[FLASH-USERS] Problem with Diffuse module

[ascenzi]

Dear FLASH users,

I am trying to simulate an expanding optically thick spherical shell of 
matter with a source in the centre (source described by user defined 
boundary conditions).

I tried to switch on the thermal diffusion using a power law 
prescription for the conductivity in the form:   sigma = K0 T**3 
rho**(-1) in order to reproduce the thermal radiative transfer of the 
star described by a diffusive approach, where the flux is F = -K0 T**-3 
rho**(-1) grad T.

Unfortunately when I switch on the thermal diffusion the timestep 
(diffusion dt) becomes extremely small, which makes the simulation 
impossible to handle. Actually I notice that my dt rise monotonically 
until I obtain a warning message (that I had several times also at the 
beginning of the simulation with no diffusion):

[gRSt] afterGeo fallback to order 1 for DIR_X at i,j=           5        
    1  in Block          10 @           0
  [dR1St] afterGeo   3.3108486768816685        9.2775874623827751E-008   
275453229333938.16       -86.817733542062342

At this point the dt drops, reaches a minimum and then rise again until 
it reach almost the same value as before and the warning message appears 
again, dt drops and so on in a sort of “saw tooth” fashion. (I notice 
also that in this “saw tooth” fashion there is a very slow drift towards 
higher values of dt, but this drift is still too slow to be useful).

I found that the warning message is probably called by the routine 
“hy_uhd_getRiemannState” and is due to the fact that the pressure is 
negative somewhere.

Do you know how can I solve this warning and why the timestep is so 
small (about 10^-8 s against 1 s of the case with no diffusion)? I put a 
diffusion cutoff at rho=5*10^-12 g/cm^3 .

Thank you very much,

Stefano

P.S. I am using the Unsplit diffusion but also with FluxBased the 
behavior is similar.