Just following the guideline vídeo for the Kelvin helmoltz instability.
Should the gravity in the vertical direction be enabled?
thanks for the output.
For KH instability, the following condition must be satisfied. Please take a look at the results slide of this simulation example (slide 6)
Here is the link:
In the simulation example video, we use two same fluids (air and air). Because rho_1 = rho_2, the RHS term (of the inequation) becomes 0. It will not matter if you turn on or off the gravity term. This is why you see that there is no gravity term enabled in the simulation video. As long as U_1 is not equal to U_2 (for such a case), the above condition for KH instability will be satisfied.
However, when we use two different fluids (in the case of air and water vapor), it is important to check if the above condition is satisfied. It is, therefore, necessary to activate the gravity term (in the negative y-direction). For the velocities and fluid properties mentioned in the results slide, I strongly recommend that you calculate your LHS and RHS terms to understand KH instability better.
I hope this answers your question.
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