沙波纹中的缺陷

Supported by the National Science Foundation for Young Scholars of China “The evolutionary pattern of aeolian sand ripples”, We obtained the data of defect type and number in aeolian sand ripples, and the possibility for a specific ripple configuration to occur because of the existence of defects is calculated; The trajectory of a defect is sinusoidal and is not uniform motion, the movement of a defect is a function of its type and location; We studied the limitation to the height of aeolian bedforms; Established a model for the collision between a sand grain and a body, derived the concept of “abrasion number”. In the implement the project, we found that defect play an very important role in aeolian sand ripples.Defects are crucial to the growth process and functional property of materials, and also supposed to play an important role in pattern dynamics. The rich spatio-temporal behavior of defects in periodic stripe patterns give rise considerable interests.For instance, the deformation processes of the helical spin stripe are accompanied by nucleation, movement, and annihilation of the magnetic defects. A smectic-A with a finite concentration of unbound dislocation loops behave like a nematic. Molecular genetic studies with zebrafish generate patterns of mutant lines. Furthermore, minutiae(defects) are distributed in fingerprints, and are considered as the most commonly used features in the fingerprint individuality studies. Similar defect population exist in many other systems. Mature aeolian sand ripples are made up of linear ridges that run parallel to one another along their length. Between the periodic stripes, however, there are always some defects such as bifurcations and terminations. Different defect type, position and direction occur in sand ripples, provide them with distinct topological appearance.We would like to explore the characteristics of defects in aeolian sand ripples.

青年科学基金项目“沙波纹的形态演化特征”。我们得到了沙波纹中缺陷种类、数目的测量数据。计算了由于缺陷的存在，对沙波纹拓扑构形的影响情况。初步掌握了各类型缺陷的移动规律，以及它们之间的相互转换规律。 某个缺陷移动的轨迹并非直线，且非匀速移动，即缺陷的移动与其所处的类型和位置有关。研究了风成床面形态的高度的限制条件。建立了磨蚀过程的物理模型，提出了“磨蚀数”概念。基于项目的执行情况，我们现申请对沙波纹中的缺陷专门进行研究。缺陷对材料的生长过程及其性能有着非常重要的影响，并且们在斑图动力学中起着重要的作用。其中，条纹状斑图中的缺陷有着丰富的时空行为。比如，在磁性薄膜、液晶、斑马鱼、指纹中，观察到了周期性条纹中缺陷的形态，以及它们的产生，移动，湮灭等过程。我们计划用实验，数值模拟等手段，详细研究风成沙波纹中缺陷的产生，移动轨迹，相互转变情况等。

缺陷对材料的生长过程及其性能有着非常重要的影响，并且它们在斑图动力学中起着重要的作用。沙波纹中的缺陷有着十分丰富的行为，而且它们对沙波纹的形态有着显著影响。在本研究项目中，我们通过野外沙漠中沙丘表面沙波纹的测量，得到普通沙丘表面沙波纹缺陷发生的密度和类型分布。由此进一步计算出带有不同缺陷的沙波纹发生的概率。我们进行了野外观测及风洞实验，跟踪了沙波纹中缺陷移动的轨迹。在沙波纹的移动过程中，缺陷也随之前移，其轨迹为S形曲线，并逐渐横向缩短。缺陷最后一般会成对的湮灭消失。偶然会发生缺陷向后移动的情况。如果预先在平坦沙床上刻留平行条纹、方形纹，或蜂窝形等不同形态、不同几何尺寸的印痕，则这些痕迹在风力条件下最终向平行波纹转变。另外，用高速相机和激光光源，观察沙波纹横截面在风沙流作用下的形成和演化过程。我们进行了计算机模拟。利用计算机给出的随机初始平坦但有细小不同起伏的床面条件，观察后续演化形成的沙波纹缺陷形态，数目的变化情况。通过大量的重复模拟，发现在相同外部条件下，经过相同的时间演化形成的缺陷个数变化很大。同时还大量模拟了在大幅面积上缺陷的聚积情况。发现它们并不是均匀分布的。最后进行了人工位错计算机模拟。预先给定规则的正弦形态的成熟沙波纹，然后移动整排的小段沙纹，使得人为产生整体上的微小错开。当这些错开的沙脊正好位于原来的沙波纹中间时，随着时间的变化，它们会随机选择上风方向或下风方向的邻近沙脊进行连接，这个过程导致缺陷的产生。有时候会形成斜向的沙波纹。我们的工作较为详细地研究了对沙波纹中缺陷生成，移动，以及缺陷的数目，类型，朝向等的变化。可以为其它系统中的缺陷研究提供对照和参考。