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

Sun Feb 12 10:27:38 EST 2012

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 kindof 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/optionsand 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=3transOrder=2RiemannSolver=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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