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

[Seyit Hocuk]

I also have artifacts in my moving (orbiting) molecular magnetized 
cloud, especially in the wake of the cloud. But I still like it.
My first impression of the unsplit solver is that it is more diffusive, 
unfortunately. Everything seems to be more smooth. Can anybody confirm?

Seyit




On 02/10/2012 06:55 PM, david arnett wrote:
> The prime failing of the un-split solver, that I know of, is that it 
> produces poor results for high electron degeneracy; errors due to grid 
> artifacts appear. This is an ancient problem; are there more recent ones?
>
> Dave
>
>
>
> On Fri, Feb 10, 2012 at 10:45 AM, Seyit Hocuk <seyit at astro.rug.nl 
> <mailto:seyit at astro.rug.nl>> wrote:
>
>     Those are all good questions. I'd like to hear them myself as well.
>     You should come to the FLASH workshop in Hamburg/Germany next week.
>
>     Seyit
>
>
>     On 02/10/2012 06:36 PM, Massimo Gaspari wrote:
>>     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
>>
>
>
>
>
> -- 
> David Arnett
> Regents Professor
> Steward Observatory
> University of Arizona

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