Drag reduction of Newtonian fluid in a circular pipe with a highly water-repellent wall

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 clarif ied. Experiments were carried out to measure the pressure drop and the velo city profile of tap water and an aqueous solution of glycerin flowing in pi pes 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 profi le measured in this experiment gives the slip velocity at the pipe wall, an d it was shown that the shear stress is directly proportional to the slip v elocity. 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 equatio n, and it agrees well qualitatively with the experimental data. The main reasons for the fluid slip are that the molecular attraction betwe en the liquid and the solid surface is reduced because the free surface ene rgy of the solid is very low and the contact area of the liquid is decrease d compared with a conventional smooth surface because the solid surface has many fine grooves. Liquid cannot flow into the fine grooves owing to surfa ce tension. These concepts are supported by the experimental result that dr ag reduction does not occur in the case of surfactant solutions.