Applied Math Laboratory, Courant Institute, New York University, New York, NY10012.
New York University-East China Normal University (NYU-ECNU) Institute of Physics, New York University Shanghai, Shanghai 200062, China.
Proc Natl Acad Sci U S A. 2019 Aug 13;116(33):16180-16185. doi: 10.1073/pnas.1815133116. Epub 2019 Jul 26.
The atmospheric ablation of meteoroids is a striking example of the reshaping of a solid object due to its motion through a fluid. Motivated by meteorite samples collected on Earth that suggest fixed orientation during flight-most notably the conical shape of so-called oriented meteorites-we hypothesize that such forms result from an aerodynamic stabilization of posture that may be achieved only by specific shapes. Here, we investigate this issue of flight stability in the parallel context of fluid mechanical erosion of clay bodies in flowing water, which yields shapes resembling oriented meteorites. We conduct laboratory experiments on conical objects freely moving through water and fixed within imposed flows to determine the dependence of orientational stability on shape. During free motion, slender cones undergo postural instabilities, such as inversion and tumbling, and broad or dull forms exhibit oscillatory modes, such as rocking and fluttering. Only intermediate shapes, including the stereotypical form carved by erosion, achieve stable orientation and straight flight with apex leading. We corroborate these findings with systematic measurements of torque and stability potentials across cones of varying apex angle, which furnish a complete map of equilibrium postures and their stability. By showing that the particular conical form carved in unidirectional flows is also posturally stable as a free body in flight, these results suggest a self-consistent picture for the origin of oriented meteorites.
流星体的大气烧蚀是一个固体物体由于其在流体中的运动而发生形状改变的显著例子。受在地球上收集到的陨石样本的启发，这些样本表明流星在飞行过程中保持固定的方向，最明显的是所谓定向陨石的圆锥形，我们假设这种形状是由于姿势的空气动力稳定而形成的，这种稳定可能只能通过特定的形状来实现。在这里，我们在平行的水流中粘性物体侵蚀的流体力学术语中研究了飞行稳定性的问题，这导致了类似于定向陨石的形状。我们在自由移动的锥形物体和固定在强制流中的锥形物体上进行实验室实验，以确定定向稳定性对形状的依赖性。在自由运动中，细长的圆锥体会发生姿势不稳定，例如翻转和翻滚，而宽或钝的形状会表现出振荡模式，例如摇晃和飘动。只有中间形状，包括由侵蚀雕刻而成的典型形状，才能实现稳定的定向和以顶点为先导的直线飞行。我们通过对不同顶角圆锥的扭矩和稳定性势的系统测量来证实这些发现，这些测量提供了平衡姿势及其稳定性的完整图谱。通过表明在单向流中雕刻的特定圆锥形形状在飞行中作为自由体也是姿势稳定的，这些结果表明了定向陨石起源的一个自洽的图景。
