Speaker
Description
The reduction of skin friction drag in turbulent flows has constituted a significant field of research over the past few decades. Spanwise forcing represents one of the most extensively studied techniques for the reduction of turbulent drag. It entails the imposition of a spanwise oscillation of the wall in a turbulent wall bounded flow. Different studies (e.g., Quadrio & Ricco 2004) have demonstrated that, if the forcing parameters (i.e., amplitude and period of oscillation) are chosen properly, a reduction of up to 45\% in skin friction may be achieved. Even though the performance of such technique has been widely studied and verified, the underlying working principle remains unclear. The prevailing hypothesis is that the reduction in skin friction is a consequence of the interaction between the wall oscillation and the turbulent motions in the vicinity of the wall. In particular, the unsteady spanwise velocity profile induced by the wall motion, known as Stokes layer, is believed to play a fundamental role. The issue, however, is that in the near wall regions, turbulence is significantly influenced by the presence of the wall itself, which makes it challenging to isolate the impact of wall oscillations on the turbulent structures. The present work proposes that this obstacle may be overcome by removing the wall effect entirely. To achieve this, spanwise forcing is not applied in a canonical wall bounded flow, but in homogeneous shear turbulence instead. Homogeneous shear turbulence is an idealized turbulent flow that has been used in literature as an intermediate stage of complication between homogeneous isotropic turbulence and inhomogeneous flows, such as wall bounded flows. It is characterised by a uniform mean velocity gradient and therefore contains many of the physical properties of real wall bounded flows, while retaining the simplicity of homogeneity. Spanwise forcing is included in the homogeneous shear turbulence simulation by enforcing the Stokes layer velocity profile. Numerical computations at different forcing parameters are performed. Results show similarities with the conventional wall bounded flow performance of spanwise forcing, particularly in the variation of the skin friction coefficient with the forcing period. Moreover, the simplicity of the base flow allows to clearly identify the effect of spanwise forcing on turbulence. The influence on several statistical quantities, commonly used to describe the behaviour of turbulent flows, is studied. The objective is to establish whether this unconventional numerical configuration may offer insights into the physics of spanwise forcing.
