Mechanics of Particle Entrainment and Transport by Wind

风在地球表面无处不在，在火星等其他行星上也是如此。在存在任何松散颗粒来源的地方，风的流体阻力可能会卷绕并携带这些颗粒中的一小部分，距离从几毫米到几米不等，最终对于长期悬浮的尘埃来说，可能超过数千公里。因此，这些颗粒的迁移可能会影响空气质量、能见度、太阳辐射的吸收和传输、气候、土壤肥力，以及通过沉积、表面地形(如沙丘)。自1989年以来，我的研究项目由NSERC发现基金资助，其核心是基于对边界层风洞中风沙输送的地球物理的调查。世界上可能只有六个风洞设施在任何给定的时间从事这项工作，而我在特伦特环境风洞开展的研究项目是时间最长、最不间断和支持最好的项目之一。与目前NSERC发现拨款提案相联系的总体长期目标旨在：1)继续和扩展粒子通过风传输的物理基础研究；2)检查气流中形成的相干涡旋结构与沉积沉积物表面形成的地形特征之间的关系；以及3)模拟和观察选定的环境控制对粉尘排放的作用。虽然这些目标是分开列出的，但它们之间有很大的重叠。具体目标将包括，例如，检查温度和湿度变化对靠近地表的相对较大颗粒的传输和尘埃排放的影响。尽管粒子是从冻结的表面释放出来的，并由冬季的风携带，但地球科学家对这些过程知之甚少。我们还打算在微观尺度上研究和量化颗粒与具有不同物理性质的床面的碰撞。这种碰撞起到了破坏表面并将更多的粒子释放到气流中的作用。到目前为止，风沙科学家一直在一个二维参照系中工作；也就是说，所有的粒子都被假定沿着气流的方向运动。对于非常重的粒子，情况并非如此。拟议的实验还将考虑跨距分量。目前工作中的另一个不足是，我们把粗糙表面上的气流、泥沙输移和床面发展之间的关系看作是一条“单行道”。我们实验室最近的一些工作，使用简单的可视化方法，清楚地表明这个概念是不正确的。我们希望通过使用激光多普勒测速来扩展这些研究，以详细地测量气流中的相干结构和床面的形态动力学调整之间的变化的相互作用。如果我们能够准确地了解交通系统在本质上是如何被“关闭”的，那么或许，这种知识可以用来改进缓解策略。最后，到目前为止，所有的交通模型都假定风速是稳定的--在给定的事件中是不变的--但显然在自然界中并非如此。我们最近开始了研究阵风的作用的实验。我们希望将其推广到考虑床面形态如何响应的问题。许多分析模型假定高度理想化的条件，不能反映自然界中发生的真实过程的复杂性。我的研究计划的使命是理解和衡量更现实的基本过程，以便能够更好地描述它们并预测它们的影响。

Wind is ubiquitous at the Earth’s surface, as well as on other planetary bodies such as Mars. Where any source of loose particles exists, the fluid drag of the wind may entrain and carry some fraction of these particles over distances varying from just a few millimeters to several meters and ultimately in the case of dust in long term suspension, over thousands of kilometres. As such, the transport of these particles may affect air quality, visibility, the absorption and transmission of solar radiation, climate, soil fertility, and through deposition, surface topography (e.g. dunes). The core of my research program funded by NSERC Discovery Grants since 1989 is based upon investigation of the geophysics of aeolian transport in a boundary layer wind tunnel. There are only perhaps a half dozen wind tunnel facilities in the world that are engaged in such work at any given time, while the research program that I have carried out in the Trent Environmental Wind Tunnel is one of the longest, most uninterrupted and best supported. The over-arching, long term goals tied to the present NSERC Discovery Grant proposal aim to 1) continue and extend fundamental investigations of the physics underlying particle transport by wind, 2) examine the relations between coherent vortex structures formed in airflows and the topographic features formed on the surface of sedimentary deposits, and 3) simulate and observe the role of selected environmental controls on dust emission. Although these goals are listed separately, there is strong overlap between them. Specific objectives will include, for example, examining the effects of changing temperature and humidity upon the transport of relatively large particles traveling near the surface, and the emission of dust. Although particles are emitted from frozen surfaces and carried by winter-time winds, geoscientists know little about these processes. We also intend to examine and quantify at a micro-scale, the collision of particles with bed surfaces having varied physical properties. Such collisions serve as a mechanism for the break-down of the surface and the release of further particles into the airflow. Up to this point in time, aeolian scientists have worked exclusively in a two-dimensional frame of reference; that is, all particles are assumed to travel in the direction of the airflow. For very heavy particles this is not true. The proposed experiments will consider the span-wise component as well. Another deficiency in current work is that we have considered the relationship between the airflow, sediment transport, and bedform development on rough surfaces to be a ‘one-way street’. Some of the recent work from our lab, using a simple visualization approach, clearly shows that this concept is incorrect. We would like to extend these studies by using laser Doppler anemometry to measure in detail the changing interaction between the coherent structures in the airflow and the morphodynamic adjustment of the bed surface. If we can understand exactly how in nature the transport system is ‘shut down’, then perhaps, this knowledge can be used in improving upon mitigation strategies. Finally, all transport models to this date assume that the wind speed is steady – invariant throughout a given event - but obviously in nature it is not. We have recently embarked on experiments which investigate the role of wind gusting. We wish to extend these to consider how the bed surface morphology responds. Many analytical models assume highly idealized conditions that do not reflect the complexity of real processes occurring in nature. The mission of my research program is to understand and measure fundamental processes that are more realistic, so that they can be better described and their impacts predicted.