[FLASH-USERS] Jet ignition problems: split/unsplit Hydro (+ mass_scalar)

[Massimo Gaspari]

Dear Flash-Users,

I'm currently injecting a jet through a few cell boundaries on one side of a 3D box. 
The jet has variable velocity, regulated by some near density-temperature condition. 
And here come some problems with the split hydro (ppm).

1) If I let vary the jet every timestep, when the velocity is increasing the sim crashes, because
the timestep (CFL condition) is fixed for 2 consecutive dt. To circumvent this I let vary vjet
only in the "second" dt (essentially fixing it also for the next one).
First question: is there another (clever) simple method to avoid this and let vjet vary every dt?

2) When vjet reaches 10^4 km/sec and also when I increase resolution, the sim easily becomes
unstable at some dt (under smallest min values possible). I have to lower CFL number to 0.1 and it goes
better, but it can in any case crash in longer evolutions. Now, am I doing something wrong with jet
injection (and above set up)? 0.1 and lower make my sims almost impossible to evolve for some Gyr, is
it really possible that with ppm strang split method I have to adjust CFL to 0.1 or lower?
Again, is there any clever way to avoid this? (I know someone else has done jets with FLASH).

3) To try another way I've begun to use unsplit hydro solvers. This way the jet can be regulated
every timestep. What kind of solver do you suggest for Jet ignition (I have not much knowledge of them)? 
I'm using HLLC and with 0.3 CFL the sims never crashes, also with high vel and resolution.
How can it be possible such a difference between split and unsplit method?

4) How much are the unsplit solvers in FLASH "reliable"?
Have you done some tests or are they still in a "beta" phase?
Again, are they adequate to study a jet ignition? 

5) Last point. I need also the evolution of a MASS_SCALAR and it seems that it is not
evolving with the flow with the unsplit solvers (HLLC). How can I use them? (In Flash.h it is present)
Is it possible? If not, is it easy to implement (or someone else has done this)?

Thank you very much in advance for any help.
In literature I've seen many flash-users implementing jets; hope that someone has interesting way to attach
the problems I mentioned (maybe yet solved), or also completely different approaches for jets.



Max


(Massimo Gaspari, University of Bologna, ITA)
 		 	   		  
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