To define a dimensionless Objective function for determining how peaked a curveis
 

I have attached 2 plots (FFT spectrums). One is considered good and one is bad (See file names).

If one is curious, the FFT spectrum is for the flow behind a cylinder.

The good one is classified on the basis on how closely spaced the frequencies and the bad is based on how multiple frequencies are present.

I am trying to determine a dimensionless objective function which could be maximized or minimized so as to lean towards which is a better plot (Good FFT).

One function which is consistent for a particular scale is: Max.Power/standard deviation But this is not a dimensionless quantity.

I would like to determine a dimensionless quantity.

Thanks in advance.

Attachments for the good and bad FFT
 

Attachments for the good and bad FFT