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Dear All,

I have got a simple geometry: a 1mm thick piston-disc is vibrating with a frequency of 1kHz at 1cm above the closed bottom of a (L=353mm long, Rc=38mm radius) cylinder. The maximum acceleration of the piston is a0=10m/s^2. The top of the cylinder is open and it is an acoustically matched layer, which means there is no wave reflection from this boundary. The fluid is air within the cylinder.

The study was performed in 2D Axisymmetric; Pressure-Acoustic, Frequency domain, and also in Acoustic-Solid Interaction, Frequency domain. The aim is to test the validity of the results with this simple setup, and also to compare the radiated acoustic power that leaves the cylinder with the mechanical input power required to move the piston.

It is simple to get the radiated power by multiplying the Rc^2*pi area with the Intensity magnitude (RMS), which gives about 2.37 uW.

It is a bit more complicated to calculate the mechanical power input, or at least I couldn't figure out a more simple way yet. In the variables used the Fp=F_int(2*pi*r*nz*(down(p)-up(p))) definition where F_int is a boundary integral applied on the piston's surface. After solving the study, the Fp acoustic pressure force on the piston is displayed on a Global 1D Plot using phase as the Parameter on the x axis. This plot is exported into Excel and now we have got all the values of the Force as the function of phase accessible for integration. The piston's displacement as the function of phase is calculated as Zph=-a0/(2*pi*freq)^2*cos(phase). By summing up all the F(phase)*dz we get the consumed mechanical energy during a full oscillation period which is -2.37E-9 J. Multiplying this with the frequency we obtain the consumed mechanical power of -2.37uW. As you can see it is not easy and simple to calculate the mechanical power this way.

My question is, whether there is any easier and simpler way to get the invested mechanical power in a frequency domain study?

I have not done this project in a transient study yet, but I suppose it should be straightforward there to get the mechanical power directly from COMSOL. Any suggestions how to do such "power-In vs. power-Out" analysis in the simplest way possible, without the need to export and externally calculate?

Thanks for suggestions,
Joe