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

[Alexander Wagner]

Hi Massimo,

I have been using FLASH's relativistic solver (RHD unit, Mignone et al 
2005) to simulate astrophysical jets. Initially I did some tests with 
the ppm split solver too. I didn't experience problems of instability, 
except when I tried using an outflow boundary rather than a reflective 
boundary on the side of the 3D box at which the jet is injected (the 
outflow condition was for all rows except the jet inlet of course). I 
suppose you're using a reflective boundary for the side of the 3D box 
where the jet is injected? My jet parameters were constant in time. The 
largest density contrast between jet and ambient medium i tried was 
several 10^-5 and it was stable. I haven't tried the unsplit solver, but 
I i'll let you know if I get a chance to do so.

I added support for mass scalars for the RHD unit, and it was a matter 
of emulating the density advection through the entire Godunov scheme. 
Tedious but it in the end it seemed to work. Don't know how difficult it 
would be to include mass scalars in the unsplit hydro solver, if they 
aren't already supported.

Sorry I can't provide more answers.

Best wishes,
Alex


On 21/04/10 9:38 PM, Massimo Gaspari wrote:
> 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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