Turblent Drag Reduction by Riblets Coated with Drag Reducing Additives

在湍流流动中，沟槽是一种减阻装置，但在高于有效区的流速下，沟槽会像粗糙表面一样增加阻力。虽然长链聚合物是可降解的，但几ppm的非常低的浓度足以减少湍流阻力。减阻效果仅限于近壁面的一个狭窄过渡区，最大减阻率可达80%以上。聚合物添加剂和肋的组合预期引入有利的协同效应。对于肋管和聚合物添加剂的组合系统，测量了压力损失和速度分布。当h^+浓度较高时，发现了协同效应，并对协同效应的原因进行了讨论，推测添加剂可以抑制脊状结构的增阻作用。由于添加剂对壁层r起增粘作用，因此h相对于壁层r的厚度δ的相对高度降低。结果表明，对于组合系统，减阻率近似为h^+/delta^+的函数。因此，添加剂的壁层增厚是高h^+协同效应的主要原因。如果聚合物添加剂有效地涂覆在壁上，这种方法也有望用于外部流动。从这一角度出发，我们研究了添加剂在壁面上的涂覆方法及其减阻性能。为了长时间保持聚合物涂层的效果，粘合剂的作用很重要。本文选用聚乙烯醇（PVA）作为粘结剂，利用PVA的分散性，在光滑的壁面上进行涂层，减阻率可达5%~ 6%。当添加剂被涂覆在肋片上时，减阻率可达20%以上。有效雷诺数的范围也从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.