[FLASH-USERS] Viscosity and PPM

[Klaus Weide]

On Sun, 12 Sep 2010, Samuel Friedman wrote:

> I'm looking at running a simulation with viscosity.  I am running FLASH3.2 and
> would like to use the PPM hydro unit with the viscosity unit. However, the
> user's guide is not 100% clear about whether or not I can do this.  

Sam,

The short answer: Yes, you can do it with FLASH3.2.

(Of course you should check whether the provided implementation is 
sufficient for your needs.)

> And from
> what I can tell of the source code, FLASH does not run PPM with
> (non-artifical) viscosity.

See below...

> Sec 19.3 reads:
> 
> The Viscosity unit implements a prescription for computing viscosity
> coefficients used by the Hydro PPM, the unsplit hydro and MHD solvers.
> 
> Sec 13.1.3 reads:
> Diffusion terms: Non-ideal terms such as viscosity and heat conductivity can
> be included in the unsplit hydro solver for simulating diffusive process.
> 
> There is no mention about being able to run viscosity with PPM though in the
> hydrodynamics unit (Sec 13).

You missed 

    15.4 Diffuse Unit

Please see whether that section answers your questions.

(Apparently the location of that information was not obvious; please let 
us know if you have suggestions for how we could make it more visible for 
someone looking for this particular information like you did.)

> As for the code, I can see both unsplit solvers asking for the value of the
> runtime parameter "useViscosity", but PPM does not.

You will find

   call Diffuse_visc

in hy_ppm_sweep.F90.  The Hydro unit (in the PPM case, at least) does not 
really need to know whether useViscosity is .TRUE.; it makes the call to
Diffuse_visc (..) anyway, but Diffuse_visc may return immediately if 
nothing needs to be done.

Klaus