Determination of periodicity in data and finding mean I have to find whether there is any pattern (I mean periodicity or close to periodicity) and if there is, for one cycle i have to perform numerical integration to determine mean. In the first picture I have done moving average(every 254 data points) using excel.

Then in the next picture i have done again moving average(with 250 points) using the data got after performing first moving average.


I have also thought about doing Fourier transform.
These datas are values of cl (lift co-efficient) vs time in a unsteady solution of CFD simulation.Variation of values at initial time steps are not significant as some time is needed for simulation to give exact value.  DATA file is here (drive.google.com/open?id=0B1KEir1gFkaMQW5GbmFoZlpRUVk)
To determine periodicity, do an autocorrelation. The autocorrelation will have peaks at time offsets consistent with the periodicity.

You mentioned using the Fourier Transform, if convenient you can do a circular autocorrelation using the FT as follows:

$$Corr = ifft(fft(x) fft(x)^*)$$ 

Where $fft(x)^*$ is the complex conjugate (if you signal is complex).