# Drag Reduction by Herringbone Riblet Texture in Direct Numerical Simulations of Turbulent Channel Flow

## TL;DR

This study investigates a bird-inspired herringbone riblet texture for turbulent drag reduction using direct numerical simulations, revealing that certain patterns can slightly reduce drag while others increase it significantly due to secondary flows.

## Contribution

It extends the drag decomposition method to textured walls and analyzes how herringbone riblet patterns affect turbulent drag, providing insights into their effectiveness.

## Key findings

- Herringbone riblet texture with large spanwise wavelength reduces drag by about 2%.
- Small spanwise wavelength increases drag up to 73% due to secondary flows.
- Convergent/divergent riblet patterns are generally detrimental to drag reduction.

## Abstract

A bird-feather-inspired herringbone riblet texture was investigated for turbulent drag reduction. The texture consists of blade riblets in a converging/diverging or herringbone pattern with spanwise wavelength $\Lambda_f$. The aim is to quantify the drag change for this texture as compared to a smooth wall and to study the underlying mechanisms. To that purpose, Direct Numerical Simulations of turbulent flow in a channel with height $L_z$ were performed. The FIK-identity for drag decomposition was extended to textured walls and was used to study the drag change mechanisms. For $\Lambda_f/L_z \gtrsim O(10)$, the herringbone texture behaves similarly to a conventional parallel-riblet texture in yaw: the suppression of turbulent advective transport results in a slight drag reduction of 2%. For $\Lambda_f/L_z \lesssim O(1)$, the drag increases strongly with a maximum of 73%. This is attributed to enhanced mean and turbulent advection, which results from the strong secondary flow that forms over regions of riblet convergence/divergence. Hence, the employment of convergent/divergent riblets in the texture seems to be detrimental to turbulent drag reduction.