What does low frequency oscillation phenomena tell in residuals
 

Hey!

I was wondering what creates a low frequency oscillation in residuals like in the particular image as an attachment.

I would bet that it is due to a coarse mesh x-direction, because y-direction velocity is not oscillating so much and because turbulence quantities are no converging as quickly. I just have never seen such a low frequency phenomena, though beginner I am.

If you think that the computational grid acts as a cut-off filtering and the numerical schemes smoothes high resolved frequencies, then it is quite common that the spectrum of the error will be dumped quickly at high frequancy and slowly at low frequencies.
In some way, this is at basis of the multigrid ideas, resolve the equations on wider grids to smooth more quick the low frequencies of the error.

In your case I suppose that the error is more "energetic" along x than along y

Looks like a separation bubble trying to converge.