FFT plot significant peak near 0 Hz

I am analyzing the vortex shedding frequency using fft and I get high peaks near 0 Hz and it is definitely wrong. The raw data is velocity with respect with time. What could this mean?

This is the code I was using to get the fft plot. data=xlsread('profile 63.xlsx'); v=data(:,5); Fs=100; T=(1/Fs); t=(0:L-1)*T;

NFFT = 2^nextpow2(L); Y = fft(v,NFFT)/L; f = Fs/2*linspace(0,1,NFFT/2+1); plot(f,2*abs(Y(1:NFFT/2+1))) grid ylim([0 0.1]) title('Single-sided Amplitude Spectrum of (y(t)') xlabel('Frequency (Hz)') ylabel('|Y(f)|')

Answer 1

[Adam]

The 0 Hz frequency value is relative to the mean of the input data. For signals I work with this therefore represents how badly (or well) 0-centred the data is since a properly centred signed signal will have close to a 0-mean.