Turblent Drag Reduction by Riblets Coated with Drag Reducing Additives

波纹管在湍流中是一种减阻装置，但在速度高于有效区域时，波纹管会像粗糙表面一样增加阻力。虽然长链聚合物是可降解的，但非常低的浓度几ppm就足以减少湍流阻力。效果仅限于壁面附近狭窄的过渡区域，最大减阻可达80%以上。聚合物添加剂与小颗粒的组合有望产生良好的协同效应。对于波纹管和聚合物添加剂组合体系，测量了压力损失和速度分布。对于较高的h^+，发现了增效作用，并对增效作用的原因进行了讨论，并推测了添加剂会抑制波纹的阻力增加。由于添加剂使壁层增厚，h与壁层厚度δ的相对高度降低。结果表明，复合体系的减阻率近似为h^+/ δ ^+的函数。因此，添加剂使壁层增厚可能是高h^+产生协同效应的主要原因。如果聚合物添加剂被有效地涂覆在壁上，这种方法也有望用于外部流动。从这个角度出发，我们研究了如何在壁面涂覆添加剂及其减阻能力。为了长时间保持聚合物涂层的效果，粘合剂的作用是很重要的。我们选择聚乙烯醇（PVA）作为粘合剂来分散聚乙烯醇海洋中的精细聚合物岛。光滑壁面的涂层可使阻力降低5 - 6%。当添加剂被涂覆在波纹上时，阻力降低20%以上。有效雷诺数范围也由10000扩展到20000。

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.