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

[Seyit Hocuk]

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
>

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