CFD Online Discussion Forums - Marangoni VS Rayleigh-Bernard

Dear Community:

I am trying to model and observe the competition between the Marangoni/buoyancy driven flow inside a small droplet (3mm dia) sitting on a heated solid support. From a simple solid-free boundary condition, Rayleigh number may be sufficient to kick the thermal instability. However, thermo-capillarity effect dominates in real life and buoyancy effect is not well observed. This is pretty much what I am trying to show with Fluent by using different thermo-capillary coefficient values(d(sigma)/dT).

I am considering thermocapillarity to be modeled through UDF, and buoyancy with Boussinesque approx.

The initial idea was to place the half-hemispherical drop on the solid and let it "sit" and deform under gravitaty until the "steady-state" is achieved. Then switch on the energy equation with appropriate boundary condition to observe the thermally and capillarity-driven flow inside the dorp.

The problem is that the solution never reaches the steady state, but oscillates due to the "parasitic error" at the interface. The magnitude of the error is O(mm/s) at the surface, and this is comparable to the magnitude of the thermo-capillary induced velocity inside the drop (which I observed from an experiment) of O(mm/s). Does this indicate that Fluent is not capable of modeling Marangoni flow ?

Or is there any other way to model this with Fluent?

I am considering to investigate different parameters but it seems that the parasitic error is always present...

Also, I have run some numerical testing: I placed a hemispherical drop (2D-axisymm) with no gravity and let it iterate. The parasitic error deformed the droplet until the liquid drop splits and eventually the solution diverged.

time step is taken to be very very small (1e-6) which is 2 orders magnitude smaller than the capillary time scale.

Thanks for the help