Talk:Magnetohydrodynamics

message: Would be great if these could be extended. For instance, in the Waves section, there are diagrams of v_Fast, v_Slow and v_Shear - I suspect that these correspond to the three types of waves in the table but it's not clear which is which, or even if that is indeed the case. As it's not my area I can only kindly ask for someone from the field to improve the description / annotation of the figures. 80.109.126.25 (talk) 12:43, 19 December 2017 (UTC)
replies: text: There is in fact a typo in the legends of those two figures. The legend entries for v_Slow and v_Shear (the pure oblique Alfven wave) should be swapped. The phase velocity of the slow wave is always less than the Alfven wave. See https://www2.mps.mpg.de/solar-system-school/lectures/space_plasma_physics_2007/Lecture_7.pdf for example OpenEyeSignal (talk) 04:41, 17 May 2023 (UTC)
thread_title:  Annotation / Description of figures 

message: Would someone do this calculation? I did it about forty years ago at school and remember the answer as one volt, but when I do it now I get a couple of orders of magnitude less.  However I am now rusty on these things and they have changed to SI units, so someone should recheck my calculation. Assume that the water is flowing at one metre per second and the river is 200 metres wide.  In London the earth’s magnetic field is 55,000 nanotesla and the dip angle is 67 degrees (it is zero at the equator and 90 degrees at the magnetic poles).  You then have to invoke Fleming's right hand contortion to get the direction. JMcC 11:07, 20 June 2006 (UTC)
thread_title: Voltage at Waterloo Bridge in trivia

message: What's the difference? — Omegatron 20:22, 26 June 2006 (UTC)
replies: text: I don't forget gravitoelectrodynamics --Iantresman 23:46, 26 June 2006 (UTC)
replies: text:  That's not really related.  These three are all involved with electrically conducting fluids. — Omegatron 23:56, 26 June 2006 (UTC)
thread_title:  [[Magnetohydrodynamics]], [[Electrohydrodynamics]], and [[Electrokinetics]] 

message: I think a picture without a scale and a legend (what is plotted?) is really not something that should be on wikipedia USferdinand 21:38, 16 January 2007 (UTC)
thread_title: Image on the MHD page

message: I think a form of the ideal MHD equations should be on the MHD page. The references to the single equation pages is confusing. In fact, the momentum equation page does not have the differential equation, and the Ampere's Law page only refers to the integrated form of the equation. I am fine with the Priest equations even though I prefer the divergenceless form of the MHD equations. 
 {\partial \rho \over \partial t} + \nabla \cdot (\rho \mathbf{u}) = 0
 {\partial \rho \mathbf{u} \over \partial t} + \nabla \cdot (\rho \mathbf{u} \mathbf{u} + (p +{B^2 \over 8 \pi} )\mathbf{I} - {\mathbf{B}\mathbf{B} \over 4 \pi}) = \mathbf{F}
 {\partial \mathbf{B} \over \partial t} + \nabla \cdot (\mathbf{u} \mathbf{B}-\mathbf{B}\mathbf{u}) = 0
where  \rho  is density, t is time, B is the magnetic field, u is fluid velocity, and  F is the forces by unit volume (such as  \rho \mathbf{g}  for the gravity).
Also a reference to the divergenceless nature of B should be added. A entry on numerical MHD (8-wave formulation) is, in my opinion, needed. USferdinand 21:54, 16 January 2007 (UTC)
thread_title: Ideal MHD equations

message: I erased the reference to "Magnetohydrodynamics (MHD)":
- I) Its author has poor credentials in the field: a search for ("Petit,JP" MHD) by Google Scholar yields a ten of answers with only 3 citations in all (whereas a same search for the other authors quoted in the list of references typically yields hundreds of citations).
- II) The scope of the referenced article is very specific, limited to shock wave anihilation by lorentz force field.  —Preceding unsigned comment added by 137.138.5.237 (talk) 11:18, 7 August 2008 (UTC)
thread_title: References

message: I looked through the page and I found the following statement at Hall MHD which I could not really understand:
The most important difference is that in the absence of field line breaking, the magnetic field is tied to the electrons and not to the bulk fluid.
I guess someone either wanted to talk about reconnection ("breaking" of field lines) or is talking about the slowing down ("braking" of the field lines). Anyway, can the person who wrote this make a better statement? Tusenfem (talk) 18:21, 5 February 2009 (UTC)
thread_title: Question about the text at Hall MHD

message: Hi, I just noticed footnote number 7 has linkrotted. Not sure what the original paper was being referenced.  —Preceding unsigned comment added by 76.118.178.33 (talk) 01:32, 1 December 2009 (UTC)
replies: text: Fixed it. --Art Carlson (talk) 09:48, 1 December 2009 (UTC)
thread_title: Busted references

message: It is beautiful, but neither cited nor described. Also I suspect it is fundamentally irrelevant as a tutorial guide, since it is an MHD simulation of a collisionless plasma. Maybe such a thing is useful in some practical sense, but it would be wrong to give the impression that such a plasma should be approached in this manner as a matter of principle.  —Preceding unsigned comment added by Hugh Hudson (talk • contribs) 13:49, 21 April 2010 (UTC)
thread_title:  What's the point of Fig. 1? 

message: - The history section needs to go near the top. It will improve the flow of the article. 
- I was planning to make some additions to the geophysics section. 
Please let me know if the changes are alright. I will check back soon.  
ShashankSawant (talk) 16:55, 23 April 2011 (UTC)
thread_title:  I was planning to rearrange some stuff 

message: Ferrofluids have nothing to do with MHD, so it's a bad example of applications for MHD.  — Preceding unsigned comment added by 82.130.106.41 (talk) 15:19, 20 October 2011 (UTC)
thread_title:  Magnetic Drug Targeting 

message: Regarding these two sentences in the "Engineering" section:
   Two widely-reported new applications for MHD drives are the hydrino reactor developed by Dr. Randell Mills of BlackLight Power in Cranbury, N.J., and the Energy Catalyzer, or E-Cat, developed by electrical engineer Andrea Rossi of Miami, Fla. Both of these inventors' devices can produce enormous amounts of heat that theoretically can then be converted to electricity by a MHD drive.
I'm actually hoping that such LENR devices will eventually be shown to be revolutionary energy devices, but the present sentences appear to state, without reservation, that these devices "can produce enormous amounts of heat."   In fact, (as of this writing) such claims are quite controversial.
Also, the two sentences appear just before a sentence regarding coal-powered MHD abrasion problems -- seems out of place to me.
Dfgriggs (talk) 14:50, 26 March 2014 (UTC)
thread_title:  Engineering -- controversial items 

message: Consider:
: ... is the study of the dynamics of electrically conducting fluids. Examples of such fluids include plasmas, liquid metals, and salt water or electrolytes. ...
If the four states of matter are solid, fluid, gas and plasma, why are plasmas considered fluids? --Mortense (talk) 16:16, 9 July 2014 (UTC)
replies: text: The four states of matter  are solid, liquid... --Musaran (talk) 21:16, 23 July 2014 (UTC)

text: The four states of matter are solid, liquid, gas, and plasma. Fluid is a broader concept that encompasses anything that can flow. That usually includes liquid, but in aerodynamics gas is a fluid, in geophysics the solid parts of the earth can behave as fluids over sufficiently long period of time. Zyxwv99 (talk) 21:59, 23 July 2014 (UTC)
thread_title:  How can a plasma be a fluid? 

message: Isn't J just ρ v ? If so, this should be said when J is intrduced. 89.217.22.3 (talk) 16:13, 8 November 2014 (UTC)
thread_title: Define J

message: If  is not directly related to Magnetohydrodynamics why is MHD sensor, which redirects to Magnetohydrodynamics, included on ? Peter Horn User talk 03:04, 24 October 2016 (UTC)
replies: text: MHD sensor is listed as an navigation instrument. Peter Horn User talk 03:20, 24 October 2016 (UTC)
replies: text: Image added for good measure. Peter Horn User talk 11:37, 24 October 2016 (UTC)
thread_title:  MHD sensor 

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thread_title:  External links modified 

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thread_title:  External links modified 

message: Hello all. Going over the Magnetohydrodynamics#Ideal and resistive MHD section, I have noticed that the two provided images do not seem very relevant per MOS:IMAGERELEVANCE. One shows an MHD simulation of the solar wind and the other shows an MHD flow simulation. I fail to see how either are "signficant and relevant in the topic's context", so I will be removing them shortly. I bring this up here, as I might be missing something; however, if I am not seeing the significance and relevance as someone who is familiar with the topic, I do not think a reader would. CoronalMassAffection (talk) 14:15, 31 January 2023 (UTC)
thread_title:  Relevance of images in "Ideal and resistive MHD" section 

message: Hello all. In the lead, MHD is presented as "the study of the magnetic properties and behaviour of electrically conducting fluids" (a statement that is supported in the given source). However, I was under the impression that MHD is just a model of fluid/plasma behavior (like Vlasov and two-fluid), not the study of magnetic properties and behavior of these fluids generally. My experience is mostly limited to MHD as it is used in astrophysics, so maybe it is used differently in other fields? CoronalMassAffection (talk) 16:26, 8 February 2023 (UTC)
replies: text: Update: The statement in question was added unattributed in 2002. It has since changed in wording, but the meaning has not changed. Added later, the first supporting source is WP:CIRCULAR; the source, a book published in 2018, directly copies the lead paragraph. The second source, a 1967 thesis, is the only reference that supports this statement at the moment. CoronalMassAffection (talk) 17:09, 8 February 2023 (UTC)
replies: text: I have found an RS, An Introduction to Magnetohydrodynamics by P. A. Davidson and published in the American Journal of Physics, that apparently introduces MHD as "the study of the interaction of magnetic fields and conducting fluids" according to a review from David J. Griffiths written in 2001. I unfortunately do not have access to this text. CoronalMassAffection (talk) 06:31, 1 August 2023 (UTC)
replies: text: I now have access. From page 1: "Magnetohydrodynamics (MHD for short) is the study of the interaction between magnetic fields and moving, conducting fluids." Regardless, MHD is presented as a model or set of equations almost everywhere else in the literature, so I think we should continue to describe it as such here. CoronalMassAffection (talk) 09:17, 22 May 2024 (UTC)
thread_title:  MHD as "the study of ..." 

message: I think that it is worth pointing out that some authors tend to use the term magnetosonic wave or magnetoacoustic wave to describe only the (fast) modes with wavevector perpendicular to the magnetic field, while others tend to use the term to describe all compressible MHD modes (i.e., both the fast and slow modes at all angles). Currently, some of the article follows the former convention, but I think we should follow the latter (like the Magnetosonic wave article). I will be MOS:BOLD and carry out this change. CoronalMassAffection (talk) 15:32, 31 May 2023 (UTC)
thread_title:  Use of the term ''magnetosonic''