It's an interesting question and already lots of studies have been performed on this topic. I will recommend you to... have a look at the recent paper of Wang et al. (2021)... published in Ocean Engineering ("0.1016/j.oceaneng.2020.108412" ). You will get some idea of how SPD graphs are useful to represent the Strouhal number.

Vortex shedding involves alternate shedding of vortices from the top and bottom sides. At a moderate Reynolds number such as yours (Re = 100), the vortex shedding is laminar and the flow behavior is time-periodic. The lowest peak in your plots represents the frequency at which vortices are shed. The next peak corresponds to the harmonic of the shedding frequency, i.e., twice the shedding frequency, and so on. The harmonics represent the effects of flow nonlinearity.

In general, it would be best to plot the results in terms of nondimensional quantities, e.g., PSD of some non-dimensional flow metric (such as the lift and drag coefficients) against nondimensional frequency (i.e., Strouhal number) and specify the exact definition of the quantity being plotted, instead of simply "Magnitude". That way, others working with a similar shape but a different body size or flow speed can easily make comparisons with your results. However, if most of the signal energy is contained within the first three or four frequencies (depending on your tolerance), you can easily fit the time history of CL to a summation of those harmonics (e.g., CL = C0 + C1 sin (omega*t + phi1) + C2 sin (2*omega*t + phi2) + ....) and report the amplitudes and phases of the leading harmonics. Note that C0 represents the time-averaged lift coefficient and would be zero for a symmetric bluff body.

Dear Meelan Choudhari Meelan Choudhari Sir I have prepared this graph for Lift coefficient vs Time variation data

Dear Santosh Kumar Singh please upload the full details of the paper you mentioned in your answer

thanks