Direct numerical simulation of turbulent flow in a pipe oscillating around its axis

The turbulent flow in a cilindrical pipe oscillating around its lungitudinal axis is studied via direct numerical solution of the Navier-Stokes equations, and compared to the reference turbulent flow in a fixed pipe and in a pipe with steady rotation. The maximum amount of drag reduction achievable with appropriate oscillations of the pipe wall is found to be of the order of 40%, hence comparable to that of similar flows in planar geometry. The transverse shear layer due to the oscillations induces substantial modifications to the turbulence statistics in the near-wall region, indicating a strong effect on the vortical structures. These modifications are illustrated, together with the implications for the drag-reducing mechanisms. A conceptual model of the interaction between the moving wall and a streamwise vortex is discussed