Turblent Drag Reduction by Riblets Coated with Drag Reducing Additives

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Riblets is a drag reducing device in turbulent flow, but increase the drag like rough surface at the velocity higher than the effective region. Although the long-chained polymer is degradable, very low concentration of a few ppm is enough to reduce the turbulent drag. The effect is limited to a narrow transitional region near wall, and the maximum drag reduction reaches more than 80%. The combination of polymer additives and riblets is expected to introduce a favorable synergistic effect. For the combined system of a riblet pipe and polymer additives, the pressure loss and the velocity profiles were measured. For higher h^+, the synergism was found out, and its cause was discussed on the speculation that the drag increase by riblets would be suppressed by the additives. Since the additives thicken the wall layr, the relative height of h to the thickness delta of a wall layr is lowered. It was shown that the drag reducing rate was approximately a function of h^+/delta^+ for the combined system. Therefore, the wall layr thickening by additives would be a main reason for the synergisitic effect for higher h^+.If the polymer additives are effectively coated on the wall, this method is also expected to be useful for extemrnal flow. From this point of view, we investigated how to coat the additives on the wall and its drag reducing ability. To maintain the effect of the polymer coating for long time, the role of a binder is important. We chose Polyvinyl Alcohol (PVA) as a binder which dispersed fine polymer islands in sea of PVA.The coated surface on smooth wall produced the drag reduction of 5 to 6%. When the additives were coated on riblets, the drag reduction more than 20% was obtained. The range of effective Reynolds number was also extended from 10000 to 20000.