<div dir="ltr">Hi Jiri,<div><br></div><div>Thank you for reporting this bug! You are correct that those calculations should be using the value calculated from "PlasmaState_tau".</div><div><br></div><div>I'm attaching a patch that users can apply to the FLASH4.7 source code. Copy it into the FLASH4.7 directory (same level as source & RELEASE-NOTES) and run</div><div><br></div><div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-style:solid;border-left-color:rgb(204,204,204);padding-left:1ex">patch -p0 < XMHDdt_Patch.diff </blockquote><br clear="all"><div><div dir="ltr" data-smartmail="gmail_signature"><div dir="ltr"><div dir="ltr" style="color:rgb(0,0,0);font-family:Helvetica;font-size:12px">*************************************************************************</div><div dir="ltr" style="color:rgb(0,0,0);font-family:Helvetica;font-size:12px">Adam Reyes<div><img src="https://flash.rochester.edu/site/common/images/email_signature/FLASH.jpg"><br></div><div>Code Group Leader, Flash Center for Computational Science <span> </span><br></div><div>Research Scientist, Dept. of Physics and Astronomy</div><div>University of Rochester</div><div>River Campus: Bausch and Lomb Hall, 369 <span> </span><br></div><div>500 Wilson Blvd. PO Box 270171, Rochester, NY 14627</div><div dir="ltr">Email<span> </span><a href="mailto:adam.reyes@rochester.edu" target="_blank">adam.reyes@rochester.edu</a></div><div>Web<span> </span><a href="https://flash.rochester.edu/" target="_blank">https://flash.rochester.edu</a><br></div><div> (he / him / his)</div><div><img src="https://flash.rochester.edu/site/common/images/email_signature/FLASH-pride-sml.png"><br></div>*************************************************************************</div></div></div></div></div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Tue, Apr 11, 2023 at 11:11 AM Loffelmann, Jiri <<a href="mailto:loffejir@fjfi.cvut.cz" target="_blank">loffejir@fjfi.cvut.cz</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-style:solid;border-left-color:rgb(204,204,204);padding-left:1ex"><div>
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Hello FLASH users,</div>
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I discovered what seems to be an implementation bug in the new FLASH version 4.7. </div>
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It concerns the computation of flux velocities and corresponding timesteps in the Extended MHD model. This is carried out in the function "hy_uhd_computeDtExtMHD.F90" inside the folder ".../source/physics/Hydro/HydroMain/unsplit".</div>
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The bug is on line 191, where an uninitialised variable 'tauCent'<span style="background-color:rgb(255,255,255);display:inline"><span> </span>is used<span> </span></span>for
the computation of the magnetization 'chi'. This results in the chi being zero. The same problem is on line 204 for the computation of 'etatau', which also ends up always being zero.</div>
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Further use of the variables 'tauCent', 'chi' or 'etatau' in the following computations results in wrong values (usually zero) of some of the flux velocities (Nernst, Hall, Resistive, ..) and incorrect computation of the time step limitation. Finally, this
can lead to instability during the simulation <span style="background-color:rgb(255,255,255);display:inline">
for some problems</span>.</div>
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I used a quick fix that corrected the computation of the velocities and erased the instability in my simulation: </div>
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Insert line "tauCent = tau" after line 189.</div>
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I am happy to provide further details and the setup of the simulation where the instability appears. But I believe that the problem in the implementation should be obvious.</div>
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Regards,</div>
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Jiří Löffelmann</div>
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