No subject

> transient advection-
> diffusion problems, in Comput. Methods Appl. Mech Eng, v. 193, 2004, p
> 2301-2323),
> they point out that:
> "Regarding the small localized oscillations in SUPG solutions we recall
> that SUPG is not monotonicity
> preserving, and that such oscillations can be expected in the vicinity of
> discontinuities and internal layers.
> Therefore, their presence cannot serve as an indication of a
> destabilization. Moreover, as the data in
> Tables 1=963 show, smaller time steps do not lead to an increase in the H=
1
> seminorm of the solutions,
>  i.e., these oscillations remain bounded for small time steps. An
> application of a discontinuity capturing
> operator [16] is recommended for a further suppression of these
> oscillations.
>
> (I've attached the paper). This seems to indicate that one should expect
> these small oscillations where
> there are rapid changes in flow, and that using something like the
> Lenardic filter is a reasonable thing
> to do as long as your confident you have a good solution of the flow.
>
> Any you advice or insight you have would be helpful...
> Magali
>
>
>
> --------------------------------------------------
> Associate Professor & Chancellor Fellow, U.C. Davis
> Chair, Geology Graduate Program
> Department of Geology & KeckCAVES
> 2129 Earth & Physical Sciences Bldg
> Davis, CA 95616
>
> E-mail: mibillen at ucdavis.edu
> Phone: no extension, please e-mail
> --------------------------------------------------
>
>
>
>
>
>
>
>
> _______________________________________________
> CIG-MC mailing list
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>
>


--=20
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener

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On Fri, Apr 6, 2012 at 5:49 PM, Magali Billen <span dir=3D"ltr">&lt;<a href=
=3D"mailto:mibillen at ucdavis.edu">mibillen at ucdavis.edu</a>&gt;</span> wrote:=
<br><div class=3D"gmail_quote"><blockquote class=3D"gmail_quote" style=3D"m=
argin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
Hello Shijie, Eh, and others...<br>
<br>
Following on the recent question on the list about max temperatures in Citc=
omS.<br>
I wanted to ask about the SUPG solver for the energy equation.<br>
<br>
My basic questions are:<br>
<br>
- Do you always expect to have some &quot;small&quot; temperature oscillati=
on from the SUPG method?<br>
- If so, in your opinion when is it reasonable to use the Lenardic filter t=
o prevent these small<br>
oscillations from growing?<br>
- If not, what strategy would you use to eliminate the temperature overshoo=
ts when decreasing<br>
the time-step doesn&#39;t eliminate them?<br>
<br>
For example, the slab benchmark in the manual gets temperature overshoots i=
f you run that<br>
forward in time. Decreasing the time-step helps, but doesn&#39;t eliminate =
them.<br>
<br>
I&#39;ve attempted to read some recent papers related to SUPG (after readin=
g the earlier Brooks<br>
and Hughes papers), and I&#39;m finding it difficult to understand the meth=
od well enough to know<br>
what its limitations are in non-steady-state problems (with or without stro=
ng flow gradients).<br></blockquote><div><br></div><div>Amazingly, the firs=
t Google hit looks good:=A0<a href=3D"http://rzbl04.biblio.etc.tu-bs.de:808=
0/docportal/servlets/MCRFileNodeServlet/DocPortal_derivate_00001549/Documen=
t.pdf">http://rzbl04.biblio.etc.tu-bs.de:8080/docportal/servlets/MCRFileNod=
eServlet/DocPortal_derivate_00001549/Document.pdf</a></div>
<div><br></div><div>SUPG just introduces some diffusion in the streamline d=
irection to prevent oscillation. Its a little</div><div>goofy because you h=
ave the tune the amount of diffusion carefully. It does not guarantee monot=
onicity.</div>
<div>The right thing to do is implement a monotonic method like TVD or WENO=
. There are now easy, open</div><div>source implementations in=A0<a href=3D=
"http://numerics.kaust.edu.sa/pyclaw/">http://numerics.kaust.edu.sa/pyclaw/=
</a>. You can use a low-pass filter (Lenardic)</div>
<div>to get rid of the oscillations, but the implications for accuracy are =
not good and forget about conserving</div><div>anything.</div><div><br></di=
v><div>I have said the simplest thing. Jed knows more about this.</div>
<div><br></div><div>=A0 =A0 Matt</div><div><br></div><blockquote class=3D"g=
mail_quote" style=3D"margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-l=
eft:1ex">