Measurement of Flow-Induced Nanoparticle Rotation with Luminescence Anisotropy

Abstract

Flow-induced rotation, the vorticity, is a fundamental characteristic of turbulent flows. Despite its importance, the direct measurement of vorticity is still limited spatially or temporally depending on the measurement method. This thesis examines luminescence anisotropy as a physical working principle for the direct measurement of vorticity. A theory is developed which relates changes in anisotropy to deterministic rotations. The results are limited to acquisitions of the complete luminescence decay for four polarization signals. The presented model enables the computation of absolute vorticity. Experimental procedures are described to provide a standard for anisotropy measurements in flow diagnostics. Xanthene-stained particles, commonly used in popular methods such as Particle Imaging Velocimetry (PIV), are analyzed concerning their potential use in the proposed method despite their low quantum yield and short luminescent lifetimes. As a reference scenario, a solid body rotation around the axis of observation is captured with fixed particles, and the results are successfully analyzed with the newly developed theoretical framework. Finally, the average absolute vorticity of a round turbulent jet at Re = 12000 and 14400 is measured. The results are compared to the theory in the self-similar region of the jet.