Analysis of drag reduction effects in turbulent Taylor–Couette flow controlled via axial oscillation of inner cylinder

Analysis of drag reduction effects due to axial oscillation of an inner cylinder in a turbulent Taylor–Couette (TC) flow is performed in the present study. The frictional Reynolds number on the inner cylinder is 218, and the non-dimensional oscillating period is varied from 8 to 32. By examining turbulence statistics, we uncover different impacts of the long- and short-period oscillations on the circumferential ( θ) and radial ( r) velocity fluctuations in large ([Formula: see text]) and small ([Formula: see text]) scales. One of the most surprising findings is that the short-period oscillation increases the large-scale Reynolds shear stress [Formula: see text] by the strong intensification of [Formula: see text] exceeding the suppression of [Formula: see text]. To understand the phenomena, the spectra of each term in the transport equations of the Reynolds normal stresses [Formula: see text] and [Formula: see text] are analyzed. First, it is shown that the short-period oscillation weakens the productions of [Formula: see text], and [Formula: see text] while it enhances that of [Formula: see text]. In contrast, the long-period oscillation reduces the productions of [Formula: see text] and [Formula: see text] while it mainly intensifies that of [Formula: see text]. Second, the investigations of the pressure–strain terms indicate that the short-period oscillation mainly impedes the inter-component energy transfer originating from the small-scale background turbulence. However, the long-period oscillation benefits the small-scale inter-component energy communication while it hinders the large-scale one. In addition, the inverse energy transfer in the turbulent TC flow is confirmed by inspecting the inter-scale energy transfer terms. The hindrance of the inter-scale energy transfer by the inner-cylinder oscillation plays a non-negligible role in the reduction of the wall friction drag.