# Hierarchical Riblet Structures for Enhanced Drag Reduction and Broader Operational Range in Water Pipelines

## TL;DR

This paper introduces a new riblet design that reduces drag in water pipelines more effectively under varying flow conditions.

## Contribution

The novel Hierarchical Step-shaped Riblet (HSR) design achieves broader drag reduction performance in turbulent pipe flows.

## Key findings

- HSR achieved a peak drag reduction of 11.2% in experiments.
- HSR outperformed conventional and hierarchical riblets across a wider range of flow conditions.
- Multiscale geometric tuning improved performance in turbulent flows typical of water pipelines.

## Abstract

In drinking water distribution systems (DWDS), skin-friction
drag
in turbulent pipe flows contributes significantly to energy losses.
Passive drag-reducing surfaces, such as shark-skin-inspired riblets,
have shown promise in controlled environments but often underperform
under variable flow conditions. This study addresses this limitation
by developing and experimentally evaluating a Hierarchical Step-shaped
Riblet (HSR) design aimed at sustaining drag reduction under the variable
and fluctuating flow conditions typical of water pipelines. Building
on conventional riblets (CR) and hierarchical riblets (HR), the HSR
configuration introduces progressively tapered riblet tips to the
hierarchical design to reduce shear-exposed surface area while maintaining
effective interaction with vortices of varying size. Riblet designs
were fabricated using high-resolution 3D printing and tested in variable
flow conditions, simulating the flows in typical drinking water distribution
networks. Drag reduction performance was evaluated across a Reynolds
number range of 4200 to 20,000 using friction factor analysis and
nondimensional riblet spacing. The HSR design achieved the highest
peak drag reduction of 11.2% and sustained favorable performance across
a broader range of flow conditions than the CR and HR designs. The
results demonstrate that multiscale geometric tuning, combined with
reduced shear exposure, enhances drag reduction across a broadened
operational range suitable for drinking water distribution pipes.