The effect of porosity on the drag of cylinders

Abstract It is well known that perforation of a flat plate reduces its drag when exposed to a flow. However, studies have shown an opposite effect in the case of cylinders. Such a counterintuitive result can have significant consequences on the momentum modelling often used for wind turbine performance predictions, where increased porosity is intrinsically linked to lower drag. Here, a study of the drag of various types of porous cylinders, bars and plates under steady laminar inflow is presented. It is shown that, for most cases, the drag decreases with increased porosity. Only special types of perforations can increase the drag on both cylinders and bars, either by enhancing the effect of the rear half of the models or by organizing the wake structures. These rare occurrences are not relevant to wind turbine modelling, which indicates that current momentum models exhibit the qualitatively correct behaviour.

摘要 众所周知，平板穿孔在受到气流作用时会降低其阻力。然而，研究表明在圆柱体的情况下会产生相反的效果。这种违反直觉的结果可能对常用于风力涡轮机性能预测的动量模型产生重大影响，在该模型中，孔隙率增加与阻力降低有内在联系。在此，对稳定层流流入条件下各种类型的多孔圆柱体、杆件和板材的阻力进行了研究。结果表明，在大多数情况下，阻力随着孔隙率的增加而降低。只有特殊类型的穿孔会通过增强模型后半部分的影响或通过组织尾流结构来增加圆柱体和杆件的阻力。这些罕见的情况与风力涡轮机建模无关，这表明当前的动量模型表现出定性正确的行为。