[FLASH-USERS] Several questions about the Spitzer resistivity used in Flash

[Hansen, Eddie]

Hello,

Yes, the SpitzerHighZ magnetic resistivity implementation is intended for ionized plasmas. It will still work for unionized, room temperature gases, but you should keep in mind that the values may not be physically realistic.

A perpendicular and parallel component for resistivity are calculated for anisotropic magnetic diffusion, however anisotropic magnetic diffusion is not yet fully implemented. So the code will use one or the other based on the runtime parameter resistivityForm (the default value is “perpendicular”). You can use “parallel” if you like, but the SpitzerHighZ implementation is hard-coded to only be accurate for strongly magnetized hydrogen.

What FLASH calls magnetic resistivity is actually, by default, the magnetic diffusivity in cm^2/s, when the unitSystem is unchanged from its default value of “none”. This is the recommended mode of operation as we cannot guarantee that non-ideal MHD terms are implemented for other unit systems.

Hope that answers your questions!

--
Eddie Hansen
Applications Group Leader
Flash Center for Computational Science


From: flash-users <flash-users-bounces at flash.rochester.edu> on behalf of lizy <3287940670 at qq.com>
Date: Monday, October 14, 2024 at 9:05 AM
To: flash-users <flash-users at flash.rochester.edu>
Subject: [FLASH-USERS] Several questions about the Spitzer resistivity used in Flash
Dear Flash developer, hello
I have some questions about the Spitzer resistivity used in Flash, and I sincerely hope to receive your answers.
1. Should Spitzer resistivity be applicable to completely ionized substances? So, is it unreasonable to study a metal that is driven from room temperature to a fully ionized state (such as in zpinch)? After all, the temperature in the early stage was too low, and the ionization level was very low.
2. The Spitzer resistivity expression I found is: η=1.03 × e − 4 Zbar ln ll/tele_eV * * 1.5 (ohm * m)
And you provided two resistors here, the original text in the program is as follows:
resPerpLoc = 8.21876126127e5*zbar*ll/tele_eV**1.5  !! In CGS -- Here tele has to be in eV
resPar = resPerpLoc/1.96
And there is the following annotation: This formula is only valid when the magnetic field is strong (and only for hydrogen).
2.1. Are the two resistivity values you provided the components of vertical current and horizontal current resistivity? If I want to calculate the  Isotropic aluminum , should I only consider resPar? When  the unit system is converted, is the conversion parameter in the flash documentation also the ratio between the parameter to be converted and resPar?
2.2. As mentioned above, can the resistivity formula you provided only calculate hydrogen? Are the coefficients in front of the Spitzer resistivity different for different substances?
2.3. The issue of unit system conversion. What I found is an MKS unit system, while the one provided in Flash should be in the form of UnitSystem="none", right? How does the coefficient not match the parameters provided in the documentation? not be 4π/c^2,,or 1/μ0？？？

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