[FLASH-USERS] Unsplit Hydro Solvers: Why and When?

Mon Feb 13 05:51:15 EST 2012

Hi,

I use:

order=3
transorder=4
rimannsolver=HLLD
slopelimiter=mc
eosforrieman=.true.

Default for the rest

I use order=3, because I want similar results with the simulations I did 
with PPM.
TransOrder seems less important in my runs. I've played around with 
that, but I don't think it's a critical parameter. TrOr=3 seemd to 
produce less artifacts. As I understand, it's kind of averaging method, 
like 'A-B' or '4A-3B-C' or '6A-3B-C-2D' or minmod('6A-3B-C-2D','A-B').
I think the Riemannsolver is quite important. 'Roe' never worked for me. 
I've seen several papers where they say that 'HLLD' is one of the best, 
so I use that one as well. Slopelimiter was a suggestion by Dongwook.

On 02/12/2012 04:27 PM, Massimo Gaspari wrote:
> Hi guys,
>
> I am glad that you share my interest on such an important topic, i.e. 
> pros and cons of the unsplit hydro solver.
>
> Seyit:
> The workshop would be really interesting, but unfortunately I am 
> unavailable for that date.
> When you say more diffusive ... which order/riemann/slope are you 
> using? (and why?)
> In theory I should expect the unsplit solver to produce *less* 
> artifacts compared to the split formulation
> (otherwise why bothering with a more complex scheme?).
>
> Tomek:
> Can you elaborate more on the "artifacts linked only to PPM"... this 
> is kind of new for me (any reference?).
> I totally agree for the database of the verification tests, in 
> particular comparing all the options of the unsplit module.
>
> Paul/Sean:
> The "FLASH knowledge base" seems a great idea.
> I do not totally agree about "in general they depend on the flow 
> problem". I think that some kind
> of quality must be *independent* of the specific problem solved, 
> leading to a solver better than another, also in general.
> Evidently some detail will be different, but I think we must find one 
> (or more) scale to properly evaluate them.
>
> Dongwook:
> A verification paper would be really great, certainly more than the 
> method paper, at least for the community.
> In particular the split vs unsplit comparison, testing different options.
> I totally agree with you that providing in advance a summary document 
> with the *when & why*
> is really necessary. Can't wait to read that.
> I think in fact that only the FLASH developers of the unsplit module 
> really know the behaviour of all the solvers/options
> and perhaps they have already a scale of preferences.
> In the meantime, what is *your* best hydro setup?
> Dongwook, what do you suggest to normally avoid?
>
> All:
> Thanks again for all your positive interest. On the other hand, may 
> you kindly provide some specific example of your current unsplit setup?
> For instance:
> order=
> transOrder=
> RiemannSolver=
> ...
> 2 lines of explanation (e.g., seems more stable... seems more 
> precise... less artifacts... I don't know, I like it!)
>
> Sadly, I am just starting to test the module, so I do not have much 
> experience to share, aside that I've found this setup to be rather stable:
> order=3
> transOrder=2
> RiemannSolver=Roe
>
> TransOrder seems to be a critical switch in order to loose/gain 
> stability. It is strange that with transOrder 1 crashes more than 
> 2.... but I need further tests.
> I am particularly interested in hydro simulations with multiphase gas 
> in it (i.e., very dense and cold clumps inside hot and diffuse 
> regions). Hope you have some experience in that direction. Strong 
> gradients in density (and temp) usually make the code suddenly 
> unstable. Any hint?
>
>
> Until next time,
>
>      Max
>
>
>
>
>
>
>
>
> > Subject: Re: [FLASH-USERS] Unsplit Hydro Solvers: Why and When?
> > From: smc at flash.uchicago.edu
> > Date: Fri, 10 Feb 2012 16:22:07 -0600
> > CC: dongwook at flash.uchicago.edu; flash-users at flash.uchicago.edu; 
> gaspmax at hotmail.com
> > To: pmricker at illinois.edu
> >
> > Paul,
> >
> > I love this idea. One word: FLASHBook. Unfortunately, that domain 
> name is already occupied (and for something far less unsavory than I 
> would have guessed). But we can work around that.
> >
> > Sean
> >
> > On Feb 10, 2012, at 2:00 PM, Paul M. Ricker wrote:
> >
> > > Dongwook,
> > >
> > > I'd like to add my support for something like what Massimo is 
> requesting. The answers to his questions can be obtained from 
> textbooks and the applied math literature, or informally from 
> experienced algorithm developers, but in general they depend on the 
> flow problem being solved. Documenting a group of test problems that 
> demonstrate the pros and cons of the different switches would 
> therefore be very useful. Perhaps the most feasible way to obtain this 
> end, given limited resources, would be to create an online space of 
> some type to allow users to submit this kind of information from their 
> own experience. A FLASH Knowledge Base, if you will.
> > >
> > > Paul
> > >
> > >
> > >
> > > On 02/10/2012 01:20 PM, dongwook at flash.uchicago.edu wrote:
> > >> Dear Massimo,
> > >>
> > >> This is a really good case I can provide more useful information 
> on the
> > >> unsplit hydro/MHD solvers.
> > >> I understand that, ultimately, it will be ideal to have a couple 
> of useful
> > >> reference papers including at least:
> > >>
> > >> (a) a method paper, and
> > >> (b) a verification paper along with comparisons with other methods
> > >> (e.g., split vs unsplit; FLASH vs other codes)
> > >>
> > >> And unfortunately, these papers are in preparation and one of my 
> todo list
> > >> in a near future.
> > >>
> > >> In the meantime, I think it will be a good idea for FLASH users 
> that I
> > >> provide a summary document that describe several key features/usages
> > >> (e.g., answering when& why) in more detail (that what's done in 
> the FLASH
> > >> users guide) and make that available to community.
> > >>
> > >> I will think about coming up with an idea how to provide such an 
> useful
> > >> information in order to help users to use the FLASH's unsplit 
> hydro/MHD
> > >> solvers for their research.
> > >>
> > >> Thank you for your interest in using the FLASH's unsplit solvers!
> > >>
> > >> Best,
> > >> Dongwook
> > >>
> > >> =========================================
> > >> Dongwook Lee, Ph.D., Research Scientist
> > >> FLASH Center for Computational Science
> > >> University of Chicago
> > >> 5747 S. Ellis Ave., Room 319
> > >> (773) 834-6830
> > >>
> > >>>
> > >>> Dear FLASH users/developers,
> > >>> I write this email in order to open a discussion on the unsplit 
> hydro
> > >>> solver.A preamble.I have mostly used the basic split PPM module 
> and I
> > >>> think it works pretty well.The only real "issue" is the 
> splitting, which
> > >>> complicates the implementation of several physical 
> modules.Therefore, I am
> > >>> right now testing the unsplit module (FLASH 4b).However... there are
> > >>> endless options! This is certainly a good thing. On the other 
> hand, it is
> > >>> pretty daunting to test every single option/solver, plus several 
> different
> > >>> combinations, even for a single problem.
> > >>> I am thus wondering if you (developers and users) may kindly 
> provide more
> > >>> comments/experiences on the methods used in the unsplit solver. 
> I don't
> > >>> want to know how the solver is written (I have Toro's book for 
> that), but
> > >>> *when* and *why* we have to use a particular solver/option, and 
> *which are
> > >>> the pros and cons/risks*?
> > >>> For instance, ...1) Using PPM (order=3) over Godunov (order=1) 
> is pretty
> > >>> trivial, but why and when using MUSCL (order=2) over PPM?
> > >>> 2) I also do not understand the default value of several options...
> > >>> shouldn't be the default value the best (in theory) option? If 
> so why
> > >>> transOrder=1? transOrder=2 seems to me a more appropriate choice, in
> > >>> general.When do you use transOrder=3? The same can be said for 
> the half
> > >>> gravity update: if it is second order, why the first order is 
> set to be
> > >>> the default?
> > >>> 3) I am puzzled by the several Riemann solvers. What is the 
> hybrid solver?
> > >>> Why should I use hybrid, LLF or Marquina solver over the more 
> standard Roe
> > >>> or HLLC?
> > >>> 4) The same can be asked about the different slope limiters...
> > >>> 5) ... or use_upwindTVD and use_3dFullCTU.
> > >>> Summarizing:a) what are your - general and specific - 
> suggestions?b) why
> > >>> should I avoid or use a specific solver/option over other standard
> > >>> implementations?
> > >>> I think the unsplit module is a great part of FLASH, but all the
> > >>> parameters/options/solvers need to be better clarified. I hope 
> you can
> > >>> help in that direction.Thank you in advance.Best,
> > >>>
> > >>> Max
> > >>>
> > >>
> > >
> > > --
> > > Paul M. Ricker
> > > Associate Professor of Astronomy
> > > University of Illinois
> > > pmricker at illinois.edu / 217-244-1187
> >

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