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<p class="MsoNormal"><span style="font-size:11.0pt">This could be a result of how the fast Alfven speed is being calculated. We have seen that in rare cases this form can lead to non-positive dt (in Hydro_computeDt):<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">cfx2 = .5*((sndspd2+b2)+sqrt((sndspd2+b2)**2-4.*sndspd2*bbx2))<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">We believe this is due to rounding error giving a very small but negative value inside the sqrt(). An equivalent form, but one that guarantees a non-negative value is:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">cfx2 = .5*((sndspd2+b2)+sqrt((sndspd2-b2)*(sndspd2-b2)+4.*sndspd2*(bby2+bbz2)))<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">This is around line 267 in Hydro_computeDt.F90. Your problem might be resolved if you comment out the old form and incorporate this new form (with analogous equations for cfy2 and cfz2).<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;color:black">--</span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Arial",sans-serif;color:#181A1B">Eddie Hansen</span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Arial",sans-serif;color:#181A1B">Research Scientist</span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Arial",sans-serif;color:#181A1B">Flash Center for Computational Science</span><span style="color:black"><o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt"><b><span style="font-size:12.0pt;color:black">From:
</span></b><span style="font-size:12.0pt;color:black">flash-users <flash-users-bounces@flash.rochester.edu> on behalf of Sheth, Nishka <phc027@usask.ca><br>
<b>Date: </b>Friday, June 30, 2023 at 7:24 PM<br>
<b>To: </b>flash-users@flash.rochester.edu <flash-users@flash.rochester.edu><br>
<b>Subject: </b>[FLASH-USERS] Need Help with Non-positive/Zero dt in MHD rz Simulation.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;color:black">Hello Flash users, <o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;color:black"><br>
I am currently trying to simulate an MHD flux tube using FLASH 4.6.2. The simulation is set up in a 2D rz cylindrical geometry, with initial conditions consisting of uniform density and pressure. I have introduced magnetic field components, B_r(r,z) and B_z(r,z),
while the velocity field has not been incorporated yet.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;color:black"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;color:black">I am facing an issue of dt getting non positive/ zero after few steps. I want to simulate my problem till t = 20 while I am receiving this error at around t = 1e-02.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;color:black"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span class="contentpasted2"><span style="font-size:12.0pt;color:black">I have made attempts to address the problem by adjusting the CFL number (like 0.1, 0.5, 0.8), simulating in the normalized units and experimenting with different Riemann
solvers, but the issue remains unresolved. I am seeking guidance on how to overcome this error.</span></span><span style="font-size:12.0pt;color:black"><o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;color:black"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;color:black">Furthermore, I have observed that this error consistently arises when I include the B_z field in my simulations, regardless of the magnetic field profile. Could anyone offer suggestions or insights
on important considerations when incorporating the B_z field?<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;color:black"><o:p> </o:p></span></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt"><span style="font-size:12.0pt;color:black">I would greatly appreciate any assistance in resolving these challenges.<br>
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Thank you in advance. <o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;color:black"><br>
<span class="contentpasted4"><span style="background:white">Nishka Sheth </span></span><br>
<span class="contentpasted4"><span style="background:white">Physics Graduate Student</span></span><br>
<span class="contentpasted4"><span style="background:white">University of Saskatchewan, Canada</span></span><br>
<span class="contentpasted4"><span style="background:white">phc027@usask.ca</span></span><o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;color:black"><o:p> </o:p></span></p>
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