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Andrea Conforti
The aim of this study is to advance understanding of the friction drag reduction phenomenon induced by transverse forcing. The traditional wall oscillation technique generates a transverse boundary layer, known as the Stokes layer, in which the velocity profile is analytically described by the solution of the second Stokes problem. However, it is challenging to understand whether the reduction in friction is mainly determined by the wall-normal spatial characteristics of the Stokes layer velocity profile or by its temporal variation, as both are related to a previously chosen value of the period of wall oscillation.A novel approach is suggested that imposes the transverse velocity profile directly on the flow, keeping the wall still. Through the proposed method, the spatial and temporal variations are separated into two parameters: one that measures the wall-normal thickness of the spanwise velocity profile and the other that represents its temporal period of oscillation. This allows for the evaluation of their separate influence on friction drag reduction and gain a more comprehensive understanding of the underlying physics.
A DNS-based parametric study reveals that the temporal variation of the Stokes layer dominates the skin friction reduction, while the spatial variation contributes to a lesser extent. The optimal period of wall oscillation for maximum friction drag reduction is found to be smaller than the traditionally chosen value in wall oscillation techniques. Evidence is presented through the study of Reynolds stresses and the behavior of tracer particles varying with the two parameters on why some velocity profiles are more efficient in providing greater friction reduction. Specifically, certain values of the Stokes layer oscillation period are found to be more effective in reducing vertical velocity fluctuations, thus reducing turbulent activity within the transverse boundary layer.