In the race to curb energy and oil consumption, zeroing of wall frictional forces is highly desirable. The turbulent skin friction drag at the solid/liquid interface is responsible for substantial energy losses when conveying liquids through hydraulic networks, contributing approximately 10% to the global electric energy consumption. Despite extensive research, efficient drag reduction strategies effectively applicable in different flow regimes are still unavailable. Here, we use a wall-attached magnetic fluid film to achieve a wall drag reduction of up to 90% in channel flow. Using optical measurements supported by modelling, we find that the strong damping of wall friction emerges from the co-existence of slip and waviness at the coating interface, and the latter is a key factor to obtain almost complete wall drag reduction across laminar and turbulent flow regimes. Our magnetic fluid film is promising and ready to be applied in energy-saving and antifouling strategies in fluid transport and medical devices.

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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 set-up, 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 (Reynolds number Re<4000) 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, with observed drag reduction rates up to 95 %. 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 outcompetes the extra turbulence so that drag is still reduced.@en

We revisit the classical entrainment experiments for gravity currents on inclined slopes (Ellison and Turner, J. Fluid Mech. 6, 423-448, 1959). We derive an entrainment relation that couples the entrainment rate E to the production of turbulence kinetic energy, the net effect of buoyancy and inner layer. Using direct numerical simulations that are run for durations long enough for the flow to reach universal self-similarity, we show that the net effect of inner layer processes on entrainment is very small and that buoyancy has an almost negligible effect on E. It is demonstrated that the dominant process causing entrainment is turbulence production due to shear. Second, we observe that for all simulations the eddy diffusivity and dissipation rate can be parameterised using the turbulence kinetic energy and shear parameter. This information can be used to derive an entrainment law which is in good agreement with the Direct Numerical Simulation (DNS) results. We discuss the potential reasons for why this result is significantly different from experiments and the classical entrainment law introduced by Ellison and Turner.@en