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.
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.
