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Drag reduction of Newtonian fluid in a circular pipe with a highly water-repellent wall
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1999
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EngineeringPressure TransducerDrag ReductionLiquid-liquid FlowFluid MechanicsMechanical EngineeringFluid SlipNewtonian FluidGas-liquid FlowFluid PropertiesWater-repellent WallTransport PhenomenaHydrodynamic StabilityParticle-laden FlowPipe FlowFlow PhysicHydromechanicsDisperse FlowMultiphase FlowDrag Reduction PhenomenaCivil EngineeringFluid-solid InteractionFlow Measurement
Fluid slip on highly water‑repellent pipe walls arises from reduced liquid–solid attraction and the inability of water to penetrate fine surface grooves, leading to a low contact area. The study measured pressure drop and velocity profiles of tap water and glycerin in water‑repellent pipes using a pressure transducer and hot‑film anemometer to characterize slip behavior. Experiments revealed a 14 % drag reduction in laminar tap water flow, confirmed in degassed water, with slip velocity proportional to shear stress and a friction‑factor formula that matches data; drag reduction vanished for surfactant solutions.
Drag reduction phenomena, in which 14% drag reduction of tap water flowing in a 16 mm-diameter pipe occurs in the laminar flow range, have been clarified. Experiments were carried out to measure the pressure drop and the velocity profile of tap water and an aqueous solution of glycerin flowing in pipes with highly water-repellent walls, by using a pressure transducer and a hot-film anemometer, respectively. The same drag reduction phenomena also occurred in degassed tap water when using a vacuum tank. The velocity profile measured in this experiment gives the slip velocity at the pipe wall, and it was shown that the shear stress is directly proportional to the slip velocity. The friction factor formula for a pipe with fluid slip at the wall has been obtained analytically from the exact solution of the Navier–Stokes equation, and it agrees well qualitatively with the experimental data. The main reasons for the fluid slip are that the molecular attraction between the liquid and the solid surface is reduced because the free surface energy of the solid is very low and the contact area of the liquid is decreased compared with a conventional smooth surface because the solid surface has many fine grooves. Liquid cannot flow into the fine grooves owing to surface tension. These concepts are supported by the experimental result that drag reduction does not occur in the case of surfactant solutions.