Visualization of instantaneous and filtered turbulent pipe flow data with and without drag reduction by spanwise wall oscillations at moderately high Reynolds number

Turbulent pipe flow with and without azimuthal wall oscillations has been simulated. Drag reduction manifested by an increased volumetric flow rate is observed with the prescribed wall oscillation parameters. Instantaneous realizations of the flow field are sampled and filtered to observe changes to the structure of turbulence in a drag reduced as opposed to a non-drag reduced pipe flow. Optimal filtering in space is taken as the streamwise-azimuthal power spectra of the streamwise velocity fluctuation, and the results are presented in both the standard and drag-reduced case. This filter acts as a low pass filter in space allowing only the most energetic large scales of motion through. This filter can be inverted to act as a high pass filter allowing for the visualization of most energetic small scales. The organization of large-scale structures with and without drag reduction is analyzed to understand how the reduction of small scales effects the convection of structures downstream.