[aspect-devel] cookbooks/composition-active.prm

[Timo Heister]

Hey,

it turns out cookbooks/composition-active.prm broke a long time ago
(r1370, more or less one year ago). Today I finally managed to take a
look at it. The change modified how the entropy viscosity is computed:
since then we are computing and using max_density and
max_specific_heat on each cell. Before, the entropy viscosity was at
the very end multiplied by the local density and specific_heat. This
change makes a lot of sense because it fixed some inconsistencies with
respect to the scaling with density and specific_heat.

Now to the problem. It turns out that the density becomes negative in
some quadrature points, while the max_density is still a positive
number. Example:

max dens = 69.7217
loop 69.7217
loop 69.7217
loop 69.7217
loop 0.992031
loop 0.992031
loop 0.992031
loop -7.73768
loop -7.73768
loop -7.73768

The density becomes negative, because the composition has a value of
about -0.08 here.

Suggestions:
1. Density should always be positive and we should enforce this in
debug mode with an Assert. What to do in release mode? Should we
restrict the density to something sensible?
2. The material model 'simple' should simply clamp the compositional
fields to [0,1].
3. If the user specifies that a compositional field is supposed to be
in [0,1], should we postprocess the field and restrict it to [0,1]? I
think not, because we would lose mass this way (we could restrict it
only for output and material models but use the original one in the
next timestep).

Opinions?

-- 
Timo Heister
https://urldefense.proofpoint.com/v1/url?u=http://www.math.clemson.edu/~heister/&k=dsHy%2FVymniCD0osh6tze%2Fw%3D%3D%0A&r=WLGUy1n%2F1Vzsqown7KFPcAEzR5DIdbP771jYw4Spy%2Bk%3D%0A&m=S11NmahkaA8sPelibOGff%2FjUvWqX3xTwDWFpjCX9Hko%3D%0A&s=1a44e5483f646199cc1b0d3d4f2c55799e3902613ce8c863e9bc3b4350fad75c