标题： 自然颗粒材料中的冲击和溅射物 显示英文标题

标题： Impacts and Ejecta in Natural Granular Material

摘要： 通过实验室实验，我们研究了低速（~m/s）撞击多尺度颗粒介质产生的抛射物，并将其与撞击单尺度介质产生的抛射物进行比较。 撞击是针对一个直径为50厘米的镀锌洗衣桶进行的，洗衣桶内装有细沙，表面下埋有较大直径的砾石，该过程由两台高速摄像机拍摄。 产生的抛射物幕主要由细沙组成，并具有复杂的不对称结构，该结构取决于抛射物的位置及其与混合在沙子中的较大砾石颗粒的相互作用。 为了表征高度不均匀的抛射物幕，我们结合了三种分析技术：粒子追踪在低密度区域中最佳地测量抛射物速度和抛射角度，而粒子图像测速（PIV）则阐明密集区域中的平均运动，方向梯度直方图（HOG）则可捕捉相对于图案背景的运动方向。 我们发现，与撞击单尺度介质产生的抛射物所表现出的对称性相比，多尺度抛射物的速度分布和抛射角度存在显著的不对称性。 我们的实验表明，在撞击过程中，表面下的较大颗粒会阻碍并引导抛射物沿优先路径运动。 显示英文摘要

摘要： With laboratory experiments we investigate the ejecta of low-velocity (~m/s) impacts into multi-scale granular media and compare them against ejecta from impacts into mono-scale media. Impacts are into a 50 cm diameter galvanized washtub filled with fine sand that has larger diameter gravel buried below the surface is filmed with two high-speed cameras. The resulting ejecta curtain consists mainly of fine sand, and has a complex asymmetric structure that depends on the location and interaction of the ejecta with the larger gravel grains mixed into the sand. To characterize the highly heterogeneous ejecta curtain we combine three analysis techniques: Particle tracking measures the ejecta velocities and ejecta angles best in low density regions, while particle image velocimetry (PIV) elucidates average motion in dense regions, and histogram of oriented gradients (HOG) which captures directions of motion against a patterned background. We find significant asymmetries in the multi-scale ejecta's velocity distributions and ejection angles compared to the symmetry seen in the ejecta from impacts into mono-scale media. Our experiments show that larger grains under the surface impede and direct ejecta along preferential paths during the impact process.