[FLASH-USERS] Magnetic Resistivity Davies Wen

[Hansen, Eddie]

Hi Markus,

That’s an error, and thank you for finding it. The call to PlasmaState_tau should be before that if statement. This has already been corrected in a newer version of the code (to be released in the near future).

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


From: flash-users <flash-users-bounces at flash.rochester.edu> on behalf of Dr. Markus Nöth <markus.noeth at marvelfusion.com>
Date: Wednesday, February 26, 2025 at 5:15 AM
To: flash-users at flash.rochester.edu <flash-users at flash.rochester.edu>
Subject: [FLASH-USERS] Magnetic Resistivity Davies Wen
Dear all,

I’m running a modified version of the Z Pinch example, still using unsplit hydro etc, but without the electric current and hence with basically zero Magnetic field. For many initial conditions I’m running into trouble. At the second timestep flash complains about temperature and pressure being NaN. I’ve traced the NaNs back to subroutine res_davieswen in the file

source/physics/materialProperties/MagneticResistivity/MagneticResistivityMain/DaviesWen/res_davieswen.F90

which is being called with only the two mandatory arguments solnVec and resPar by the unsplit hydro solver which resides in the file

source/physics/Hydro/HydroMain/unsplit/hy_uhd_unsplitUpdate.F90


With this combination the code for magnetic resistivity declares the variable tau and uses it to initialize the value of resPar, but itself is never initialized (it is only initialized if magnetic resistivity is called with at least three arguments). Then, in many instances, tau is zero after declaration which results in resPar==0 which results in infinite magnetic resistivity. The infinity is then caught by the minimum located towards the end of the implementation of magnetic resistivity and replaced by a very large number. However; this number is so large that an average over two magnetic resistivities of such a size yields infinity again which cause the NaNs in the rest of the code.

So my question is: Is the if clause in the code for initializing tau:

if (present(resPerp) .or. present(resCross)) then
call PlasmaState_tau(ELE_ION, tele, llei, zbar, nion, tau)
omega = res_dw_qele*xmag / (res_dw_mele*res_dw_speedlt)
…
end if

really necessary or useful? If so, should there not be a different initialization of tau in case the body of this if clause is not executed?

Best,
Markus


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Dr. Markus Nöth
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