基于轨迹图像的非均匀沙风沙流颗粒跃移运动参数研究

Wind-blown sand has an important influence on the process and evolution of aeolian landforms, thus as sand ripple and dunes. The sand grains in natural world are all mix-sized grains. Most studies pay attention to uniform size grains and usually investigate the motion parameters of saltating sand grains (velocity, size and concentration) separately due to its complexity and the limitation of measurement methods and technique. Wind-blown sand of mix-sized grains is taken as the study object. Firstly, movement of saltating grains of different height is measured by using a high-speed digital camera system in a wind tunnel, and then the movement parameters of grain velocity, size and concentration is analyzed simultaneously to reveal initial lift-off velocity distribution function P(u,α,d) and volume concentration distribution With the aid of image processing technology. Secondly, the size distribution and size statistics (mean size, sorting, skewness and kurtosis) of all sand trap compartments are analyzed to reveal the spatial distribution laws of each size group. Thirdly, The above results are compared with the results of wind-blown sand movement model in which P(u,α,d) is as an initial condition to modify P(u,α,d), as there is some data loss of P(u,α,d) due to the very high concentration at the near surface area. Finally, the movement law of mix-sized grains among different size groups is revealed. This research also has important significance both theoretical and practical, including perfecting the theory of aeolian physics, accurately predicting the evolution of aeolian landform and making reasonable measures of prevent and control sand erosion.

风沙流运动对沙波纹、沙丘等风沙地貌的形态和运动具有重要影响。自然界风沙流都是非均匀沙风沙流，然而由于风沙流运动的复杂性以及测量技术和方法的限制，目前风沙流及其运动参数研究主要针对均匀沙风沙流，而且缺少沙粒速度、粒径和浓度的同步测量研究。本项目拟在室内风洞中利用数字高速摄像技术对非均匀沙风沙流跃移沙粒运动轨迹进行拍摄，结合图像处理技术对沙粒的速度、粒径和浓度开展同步测量，进而建立非均匀风沙流中跃移运动沙粒起跳速度概率分布P(u,α,d)和体积浓度分布函数；分析风沙流通量的质量和粒径统计参数，获得各粒级组分输沙率分布规律；通过风沙流模型模拟计算输沙率分布和粒径统计参数并和风洞实验数据对比，修正近床面运动沙粒测量时的部分数据缺失对P(u,α,d)的影响，最终揭示非均匀沙各粒级组分沙粒的运动规律。这对我们完善风沙流动力学理论、准确预测风沙地貌形态变化以及合理制定防沙治沙措施等具有重要意义。

在风洞中通过测量跃移沙粒速度、输沙率分布、浓度分布和稳态风沙流输沙率分布数值模拟，研究了风沙流中各粒径组分间的相互作用规律。主要结果如下：（1）沙粒冲击速度和起跳速度的概率分布都符合对数正态函数分布，冲击角度和起跳角度的概率分布都符合指数函数分布。所有速度对应的角度概率分布也都符合指数函数分布，该指数函数的拟合参数和冲击速度呈对数函数关系，和起跳速度呈线性关系，最后给出了沙粒速度和角度的独立假设联合概率分布和条件联合概率分布函数公式。（2）以两种联合概率分布作为稳态风沙流模型中沙粒起跳初始条件数值计算了输沙率分布，并和实验结果进行了对比。条件联合概率分布的计算结果和实验结果更加吻合，4组实验的平均误差为5.8%，而独立假设的为39.2%，由此证明了采用条件联合概率分布的必要性。（3）通过跃移沙粒运动图人工解析，发现沙粒总体积浓度与各粒级组分浓度都随着高度增加而呈负指数趋势减少，各粒径组分的浓度在高度上的衰减速率随粒径增大而增大，即大粒径组分衰减的更快。（4）在风洞中测量了6个风程长度的输沙率分布，研究了风程效应对总输沙率、输沙率分布和各组分的输沙率分布等影响。总输沙率分布和各粒级组分输沙率分布的衰减指数都在很短距离的快速衰减后趋于稳定，且显著小于总输沙率的饱和风程长度。随风程长度的增加，各粒级组分的参与程度和运动规律也有变化，风沙流中各粒级组分比例和实验用沙中各粒级的比例顺序不完全一致。最大主成分粒度176.0μm明显小于实验用沙的主成分的粒度209.3μm。这表明风沙流中存在明显分选作用。同时，沙床面大粒级组分对小粒级组分有遮蔽保护作用。本项目研究不仅结果揭示了非均匀沙运动规律，也为合理选择风洞参数和实验条件提供了依据。