The Near-surface Turbulence Structures And Their Influence To Sand/dust Transport

The thesis carries out experimental studies of the near-surface wind flow and the dust concentration under the sandstorm conditions. First, continuous and synchronous observations of the near-surface flow field and the dust concentration were made utilizing the ultrasonic anemometers, the dust monitors and other devices arranged on the3#sandstorm observation tower in Minqin area. The effective data are obtained through selecting the measured experimental data, excluding the spikes and the direction adjusting pretreatment. On this basis, analysis of the mean profile, the statistical characteristics and the spectra of the near-surface velocity were made using the statistical analysis method and the spectral analysis method, and the statistical information of the near-surface flow field and their variations with the existence of dust particles are given. Further, through analyzing of the measurement data, the characteristics of turbulent coherent structures in the near-surface flow field, their spatial and temporal scales, their variations with Reynolds number are revealed, which can also be verified by wind tunnel experiment. Finally, statistical analysis of unsteady sediment transport and the horizontal and vertical transport of dust particles are made, and a random wind speed prediction model is given which can be used to forecast the random wind variations related to saltation transport. The main works are concluded as follows:1. In the aspects of wind power spectra:It was found that the widely applicable Kaimal spectrum is not suitable for describing the low-frequency part of the wind spectrum below2m, then the thesis gives a new empirical expression that can be applied to the power spectrum from the surface to50meters;2. In the aspects of wind structures:1) very large-scale motions(VLSMs) are widely existed in the near-surface flow field, and their spatial scales reach up to several kilometers and the time period of these flow structures passing through the measuring point was more than1minute.2) The turbulence structures showing a regional feature from the surface to the edge of the near surface layer:the very large-scale motions (VLSMs) existed above eight meters, and the large-scale motions (LSMs) existed below two meters and these two How structures coexisted between2m to8m.3.In the aspects of the temporal-spatial variations of PM10: VLSMs have a significant influence to the dust transport in the near-surface How field. i.e. they not only play a dominating role to the transport of dust particles above8meters, but also affects the near-wall dust transport below2m.4.In the aspects of the surface wind-blown sand flow: A prediction model was given for forecasting the random wind velocities below two meters which having an impact to the surface transport of sand particles, and it is found that the prediction model can be more accurately reflects the statistical characteristics of the wind field comparing with the experimental results.