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

[Massimo Gaspari]

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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