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

[Tomasz Plewa]

The problem with artifacts in degenerate conditions was initially 
related to the PPM solver, not an unsplit solver (which was not 
available at that time). Perhaps this has changed over time?

Still, the best way of learning about particular method is to review 
published test cases. Perhaps FLASH documentation could offer some 
references and basic information even if in condensed form, but this 
will never be as complete as simply reading the original method paper 
(assuming the code has been verified).

In this context, it would be helpful to construct a consolidate summary 
of classic verification tests similar to the one produced by Liska & 
Wendroff in their 2003 paper. I understand such a database can be quite 
easily constructed and made available online for a review on a daily 
basis (with access restricted to licensed FLASH users, if such a 
restriction is deemed necessary for any reason).

Tomek
--
On 02/10/2012 01:02 PM, Seyit Hocuk wrote:
> 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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