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A turbulent pipe flow experiment was conducted where the surface of the pipe was oscillated azimuthally over a wide range of frequencies, amplitudes, and Reynolds numbers. The drag was reduced by as much as 35%. Past work has suggested that the drag reduction scales with the velocity amplitude of the motion, its period, and/or the Reynolds number. Here, we find that the key parameter is the acceleration, which greatly simplifies the complexity of the phenomenon. This result is shown to apply to channel flows with spanwise surface oscillation as well. This insight opens potential avenues for reducing fuel consumption by large vehicles and for reducing energy costs in large piping systems. [ABSTRACT FROM AUTHOR]

Liquid-infused surfaces present a novel coating for passive drag reduction in turbulent flows. Conceptually similar to superhydrophobic surfaces, which are composed of air pockets trapped within hydrophobic roughness, liquid-infused surfaces instead rely on a preferentially wetting, liquid lubricant to create a heterogeneous surface of fluid–solid and fluid–fluid interfaces. The mobility of the lubricant within the textures allows the fluid–fluid interfaces to support a finite interfacial slip velocity. In the proper configuration, the collection of slipping interfaces can result in a significant reduction in skin friction drag. However, sustaining this drag reduction is predicated on maintaining the integrity of the lubricating layer. While liquid-infused surfaces are robust to the most common sources of superhydrophobic surface failure, they exhibit a distinct susceptibility to shear/slip-driven drainage. Here, the robustness and behavior of lubricant-infused streamwise microgrooves are studied, in a turbulent channel flow facility. In the presence of external turbulent shear flow, a finite length of lubricant is found to be retained within the microtextures by a mechanism analogous to capillary rise, consistent with the observations of Wexler et al. (Phys Rev Lett 114(16):168301, 2015b), while the remainder of the lubricant is driven out of the surface. This retention mechanism is exploited to maintain a lubricating layer over a larger surface area by using a novel chemical patterning technique. Chemical barriers are scribed along the streamwise length of the grooves. These periodic barriers disrupt the continuity of the streamwise groove and inhibit the downstream drainage of the lubricant. The effectiveness of this approach is evaluated in a turbulent channel flow with promising results. [ABSTRACT FROM AUTHOR]