Discussion Forum

Hi everyone,

I am trying to perform a simulation in which I first electrostatically bias a membrane, and then apply an AC signal to the biased membrane, which should in turn cause the membrane to vibrate and radiate a pressure wave into a surrounding fluid medium (water). I have been able to create simulations that statically bias the membrane using electrostatics, plain strain, and ALE (similar to the ALE cantilever example). I have also been able to create a simulation that applies a pressure loading to the membrane which causes it to radiate waves into a fluid half-space. However, coupling these two scenarios seems much more complex.

The ALE cantilever beam describes a method involving a femssr wrapper which should effectively do everything that I need (take a biased solution and superimpose a small-signal AC voltage). Following this process, I get somewhat expected results. Specifically, there is a phenomenon known as spring-softening which essentially implies that as you apply higher bias to a capacitive membrane, its center frequency will shift lower. I observe this effect using the femssr wrapper. Also, when I compare my results using water and air as the fluid medium, I see a pronounced fluid loading effect that shifts the water-loaded membrane to a lower center frequency, as expected. However, I have noticed that the frequency response in both water and air are very low bandwidth (high-Q) systems. I would somewhat expect this in air, but water should be more "damped" and I do not observe this using the femssr method. On a related note, I do observe the high-Q in air, low-Q in water effect when I run my more basic harmonic analysis with no electrostatic bias, so I believe something is wrong with this femssr method.

I have also tried a multi-step approach in the Comsol GUI. First, I bias the membrane as usual. Then, I define a new plain-strain application and a new electrostatic application, and use the biasing simulation results as my linearization point for a harmonic analysis. Thus far, this has only given me results similar to a typical harmonic analysis, in that I do not observe spring-softening. But it is possible that I have not coupled the two simulations together correctly.

In summation, I really just want to know the basic approach to accomplish what I am requesting--to bias a membrane electrostatically, and then superimpose an AC signal such that it couples to the structural domain and causes pressure waves to be radiated. From reading through the various threads, it seems that other people are having similar problems, but as of yet I haven't found a definitive solution. Any advice or recommendations would be appreciated. Thanks!