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

[Hix, William Raphael]

Hi All-

I second Paul's suggestion. Seems like this is tailor-made for a Wiki, in the original sense.  If Dongwook posts his test problems (setups and results) as a Wiki, others can add additional problems with results.  Extend this to other issues, and you turn the tremendous anecdotal wisdom of the FLASH community into a much easier to tap resource.

						Raph

On Feb 10, 2012, at 3:00 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

------------------------------------------------------------------------
William Raphael Hix                       Email:  raph at ornl.gov
Oak Ridge National Laboratory             Voice:  (865) 574-4716
P.O. Box 2008                               Fax:  (865) 576-8746  
Oak Ridge, TN 37831-6354