|M.Sc Student||Kuperman Sofia|
|Subject||Rotational and Translational Dynamics of Inertial, Heavy|
Fibers in Isotropic Turbulence
|Department||Department of Mechanical Engineering||Supervisor||Professor Rene Van Hout|
|Full Thesis text|
Particle-flow interactions are important in many environmental, industrial and health related processes, e.g. plankton movement in the ocean, combustion processes and spray-based oral drug delivery. Most flows occurring in industrial applications and in nature are turbulent, and in many applications particles are non-spherical. The interaction between anisotropic particles (such as fibers) and isotropic turbulence is significant in high Reynolds number turbulent flows at the smallest scales where turbulence is considered isotropic. The majority of prior research regarding 3D orientations of non-spherical particles in turbulence are numerical simulations and most of them focus on the simple case of inertialess particles without gravitational effects. There is a lack of experimental data on inertial fiber dynamics in isotropic turbulence.
In this research, rotational and translational dynamics of rigid, heavy inertial fibers in air isotropic turbulence were measured using two orthogonal view, holographic cinematography. Measurements were conducted in a turbulence chamber having transparent windows. Woofers placed on each corner of the chamber creating air jets, were activated at a random frequency and phase thereby generating isotropic turbulence (Re λ = 115) at the chamber’s center. Fiber inertia was characterized by the Stokes number, St, defined as the ratio between the fiber response time and the Kolmogorov time scale. Fibers were dispersed from a specially designed dispenser located at the top of the chamber. Holograms were recorded at the center of the chamber where the fibers' motion was tracked. The holograms were digitally reconstructed to find the location and orientation of the fibers in each camera. Combining the information from both cameras, the fiber 3D location and orientation were determined. Using the fibers’ in-plane rotation rates and their polar angle rotation rates, fiber tumbling rates were calculated. Several batches of nylon fibers with different diameters and lengths were investigated having Stokes numbers ranging between 1.0 < St < 32.5 and fiber length to Kolmogorov length scale ratios ranging between 3.6 ≤ L / η k ≤ 17.3.
Results indicated that fiber inertia decreased the translational response of the fibers to the fluctuating air velocities. Furthermore, the probability density functions of the fluctuating fiber centroid velocities showed that when length effects were eliminated, the translational response narrowed with increasing St.
In the absence of significant length effects, fiber rotation rates were mainly governed by fiber inertia. Our results indicated that the fibers’ tumbling rates peak around St ≈ 4 . With increasing St, tumbling rates decreased due to increasing fiber inertia. The statistical correlation between the fluid angular velocity (through the measured vorticity) and the normalized fiber tumbling rates was also investigated. For extreme cases, it was found that the ratio between them was dictated by fiber inertia and the tumbling rate may exceed the angular fluid velocity for high inertia fibers (St > 20). Current models on fiber-turbulence interactions don’t consider the effects of inertia on the fiber orientations and rotation rates, and the present results can be used to improve these models.