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

[Paul M. Ricker]

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