# Magnetohydrodynamics

• Emanuel Santos asked a question:
Open
How can one estimate the maximum fluid flow velocity inside the forming droplet in GMAW? Or the momentum of the forming droplet?

Dear colleagues,

Is there a way that I can use the value of welding current to estimate the maximum fluid velocity inside the forming droplet in GMAW, other than by performing CFD analysis? I am looking for a way of estimating the magnitude of the fluid flow induced inside the droplet by the Lorentz Force. There could be the a way to estimate the momentum as well of the droplet as well.

Much appreciated,

Emanuel Santos

2
How to calculate the dependence of the tearing mode 'Delta' parameter on the mode frequency?
In tearing-mode theory, how to calculate the dependence of the 'Delta' parameter on the mode frequency in the resistive layer?

I realise it is an old question, but just like to add, that the growth rate depends on the delta prime, physically. As for the functional dependence, it has been covered by the previous answer. I'm curious though, since I have only seen an expression for the growth rate in the slab and cylindrical case, and have not seen any dependence on real frequency. Neither have I encountered a formula for the two fluid case. Therefore, I think there is no such dependence to answer your question.

6
How to find L conditions in ionograms?

F1 layer is categorized into two types:

(1) foF1 and (2) ledge.

The foF1 type is defined when a well-developed cusp is formed between the E and F2 layers, or a distinct transition point is formed between the end of the F1 layer and the beginning of the F2 layer. For the other type, the F1 layer is not fully developed, but a ledge (L-condition) is observed below the F2 layer.

Now, the question is, other than manually inspecting each and every ionogram, is there any less tedious way to find the L-condition?

ram

4
How can I calculate Modified Hausdorff Distance (MHD) for Gray images (JPEG) in MATLAB?

I have gray images (.jpg) of broken characters and I want to compare them using MHD in MATLAB, but I faced error because type of images is JPEG.

The hausdroff distance between two Sets A and B, the ouput obtained using HD is scalar or vecter??

A=0.5

B=[1 2 3 4 5 6]

HD is scalar or vector

3
I have an interest in changing the angular momentum using a gyroscope?

I have an interest in angular momentum; I see a gyroscope as an operating device that exists by coupling rings of angular motion in the x, y, and z, axis’s. If one of those axis’s were to alter their speed of rotation what would occur? I bet no one has ever tried to accomplish that task?

Clint,

>Where does one find the real z axis

I confess, I don't know what you mean.

If I have an object, or ensemble of objects, rotating in any number or combination of planes, the sum angular momentum is still a unique and solitary vector.

(I'm reminded of Heinlein's 'Number of the Beast' story in which the device that was used to hop from parallel universe to universe was a nested set of three orthogonal gyroscopes that were simultaneously rotated - a true macguffin - that is readily dispelled by a simple undergraduate analysis)

4
Physical data for an aqueous solution of cetylpyridinium chloride and sodium salicylate (CPyCl/NaSal) are required. 40: 60 or 60: 40 Any help?

We are planning to use this viscoelastic fluid in our heat transfer problem.  We are not able to get the physical properties of this fluid.  Can anyone help us with linear mixture formula or nonlinear mixture formula to interpolate these values from the individual values of the two components?

We got the required data...how the viscosity varies with relative concentration of cetylpyridinium chloride and sodium salicylate aqueous solution.  It behave like a Maxwellian fluid  and its viscosity varies orders of magnitude.  However, we are not able to get any information regarding the variation of thermal conductivity of this aqueous solution depending on the relative concentration of its constituents.  Without this information we do not know how to predict the variation of Prandtl number.

6
Where can one find a conservative form of the Chew-Goldberger-Low MHD equations?

I wonder if is it possible to bring Chew-Goldberger-Low MHD equations to a conservative form? Conservative form of the system of equations of ideal MHD for plasmas with isotropic pressure is well known and can be found in many textbooks. What is the analog of plasma energy in the Chew-Goldberger-Low MHD (in the MHD theory with two adiabatic invariants)?

Chapter 3 in: Blokhin A., Trakhinin Y. Stability of strong discontinuities in magnetohydrodynamics and electrohydrodynamics. New York: Nova Science Publishers, 2003.      (I attached its pdf)

In Chapter 4 of the above monograph you can also find symmetrization of the CGL equations (they are written in the form of a symmetric hyperbolic system).

3
How to determine the effect of MHD on the temperature profile of a couette flow?

How to determine the effect of MHD on the temperature profile of a couette flow..Please help..Any suggestions are welcomed .Please if possible supply yhe equations.It will be of great help..Thank you

See my PhD Thesys in Researchgate

2
How to calculate the Alfvén wave experimentally ?

How to calculate the Alfvén wave experimentally ?

Thank you very much for help.

14
How can I do 3D rigid image registration for multimodal CT/MRI images?

Could any one suggest me how to do 3D rigid image registration for multimodal CT/MRI images in matlab or other Software? My data formats are .mhd- .raw/.hdr-.img. Thanks in advance for helping me.

Hello All,

I would like to register 2 CT images with different resolution. Could you please suggest a software to do image registration? My data in .raw format.

Thank you.

• G. G. J. Ernst asked a question:
Open
Have blue jets, red sprites, elves above significant explosive volcanic eruption clouds been seen from the International Space Station ?

Hello everyone,

A bit of a forum started about the expectation that blue jets, red sprites and elves should be observable above significant eruption clouds associated with explosive volcanic eruptions. As significant explosive volcanic eruptions, typically those that pose a significant threat to international air traffic each year (VEI 3 events or stronger), with clouds reaching above 10km elevation are numerous, such phenomena above eruption clouds should be common.

Yet it does not seem that they have been reported yet for volcanic clouds.

All the reports since 1994 seem to focus on, and concern exclusively thunderstorm clouds.

However the input from the thunderstorm expert community or atmospheric physicists was minimal.

I am writing to ask those who are familiar with observations made from the International Space Station, or even better those who have been on ISS missions if they may have witnessed blue jets, red sprites and elves above volcanic eruption clouds.

I would be grateful for any reply or suggestions.

With best wishes and kindest regards to all,

Gerald

10
Is there anyone who has worked on pulsatile flow through tube of elliptic cross section and handled the complexity of Mathieu functions?

I am working on pulsatile flow of blood in an artery with slightly elliptic cross section and landed up with Mathieu Functions. The computational complexity of dealing with continued fractions and recurrence relations poses a great threat.  There are some classical work on these lines way back in 1950's but due to poor computational facility at that time, the authors have just provided mathematical solutions without carrying out any computational work to understand the flow characteristics.  Any comments on this? References?

My e-mail maria.bubnyak@gmail.com.

16
Has anyone ever documented blue jets, elves or sprites above volcanic eruption clouds ?

Hello everyone,

I am re-asking a question I have recently asked as I believe I used the wrong keywords to attract the attention of those who may have the answer.

In the early to mid 1990s, my research led me to anticipate that blue jets, elves and sprites should be generated above 10km+ explosive eruption clouds many times per year.

I went to a special session on "blue jets, sprites and elves in the ionosphere above thunderstorms" at AGU San Francisco to ask the above question to experts of this in the atmospheric science community, but no one seemed to have paid attention at the time to the possibility that they may commonly be generated above volcanic eruption clouds.

Much time has flowed by and I wonder if atmospheric scientists may now have detected such ionospheric phenomena about explosive eruption clouds ?

If not, I believe studying them above eruption clouds may advance both understanding of explosive eruption physics and of  the link between these ionospheric phenomena and thunderstorms.

Thanking you in advance for any info or  insights.

Best wishes to everyone,

Gerald

Dear all,

Just to add a little info and to correct mistakes I made in my previous question on the same subject:

Blue jets have been documented (from 1994, with reports at AGU SF) above thunderstorms (up to 20km elevation or so) as to extending to typically 50km elevation, together with upward thunderbolts, ie. they are in the main stratospheric it seems.

Above this, purple tendrils have extended up into red sprites up to about 80-90km elevation (ie. from base of stratosphere into mesosphere and ionosphere).

Around a cluster of red sprites extending lower tendrils annular green elves, much broader in spatial extent have been documented above and around those sprites at 90-100km elevation.

This is for meteorological thunderstorms and typically observations made from airplanes. Some of those phenomena are flash-like or discharge-like in their initiation and when they are discharged this is accompanied by radio acoutic signals that can be heard from a recorder on the plane for instance.

I am still awaiting to hear if anyone may have documented the equivalent phenomena expected above thunderstorm-scale explosive eruptions (which are common) or from larger eruptions ?

Kind regards to all,

Gerald

6
Can anyone help with simple MHD simulation setup in Fluent?

Hello everyone,

I am trying to simulate a simple pipe flow with conductive fluid under the influence of a constant magnetic field.
I went through the MHD module of fluent, but failed to set up the problem correctly.
If anyone has done MHD simulations with CFD solvers, please guide me.

Thanks

Hey Amir

My case is simple and I want to simulate something like flow in a backstep channel where magnetic field is acting on the region near the backstep.
The objective is to see effect of magnetic field on separation.

I am trying to code this in Matlab as of now but if I can run in CFD solvers, I can very easily switch to 3D geometry

Thanks again

3
If the magnetic flux tube is not empty but contains a plasma that provides a pressure, how must that pressure look as a function of radius?

Assume that a cylindrical magnetic flux tube is embedded in a plasma with   P = 108 Pa. Assume that the field is along the z-axis in a cylindrical coordinate system, that the flux tube has a radius of 1 Mm and that there is no plasma inside the flux tube. What does the magnetic field need to be in order for the system to be stable? If now the magnetic flux tube also is twisted so that the field lines go around the flux tube once for every Mm in z, can the flux tube be stable? If the magnetic flux tube is not empty but contains a plasma that provides a pressure, how must that pressure look as a function of radius?

Note that some flux tubes are not in force-free equilibrium.  For example, a flux rope from a coronal mass ejection (a type of solar storm) is not in equilibrium and overexpands relative to the solar wind.  In the interplanetary medium, the dynamics are in some sense dominated more by the initial conditions than by a balance of forces.

1
How can I read a 3d .mhd image?

How can I read a .mhd 3d image? Can you please provide a MATLAB example code.

Hi..

Gud luck...!!!

3
How can I fix the electrical field in Fluent MHD model which does not work appropriately?

I try to model charged nanoparticles moving in two parallel plates with voltage on boundary condition. However, the electrical field setting in MHD model in Fluent does not affect the charged particles at all.

MHD->Magnetohydrodynamics

5
Any suggestions for the boundary conditions of the magnetic field equation in MHD?
There is an equation: dB/dt - curl( v x B) = - curl ( L curl B) for the magnetic field B in MagnetoHydroDynamic domains. For the boundary conditions we have curl B = -mu j (j is the electric current density)

Boundary conditions are obtained by integration of equations over a small domain containing boundary. You can find example of such calculation in available book on electrodynamics. In MHD some small terms are neglected.
Seems you are searching solution with resistive conductors presented somewhere. In this cases it is better to estimate initially whether you conductor thin or thick. If characteristic time of your process is much larger than characteristic skin time of conductor, you need not solve problem on current distribution inside conductor and take it uniform.

3
Does charged particle drift perpendicular to the direction of applied magnetic field which has shear terms as well?
Assuming there is no external electric field exerting on the system.

Where is an online copy of Morozov, Solov'ev 1966? I give details in the discussion at the link below

3
Do you have information on long-lived internal non-resonance kink mode in Tokamak?

NA

This mode attracted attention in the late eighties as a possible explanation for rapid sawtooth crashes but it was shown to saturate in "Nonlinear growth of the quasi-interchange instability" F. L. Waelbroeck, Phys. Fluids B 1, 499-505  (1989). It has recently received renewed attention in the context of the long-lived mode of your question. A good reference is "Saturated ideal modes in advanced tokamak regimes in MAST," Chapman, I. T., et al., Nuclear Fusion 50.4, 045007 (2010).

5
Which is the most accurate technique for solving MHD equations ?
I want to know which technique gives more accurate result for solving MHD equations ?

Let me second Sergey's and Rui's comments on the many ways of solving the MHD equations. Generally many of the same considerations applied to computational fluid dynamics apply equally to solving the MHD equations. For those I recommend one of the many textbooks out there on computational fluid dynamics. It would be best to find one from your specific area of research (e.g., astrophysics, plasma physics)

There is one additional consideration, however, with the MHD equation which is the divergence constraint on the magnetic field. There are generally three ways to enforce the div(B) = 0 constraint - divergence cleaning, constrained transport, and the streamfucntion or vector potential formalism.

In divergence cleaning one solves an additional diffusion equation for div(B) which pulls it back toward zero. The drawback is that diffusing div(B) is unphysical.

Constrained transport uses an additional step of calculation the electric field along each cell face and then calculating B from Faraday's Law. This requires a staggered mesh and can produce some odd results in some applications.

Streamfunction methods write B in terms of it's vector potential, thus requiring div(B) = 0 at all times. The drawback is that one must take additional spatial derivatives.

2
Can anyone advise on electrostatic charge distribution?
I am designing a simple electrostatic simulator see attached diagram, the blue line is the nozzle and the red piece is the electron emitter.
I suppose that the charges will be distributed with the same surface density in the blue line as long it is at the same potential.
The first question is:
What is the simplest way to calculate the charge?
If it is considered as a capacitor, Q=CV, where C is the capacity of the “capacitor” that I have made, but I do not know if it right as long as the plates have finite dimensions and the distance between them is different.
The second question is where to set the ground, because the field would be different. Does it depend on the electrode area?
Here are the test report. I had to modify the classical equation adding a constant to fit with test data at short range
• Source
##### Article: About the Electric field potential and charge distribution in a two plate setup during Pulsotron Plasma Thruster test programme
[Hide abstract] ABSTRACT: June 2014.-Two different conductive squared plates of different sizes both in parallel were installed. A power supply between both plates was also installed. The Electric Field Potential between both plates was measured as a function of the distance between them. It is demonstrated that the potential does not correspond to the classical theory and a new equation that fits with the experimental setup is proposed. The test is performed in the Pulsotron Plasma Thrusters test framework.
Full-text Article · Jun 2014
8
Difference between B,H and M in magnetics
I am somewhat confused with what is B, H and M in magnetism. Can someone explain their equivalence in terms of electrostatics
Bio Savart law gives us B (which I suppose is magnetic field). But I have read in many places H is magnetics field and is defined as and we have relation as B=mu0*H where B is magnetic flux density. This is somewhat confusing.
It would be really helpful if someone may explain the following case for the following terms magnetic field, magnetic field strength magnetic flux density and magnetization. Also how mu and mu0 are related in each aspect

Consider a current carrying conductor in free vacuum. Then at any point Pin free space some B will be felt (I am not giving any name as I am not clear what exactly it is).
Now suppose the space is not free space but some gas which poses some restriction to flow of magnetic field’s lines through it then if UI am correct, the magnetic field at same pointy will decrease. Is that factor of reduction mu (permeability of gas.
The DOI is:

DOI 10.1108/03321641111101212

But it is better that you go to my papers because you can find more things related with your interests.
5
Do you have a formula, algorithm, routine c ++, or another programming language that calculates the focus of one or two magnetic lenses?
I would add it to the next Excel table to upload it to ResearchGate.
• Source
##### Dataset: Useful formula and excel tables for plasma physics, plasma thrusters, fusion and particle accelerators V01
Full-text Dataset · May 2014
My idea is to use two thin lens with different (high) currents to reduce as much as possible the defocusing in the perpendicular and in the axial axle to collide high density ion beams again HLi6 target. It must be impossible to have a little defocusing due dense ions repulsion.
6
What is the best workflow for developing a good cfd code?
I will be more specific here. I am trying to simulate a two dimensional mhd flow through an MPD thruster. I have the euler equations with coupled electric and magnetic field. I prefer using opensource products and I am comfortable with python and fortran. I want to know if a mesh from gmsh or netgen could be directly used for the code. I have written codes only using basic fdm. I haven't even written any complex fvm codes. But I am planning to develop three codes using mhd, DSMC and PIC methods. And I am not an expert in any of these. I am conducting experiments too although I don't need any help in that part. I would really like some expert input on the problem that I have on hand. Expecting some good answers.
Here it is solved the issue on windows installation.

But I heard that openfoam is an "incompressible magnetohydrodynamic equation solver" that works only with non compressed fluids, perhaps it is not the best suite for MHD.
4
What electric isolation is recommended for the Rogowski probe at Pulsotron Z-pinch?
I have to measure ion speed (to be used in direct electricity generation using MHD). I install the probe very close to the plasma ball, the problem is at the end when the plasma ball generates an explosion burning any carbon based coating and breaking any ceramic material nearby.

Actually the Rogowski coil is a copper wire coated by a thin PTFE coating.

What electric isolation would be recommended?
It is right but in such heavy duty place I used needle sensors. I have submited following article:
Z-PINCH HIGH POWER NEEDLE FILTER
Unfortunately I obtain light/no light, no photos of every particle
Here is a photo of the environment, where some megawatts of light are released
• Delara Soltani asked a question:
Open
What is the application of electrohydrodynamics in blood devices and how does it work?
I want to know the EHD flow uses for different kind of blood devices.
8
Are solar/space physics in a mature state where all "easy" problems have been done? Are we reduced merely to reducing data and running models?
Current work in solar physics is dominated by data acquisition, reduction, and numerical modeling. Has the tool between the ears become less important than those catching photons, particles, and those that move electrons around on electrical devices in computers? Is the infrastructure now guiding our research, or are scientific questions guiding the infrastructure we have?

To see how we might make real progress and avoid some potential traps in relying on external tools, it might be useful to compare thoughts on the major achievements, questions that have been answered in solar physics since, say 1900, and how these were achieved. I might start it off by saying that the development of the Saha equation allowed us to begin to understand solar spectra for the first time, in a quantitative fashion, for example.
A gripping discussion. I'm not sure that I can add to it, other than to say that I am sympathetic to Bo's point of view, and have been for years.

Space Physics has a number of fundamental obstacles to scientific profundity (as Bo put it). Space plasmas occupy a middle ground --- they're not particularly hot, not particularly cold, not particularly dense (they are the most tenuous). They are resistant to laboratory simulation (e.g. length scales too large for vacuum chambers). They are difficult to study in situ (satellites are expensive, have very long program duration, and often travel at supersonic speeds through the plasma). Space plasmas are difficult to study remotely -- the most useful radar instrument (incoherent scatter) is quite expensive to build and operate. And of course, space plasmas are difficult to simulate computationally because of the range of mass, interaction length, and interaction times involved.

Yet another challenge is that Space Weather has not risen in the mind of the public to the level of Tropospheric Weather. Everyone knows what it's like to get rain on their picnic, but hardly anyone notices when the Kp index exceeds 7, or when there has been an X30 solar flare.

These are challenging times for space physicists.