  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.
