Spectral Dynamic of Pulsating Turbulent Pipe Flow

The present work investigates the effects of a forced harmonic oscillations of fixed frequency on the characteristics of a turbulent pipe flow with a mean Reynolds number based on the cross sectional bulk velocity and pipe diameter of 5900. The setup refers to an extremely thin Stokes layer so that the vorticity associated to the oscillating motion is generated in a small near wall region. When the ratio of the amplitudes of the oscillating and bulk velocities is increased from 1 to 11, the turbulence is affected by the harmonic forcing so much that the near wall coherent structures are substantially modified, although not suppressed through a reverse transition process. This results in an overall space and time averaged resistance reduction which, for the largest amplitude case, adds up to 33 % of the non pulsating flow at the same bulk Reynolds number. The present contribution is focused on the analysis of the phase locked averaged statistics of the Reynolds stress tensor components and velocity spectra. The analysis is carried out processing a set of statistically independent samples obtained from wall resolved Large Eddy Simulations with a spectrally accurate Navier-Stokes code