Drag reduction induced by superhydrophobic surfaces in turbulent pipe flow

TL;DR

This study investigates how superhydrophobic surfaces with specific textures can significantly reduce drag in turbulent pipe flow by trapping air bubbles, leading to increased flow rates through slip effects.

Contribution

It models the effects of streamwise grooves with alternating boundary conditions, demonstrating the impact of surface texture and solid fraction on drag reduction in turbulent flow.

Findings

Substantial drag reduction observed with superhydrophobic textures.

Flow rate increases due to mean slip velocity at the wall.

Turbulent structures are altered, affecting flow dynamics.

Abstract

The drag reduction induced by superhydrophobic surfaces is investigated in turbulent pipe flow. Wetted superhydrophobic surfaces are shown to trap gas bubbles in their asperities. This stops the liquid from coming in direct contact with the wall in that location, allowing the flow to slip over the air bubbles. We consider a well defined texture with streamwise grooves at the walls in which the gas is expected to be entrapped. This configuration is modelled with alternating no-slip and shear-free boundary conditions at the wall. With respect to classical turbulent pipe flow, a substantial drag reduction is observed which strongly depends on the grooves' dimension and on the solid fraction, i.e. the ratio between the solid wall surface and the total surface of the pipe's circumference. The drag reduction is due to the mean slip velocity at the wall which increases the flow rate at a fixed pressure drop. The enforced boundary conditions also produce peculiar turbulent structures which on the contrary decrease the flow rate. The two concurrent effects provide an overall flow rate increase as demonstrated by means of the mean axial momentum balance. This equation provides the balance between the mean pressure gradient, the Reynolds stress, the mean flow rate and the mean slip velocity contributions.