[FLASH-USERS] "jet-by-hand" in unsplit

[Hal Jacques Cambier]

   Hi fellow flash-users,

   I have been using FLASH 3.2, and the PPM split solver, 
to simulate the evolution of the bound remains of a 
tidally-disrupted star around a black hole (in cartesian 
3D).  I'm currently only interested in the aftermath, so 
the stellar remains enter my simulation via source term : 
a rectangular yz-plane (1 cell thick), of dens., temp. 
(20K)*, and x-velocity values (~ -1e9).  This is set 
inside a frame of buffer cells given ambient values to 
prevent spurious backflow.  Maximum source--ambient 
density contrast is typically 12 orders of magnitude in my 
simulations (10^-20 vs 10^-12)...
   So far this source term has worked nicely using the 
gamma-law or even the Helmholtz (with some complaints) 
EOS.
   However, when I switch to the unsplit solver (with 
gamma-law), I notice that after a few iterations, cells 
with smalle values and opposing velocities form at the 
leading edge of the jet and then spread everywhere...
*I have tried using the unsplit "shock fixes" but saw no 
effect
*I have tried reducing the density contrast (in hopes of 
smoothly ramping it up) -- which didn't work by itself, 
nor with increased or decreased values of the eintswitch 
toggle.  Just to be thorough, removing the point mass (the 
only gravity here) does not remove the error.
*I also found that inputting density at the maximum 
contrast with no velocity seems to cause no problems
   I tried to be comprehensive yet brief, so let me know 
if you'd like more detail.  Thanks for your time,

Hal
hcambier at ucsc.edu

* such low T was just chosen to keep the problem as 
initially 'ballistic' as possible

(Figures of upper xyz corner of source & buffer cells)

/|\+y
  |
o o o o
o o o o
o o o o
o o o o
x o o o
x o o o ----->+x
...etc...

/|\ +z
  |
o o o o o o o o
o o o o o o o o
o o o o o o o o...etc...
o o o o o o o o
o o o o x x x x
o o o o x x x x ---->-y
... etc ...