Twelve-hour real-time and synchronous measurements of the three-dimensional components of the near-surface turbulence at four heights(8,16,32 and 47 m)during a dust storm are obtained by sonic anemometers for the first time.The experimental results show that(1)the mean vertical velocities at all four heights have considerably positive values during the dust storm,illustrating a strong upward flow near the surface;(2)during the dust storm,the mean vertical velocity is higher at 8 m than at 16 and 32 m,implying that the convection is stronger at 8 m than at the other two heights;and(3)when the longitudinal velocity is a maximum,the mean of the vertical components of wind velocities at 8 m is a minimum and the standard deviation is a maximum,which reveals weaker vertical convection during the period of greatest dust storm intensity,while the turbulence intensity contrarily increases,which may result in more entrainment of surface dust.The analysis results also show that the standard deviations of the longitudinal and vertical velocities increase as the mean inflow velocity increases,but the standard deviation of the lateral velocity is hardly affected by the mean inflow velocity.Based on the calculated skewness and kurtosis of the longitudinal,vertical and lateral velocities every 20 min at different heights,it is seen that all three components obviously fluctuate with height,which further indicates the importance of performing real-time and synchronous measurements at different heights near the surface during a dust storm so as to reveal the three-dimensional turbulence structure accurately.
The discrete material, which belongs to the category of soft materials, is one of the most prevalent forms of matter in nature and engineering fields. These materials often exhibit abundant and complex mechanical properties which are still far from being perfectly understood. From the view of multi-scale framework concentrated on the ＇bridge＇ role in the macro-micro relation, this review mainly introduces some theoretical investigations of mechanical behaviors in discrete materials, including the continuum constitutive model based on the macroscopic phenomenological approach and coupled micro-macro approach, the statistical analysis of some microscopic physical quantities involved contacted forces between particles and its transmission within the whole system, and the statistical analysis for some microscopic processes in aeolian landform systems involving the grain-bed impact, the transportation and sedimentation of wind-blown sand flux, et al. Finally, some further worthwhile challenges in these fields are suggested.
Xiaojing Zheng Dengming Wang (Key Laboratory of Mechanics on Western Disaster and Environment,the Ministry of Education of China,Lanzhou University,Lanzhou 730000,China)
