风沙流中风速廓线的数值模拟与实验验证

如何描述风沙流中被风沙运动改变了的风速廓线是风沙相互作用研究中的关键问题之一.该文中将跃移风沙流视为一种颗粒拟流体,将跃移颗粒对气流产生的阻力用颗粒流的阻力系数来表达,建立了描写两场相互作用的数学模型.颗粒流的阻力系数采用了前人在液态流化床研究中得出的阻力系数表达形式,通过引入一个修正系数,使其适用于风沙流(气-固两相流).将风沙边界层划分为跃移颗粒所产生的阻力不可忽略的内边界层和跃移颗粒阻力可以忽略但受内边界层影响的外边界层,分别建立了内边界层和外边界层的风速廓线表达式.应用所建立的数学模型,根据由风洞实验测定的跃移风沙流的浓度分布和速度分布资料,计算了跃移风沙流中的风速廓线,并与风洞实测结果进行了对比.结果表明,计算风速廓线与实测风速廓线吻合得比较好,在半对数图上均为上凸的曲线,有别于无风沙运动时的直线.跃移边界层外风速分布可较好地用对数函数来描述.对风沙流中风速廓线的进一步分析证实了风沙物理学奠基人Bagnold在其早期观测风沙流中的风速廓线时提出的"结点现象"(Bagnold结),该结点的高度随风速的增大而升高,随颗粒粒径的增大而降低.根据数值模拟和模拟实验,可以认为有风沙运动的动床剪切风速是综合反映风场与跃移层以及地表之间相互作用的物理量.

Numerical and experimental simulation of the wind velocity profile with a blowing sand cloud

Describing the wind velocity profiles modified by blown sand movement is among the central issues in studying the wind-sand interaction in a blowing sand cloud. This paper develops the mathematical models of the wind-sand interaction, taking the saltating cloud as fluidized flow. The force exerted on the airflow by saltating particles is expressed in terms of the drag coefficient of the fluidized particle flow. The drag coefficient is introduced by modifying the expressions for the drag coefficient of the fluidized liquid-particle flow obtained by previous researchers by a correction factor. The saltation boundary layer is divided into an inner boundary layer in which the saltating particles exert significant force on the airflow, and an outer boundary layer in which saltating particles exert insignificant force on the airflow, but is affected by the inner boundary layer. Expressions for the wind velocity profiles of the inner boundary layer and outer boundary layer are developed. The simulated wind velocity profiles based on the velocity and concentration profiles obtained from wind tunnel tests are compared to those directly measured. They show reasonably good agreement. It is revealed that the wind profiles with saltation cloud are up-convex curves on the log-linear plot rather than the straight lines of the clean wind. The up-convexity of the curves increases as wind velocity increases. The wind velocity profiles above the saltation boundary layer approach the logarithmic law. The results confirm the existence of the so-called Bagnold's kink, which is found to get higher with increasing wind velocity and is lowered with increasing particle size. It is suggested that the wind shear velocity with a saltating cloud reflects the response of airflow to the ground surface and the movement of saltation movement.