An energy-efficient pathway to turbulent drag reduction

Abstract

Simulations and experiments at low Reynolds numbers have suggested that skin-friction drag generated by turbulent fluid flow over a surface can be decreased by oscillatory motion in the surface, with the amount of drag reduction predicted to decline with increasing Reynolds number. Here, we report direct measurements of substantial drag reduction achieved by using spanwise surface oscillations at high friction Reynolds numbers ($${{{\mathrm{Re}}}_{\tau }}$$ Re τ ) up to 12,800. The drag reduction occurs via two distinct physical pathways. The first pathway, as studied previously, involves actuating the surface at frequencies comparable to those of the small-scale eddies that dominate turbulence near the surface. We show that this strategy leads to drag reduction levels up to 25% at $${{{{{{{{\mathrm{Re}}}}}}}}}_{\tau }$$ Re τ = 6,000, but with a power cost that exceeds any drag-reduction savings. The second pathway is new, and it involves actuation at frequencies comparable to those of the large-scale eddies farther from the surface. This alternate pathway produces drag reduction of 13% at $${{{{{{{{\mathrm{Re}}}}}}}}}_{\tau }$$ Re τ = 12,800. It requires significantly less power and the drag reduction grows with Reynolds number, thereby opening up potential new avenues for reducing fuel consumption by transport vehicles and increasing power generation by wind turbines.

Exact coherent states in plane Couette flow under spanwise wall oscillation

“…2014; Gatti & Quadrio 2016; Yao, Chen & Hussain 2019; Marusic et al. 2021). In this respect, an interesting variant of the spanwise wall oscillation has been proposed recently by Marusic et al.…”

“…In this respect, an interesting variant of the spanwise wall oscillation has been proposed recently by Marusic et al. (2021), who obtained a considerable amount of skin-friction reduction with net energy savings by applying a spanwise wall oscillation for large-scale outer structures at high Reynolds numbers. This new finding is certainly beyond the analysis of the present study, although a suitable extension of the dynamical systems approach to such high Reynolds numbers may enable one to analyse the dynamics associated with the drag reduction mechanism (Hwang et al.…”

“…2015; Bird, Santer & Morrison 2018), and the emerging evidence suggests that there is an important Reynolds number effect (Touber & Leschziner 2012; Chung, Hurst & Yang 2013; Gatti & Quadrio 2013; Agostini, Touber & Leschziner 2014; Hurst, Yang & Chung 2014; Agostini & Leschziner 2021; Marusic et al. 2021). The time-harmonic spanwise wall oscillation can also be extended to a spatial form, i.e.…”

Drag Reduction of Turbulent Boundary Layers by Travelling and Non-Travelling Waves of Spanwise Wall Oscillations

“…The only experimental data to date for this type of wall forcing are provided by [17], who applied the streamwise travelling wave on the pipe flow, and by [18] for the high-Reynolds number boundary layer.…”

Genetic Algorithm-based control of the wake of a bluff body

“…Some examples are reported in [19,20,21,22,23,24]. Other examples are related to the efficient implementation of the spanwise wall oscillation, as reported in the recent DNS investigation by Marusic et al [25].…”