Sloshing frequency determination
 

Hi,

I am trying to determine the sloshing frequency in a cube.

I realize that one way to do this is to plot the variation of the force on the container, however there are two phases air and water that I am simulating and the walls have been modeled as one part, how accurate would the result be if I determine the sloshing frequency by applying a FFT to force variation as function of time which will include forces applied by both air and water.

The question is whether the force applied by air be so small that frequency will be obtained to a reasonable degree of accuracy?