Fig. 3: Drag reduction and net power savings increase with friction Reynolds number far above predicted values when oscillatory surface actuation couples to large eddies.
From: An energy-efficient pathway to turbulent drag reduction
DR(%) and NPS(%) for small-eddy actuation (a, b) confirming an \({{{{{{{{\mathrm{Re}}}}}}}}}_{\tau }\) trend of DR predicted by the formulation of Gatti and Quadrio13,GQ and resulting in negative NPS. In contrast, large-eddy actuation (e, f) leads to DR increase at higher \({{{{{{{{\mathrm{Re}}}}}}}}}_{\tau }\) (dark red) to 10−15%, ~ 10-fold greater than predicted and resulting in positive NPS ≈ 10%. c, d Trends of DR(%) and NPS(%) for an intermediate actuation corresponding to the large-scale cut-off \({T}_{{{{{{{{\mathrm{osc}}}}}}}}}^{+}\approx 350\). Open squares indicate large-eddy simulations (LES), circles indicate hot-wire anemometry experiments, triangles indicate drag balance experiments and crosses indicate direct-numerical simulation (DNS) result of Gatti and Quadrio13. Error bars indicate one standard deviation estimated uncertainty ranges. Dashed lines connecting experimental and computational measurements are included as a visual aid only. See Supplementary Table 1 for details of the flow and actuation.
