Creating an expression for finding droplet generation frequency in CFD-post
 

I am simulating the droplet generation process in a flow-focusing (cross-junction) microfluidic device. I have done my simulations and I know that the droplets are formed in the system. I want to create and expression in cfd-post in order to find droplets generation frequency using dispersed phase volume fraction and CELs in cfd-post. I know that I can introduce a point in the main channel and plot the dispersed phase Volume fraction Vs. time and then, use FFT (fast Fourier transform) provided by the cfd-post to find the frequency, but I want to use this frequency for another purpose. Can anyone help me define the expression?