counter intuitive to things that people commonly see, such as Doppler radar images on www.weather.com, MRI's, traffic flow, etc.

Also, do you still believe that it is sufficiently likely that GEWP is a hoax, such that a editor like you should not list the link on the main article to help a fellow editor who has disclosed a conflict of interest? Do you see anything in the calculations that suggests GEWP is a hoax? 100TWdoug (talk) 23:20, 15 March 2008 (UTC) 100TWdoug (talk) 23:20, 15 March 2008 (UTC)

linearized Euler equations
Thanks for pointing out improvements for my contributions about the flux jacobians, I've put a minor edit to say that the matrices are only for the ideal gas law, however if you feel the need to change more things, feel free to do so! I'm afraid I don't have that much time to edit, so your help is much appreciated. Thanks for the tips as well.

Arthurv (talk) 12:08, 26 March 2008 (UTC)

Bernoulli's principle
Hi again Crowsnest. Thanks for your two messages on my User:Talk page. Thank you also for restoring the deleted text. I started a new topic in Talk:Bernoulli's principle to highlight the problem of hacking on the subject of Bernoulli and wing lift. After I saw that you had kindly restored the paragraph I went back into my new topic and removed the reference to "Crowsnest" to de-identify the two editors. Happy editing! Dolphin51 (talk) 12:37, 25 March 2008 (UTC)

Your excellent edit on Wave Power
Thanks for improving Wave_power so much with the new equations you just placed in the article. I think my simpler calculation will give the same result, considering that 1/2 mv^2 is extremely sensitve to v. Your formalism is much better than mine. My guess is that many wave power efforts have been derailed by bad physics. Just think, the undefined constant k in the equation of a day ago (removed) could have been -1 or 0! I will now try to derive all the equations you just put up from scratch, looking at no other sources other than elementary physics, then publish on www.gewp.org with no links from the Article. I'll let you know how that goes if you want. 100TWdoug (talk) 17:42, 26 March 2008 (UTC)

Also, unless you know the NOAA global maps on wave height and period are in error, could we put those back into the Article, high up, so as to motivate newcomers and students? I would suggest replacing the top image in the Article with a NOAA image. (The current rolling ball figure is not very informative or motivating.) Perhaps an animated NOAA image could be running. For graphical resolution on a small scale, Zealandia is a nice region to display on NOAA imagery / forecasts.100TWdoug (talk) 18:13, 26 March 2008 (UTC)


 * I put the NOAA maps plus a link to the KNMI wave climate to the end of the article, in Wave power. I did not know the NOAA site before you put it in, and I think it is giving a very nice view on the wave dynamics on a global scale. While the NOAA site gives forecasts, at ECMWF they have done hindcasts, the statistics of these are summarized in the KNMI wave atlas. There are also commercial providers of such information, for the oil and dredging industries, as well as for infrastructure planning (ports etc.) along the coast. Some of these use satellite data to estimate ocean wave statistics.
 * Your estimates seem to be in the correct order of magnitude, but as wave energy transport is horizontally, once it's energy is tapped by a line stretch of wave power devices perpendicular to the wave direction, the sea is calm behind and it will take space (fetch), or time, for the wave height to rebuild. This will put limits on the amount of available wave energy, if a considerable part of the energy flux is transformed into other forms (e.g. electricity). It puts limits on how dense "lines" of wave power devices can be put behind each other, before they come in the shadow zone of "upwave" devices. Crowsnest (talk) 20:07, 26 March 2008 (UTC)

Yes, that was the error in the 100TW calculation - I can't get sufficient linear length if the entire wave is harvested up front. The NOAA images are incredible, and I am pleased they are still up. I might experiment with an image that is thumbnail sized. It would appear that one good direction for the Article would be to look into small scale wave energy harvesting (perhaps into Hydrogen) for self-powered sail boats / yachts. That would be an expensive endeavor, but it would push the high-tech limits into prototyping that might someday be used on a larger scale for global power. I'll run some searches so I don't go off into prophecy again. I am also wondering if anyone has looked into butterfly (chaos) seeding upwind from a wave power plant.100TWdoug (talk) 21:14, 26 March 2008 (UTC)

I am now running into my first problem with calculating wave potential energy and wave kinetic energy from height - from scratch. It appears that the literature going back at least three decades assumes that the wavelength is 2pi times the wave height. Do you know why such a factor would be assumed, other than a basic description of a sine wave for teaching mathematical integration?

Also, this is an informative site: http://www.seafriends.org.nz/oceano/waves.htm. There is an interesting comment concerning waves picking up energy at the fourth power of wind speed. KE for wind is certainly v^3 (from (mv^2)/2 with v-times more mass transport), but it is unclear where a 'clean' extra power of v comes from to make it overall v^4.100TWdoug (talk) 20:44, 27 March 2008 (UTC)


 * The only thing I am aware of, is that in deep water there is a maximum steepness: wave height divided by wave length, above which waves break. For individual waves in a sea state, the maximum is about 1/7, close to the 1/(2&pi;) you mention. However, the statistical average: significant wave height divided by characteristic wave length (of the most energetic) waves will have a maximum due to wave breaking less than 1/7 (I believe somewhere near 1/16, but that may be wrong). Anyway, in energetic sea states, interesting for wave power, the significant wave height divided by the mean wavelength is most often (far) below either of these breaking limits. For instance, in 15 second waves the wavelength is about 350 meter and with a significant wave height of 15 meter the steepness is about 1/23. So you will need besides the wave height an independent measure of wave period, in order to get the wavelength. I know that the KNMI wave atlas, for instance, also gives global information on the mean wave period, and NOAA will probably do the same.
 * Unless you are interested to capture wind energy at the same time, knowledge of the wave potential energy (½ &rho; g times the variance of the vertical water surface excursions) is enough to derive the wave power, since the mean kinematic energy of the waves is equal to the mean potential energy.
 * Lecture notes on ship hydrodynamics and ocean waves can for instance be found at http://www.shipmotions.nl/LectureNotes.html. Crowsnest (talk) 21:24, 27 March 2008 (UTC)

Thank you. It is beginning to sound like the model for the Article's equations (and elsewhere) is: 1) sinusoidal, 2) KE = PE, and 3) wavelength = 2pi x (peak amplitude). KE = PE makes (triangulated) tethered buoys more attractive, again. KE = PE might be an equilibrium condition to maintain sinusoidal waveform, else energy will be dissipated into turbulence and heat.  That is also the partioning of kinetic and potential energy for the Bohr atom and orbital mechanics, as opposed to the KE and PE exchange in a pendulum over time.  However, the time average for a pendulum would still have KE = PE.100TWdoug (talk) 22:17, 27 March 2008 (UTC) More on stability: http://www.cs.cmu.edu/~cfr/talks/2003-Jul-9.ppt 100TWdoug (talk) 02:41, 28 March 2008 (UTC)


 * Deep water waves are quite linear, especially in the Lagrangian frame of reference, moving with a fluid particle. When looking at the mean statistics, like mean wave KE and PE averaged over a random sea state, nonlinear contributions in general tend to average out, driving KE and PE closer together than in a simple periodic motion (like that of a pendulum). Crowsnest (talk) 10:08, 28 March 2008 (UTC)
 * Wavelength is not 2&pi; x (peak amplitude), see wave power. Crowsnest (talk) 10:10, 28 March 2008 (UTC)

Thanks again. In addition to: is there another fundamental equation I need to use to start a derivation of the energetics in wave power so as to match the current equations in the Article? 100TWdoug (talk) 12:05, 28 March 2008 (UTC)
 * Definition of sine wave
 * KE = mv^2/2
 * PE = mgh
 * KE = PE


 * Now you mentioned pendulums, see: Crowsnest (talk) 12:15, 28 March 2008 (UTC)
 * For a sine wave of vertical amplitude a at the free surface: KE = PE = ¼ ρ g a², so E = ½ ρ g a², and the wave height is H = 2 a. Remember water is a continuum consisting of many particles. KE and PE are phase averaged over a wave length, or wave period. See wave power or any textbook on water waves, for instance around page 97 of . Crowsnest (talk) 12:25, 28 March 2008 (UTC)

OK, that will keep me busy! Is there any chance you use either Mathematica or SAGE? I am thinking that we might want to eventually load wave power equations into code that will run in an on-line calculator. WebMathematica requires more licenses from Wolfram, but a local server can run a Mathematica kernel within public domain SAGE with output in MS Internet Explorer and other browsers. The other possibility is for me to fully port from Mathematica to SAGE and beg or buy some SAGE hosting on a server. Tell me when I have bugged you to death, but my questions should help clarify your editing of the Article as you see where I stumble. 100TWdoug (talk) 12:51, 28 March 2008 (UTC)

I think this is the answer to my question (struck): http://en.wikipedia.org/wiki/Shallow_water_equations 50MWdoug (talk) 14:47, 29 March 2008 (UTC) That graphic is an animated gif, which can be made in Mathematica and played on any browser, including (within) a WP Article, but it is huge - 7MB.50MWdoug (talk) 14:58, 29 March 2008 (UTC)

The Virial_theorem would have KE / PE = -1/2 as in orbital mechanics, Bohr atom, etc. Why is KE = PE for waves?50MWdoug (talk) 00:15, 30 March 2008 (UTC)

Also, see pendulum wave machine: http://pirt.asu.edu/news%20Pendulum%20Wave.asp 50MWdoug (talk) 00:50, 30 March 2008 (UTC) Also, http://books.google.com/books?id=phbQAHHjyEUC&pg=PA5&lpg=PA5&dq=pendulum+virial&source=web&ots=SyGPGSJnQp&sig=aW_tAQ7nYpMce4yskPaYX9XsYx0&hl=en#PPT1,M1 50MWdoug (talk) 00:58, 30 March 2008 (UTC)

Nevermind, KE = PE, just like a pendulum, just like the coupled ASU device. Done 50MWdoug (talk) 05:02, 2 April 2008 (UTC)

I have two questions for you that might lead to encyclopedic quality material being entered into Wave Power:

1) Is your animated gif of the floating ball using industry-specific canned software or is it code that can be deposited somewhere?

2) Is there a reference (or can you produce another animated gif) of the same animation with a "ball" of finite and significant volume that takes into account weight, displacement, etc., and the resultant distortion on the wave? Doug Youvan (talk) 18:40, 19 April 2008 (UTC)

Talk:Wave power/Archive 1
Hi- archives should be in the talk space only. You had copied the archive into the article space also- I deleted the article space version, leaving only the talk space one. J Milburn (talk) 12:52, 28 March 2008 (UTC)
 * Oops! Thanks a lot! Crowsnest (talk) 13:00, 28 March 2008 (UTC)

Sorry for not replying sooner: Yes, that’s fine by me; I haven’t archived anything before, so I thought I’d better ask first, but then ran out of time. So thanks for doing that.

As far as the content goes, what is the position on this GEWP business? Is it a hoax? It all sounds far-fetched to me, and even the name is iffy. I thought I’d seen a link to a newsletter piece somewhere about it, but can’t find it now.

And is Nukeh just messing around? I really can't tell.

Moonraker12 (talk) 17:28, 1 April 2008 (UTC)

Lift
I agree my words were cumbersome but I don’t agree that there is no value in demonstrating that Equal Transit Time implies no lift in order to demonstrate the fallacy. If there is lift at ETT then ETT is not a fallacy, it is merely a special case. Rolo Tamasi (talk) 22:15, 28 March 2008 (UTC)
 * I very much value your work on improving the section on the equal transit time fallacy.


 * I have not done a lot of work on it! - The problem is that so many people start building their understanding of lift from it and yet there are many things wrong with it and it developed the thinking down blind alleys. For example it encourages thinking in terms of velocity/pressure relationships. Indeed many believe velocity changes create pressure changes - IMHO an unhelpful conceptual model.


 * I far prefer a conceptual model constructed around pressure changes. Velocity changes are merely secondary effects as an inevitable consequence of pressure gradients.


 * I strongly disagree with a notion of velocity changes being an mere secondary effect of pressure changes, see the recent discussion at Talk:Bernoulli's principle. Fluid flow is in general an interactive balance of forces (including pressure) and momentum changes, and one cannot say that it is only pressure changes that create momentum changes, or the other way around. They interact in a dynamic play of forces and momentum (and in general with other quantities like heat, entropy, etc.). Crowsnest (talk) 08:53, 29 March 2008 (UTC)


 * ETT is a fallacy, just because it only occurs in special cases, which are mostly uninteresting from the point of view of generating enough lift for flight. In the ETT explanation of lift, it is assumed that there is always ETT, i.e. fluid particles splitting at the stagnation point join at the TE, which is wrong.
 * Further, there may be lift at zero angles of attach, see e.g. the second figure at Angle of attack.


 * This is just a feature of the decision to calculate angle of attack from the chord line (which is itself a poorly defined line - for example consider a section where the radius of the leading edge were to be the chord). From an aerodynamics perspective the chord line is an arbitrary line a non-symmetrical cross section. The angles calculated from it are also arbitrary. It is therefore unhelpful when describing aerodynamic principles.Rolo Tamasi (talk) 00:15, 29 March 2008 (UTC)


 * You are free to define what you call the zero angle, and therefor the zero angle-of-attack (AOT), in different ways. One definition could be that there is no lift at zero angle of attack. But that would, in general, be a very cumbersome one, since then which direction is the zero AOT will depend on the Mach number, apart from low Mach-number flow. So that is not commonly done, and Wikipedia is an encyclopedia, adhering to common notions in this respect. It is not for posing original research. So in general there is lift at zero AOT. Crowsnest (talk) 08:53, 29 March 2008 (UTC)


 * There can be lift at ETT, since the pressure depends nonlinearly on the velocity. For non-symmetric airfoils, in a situation of ETT the lengths from stagnation point to trailing edge may differ above and below. In that case it is easy to see that there will be a non-zero lift, due to the different mean velocities.
 * But even in the case of ETT and equal lengths above and below, for an asymmetrical airfoil there may be lift, because the fact that by Bernoulli's theorem the pressure changes are proportional to the velocity squared. And the mean of the squared velocities will in general be different. e.g. the pairs (2;4) and (1;5) have both a mean value of 3, but the mean of their squares is 10 and 13 respectively. Crowsnest (talk) 22:57, 28 March 2008 (UTC)


 * Now there you go - a velocity/pressure model! ;-)
 * How about looking at it this way, in an ETT condition. The upper stream and lower stream are coincident at a point in front of the wing and have equal momentum. The same is true behind the wing. Therefore the two streams may have a different profile of momentum changes but must have exactly the same net total momentum change between these two points. Therefore the net forces applied to the wing must also cancel out.
 * Not the most eloquent explanation of the principle but I hope, in my rush, it is adequate.Rolo Tamasi (talk) 00:15, 29 March 2008 (UTC)


 * Well yes, I think it is unavoidable to think in terms of a velocity/pressure model, when looking at forces (pressure, lift) in fluid dynamics (having velocities/accelerations).
 * In the case of ETT, the momentum of the two halves of a fluid parcel splitting just before (position A), and rejoining just after (position B), the airfoil are the same. Therefor, the momentum changes from A to B of these parcels are the same, and the work in between A and B by pressure forcing will have been the same on them. But this does not say anything about lift, since that can only be derived by determining the momentum balance of the system, not from that of two individual particles at two individual locations A and B just before and after the airfoil. Deriving forces from momentum balances is in general a delicate affair, which may easily result in wrong answers when not done properly. That's why people prefer, if possible, to determine the forces directly, either by direct measurement or integration of the pressures. Crowsnest (talk) 08:53, 29 March 2008 (UTC)

Please look at ASU device
I don't think it is coupled, and I can't find a sufficient description. Please delete my last edit to Wave Power if you think this is misleading.50MWdoug (talk) 00:38, 3 April 2008 (UTC)
 * Not misleading, appears to be a nice and well-made demonstration apparatus. But I do not see a connection with wave power from ocean waves. Crowsnest (talk) 06:59, 3 April 2008 (UTC)

Actual forces on a large, heavy, bouyant ball must be simulated?
Is there a way for you to clearly point out in the wave power article that the sinusoidal modeling of deep ocean waves places (all) KE and PE in a plane or a line that is perpendicular to sea level? If I have understood this correctly, very simple language would be helpful to new readers and students. I would suggest using words or a diagram, not equations.50MWdoug (talk) 17:07, 3 April 2008 (UTC) I should add to this question that the overall vector would be a time-average. My concern is that the group velocity knocks the average energy off the normal.50MWdoug (talk) 00:24, 4 April 2008 (UTC) Overall forces and energy harvesting is looking like a hard discrete mathematics calculation involving the motion of a "ball" whose diameter is significant relative to the wavelength of a trochoid, which is actually a "heavy ball" with water displacement that will significantly change the trochoid's shape. Make sense? 50MWdoug (talk) 07:34, 4 April 2008 (UTC)


 * The surface shape of periodic deep-water waves is to good approximation of trochoidal form. While the surface shape is trochoidal, and a trochoidal shape can be produced by a rolling wheel or ball, the particle motion in water waves is different, see e.g. the animations at Stokes drift of (nonlinear) water waves. In a rolling ball, a particle on or in the ball has an average velocity equal to the propagation velocity of the ball. In water waves, particles move in orbital motion, with an average horizontal velocity of only a fraction of the propagation velocity of the waves. The mean momentum is small, it is mainly energy being transferred with the wave. And the average wave energy transport speed is the group velocity. Crowsnest (talk) 08:08, 4 April 2008 (UTC)

George Gabriel Stokes
i'm sorry, but i have very little to offer on that subject. nothing in wikipaedia engenders more warring and vitriol than nomenclature of the nationals of the british isles, and i have learned the hard way to never ever get involved. --emerson7 17:09, 3 April 2008 (UTC)

About removing old discussion topics
Thanks for your comment (about R.e.b.'s "List of nonlinear PDE's).  My contributions have been minor; this is  R.e.b.'s "baby".  I have mainly been making some  (hopefully) "helpful" remarks, and deleting them once they have been acted upon. It is only my own remarks that I have deleted (plus, perhaps, his acknowledgments of having acted on them.) So, I don't think anything is lost, since these are just notes about "work in progress", and removing them when they are no longer applicable helps keep the discussion page relatively clean and unconfused. (It could equally be done via e-mail, of course, but this way others, like yourself, who may be interested in assisting, are made aware of it.) Please check with him. If he prefers to keep these "work in progress" remarks, I have no objections - but I am one of those who prefers to clean up the dishes while cooking the meal. (See  R.e.b.'s (and Bismarck's) comment about "sausages" earlier in the game.) 24.202.238.172 (talk) 17:17, 6 April 2008 (UTC)

Regarding such a work in progress, I am not sure what is common on Wikipedia regarding "cleaning up" the talk page of a "work in progress". Personally I think it may be valuable to keep such notes, so people may see afterwards how and why things are made as they are. If the article is more or less ready, this can be archived, in order to have a cleaned-up talk page. Best regards, Crowsnest (talk) 17:39, 6 April 2008 (UTC)

Not everything that is written is worth "preserving for posterity" (and most certainly, not everything written in Wikipedia discussion pages!). If it is about a correction to some minor error or oversight that has already been attended to I, think it should be up to the author of the remark whether or not they feel it is worth keeping, and one should simply abide by that. I agree that no-one should presume to erase the remarks of another, made in a common "discussion" page, but everyone is entitled to remove their own, when they feel it is no longer relevant. I have chosen not to make my inputs through a "user" account, and hence do not keep anything on the "talk" page that has been automatically assigned to inputs from an account identified simply by an IP address. (And, naturally, these could appear under more than one IP address.) In short, I am not intending any such things "for posterity"; I am only trying to be a bit helpful to the author who has decided to create this article. 24.202.238.172 (talk) 17:54, 6 April 2008 (UTC)


 * Personally I agree with you. But it to me it would be more elegant to do such a one-to-one exchanges in the user-space of the persons involved. Or (a bit less elegant), mark in the top of this "Comments" section that that section is used as scratch/communication space, and regularly cleaned up. Otherwise, other people may have the same feeling as I had: things from the talk page are being removed, which is unusual and in many instances undesirable. Resulting the same type of remarks or questions I made. Crowsnest (talk) 18:23, 6 April 2008 (UTC)

Perhaps you are right. I only placed my comments in the article's "discussion" page, because R.e.b. had made an announcement there of his intentions. I thought it would be a little less imposing if I just place my remarks there, and let him decide whether or not to act upon them, rather than at his personal "talk" page. But perhaps, in future, if I have any more remarks to communicate about this, I will rather place them on his "talk" page. 24.202.238.172 (talk) 18:41, 6 April 2008 (UTC)

Wave Power
Thanks for being kind to me. I like math, but am only at middle under-graduate level. I got A's on the first 2 levels of calculus, but I failed vector. I liked your recent edits, especially putting the simple math first. As an average math guy, I want to help folks understand how pressures work: How liquid-to-liquid pressures are highly constant, how gas-to-gas pressures are highly dissipative, and how ocean waves balance these two. I only wanted the "physical concepts" section to have a simple equation (early on, or in the subsection): after that ... anything. Even though I was wrong, my heart was in the right place. You fixed or reversed my mistake, but more is needed. In the subsection, you must add a "more", a more that only someone like you, i.e. someone that understands the math of hydrodynamics, can do. You must explain how waves work ... and do it in the "Wave power" article, even if that is not the purpose of the article. Anthony717 (talk) 05:58, 10 April 2008 (UTC)

Accession or Acension?
Re the edit on the Euler page, I think you will find that "accession" = "the act of coming to high office or a position of honor or power" and might be preferable to "ascension" in this context. Unless there's a subtle distinction in the Russian monarchy that I'm missing? Radagast3 (talk) 08:31, 15 April 2008 (UTC)
 * In fact, I do not like both terms standing on their own, ascension and accession. You access or ascend to something. How about accession to power, or accession to the throne? Best regards, Crowsnest (talk) 10:42, 15 April 2008 (UTC)

Reversions of identities using Feynman notation
Hello Crowsnest: You reverted these entries in Vector_calculus_identities why?

Point 1: The notation is defined and is useful - why delete it? It's only notation - use it or don't; suit yourself. So why not show how it works and give the reader a choice?

Point 2: Feynman's notation is used; for example, see: "following Feynman, introduce a partial operator &nabla;A in the latter identity (14), where the subscript denotes the quantity to be differentiated. p. 4

Point 3: Quote from Feynman: "Here is our new convention: we show by a subscript, what a differential operator works on; the order has no meaning." etc. The guy introduced it in his undergrad lectures - he must think its useful. Hey, he's a Nobel Prize winner; why trust his judgment, eh?? Reference:

Point 4: They are identities, for Pete's sake. They are here for reference only - no great wisdom attaches. You can prove them yourself, there is no question of validity. It's just to save some time, or provide a little smörgåsbord for browsing when solving a problem. What possible purpose or rationale is there for these reverts??? Brews ohare (talk) 05:46, 23 April 2008 (UTC)
 * My answer is on Talk:Vector calculus identities. Crowsnest (talk) 10:00, 23 April 2008 (UTC)

Froude Number
"Hello, you suggested another form, using the Froude-number squared, but not the left-hand side squared. If you have reasons to do so, then perhaps another symbol than Fr should be used. Otherwise this may be confusing."

Yes. It is totally confusing. I'm not sure what the best course of action would be for the article, since both forms appear in literature, and the form I added is not called 'Fr^2' when it is used. (The preferred equation seems to depend on the field of study.) --Hawryluka (talk) 19:37, 28 April 2008 (UTC)

Archiving old material on Talk pages
Hi Crowsnest. Thank you for your message on my User Talk page about deletion of the article on L.J. Clancy. I tried my hardest to keep the article but in the end there were too many users voting against me! (See Articles for deletion/Laurence Clancy)

On 30 March you did an excellent job of archiving the old material from the page Talk:Bernoulli's principle. I have read some of the information on your User page about archiving, and I must admit it looks a bit daunting. Over the past 6 months I have participated in a debate on the page Talk:Static pressure. The outcome was that I made a comprehensive re-write of the article Static pressure and no-one has challenged it so I imagine it will remain in place for a long time. The debate on the Talk page is now redundant and would be better placed in an archive. Could I interest you in doing some archiving of old material on Talk:Static pressure? All that should remain is the most recent topic titled Incidental advertising. I would be very grateful if you would do that small task. Best regards. Dolphin51 (talk) 11:31, 20 May 2008 (UTC)


 * Sure. I will do that within the next day. Best regards, Crowsnest (talk) 11:55, 20 May 2008 (UTC)


 * Thanks very much Crowsnest! That is a great improvement.  You have inspired me to study archiving so I can do it myself.  I see lots of Talk pages that need to be tidied up by archiving.  Regards.   Dolphin51 (talk) 23:28, 20 May 2008 (UTC)

Overuse of minor
It's not a really big deal, but you appear to be significantly overusing the minor flag with your edits. Many of your edits appear to contribute material, or delete material that is other than a vandalism, yet they are frequently marked as minor. One random example: an entire paragraph appeared, apparently from nowhere.

You might want to look at trying to break this habit- marking none of your edits as minor isn't going to hurt anything, but the converse is slightly frowned upon, arbcom considered it sort of slightly underhand or something and many people's watchlists may be set to ignore minor edits.- (User) WolfKeeper (Talk) 04:57, 22 May 2008 (UTC)


 * Thanks for pointing out. I was not well aware of the function of the minor flag. I will keep this in mind. — Crowsnest (talk) 07:16, 22 May 2008 (UTC)

A Barnstar

 * Thanks! Best regards, Crowsnest (talk) 16:36, 23 May 2008 (UTC)

On 23 May 2008 I awarded you the Original Barnstar, and placed it here on your Talk page. I have now transferred it to your User page. That seems to be the best place for Barnstars.

We have missed your presence on Wikipedia over the past two months. I hope you have been enjoying yourself doing something interesting elsewhere! I look forward to again seeing your User name regularly on my Watchlist. Dolphin51 (talk) 01:53, 30 January 2009 (UTC)

Bavarian B V
Hi crowsnest. Thanks, I am slowly learning the conventions. I am new to wikipedia authoring, but enjoying it. I am translating German railway info from de.wikipedia, primarily Bavarian, but also DRG and one or two other minor pages. I am trying to encourage the Railways of Germany forum on the Internet (and others) to contribute. [User:Bermicourt] Bermicourt (talk) 16:06, 28 June 2008 (UTC)

Linear stability analysis, Rayleigh-Taylor
Dear Crowsnest. Many thanks for your suggestions concerning my Rayleigh-Taylor edit. I would be very happy if you would alter the article a bit as you suggested, so that the salient points and final formula can be accessed without trawling through the analysis.

As you suggested, it will be good to retain the analysis in some form. I believe this analysis is important for pedagogical reasons, since it is the simplest two-phase stability analysis possible, and demonstrates some of the techniques needed for more complicated analyses.

I will keep an eye on the article as you change it, and make suggestions. This will also help me to write better mathematical articles in the future.Onaraighl (talk) 09:04, 30 June 2008 (UTC)

You have done a good job. Thanks. This will be a helpful template if I produce more mathematics articles.Onaraighl (talk) 09:08, 2 July 2008 (UTC)

Double redirects
You have been changing wikilinks that lead to redirect pages, while putting double redirect in the edit summaries, like for instance here. For your information, double redirects are redirect pages that redirect to another redirect page. For example, at this moment Plant disease redirects to‎ Plant Pathology redirects to‎ Plant pathology; if you click on Plant disease you don't end up on Plant pathology since in serving up pages the Wikimedia software never does more than a single step of following redirects. This undesirable situation won't persist for long, as bots are working all the time to fix such double redirects, so normally one need not bother about them – although it is harmless to fix them and, obviously, one should try to avoid introducing them.

The changes you have been making, as far as I've seen, are not to double redirects but to wikilinks leading to "simple" redirects. Here the basic rule is: do not "fix" links to redirects that are not broken. In the diff linked to above, the redirect was actually specifically to a section on the target page and superior to the "fixed" wikilink. --Lambiam 08:06, 2 July 2008 (UTC)
 * Thanks for the clarification and pointing this out. That also saves me work :). -- Crowsnest (talk) 08:11, 2 July 2008 (UTC)

H.P. Lovecraft
Hello,

I would have sent you a private message about this, but I'm not sure how.

You reversed my changes and suggested I use the sandbox; that makes me think I violated a style convention for wikipedia when I edited that article, but you didn't go into detail.

Anyway, I posted the changes I want in the talk page for H.P. Lovecraft, and hopefully someone else will make this change for me.

Thanks,

--75.169.135.154 (talk) 20:47, 7 July 2008 (UTC)

Wave power equations
I few months back I had a (text) discussion with you on the Wave power article. I deferred to your better physics knowledge, and you added a version of the simplified equation were k was twice what I had (notwithstanding my confusion over height and amplitude). Recently you changed it back to P~0.5h2t(kw). Are you sure it's correct now? The solution of the problem using standard seawater density and standard gravity gives <((1025*(9.80665^2))/1000)/(64*pi)=0.490270057>, which is kind of how I figured it out or verified it a year or so ago. The long equation (pg2h2t/64*pi) uses significant wave height, which seems somewhat inexact. Is the equation perfectly exact for perfect waves (i.e. waves that never change in height or period)? The source literature seems very confusing to me. [2nd edit] So, anyway ... you or one of the other editors needs to add some more math (on formula derivation) to the physical concepts section - so that the reader can understand the connection between wave energy (inc. linear wave theory) and wave power (flux). Anthony717 (talk) 00:15, 20 July 2008 (UTC)


 * Yes, we certainly need a good reference for this. I will look further for that. You were right in the first place, and I was wrong, about the value of the numerical coefficient. The confusion about the factor two is coming from the relation between wave energy density E and wave height:
 * For regular waves it is E=$1/8$ ρgH2, which is "exact" within the framework of linear wave theory (where H is twice the wave amplitude a).
 * For random seas it is E=$1/16$ ρgHm02, also "exact" according to linear theory.
 * In both cases, see below, the energy density per unit horizontal area is equal to E=ρgσ2, with σ the standard deviation of the free surface elevation η. This corresponds with E being twice the potential energy density U per unit horizontal area.
 * For random seas, the significant wave height Hm0 is defined as 4 times the standard deviation σ of the free surface elevation η. The potential energy density U of the waves is, since it is the mean value of ½ ρgη2, exactly equal to U=½ ρgσ2, so in terms of Hm0 the potential energy density is U=$1/32$ ρgHm02.
 * Now by the virial theorem, since U is the mean value of ρgηn with n=2, for random waves the kinetic energy K per unit horizontal area is equal to the potential energy: K=U, which gives E=K+U=2U=$1/16$ ρgHm02.
 * In sinusoidal waves, the standard deviation of the surface elevation η=a sinθ is equal to σ=a/&radic;2, so there H=2a=(2&radic;2)σ, giving rise to the factor 2 difference in E (also in sinusoidal waves K=U).
 * While the significant wave height H1/3 determined in the time domain is only approximately equal to 4σ, the significant wave height as determined in the frequency domain Hm0 is exactly equal to Hm0=4&radic;m0=4σ by definition.
 * For the rest, 0.5 is a good enough approximation, given all the uncertainties in measurements, statistics of random data and possible non-linear effects on the group velocity cg. Crowsnest (talk) 14:16, 20 July 2008 (UTC)

Use of Boldface
I just noticed the BOLDFACE guidelines. So I have a question: What it is the best way to emphasize a term within an article. I have been using boldface for highlighting or emphasizing terms, but perhaps italics is better. What do you think? Sanpaz (talk) 00:03, 31 July 2008 (UTC)


 * According to these guidelines, WP:MOSBOLD, as I understand it italics is better. Boldface seems to be reserved for the article name and synonyms/acronyms (as from e.g. redirects) in the lead section of an article, apart from a few exceptions mentioned in the guideline.
 * Perhaps some additional attention can be gained in this case by using "em dashes", e.g.


 * "... expressed as a continuum by assuming that — at any configuration, or geometrical state of the body — there is a region ..."


 * Best regards, Crowsnest (talk) 00:20, 31 July 2008 (UTC)


 * Thanks for adding the references to the article! Crowsnest (talk) 00:23, 31 July 2008 (UTC)

HighThrust section in Bernoulli Principle discussion
I updated my comments on this. I'll reiterate the main point here.

little change in air speed upstream and downstream in the immedicate vicinty of the virtual disc formed by a propellor - Implied by this statement: The airspeed through the propeller disk is simply the average of the free stream and exit velocities. from this web page: propeller_analysis. Note the statement isn't "The average airspeed through the propeller disk...".

What the article implies is that speed stays about the same while pressure jumps significantly for air flowing through a propeller's virtual disk. I remember a similar statement about the rotor wash of a helicopter model in a hover (high enough to be out of ground effect), in a radio control magazine, but I don't keep my old magazines.

I also removed the comment about pressure jumps near 1 atm and renamed it to high pressure jumps, since it was unrelated to the main statement about air speeds not changing significantly in the immediate vicinity of a propeller, combined with the fact that 1 atm pressure jumps are virtually non existant. I renamed it to high pressure jumps, but it could just be deleted. The source of confusion was on my part. I was thinking of greater than 1 psi jumps on some electric ducted fans (turbine) for radio control aircraft (19 lbs of thrust from a 120 mm fan, 17.5 in^2 area, or about 1.08 psi jump), while at the stame time I was thinking of shock waves (peak pressure greater than 2 atm, trough pressure can't be negative), while reading articles about db (about 191db, sound waves become shock waves), and I confused psi with atm.

Jeffareid (talk) 21:47, 6 September 2008 (UTC)


 * As far as I can see, the NASA article states, see section "Simple Momentum Theory", 3rd equation, that the trust is proportional to the velocity difference. So over larger distances (between free-stream and exit) there are substantial velocity differences. At this related page they use Bernoulli's equation in a rather crude sense to estimate the pressure difference over the propellor or rotor disc (neglecting velocity changes going from further away to near the rotor disc; the page states it is approximate).
 * Two equations for the thrust, one from a momentum balance and the other from BP, are used to solve for the velocity component perpendicular to the rotor disc and at the rotor disc. It turns out to be approximately the average of the free stream and exit velocities. But that does not mean that the free stream and exit velocities are about the same: the average of 1 and 9 is 5, but that does not men that 1 or 9 is close to 5.
 * The 2nd page clearly states Bernoulli's principle cannot be used -- in this simple approximation, only considering the velocity component normal to the rotor disc, and not the other two components of the velocity vector -- going through the disc.
 * This overall momentum consideration (relating far-away free-stream and exit velocities, and disc-averaged) is very different from what "Real world application: is talking about, which is about how an individual blade (using BP on the near-blade velocities) generates lift. -- Crowsnest (talk) 21:23, 8 September 2008 (UTC)


 * only considering the velocity component normal to the rotor disc. The article doesn't make it clear what Ve is, but Ve should include any angular velocity or radial component of flow as well as the thrust component. Spiral propwash is one component of p factor, so it's not like the angular velocity (spiral flow) is some little known aspect of propellers. Jeffareid (talk) 19:16, 13 September 2008 (UTC)


 * I am afraid I can not help you in this. It is not clear to me either where Ve is to be taken. It must be related to some rules of thumb for propeller and rotor design, or it is used in windtunnel tests to determine the trust by a momentum balance of the flow. But these are just guesses. I don't know.
 * In the momentum balance perpendicular to the rotor disc, the contributions of the angular and radial contributions average out over the rotor disc, or the corresponding discs at inflow and outflow. So Ve is just and only the velocity component normal to the rotor disc. -- Crowsnest (talk) 18:31, 15 September 2008 (UTC)

Alternate physical description of lift
I posted what I consider a reasonably simple explanation lift at the end of the lift force discussion page. This can give you an idea on where my thoughts are regarding the Bernoulli principle discussion. My concern is the fact that with a wing, rotor, propeller, or even a bus (drag only), there's always some non-Bernoulli like mechanical interaction between any solid and air flow. The interaction may closely resemble Bernoulli, but that's not the same as being a Bernoulli reaction. Even Newton's 2nd law only holds if you take into account that at the molecular level, some of that acceleration is going to wind up as a slight increase in temperature. At least Newton's 3rd law always holds.

As titled, it's just an alternate description, and I'm not proposing it be added to the main article.

Alternate physical description of lift

Jeffareid (talk) 18:58, 13 September 2008 (UTC)

Birkhoff quotes
Hi. I noticed your recent comment on the talk page for the d'Alembert paradox article. Amazing. I wouldn't have expected such a clear turnabout by Birkhoff, given the emphasis on his quotes in the first place. It raises more questions in a way. --C S (talk) 17:12, 21 September 2008 (UTC)


 * They (H&J) do not seem to be able to reflect on the possible shortcomings and relevance of their own work. As soon as, on a technical level, arguments were given which were unfavourable with respect to their book and paper, Egbertus changed subject or started asking other questions.
 * The quotes seem to be used to authorize their claims (on d'Alembert paradox, the direction of time, 2nd law of thermodynamics, Millennium prize of the Clay Institute, etc.) -- all mostly based on the results with their numerical "regularized Euler" code. The Stewartson quote, which is in support of the scientific consensus, was also taken out of its context. The same seems to be true for the Birkhoff quotes. The main emphasis in the Birkhoff book is (from the start of his preface in "Hydrodynamics. A study in logic, fact, and similitude", 1960, 2nd revised ed.):
 * "The present book is largely devoted to two special aspects of fluid mechanics: the complicated logical relation between theory and experiment, and applications of symmetry concepts. The latter constitute “group theory” in the mathematical sense.
 * The relation between theory and experiment is introduced in Chapters I and II by numerous “paradoxes,” in which plausible reasoning has led to incorrect results..."
 * In chapter I (one) he gives paradoxes for the Euler equations and potential flow (zero viscosity), and in Chapter II for viscous flows. Showing that also in viscous flows paradoxes arise due to the use of seemingly "plausible intuitive hypotheses" added to the mathematical problem description in order to be able to obtain results. So the 1st Birkhoff quote seems also to be taken out of its context, since Birkhoff's focus (in the 1960 version) is not on the resolution of the mentioned paradoxes. He just uses the paradoxes to show that you can get paradoxes starting from seemingly reasonable hypotheses, and that adding viscosity is not the panacea to prevent the occurrence of paradoxes. "The deeper root" in the quote refers to the hypotheses used in the mathematical model formulation, and not to a "clear doubt in their official resolutions".
 * H&J now use the same strategy with respect to out-of-context quotes in their Knol with respect to Wikipedia. -- Crowsnest (talk) 21:57, 22 September 2008 (UTC)

Howell Peregrine
Thank you for improving the article. -- Adipro (talk) 02:04, 23 September 2008 (UTC)

Glowworm
Hi. I blocked both edit-warrers, User:86.0.92.239 and 62.40.36.14. Thanks, Black Kite 16:43, 1 October 2008 (UTC)

For your work on d'Alembert's Paradox and related articles...
I don't know how you feel about these things (and I never hand them out) but...

MohidLand hydrology model
I do not understand why you taged MohidLand hydrology model into unclear. Can you explain main reasons for that? chambelpc
 * I found it very difficult to decipher what was meant, while being quite knowledgeable in fluid dynamics and groundwater flow. Reading it over and over again, it seems correct and I now understand what is meant, but it is very inaccessible for someone not being an expert on the field. The information density is very,very terse.
 * For instance: "… Watershed mathematical model or hydrological transport model designed to simulate drainage basin and aquifer …" is hardly a sentence and seems unconnected to the preceding and following sentences. So I think the article needs expansion and rewriting to become accessible (see e.g. Make technical articles accessible).
 * At first, when looking for MohidLand in Google and Google Scholar, I hardly found any references, thinking it was even not notable (sorry about messing around with starting an AfD procedure). Only after looking for "MOHID Land", I found a lot of (scholarly) references. Moving the article to another name, e.g. MOHID Land in accordance what is said in the 1st sentence, would also help. Best regards, Crowsnest (talk) 10:19, 30 October 2008 (UTC)

Thanks a lot for your contribution! Chambelpc

Group velocity; File:Wave group.gif
Hi! Your made and up loaded "File:Wave group.gif" (and "File:Wave phase.gif") is very attractive. Could you revise and up load File:Wave group.gif three waves with Wave A and B, and A+B (showing group velocity)having phase velocity red dot on approx. 1Hz current wave (A) and yellow dot on 1/13Hz(?) (B), then show "A+B" on top. Also have green dot and Reticle, with vertical bar over (A) and (B), on group velocity point for more understandable descriptive image. Thank you.--Namazu-tron (talk) 07:31, 23 December 2008 (UTC)

Betz - Analogy to Wind Power?
Crowsnest, I see you have a good math and fluid dynamics background. Could you take a look at http://en.wikipedia.org/wiki/Talk:Wind_turbine#Article_needs_Wind-Harvesting_Efficiency_Examples and see if you can properly phrase or understand the question I posted - in the interest of finding a reference or two? Navuoy (talk) 21:39, 3 January 2009 (UTC)