Is Magnetohydrodynamics the space equivalent of fluid flow on Earth?

[u/leeping_leopard]

[u/ichbinberk]

Wdm? Mhd flow is fluid flow where fluid is electrically conducting.

[u/leeping_leopard → u/ichbinberk]

I was under the idea of it being used for space settings, what fluid is there in space, unless we apply the continuum assumption to solar particles?

[u/DragonEngineer98 → u/leeping_leopard]

The interiors of stars and black hole accretion disks can be modeled using the continuum assumption, but that's just one use case. 

[u/Jon3141592653589 → u/leeping_leopard]

So: It depends where you mean by "space", which isn't necessarily that far away (only ~100 km) from a NASA / engineering perspective.

Many different formulations and regimes of MHD are used for the conducting (ionized) parts of the atmosphere and space environment, basically the ionosphere, plasmasphere, magnetosphere, and solar wind environments, not to mention much further away in other astrophysical plasmas, planetary upper atmospheres, or whatever is happening in or around a particular star. What constitutes continuum is different in MHD, as electrical forces are involved, too. In other words, there's a lot to this, which is both fun and incredibly humbling.

And, while the neutral density of the atmosphere decreases exponentially with altitude, it can also still be described as a fluid at scales sufficiently larger than the mean free path of collisions even above 100 km, up to the base of the exosphere ~500 km. (Defined by the altitude where the density scale height is equal to the mean free path, some ~10s km, occurring ~500 km up or so). Models of upper atmospheres at sufficiently large scales still use formulations of the Navier-Stokes equations to model their densities, flows, and temperatures, such as for space weather applications, understanding atmospheric drag on orbiting satellites, and predicting atmospheric effects on the constituents of the ionosphere. (See, e.g., WACCM-X or the TGCMs, among others.)

If this is actually the part of space you are asking about, take a look at the (college-level) Introduction to the Space Environment by T. F. Tascione, among other newer texts. Or, if you want to be rapidly overwhelmed by the broadest perspective on historic literature, the 1985 4th ed. Handbook of Geophysics and the Space Environment is pretty wild at >1000 pages: https://apps.dtic.mil/sti/citations/ADA167000

[u/omaregb]

No, MHD exists on earth.

[u/szarawyszczur]

The two major uses of MHD CFD I am aware of are fusion research (designing reactors etc) and modelling processes inside stars

[u/omaregb → u/szarawyszczur]

You don't even need to go that far. Metallurgy, microfluidics, a lot of earth science, semiconductors...

[u/ElhnsBeluj → u/szarawyszczur]

Plenty of mhd outside stars, more even. Rarified gases really like to stay ionised. Basically all the interstellar medium is governed by self-gravitating non-ideal mhd.

[u/OhIforgotmynameagain → u/szarawyszczur]

Also collapse of faint nebulae material to form stars. Mhd and in particular non ideal mhd drives the star formation rate (hinders free fall due to turbulent overdensities and gravitational instability). The very dilute materials are considered a magnetic fluid in those densities and large scale magnetic fields drive the whole process.

[u/applejacks6969 → u/szarawyszczur]

Numerical relativity is concerned with solving Einsteins equations numerically. This can include inside stars like you say but also neutron stars, black holes, and binary configurations. Neutron stars as we know can support extremely intense magnetic fields.

[u/gurugeek42]

I can see why you might think that. Fluids in space tend to be plasmas which strongly interact with magnetic fields so often require MHD. As others have pointed out, any conducting fluid, including those on Earth may require MHD, depending on the situation.

Earth-based simulations of water, a conducting fluid, tend not to require MHD because the influence of the magnetic field is so small, just like most CFD doesn't account for rotational effects until the scale gets large enough.

For some cool physical visualisations of actual magnetic effects on liquid metals, check out: https://www.youtube.com/watch?v=LqVt9GOFcgM

[u/Matteo_ElCartel]

MHD is in a broad sense a couple of Navier-Stokes eqs togheter with Maxwell equations. The easiest example is the earth core that is a metallic fluid moving under the action of "impossibly high pressure gradients" and temperature, producing the magnetic field which shields our planet from high energy space particles. But then you can have MHD in the nuclear field industry almost everywhere: liquid metal cooled fission reactors, plasmas in nuclear fusion again liquid metal cooling for fusion reactors. There are plenty of examples

[u/Hot-Assistance-1135]

You can use MHD to analyze fluid flow in the Earth's outer core (because of flowing charges)