风沙运动的理论模拟和风洞实验对比研究

Theoretical simulation and wind tunnel experiment in wind-blown sand movement

针对输沙率这一风沙运动的重要问题，在沙坡头野外风洞进行了实验观测，同时建立热对流一扩散作用下的风沙运动多场耦合模型，采用大涡模拟方法对其进行数值模拟研究。通过对比实验观测和理论模拟，结果表明：实验和理论模拟得到的风速和输沙率都比较吻合，风速近似可分为两部分，在35cm高度以上，风场受沙粒的影响较弱，风速服从对数分布，与净风场一致；在0～35cm高度区间，受大量跃移运动沙粒的阻滞作用使得风速单调减小，随着风速加强，跃移运动沙粒的数量也在增大，风速梯度逐渐减小。在风沙流发展过程中，开始阶段输沙率随时间呈指数增大，而后逐渐减小，直至达到动态稳定；随着风速加强，输沙率变大，风沙运动达到动态稳定的时间变短。风洞实验和理论模拟的输沙率结果在10cm以上吻合得很好，但在10cm以下，风洞实验和理论模拟差别较大。同一风速下，采用最小二乘法对风洞实验和理论模拟的输沙率进行拟合并得到拟合公式：输沙率沿高度呈指数规律递减。同时，在不同风速下对同一高度层输沙率对比分析表明，贴近地表处（0～10cm）高度层内输沙率随风速增大所占的百分率降低；而在10cm以上高度层内，随着风速的增大，其输沙率所占总输沙率的百分比却有明显增加。

The sand transport rate is the important data to describe wind erosion in wind-blown sand movement. In this paper the sand transport rate is measured in wind tunnel in Saopotou, and considering thermal diffusion and heat convection the multi-field coupling model is established to numerically simulate the process of wind-blown sand movement with LES （Large Eddy Simulation）. Comparing the theoretical results with the experimental results, it is found that wind speed and the sand transport rates of numerical simulation results both are good agreement with the experimental results. With height increasing, wind speed can be divided into two parts： above 35 cm height, wind speed is affected weakly by moving sands, and wind speed obeys the logarithmic distribution as net wind field; at the interval of 0-35 cm height, acted by large amount of sahating sands wind speed is blocked and decreases monoto- nously. With the wind speed strengthening, the quantity of sahating sands also increases, which causes wind speed gradient decreases gradually. During the process of wind blown sand flux, at the beginning the sand transport rate in- creases exponentially with time, and then decreases gradually until it reaches the dynamic stability. With wind speed strengthening , the sand transport rate becomes larger, but the duration time becomes shorter. The experimental and theoretical results show that above 10 cm height the sand transport rates of theoretical simulation are good agreement with the experimental results, but below 10 cm height, the theoretical results are obviously bigger in quantity than that of the experimental results. This is because in wind tunnel experiment sand sampler will interfere with the air flow, which causes that some moving sands could not entry sand sampler. Especially at the interval of 0-10 cm height air flow affected by sand sampler will form spiral vortex with undercutting for the sand bed, which is unfavor- able for sand sampler to collect the moving sands. At the same time when wind velocity is bigger, suspension quanti- ty of sands increases, which also could not entry sand sampler. There are other factors to cause the experimental re- suits to be smaller than that of true values. But the theoretical simulation solves the problems of low trap efficiency of sand sampler near the sand bed and reduces the error. At the same wind velocity the experimental and theoretical data of the sand transport rate are fitted with least squares method, and the fitting equation can be obtained, which indicates that the sand transport rate exponentially decreases with height increasing. And in order to further describe the characteristics of the sand transport rate, they are calculated at several height intervals： total sand transport rate and the sand transport rate at different height interval both increase rapidly with wind velocity enlarging. In the in- terval of 0-10 cm height the percent of the sand transport rate diminishes with the wind speed enlarging, but above 10 cm height in the same interval the percent of the sand transport rate increases obviously with the wind speed en- larging