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

[dongwook at flash.uchicago.edu]

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
>