It can't be a legacy thing, because some form of this condition was originally in underworld, though there was an assumption that diffusivity was unlikely to change which has been strangely difficult to knock out of the code.<div>
<br></div><div><div>One would generally use the local diffusivity and mesh size to give the limiting condition rather than the global extremes.</div><div><br></div><div>L.</div><div><br><br><div id="WISESTAMP_SIG_2938"><span style="font-size: 13.3px; font-family: Verdana, Arial, Helvetica, sans-serif;"><div style="margin: 0 0 8px 0">
<hr>
<table style="width: 100%; border: 1px #FFFFFF solid;">
<tbody>
<tr>
<td style="border: 0px;"><a href="http://picasaweb.google.com/lh/photo/OtKDJTTS09HqGNGw6YrhYQ?authkey=Gv1sRgCMzzrpP69p_ZtQE&feat=embedwebsite"><img src="http://lh3.ggpht.com/_LUlLz13t5HQ/S4yFc5pX0zI/AAAAAAAAAUs/oRgDVK9SjVc/s144/FlatEarthBox-Boundary-Layers.02000.png" alt=""></a></td>
<td style="font-family: arial,sans-serif; font-size: 11px; text-align: right; border: 0px;">Professor LouisMoresi<br> <a href="mailto:louis.moresi@sci.monash.edu.au"> louis.moresi@sci.monash.edu.au<br></a><a title="Go to my public google profile" href="http://www.google.com/profiles/louis.moresi">GoogleProfile</a> |<a title="Link to my public google calendar" href="http://www.google.com/calendar/embed?src=louis.moresi%40gmail.com">GoogleCalendar</a><br>
Mobile phone: +61 4 2850 1907<br><br></td>
</tr>
</tbody>
</table>
<hr></div><img src="http://s.wisestamp.com/pixel.png?p=chrome&v=2.0.3.0&t=1287617538954&u=3755747&e=13" width="1" height="1"></span></div><br><br><div class="gmail_quote">2010/10/21 Laetitia Le Pourhiet <span dir="ltr"><<a href="mailto:laetitia.le_pourhiet@upmc.fr">laetitia.le_pourhiet@upmc.fr</a>></span><br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;">Hi all,<br>
<br>
I was intrigued by all this problems with thermal diffusion...<br>
<br>
I don't understand why this limitation for the time step is not included in gale, I guess it is historical since diffusion is negligible for convection problem and gale is based on underworld but as Charmaine showed, it is not negligible for post-rift simulation.<br>
<br>
Anyway<br>
<br>
I found that FiniteElementContext.c contains a parameter we can all use for the moment to limit the time step without using the very dangerous dt parameter...<br>
<br>
context->maxTimeStepSize = Dictionary_GetDouble_WithDefault( self->dictionary, (Dictionary_Entry_Key)"maxTimeStepSize", 0.0 );<br>
<br>
<br>
So providing that function is really used in Gale, it means that everybody has to compute by hand its diffusion timestep i.e. :<br>
<br>
dt_diff = sqrt(min_length_of_a cell)/max_diffusivity_in_your_model...<br>
<br>
and add<br>
<param name="maxTimeStepSize">dt_diff</param> in your xml ....<br>
to their xml<br>
<br>
I think we should all include that in our simulation because for a 3km mesh and a diffusivity of 1e-6 it makes a timestep of 300ka which is about or a little bit less than what I get with dtfactor=0.5 for a 1cm/year crustal extension problem.<br>
<br>
hope this help<br>
Laetitia<br>
<br>
<br>
<br>
Charmaine Thomas wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div class="im">
Hi Walter,<br>
<br>
I did a series of experiments to get Gale to do pure thermal conductivity problems without having a non-zero velocity boundary applied to the right/left walls, ie velocity=0.<br>
<br>
Firstly I tried turning off the Stokes flow and the uzawa condition, and although everything ran very smoothly and quickly, it took ridiculously big timesteps, despite having an explicitly set 'dt' (I assumed later that Stokes flow has to be turned on for this parameter to kick in?). This predictably resulted in very high temperatures. I had to do this experiment because I was dealing with non-newtonian rheologies, and needed to model a time of quiescence in my crust. <br>
Finally I found the best option was to leave the Stokes/uzawa stuff turned on and to apply either a very low strain-rate, or set the right/left velocities as zero, but also have a right and left wall stress boundary condition. This second option runs smoothly even with non-newtonian rheologies, and takes more reasonably sized timesteps - I could also try changing the dtfactor to speed things up. So does this method sound more reasonable? Is there a better way?<br>
<br>
Cheers,<br>
<br>
Charmaine Thomas<br>
School of Geosciences | University of Sydney<br>
<br>
<br></div><div class="im">
On Wed, Oct 20, 2010 at 10:21 AM, Walter Landry <<a href="mailto:walter@geodynamics.org" target="_blank">walter@geodynamics.org</a> <mailto:<a href="mailto:walter@geodynamics.org" target="_blank">walter@geodynamics.org</a>>> wrote:<br>
<br>
<Guillaume.Duclaux@csiro.au> wrote:<br>
> Indeed.<br>
><br>
> But, shouldn't it be possible to solve purely thermal problem with<br>
> Gale? (let's pretend the thermal expansion is null is Nicolas'<br>
> problem). ie a sill at a temperature of 1000 K has intruded a mass<br>
> of rock at constant temperature (600 K) and I want to simulate the<br>
> thermal evolution of the system as I change the thickness of the<br>
> dyke or the radiogenic heat production of one or the other material.<br>
<br>
It is possible to do pure thermal conductivity problems with Gale.<br>
You have to turn off all of the Stokes flow stuff, but it does seem to<br>
work.<br>
<br>
> To ensure the solver timestepping is not missing the temperature<br>
> perturbation timescale, how should the time be scaled?<br>
> I guess viscosity doesn't matter if the problem is purely thermal,<br>
> but as soon as the thermal expansion is on, some body forces act too<br>
> creating some 'slow' displacement.<br>
<br>
For this particular case, the displacement is so slow that it can be<br>
neglected. If you still want to solve the Stokes flow, then you can<br>
set the timestep explicitly with 'dt' (see Appendix A.1.4). Gale<br>
should probably take the thermal diffusivity into account when<br>
deciding upon a timestep, but it does not do that now.<br>
<br>
Cheers,<br>
Walter Landry<br></div>
<a href="mailto:walter@geodynamics.org" target="_blank">walter@geodynamics.org</a> <mailto:<a href="mailto:walter@geodynamics.org" target="_blank">walter@geodynamics.org</a>><br>
_______________________________________________<br>
CIG-LONG mailing list<br>
<a href="mailto:CIG-LONG@geodynamics.org" target="_blank">CIG-LONG@geodynamics.org</a> <mailto:<a href="mailto:CIG-LONG@geodynamics.org" target="_blank">CIG-LONG@geodynamics.org</a>><div class="im"><br>
<a href="http://geodynamics.org/cgi-bin/mailman/listinfo/cig-long" target="_blank">http://geodynamics.org/cgi-bin/mailman/listinfo/cig-long</a><br>
<br>
<br></div>
------------------------------------------------------------------------<div class="im"><br>
<br>
_______________________________________________<br>
CIG-LONG mailing list<br>
<a href="mailto:CIG-LONG@geodynamics.org" target="_blank">CIG-LONG@geodynamics.org</a><br>
<a href="http://geodynamics.org/cgi-bin/mailman/listinfo/cig-long" target="_blank">http://geodynamics.org/cgi-bin/mailman/listinfo/cig-long</a><br>
<br>
</div></blockquote>
<br>_______________________________________________<br>
CIG-LONG mailing list<br>
<a href="mailto:CIG-LONG@geodynamics.org">CIG-LONG@geodynamics.org</a><br>
<a href="http://geodynamics.org/cgi-bin/mailman/listinfo/cig-long" target="_blank">http://geodynamics.org/cgi-bin/mailman/listinfo/cig-long</a><br>
<br></blockquote></div><br><br clear="all"><br>-- <br><div><font face="Verdana, Arial, Helvetica, sans-serif" size="1"><span style="font-size:9px"><div style="color:rgb(0, 0, 0);font-family:Verdana, Arial, Helvetica, sans-serif;font-size:80%;margin-top:8px;margin-right:8px;margin-bottom:8px;margin-left:8px;background-color:rgb(255, 255, 255);padding-top:0.3em;padding-right:0.3em;padding-bottom:0.3em;padding-left:0.3em">
<hr><table style="font-size:medium;border-top-width:1px;border-right-width:1px;border-bottom-width:1px;border-left-width:1px;border-top-style:solid;border-right-style:solid;border-bottom-style:solid;border-left-style:solid;border-top-color:rgb(255, 255, 255);border-right-color:rgb(255, 255, 255);border-bottom-color:rgb(255, 255, 255);border-left-color:rgb(255, 255, 255);width:541px">
<tbody><tr><td style="color:rgb(0, 0, 0);font-family:Verdana, Arial, Helvetica, sans-serif;font-size:medium;margin-top:8px;margin-right:8px;margin-bottom:8px;margin-left:8px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-top-style:dashed;border-right-style:dashed;border-bottom-style:dashed;border-left-style:dashed;border-top-color:rgb(187, 187, 187);border-right-color:rgb(187, 187, 187);border-bottom-color:rgb(187, 187, 187);border-left-color:rgb(187, 187, 187);border-style:initial;border-color:initial">
<a href="http://picasaweb.google.com/lh/photo/gQ7_To7dX8sI0sEe2ty0AaO6j9vpN3ZPghWjdy2iDOg?feat=embedwebsite" target="_blank"><img src="http://lh4.ggpht.com/_LUlLz13t5HQ/TFJicxcKudI/AAAAAAAABKE/kwovRKlv36Q/s800/Gunnedah3D-LowRes.00001.png" alt="" height="100" style="border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial"></a></td>
<td style="color:rgb(0, 0, 0);font-family:arial, sans-serif;font-size:11px;margin-top:8px;margin-right:8px;margin-bottom:8px;margin-left:8px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-top-style:dashed;border-right-style:dashed;border-bottom-style:dashed;border-left-style:dashed;border-top-color:rgb(187, 187, 187);border-right-color:rgb(187, 187, 187);border-bottom-color:rgb(187, 187, 187);border-left-color:rgb(187, 187, 187);text-align:right;border-style:initial;border-color:initial">
Professor LouisMoresi<br><a href="mailto:louis.moresi@monash.edu" target="_blank">louis.moresi@monash.edu<br></a><a title="Go to my public google profile" href="http://www.google.com/profiles/louis.moresi" target="_blank">GoogleProfile</a> | <a title="Link to my public google calendar" href="http://www.google.com/calendar/embed?src=louis.moresi%40monash.edu" target="_blank">GoogleCalendar<br>
</a><a href="http://www.underworldproject.org/" target="_blank">Underworld Geodynamics Software</a><br>Mobile phone: +61 4 2850 1907 ☎</td></tr></tbody></table><hr></div></span></font></div><br>
</div></div>