Turbulent drag reduction by spanwise wall forcing. Part 1. Large-eddy simulations

• Published in: Journal of fluid mechanics 2023-07, Vol.968, Article A6
• Main Authors: Rouhi, A., Fu, M.K., Chandran, D., Zampiron, A., Smits, A.J., Marusic, I.
• Format: Article
• Language: English
• Subjects: Actuation; Attenuation; Channel flow; Dimensional analysis; Drag reduction; Flow control; Fluid flow; Fluid mechanics; Friction; JFM Papers; Large eddy simulation; Oceanic eddies; Parameters; Reynolds number; Shear strain; Shear stress; Simulation; Traveling waves; Turbulence; Velocity; Viscous sublayers; Vortices; Wavelengths
• ISSN: 0022-1120; 1469-7645
• DOI: 10.1017/jfm.2023.499

Turbulent drag reduction (DR) through streamwise travelling waves of the spanwise wall oscillation is investigated over a wide range of Reynolds numbers. Here, in Part 1, wall-resolved large-eddy simulations in a channel flow are conducted to examine how the frequency and wavenumber of the travelling wave influence the DR at friction Reynolds numbers $Re_\tau = 951$ and $4000$. The actuation parameter space is restricted to the inner-scaled actuation (ISA) pathway, where DR is achieved through direct attenuation of the near-wall scales. The level of turbulence attenuation, hence DR, is found to change with the near-wall Stokes layer protrusion height $\ell _{0.01}$. A range of frequencies is identified where the Stokes layer attenuates turbulence, lifting up the cycle of turbulence generation and thickening the viscous sublayer; in this range, the DR increases as $\ell _{0.01}$ increases up to $30$ viscous units. Outside this range, the strong Stokes shear strain enhances near-wall turbulence generation leading to a drop in DR with increasing $\ell _{0.01}$. We further find that, within our parameter and Reynolds number space, the ISA pathway has a power cost that always exceeds any DR savings. This motivates the study of the outer-scaled actuation pathway in Part 2, where DR is achieved through actuating the outer-scaled motions.