Mechanics of Particle Entrainment and Transport by Wind

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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.