Optical measurement techniques are widely used in fluid mechanics to measure quantities like velocity, vorticity, and temperature. They are usually non-intrusive visualization techniques that provide a major advantage by not influencing the flow properties. Most of the developed measurement techniques estimate vorticity by computing the gradients of the measured velocity fields. In this project, the idea is to directly measure the mean vorticity using polarized emission of nanoparticles. The nanoparticles are excited using vertically and horizontally polarized laser. Accordingly, the emission from these nanoparticles is separated into vertical and horizontal polarized emissions respectively. Depending on the percentage of fluorophores in the nanoparticles whose absorption dipole moment is parallel to the excitation electric vector of the laser, the polarized emission is varied. The nanoparticles in the quiescent fluid state have a characteristic polarization (or anisotropy) depending on the chemical composition of the nanoparticle. However, in turbulent flows, depolarization of the particles can occur because of the rotation of the nanoparticles due to vorticity. Based on this idea, the vorticity can be estimated by quantifying the depolarization in turbulent flows. To verify the above phenomena experimentally, steady-state polarization measurements were conducted. An experimental setup was built specifically to measure depolarization from these nanoparticles. Important preliminary tests like lifetime measurements, spectroscopic studies, and photo-bleaching experiments were conducted. Based on these results, hybrid europium based chelated spherical particles were chosen. To further understand the properties of these particles in a quiescent flow, a consolidated set of anisotropy measurements were conducted to study their oxygen sensitivity in the surrounding medium, viscosity dependence and the effect of its concentration in the fluids. Analytical correlations which was developed in-house to translate the depolarization of the nanoparticles to vorticity was used in this thesis. Flow in a square duct was studied to estimate the vorticity along the edges and further compare it with the DNS data.
Two additional non-dimensional numbers ND1 and ND2 were introduced to interpret the underlying physics behind the problem. The variation of synthetic signals was studied for different Re, fundamental anisotropy r0 and the non-dimensional number ND2. These synthetic signals showed a drop in anisotropy near the edges of the square duct that has occurred probably due to vorticity. Emission polarized intensity signals and the anisotropy contours for Re = 0 and Re = 4434 were analyzed. Due to the measurement error, the contours were column averaged and respectively compared their variation along the width of the square duct. A drop in anisotropy was observed for Re = 4434. Using the ratio of emission intensities for each excitation polarization, the vorticity was estimated using the optimization algorithm. The estimated vorticity Ѡy shows similar trends compared to DNS data which is a promising evidence for the proof of concept. However, the results are not overlapping from the repeatability studies due to the high sensitivity of the optimization algorithm and the random error from the measurement equipment. To improve this technique in the future, transient experiments have to be conducted to estimate vorticity accurately and to capture the underlying physics.