[Nek5000-users] Simulating two isothermal fluids with different densities

[nek5000-users at lists.mcs.anl.gov]

Hi Nek users,

I am simulating two miscible fluids with different density and viscosity.
There is no temperature difference. I am using the Boussinesq approximation
to account for buoyancy within userf. And to calculate density, I am
transporting concentration (kmol/m3) using the "temperature" scalar field.

Attached is an example box case (4 walls with no slip and zero flux BCs).
For ICs, the left half of the box is one concentration and the right half
is another. Fluid then moves due to buoyancy. Density and viscosity are
calculated as a function of concentration.

These equations work well with larger mass diffusivity (D), specified in
p08. However when I decrease this value, the maximum and minimum
concentrations exceed the initial values. If I want to consider a real
fluid, D needs to be extremely small (eg for water-ethyl, D=1e-9 m2/s). In
the attached example, the concentration scalar diverges.

Can somebody help me understand why concentration diverges with low D
values and how to fix it? If I am approaching this problem incorrectly,
please let me know.

Thank you,
Jonathan
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/nek5000-users/attachments/20171127/a8bf9365/attachment-0001.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: boussinesq_box_example.zip
Type: application/zip
Size: 164562 bytes
Desc: not available
URL: <http://lists.mcs.anl.gov/pipermail/nek5000-users/attachments/20171127/a8bf9365/attachment-0001.zip>