Drag reduction by reconfiguration of highly flexible structures subjected to fluid flow

在大多数传统的工程应用中，结构被设计成坚固的，以至于它们必须承受的载荷不会使它们显著变形。在自然界中，情况恰恰相反。尤其是在流体负荷方面，自然结构往往是柔韧的，而人造结构则是刚性和坚韧的。植物寻求最大限度地利用它们的表面积来捕捉最多的阳光，当它们受到流体载荷时，通过改变它们的形状来利用它们的灵活性，无论是水流还是风。我们说它们是重新配置的。通过在流体载荷下弯曲和扭曲，植物减少了垂直于水流的投影面积，也变得更加流线型。通过这两个主要的重构机制，植物必须承受的阻力载荷不会随着它们所承受的水流速度的平方而增长--就像在雷诺数较高的刚性钝体上一样--而是以一种不那么明显的方式增长。关于水生和陆生植物重构的实验测量在文献中很多，对于了解植物阻力随水流速度的标度也有很大的兴趣。然而，由于植物复杂的几何形状和材料，以及样品之间的显著差异，理论解释很难从实际植物中获得。正是出于这个原因，我们通过研究受流作用的梁和板等简单结构来寻求对重构的基本理解。我们试图通过风洞实验和数值模拟对简单弹性结构的重构有一个基本的了解。我们的目标是描绘一幅完整的关于植物重组的力学方面的图景及其对阻力缩放的影响。为了设计更好的模型来理解和预测风对森林、农作物和海岸植被的损害，以及研究水生和陆生植物对其环境的适应，了解对植被的流体负荷是至关重要的。

In most traditional engineering applications, structures are designed to be stiff such that the loads they must bear do not deform them substantially. In nature, the contrary is true. Especially when it comes to fluid loading, natural structures tend to be compliant and flexible whereas man-made structures are rigid and unyielding. Plants, which seek to maximize their surface area to capture the most sunlight, make use of their flexibility by changing their shape when they are subjected to a fluid loading, whether water flow or wind. We say that they reconfigure.By bending and twisting under fluid loading, plants reduce their projected area perpendicular to the flow, and also become more streamlined. Through these two primary mechanisms of reconfiguration, the drag load that plants must support does not grow with the square of the velocity of the flow they are subjected to - as it would on a rigid bluff body at a high Reynolds number - but rather in a less pronounced way. Experimental measurements on the reconfiguration of aquatic and terrestrial plants are abundant in the literature, and a large interest exists for an understanding of the scaling of drag of plants with flow velocity. However, theoretical interpretation is difficult to obtain from real plants because of their complex geometries and materials as well as the significant variations between specimens. It is for this reason that a fundamental understanding of reconfiguration is sought by studying simple structures such as beams and plates subjected to flow. We seek to gain a fundamental understanding of the reconfiguration of simple elastic structures via wind tunnel experiments and numerical simulations. Our goal is to paint a complete picture of the mechanical aspects of plant reconfiguration with all its mechanisms and their effects on drag scaling.It is essential to understand the fluid loadings on vegetation in order to devise better models to comprehend and predict wind damages to forests, crops and shore vegetation, as well as to study the adaptation of aquatic and terrestrial plants to their environment.