Large Drag Reduction over Superhydrophobic Riblets

TL;DR

This paper presents a novel surface combining riblets and superhydrophobic features that significantly reduces drag in turbulent flows, with experimental and simulation results demonstrating over 15% drag reduction and high slip lengths.

Contribution

The study introduces a fabrication method for combined riblet-superhydrophobic surfaces and evaluates their drag reduction performance through experiments and simulations.

Findings

Achieved 15-20% drag reduction across flow conditions.

Sample with 100 μm deep grooves showed high slip length over 100 μm.

Demonstrated effectiveness in transitional-turbulent regime.

Abstract

Riblets and superhydrophobic surfaces are two demonstrated passive drag reduction techniques. We describe a method to fabricate surfaces that combine both of these techniques in order to increase drag reduction properties. Samples have been tested with a cone-and-plate rheometer system, and have demonstrated significant drag reduction even in the transitional-turbulent regime. Direct Numerical Simulations have been performed in order to estimate the equivalent slip length at higher rotational speed. The sample with 100~$\mu$m deep grooves has been performing very well, showing drag reduction varying from 15 to 20 $\%$ over the whole range of flow conditions tested, and its slip length was estimated to be over 100 $\mu$m.