Abstract
The friction drag reduction in a turbulent channel flow generated by surface-mounted rotating disc actuators is investigated numerically. The wall arrangement of the discs has a complex and unexpected effect on the flow. For low disc-tip velocities, the drag reduction scales linearly with the percentage of the actuated area, whereas for higher disc-tip velocity, the drag reduction can be larger than the prediction found through the linear scaling with the actuated area. For medium disc-tip velocities, all the cases which display this additional drag reduction exhibit stationary-wall regions between discs along the streamwise direction. This effect is caused by the viscous boundary layer which develops over the portions of stationary wall due to the radial flow produced by the discs. For the highest disc-tip velocity, the drag reduction even increases by halving the number of discs. The power spent to activate the discs is instead independent of the disc arrangement and scales linearly with the actuated area for all disc-tip velocities. The Fukagata-Iwamoto-Kasagi identity and flow visualizations are employed to provide further insight into the dynamics of the streamwise-elongated structures appearing between discs. Sufficient interaction between adjacent discs along the spanwise direction must occur for the structures to be created at the disc side where the wall velocity is directed in the opposite direction to the streamwise mean flow. Novel half-disc and annular actuators are investigated to improve the disc-flow performance, resulting in a maximum of 26% drag reduction.
Highlights
Turbulent skin-friction drag reduction has been the subject of growing interest in the fluid mechanics research community in recent decades
For medium disc-tip velocities, all the cases which display this additional drag reduction exhibit stationary-wall regions between discs along the streamwise direction. This effect is caused by the viscous boundary layer which develops over the portions of stationary wall due to the radial flow produced by the discs
Sufficient interaction between adjacent discs along the spanwise direction must occur for the structures to be created at the disc side where the wall velocity is directed in the opposite direction to the streamwise mean flow
Summary
Turbulent skin-friction drag reduction has been the subject of growing interest in the fluid mechanics research community in recent decades. Since the pioneering direct numerical simulations (DNSs) of Jung et al.[4] and the experiments of Laadhari et al.,[5] the response of wall-bounded turbulent flows to spanwise, spatially uniform sinusoidal oscillations of the wall has become one of the most studied active open-loop techniques. A generalization of the oscillating-wall and standing-wave forcing was proposed by Quadrio et al.,[10] who studied the response of a turbulent channel flow to streamwise travelling waves of spanwise wall velocity. They showed a maximum drag reduction of 47% and a maximum net energy saving of 26%. The experimental works of Gouder et al.[11] on electroactive polymers and of Choi et al.[12] on dielectric-barrier discharge plasma actuators are certainly advances in this respect
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