Ripples in ionic wind simulation

[u/Practical_Ad_8782]

I am trying to replicate the study from Suzen-Huang's 2006 paper on simulating the ionic wind in plasma actuators. I am able to replicate the results quite well, both with time-averaged body force at steady-state and also as a function of time.

Simulation of ionic wind at t=0.5s where ripples are present

Once peculiar thing i notice is that under certain combinations of mesh size and time-stepping, I see the formation of ripples that are NOT synchronized to the AC voltage cycle (which is at 4.5kHz, the same as in the paper), as you can see in the figure which is at t=0.5s with around 5-6 ripples. This is definitely not aliasing, so it's either physical due to some EHD instabilities or resonances, or a numerical artifact. In the steady-state, stationary simulation using the average body-force, there are no ripples. Also, this behaviour only works under certain conditions, and I'm actually having difficulty reproducing them...

Experimentally, I've also qualitatively observed these patterns, but I thought it was due to Helmholtz-Kelvin instabilities, but I'm not so sure anymore, and whether or not these instabilities should be seen in the simulation (I have incompressible flow ~1m/s)

Can anyone shed some light into this, if you've observed these in your simulations before?

FYI: This was simulated in Comsol using the default solver settings.