  This study explores the application of a wall-attached ferrofluid film to
decrease skin friction drag in turbulent channel flow. We conduct experiments
using water as a working fluid in a turbulent channel flow setup, where one
wall is coated with a ferrofluid layer held in place by external permanent
magnets. Depending on the flow conditions, the interface between the two fluids
is observed to form unstable travelling waves. While ferrofluid coating has
been previously employed in laminar and moderately turbulent flows to reduce
drag by creating a slip condition at the fluid interface, its effectiveness in
fully developed turbulent conditions, particularly when the interface exhibits
instability, remains uncertain. Our primary objective is to assess the
effectiveness of ferrofluid coating in reducing turbulent drag with particular
focus on scenarios when the ferrofluid layer forms unstable waves. To achieve
this, we measure flow velocity using two-dimensional particle tracking
velocimetry (2D-PTV), and the interface contour between the fluids is
determined using an interface tracking algorithm. Our results reveal the
significant potential of ferrofluid coating for drag reduction, even in
scenarios where the interface between the surrounding fluid and ferrofluid
exhibits instability. In particular, waves with an amplitude significantly
smaller than a viscous length scale positively contribute to drag reduction,
while larger waves are detrimental, because of induced turbulent fluctuations.
However, for the latter case, slip out-competes the extra turbulence so that
drag is still reduced.
