[Richard_S]

Hello everybody,

I set up cases with multiphaseInterFoam for simulative investigation of slot die coating processes.
In my cases I simulate the flow of two identical but immiscible pastes with air (see png-file; paste1 blue, paste2 grey, air red). Due the pastes are identical, the surface tension should be 0 N/m.

I observed that surface tensions close to 0 induce a blurred interface with waves (like the upper case of the png-file. The case beneath has a higher value of the surface tension).
Does anybody knows the reason for the formation of those waves? I assume the answer is hidden in equations that use the surface tension - can anybody help me?

Sincerely Richard

[wkernkamp]

1. It would be logical to see turbulent mixing in the flow, this will be suppressed by adding surface tension.


2. The waves in the interface seem connected to a rhythmic pattern on the lower boundary. This pattern also exist in the solution with surface tension.

[Richard_S]

Thx for the answer!

1. I forgot to say that I only simulate a laminar flow.
2. The lower wall is in motion and moves rightwards. The rhythmic pattern you detected is entrained air. This pattern is unwanted and I suppressed this effect in a new case (see new picture "localSlip") with a special boundary condition.
I also added a pic with velocity U and pressure p.

However, the waves on the interface remain.
In my simulations, surface tensions of dimension 0.01 N/m or lower result in those waves.
With 0.1 N/m the waves vanish. (very high surface tension! ; water-air has 0.07 N/m !)

I don't understand this phenomena and appreciate every explanation.

[wkernkamp]

Could it be grid related. Try a 2x refined mesh and see if the waves become twice as small.


Shear layers are unstable in this way, but from you velocity magnitude plot it doesn't look like there is a velocity gradient at the interface (but hard to tell).