[FLASH-USERS] reaction-diffusion problem with FLASH?

[Peter Woitke]

Dear experts,

is it possible to apply FLASH to a 1D reaction-diffusion problem as

    d/dt (rho x) = d/dz (rho D dx/dz) + R(x,z)     [1/cm3/s] ?

The equation describes chemicals in a planetary atmosphere.
z is the height in the atmosphere [cm], rho the mass density [g/cm3],
D the diffusion coefficient [m^2/s], and the vector x are the mass 
concentrations of certain gas molecules and solid particles
[rho x has units 1/cm3]. There is no bulk motion: v=0.

Both the diffusion constant D(z) and the mass density rho(z) vary
by about 8 orders of magnitude from the bottom to the top of the 
atmosphere. That's why I need the more general formulation of
diffusion, can't use  rho D d2x/dz2.

There will be a certain fixed temperature structure T(z) assumed,
and D(z) will be adjusted to turbulent mixing.

The reaction rates R depend on concentrations x. They are often
huge compared to the influence of diffusion, hence it's a stiff
reaction-diffusion problem, and the local chemistry is close to kinetic
equilibrium. I am interested in how diffusion changes the kinetic 
equilibrium over long timescales.

I am actually most interested in the t -> infinity solution
where one could drop the d/dt(rho x) term.

I noticed that there is the flash4.4/source/physics/Diffuse
directory. Does anyone has experience with using that?
Is there a test problem with diffusion that I could study
and visualise?

Any advice would be highly appreciated.

Thank you,

   Peter Woitke



-- 

Dr. Peter Woitke
St Andrews University
School of Physics & Astronomy
top floor, room 306
North Haugh
KY16 9SS
St Andrews, UK
Tel: (+44) 1334 46 1681
Fax: (+44) 1334 46 3104