Measuring rotations of 3D printed particles in turbulent fluid flow

摘要：当粒子在湍流中运动时，它们的运动受到其形状的强烈影响。以往大多数关于湍流中粒子运动的工作都考虑了最简单的情况，即球形粒子，但了解非球形粒子运动对于理解许多重要情况是必要的，包括云中冰晶，造纸中木纤维的加工，以及许多生物有机体，如浮游生物，其在湍流中的运动受其非球形形状的影响。近年来，人们逐渐认识到，各向异性粒子的动力学也为理解湍流中流体运动的一些基本特性（如涡旋拉伸）提供了一种强有力的新途径。在这个项目中，我们使用3D打印机创建具有各种形状的粒子，并使用多个高速摄像机跟踪湍流水流中的粒子运动。我们已经确定了一种特殊的粒子形状，我们称之为手性偶极子，当它被放置在随机湍流环境中时，它应该优先向一个方向旋转。这种粒子可以从湍流的特定尺度中提取能量。我们还将测量由中心连接的几根对称细杆形成的粒子的旋转运动。这些粒子的尺寸范围很广，可以测量湍流中不同尺度的旋转能量。我们将研究在流体流动中变形的颗粒，并开发制造定制设计形状的小颗粒的新方法。这项工作将为涉及湍流中各向异性粒子的工程和环境应用的工作提供有价值的基础科学。它还将提供一种新的和直观的方法来测量湍流中一些最基本的过程，包括涡旋拉伸和湍流中不同尺度上存在的旋转能量。测量颗粒受力和制造具有定制形状的小颗粒的方法也应该远远超出我们的工作范围。教育和研究培训是这个项目的核心，它将支持博士后科学家、研究生和本科生的研究指导。该项目还支持PI共同指导卫斯理科学推广计划的工作。技术摘要：紊流中各向异性粒子的动力学在造纸、冰云和环境流中微生物的运动等许多应用中都具有重要意义。最近，各向异性粒子的动力学也为小尺度湍流的基本特性提供了一个强有力的新窗口，这一点已经变得很清楚。在这个项目中，我们使用3D打印颗粒来实验测量湍流流体中各种尺寸和形状的各向异性颗粒的旋转和排列。在各向同性湍流中，由两个相反的手旋螺旋连接在中心形成的手性偶极子应该具有优先的旋转方向。这应该提供了一种优雅的方式来观察湍流的基本特性：平均而言，物质的线和漩涡会被气流拉伸。当手性偶极子被拉伸时，它们应该显示出一个实体旋转向量，其在手性偶极子向量上的投影具有非零的平均值。颗粒也将被打印成四面体对称的四臂，尺寸范围很广，以便观察湍流中旋转能量的分布作为尺度的函数。固体旋转速率的矩作为粒度的函数可能在惯性范围内显示幂律标度，由于湍流间歇性对平均场理论标度指数进行了修正。作用在粒子臂上的力可以通过臂的弯曲来测量。颗粒将由柔性聚合物打印出来，并在高粘性流体中进行跟踪，以确定这种方法测量力的可行性。最后，我们将探索使用双光子立体光刻技术来制造具有更高空间分辨率的颗粒，从而可以打印更小的颗粒。教育和研究培训是这个项目的核心，它将支持博士后科学家、研究生和本科生的研究指导。该项目还支持PI共同指导卫斯理科学推广计划的工作。

Nontechnical Abstract:When particles are transported in a turbulent fluid flow, their motion is strongly affected by their shape. Most previous work on particle motion in turbulence has considered the simplest case which is spherical particles, but an understanding of non-spherical particle motion is necessary to understand many important situations including ice crystals in clouds, the processing of wood fibers in paper making, and many biological organisms such as plankton whose motion in turbulent flow is affected by their non-spherical shape. Recently, it has become clear that the dynamics of anisotropic particles also offer a powerful new way to understand some fundamental properties of the fluid motion in turbulence such as vortex stretching. In this project, we use 3D printers to create particles with a wide range of shapes and track the particle motion in a turbulent water flow using multiple high speed video cameras. We have identified a special particle shape which we call a chiral dipole that should preferentially rotate in one direction when it is placed in a random turbulent environment. Such a particle can extract energy from specific scales of the turbulent flow. We also will measure the rotational motion of particles formed from several symmetric thin rods connected in the center. These particles can be made in a wide range of sizes and allow measurement of the amount of rotational energy at different scales in the turbulent flow. We will study particles that deform in the fluid flow, and develop new methods of fabricating small particles in custom designed shapes. This work will provide valuable foundational science for work on engineering and environmental applications involving anisotropic particles in turbulence. It will also provide a new and intuitive way to measure some of the most fundamental processes in turbulence including vortex stretching and the rotational energy that exists at different scales in turbulent flows. Methods for measuring forces on particles and fabricating small particles with customized shapes should also find use far beyond our work. Education and research training are central to this project, which will support the mentoring of a postdoctoral scientist, a graduate student, and undergraduates in research. The project also supports the PI's work co-directing the Wesleyan Science Outreach program.Technical Abstract: The dynamics of anisotropic particles in turbulent fluid flows are important in many applications including paper making, icy clouds, and locomotion of micro-organisms in environmental flows. Recently, it has become clear that the dynamics of anisotropic particles also offer a powerful new window into fundamental properties of the small scales of turbulent flows. In this project, we use 3D printed particles to experimentally measure the rotation and alignment of anisotropic particles of a wide variety of sizes and shapes in turbulent fluid flow. Chiral dipoles formed from two opposite handed helices joined in the center should have a preferential rotation direction in an isotropic turbulent flow. This should provide an elegant way to observe a fundamental property of turbulence: on average material lines and vortices are being stretched by the flow. As chiral dipoles are stretched, they should exhibit a solid body rotation vector whose projection onto the chiral dipole vector has a non-zero mean. Particles will also be printed with four arms in tetrahedral symmetry and a wide range of sizes in order to observe the distribution of rotational energy as a function of scale in a turbulent flow. The moments of the solid body rotation rate as a function of particle size may show power law scaling across an inertial range with corrections to the mean field theory scaling exponents due to turbulent intermittency. The forces acting on arms of particles can be measured from the bending of the arms. Particles will be printed from flexible polymers and tracked in highly viscous fluids to determine the feasibility of measuring forces this way. Finally, we will explore the use of two-photon stereo-lithography for fabricating particles with much higher spatial resolution allowing much smaller particles to be printed. Education and research training are central to this project, which will support the mentoring of a postdoctoral scientist, a graduate student, and undergraduates in research. The project also supports the PI's work co-directing the Wesleyan Science Outreach program.