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USING 3D-PRINTED ANALOGUES TO UNDERSTAND THE AERODYNAMICS OF COMPLEX ICE PARTICLES

使用 3D 打印类似物了解复杂冰颗粒的空气动力学

基本信息

批准号：
NE/R00014X/1
负责人：
Chris Westbrook
金额：
$ 48.2万
依托单位：
University of Reading
依托单位国家：
英国
项目类别：
Research Grant
财政年份：
2017
资助国家：
英国
起止时间：
2017 至无数据
项目状态：
已结题

来源：
https://gtr.ukri.org/projects?ref=NE%2FR00014X%2F1
关键词：
USING 3D PRINTED ANALOGUES UNDERSTAND

项目摘要

Many of the clouds in the atmosphere contain ice particles. These ice particles play an important role in the climate system, because high-altitude cirrus clouds cover around 30% of the globe at any one time, and act to warm the planet. Ice particles are also important for the development of precipitation, and not only in cold polar climates: even in mid-latitudes (like the UK), over three quarters of the precipitation that falls originates as snowflakes aloft - it is just that most of it melts before arriving at the surface.Ice particles, both in clouds and in snowfall at the surface, precipitate. In other words, they are able to grow large enough to fall through the air. This has several implications. The most obvious of these is that the rate at which the particles fall out controls the transport of water vertically through the atmosphere and to the surface. More subtly, the movement of each ice particle through the air directly influences the rate at which the particle grows, evaporates and melts. For example, if the air is humid enough, water molecules will diffuse to the ice crystal's surface and deposit there, leading to growth. If the particle is stationary, this growth occurs steadily but slowly, because the growing ice crystal depletes the vapour around it, leading to a shallow gradient in the concentration of molecules. If the particle is falling, this growth can occur much faster, because the ice crystal is constantly falling into fresh, humid air, leading to steep concentration gradients. The quantitative details of exactly how fast an ice particle of a given size and shape falls, and how much the growth rates are enhanced by, is determined by the airflow around the ice particle, or its aerodynamics. Unfortunately, this is an area of cloud physics where our understanding is extremely limited. The aerodynamics of simple shapes like spheres, spheroids and discs is well studied. However it is clear from observation of natural ice particles that they are not simple in their geometry. Instead the particles are often complex and irregular in their shape. We have almost no high-quality data on the aerodynamics of such particles. As a result, even state-of-the-art microphysical models are forced to approximate the aerodynamical effects on ice processes as though these complex irregular particles were spheres or spheroids, hoping that this is an adequate approximation. To solve this problem, experimental data is needed for the aerodynamics of particles with the complex shapes that we observe in the atmosphere. The stumbling block is that making suitable observations of natural ice particles in free-fall is extremely challenging. In snowfall at the surface the particles are small, fragile, easily blown by the wind, and likely to melt or evaporate if not handled with great care. Direct sampling of falling particles in cirrus clouds is impossible. In neither case is it possible to directly determine the airflow around the particle or the influence of that flow on the microphysical process rates.In this project we overcome these problems with the use of analogues. Using 3D printing techniques we will create plastic particles with the same complex geometry as natural ice particles. By dropping the particles in tanks of liquids, and through air in the laboratory and a vertical wind tunnel, we can determine how the fall speed of the particles is controlled by their size and geometry. Exploiting recent developments in tomographic particle imaging velocimetry we can measure the airflow around the falling analogues. From this we can directly determine how the airflow enhances the particle growth, evaporation and melting rates.

大气层中的许多云都含有冰粒。这些冰粒在气候系统中扮演着重要的角色，因为高海拔卷云在任何时候都覆盖了地球仪的30%，并使地球变暖。冰粒对降水的形成也很重要，不仅是在寒冷的极地气候中：即使在中纬度地区（如英国），超过四分之三的福尔斯降水都是由高空的雪花产生的--只是大部分在到达地面之前就融化了。云中和地面降雪中的冰粒都沉淀下来。换句话说，它们能够长得足够大，可以从空中落下。这有几个含义。其中最明显的是，颗粒脱落的速度控制着水垂直通过大气层到达地表的传输。更微妙的是，每个冰粒在空气中的运动直接影响着冰粒生长、蒸发和融化的速度。例如，如果空气足够潮湿，水分子会扩散到冰晶的表面并在那里存款，导致生长。如果粒子是静止的，这种增长会稳定但缓慢地发生，因为不断增长的冰晶耗尽了它周围的蒸汽，导致分子浓度的浅梯度。如果粒子下落，这种增长会发生得更快，因为冰晶不断落入新鲜潮湿的空气中，导致陡峭的浓度梯度。给定大小和形状的冰粒福尔斯的速度以及增长率的增加程度的定量细节取决于冰粒周围的气流或其空气动力学。不幸的是，这是云物理学的一个领域，我们的理解极其有限。简单形状如球体、椭球体和圆盘的空气动力学研究得很好。然而，从对天然冰粒的观察中可以清楚地看出，它们的几何形状并不简单。相反，颗粒通常是复杂和不规则的形状。我们几乎没有关于这种颗粒的空气动力学的高质量数据。因此，即使是最先进的微物理模型也被迫近似于空气动力学对冰过程的影响，就好像这些复杂的不规则颗粒是球体或椭球体一样，希望这是一个足够的近似。为了解决这个问题，需要我们在大气中观察到的具有复杂形状的颗粒的空气动力学的实验数据。障碍在于，对自由落体中的天然冰粒进行适当的观测是极其具有挑战性的。在地面的降雪中，颗粒很小，很脆弱，很容易被风吹走，如果不小心处理，很可能融化或蒸发。对卷云中下落的粒子进行直接取样是不可能的。在这两种情况下都不可能直接确定颗粒周围的气流或该气流对微物理过程速率的影响。在本项目中，我们通过使用类似物克服了这些问题。使用3D打印技术，我们将创建具有与天然冰粒相同的复杂几何形状的塑料颗粒。通过将颗粒放入液体罐中，并通过实验室中的空气和垂直风洞，我们可以确定颗粒的下落速度如何受其大小和几何形状的控制。利用层析粒子成像测速技术的最新发展，我们可以测量下降的类似物周围的气流。由此，我们可以直接确定气流如何增强颗粒生长、蒸发和熔化速率。