turbulent energy spectrum problem

cfd.newbie

Hi

I want to calculate energy spectrum for my LES simulation. I put a point in domain and monitored u, v and w velocities. I calculated the kinetic energy from my velocities and imported the kinetic data and time into matlab. I, then, converted my time domain into frequency domain by using fft.

How do I draw energy spectrum then? In book there is E(k) on y-axis and k (wave number) on x-axis. Is E(k) same as kinetic energy which is 0.5(u^2+v^2+w^2)? How do I calculate k (wave number) on x-axis? Thanks in advance for your answer.