The objective of this research was to investigate parameters affecting vortex ring formation and propagation and their application to mixing of fluids. To this end both empirical and numerical simulation experiments were conducted. The empirical experiments involved observations and measurement of the volume, displacement and velocity of vortex rings generated from a 5 cm diameter tube. The results revealed that there is an optimal range of generation injection velocity for various mixing requirements. The numerical simulations were done using a commercial package, FLUENT. Both tube type and plate orifice type vortex ring generators were investigated. Also the affects of a central shaft and various projections on the control of the motion of a vortex ring. All models considered a polar model cylindrical tank with a diameter to height ratio of 3:10. The average injection velocity was in the range of 0.7 m/s to 3 m/s. When simulating the tube type generator various injection velocity profiles and value were investigated, which resulted in a fitted correlations of nondimension displacement versus non-dimension time as a function of infection profile. In order to control the forward motion of vortex rings some obstructions were considered. It was found that the trajectory and energy of a vortex ring can be controlled with shape and geometries of baffles. In the simulations for the orifice plate type generator, a moving mesh technique was used. As expected a pair of vortex rings were produced per half cycle of the plate oscillation, but they did not travel as fast as expected. Recommendations have been made to improve the simulation accuracy. It has also been found that the FLUENT package will not properly simulate turbulent vortex rings. However, this may be because a vortex ring is not truly homogeneously turbulent. The use of a laminar model appears to give quite good agreement with empirical data for tube type vortex ring generator. The results of this research are expected to be useful for the optimization of the design of vortex ring mixing systems. / Thesis / Master of Engineering (ME)

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The family of III-V semiconductors is of high significance in photonics for two main reasons. First, not only they are the most practical material platforms for active photonic devices but also they are suitable for monolithic integration of passive and active photonic devices. Second, some III-V compounds exhibit high values of second and third-order nonlinear coefficients – the property useful in all-optical signal processing and wavelength conversion. This Ph.D. thesis explores the above perspectives with two candidates from the group III-V family, namely AlGaAs and InGaAsP. The dissertation consists of two main parts. The first part is dedicated to the theoretical modelling of nonlinear bianisotropic AlGaAs metasurfaces, while the second part focuses on the experimental studies of the nonlinear optical performance of InGaAsP waveguides. Concerning the first part, due to the high confinement of light supported by the Mie resonances, AlGaAs nanoantennas and metasurfaces with both high refractive index and high nonlinear susceptibility have found a unique place in planar nonlinear optics, where not only the presence of high intensity of light is of significant matter, but also the optically thin thickness of the entities releases the device from phase matching. We first describe the linear optical properties of AlGaAs meta-atoms and metasurfaces such as relatively high scattering cross-sections and the bianisotropic effect. Also, we derive and explain all required analytic formulas for this purpose. Bianisotropic metasurfaces with magnetoelectric coupling and asymmetric optical properties have sparked considerable interest in linear meta-optics. However, further in this thesis, we explore the nonlinear features of bianisotropic AlGaAs metasurfaces. In particular, we explore a second-harmonic generation in a bianisotropic AlGaAs metasurface based on the multipolar interference inside the meta-atoms and the nonlinear polarization current. We theoretically demonstrate that it is possible to obtain several orders of magnitude secondharmonic power differences for the forward and backward illuminations by adjusting the geometrical parameters of the meta-atoms in such a way that quasi-bound states in the continuum (quasi-BICs) are achievable. This research paves the way for the generation of directional higher-order waves. Concerning the second part, the research is focused on exploring nonlinear material platforms for monolithic integration of active and passive devices on the same chip. In this regard, we explore InGaAsP/InP waveguides of different geometries. First, we provide the theoretical background such as the nonlinear Schrodinger equation and four-wave mixing (FWM) equations in a nonlinear waveguide, then we solve the set of FWM equations using MATLAB to observe the qualitative behavior of the signal, idler, and the pump inside a nonlinear waveguide. Furthermore, we design and employ two waveguide geometries i.e. half-core and nanowire waveguides. We first design these waveguides so that achieving zero group velocity dispersion is possible through a suitable material composition and certain geometrical dimensions. However, for the rest of the work, we continued with the waveguides of different dimensions compared to the designed ones (due to some limitations in fabrication). We demonstrate self-phase modulation (SPM) and FWM for the half-core waveguides. For the case of the nanowire waveguides, we also demonstrate the FWM effect. We measured and extracted the effective value of the nonlinear refractive index of InGaAsP/InP waveguides to be n2 = 1.9 × 10−13 cm2/W through the relation between the idler and the pump power when the phase mismatch is negligible. Finally, we experimentally observe the two-photon absorption effect in our waveguides through the nonlinear characteristics of input and output powers of the waveguides from which the two-photon absorption coefficient of 19 cm/GW is calculated.

Bluff bodies immersed in a fluid stream are susceptible to flow-induced vibrations. Depending on the body dynamic characteristics and flow conditions, different types of flow-induced vibrations may occur. The failure of a blade in a large mixing vessel in a chemical plant raised the question of the response of a parabolic cross-section bluff body to the flow excitation. Experiments were conducted in a wind tunnel using two- dimensional “sectional” models. Models with parabolic, semi-elliptic and semi-circular cross-section were investigated. In the dynamic experiments, flow velocity was increased from 0 to 22 m\s, and the oscillating amplitude and wake response were monitored. Vortex-induced vibrations were observed with Strouhal numbers for parabolic and semi-circular cross-sections of 0.13 and 0.12, respectively. Steady lift force and fluid moment for different angles of attack were monitored in the static experiments. From these results, lift and moment coefficients were calculated. For the closed semi-circular cross-section, Reynolds number had a strong influence on the lift coefficient. With an increase in Reynolds number, the lift coefficient decreased. The largest difference was noted at an angle of attack a = 45°. In contrast, the open semi-circular model lift coefficient was independent of Reynolds number. In the experiments where the elastic axis of the model coincided with the model centre of gravity, galloping was not observed in the plunge mode. When the model elastic axis was moved to a position 90 mm behind the test model centre of gravity, galloping was observed for the semi-elliptic and parabolic models. The onset of galloping coincided with the vortex-induced resonance. Changing the model elastic axis position introduced a combination of plunge and torsional motion, and latter is believed to be responsible for the existence of galloping. The parabolic model was modified in an attempt to eliminate galloping instability. Fins were added at the separation points to widen the wake and prevent the reattachment of the flow to the afterbody. With these changes, galloping was not observed, although the oscillation amplitudes remained unacceptably high. The present investigation revealed previously unknown characteristics of semi-elliptical and parabolic cross-section bluff body behaviour in fluid flow. At the same time, it laid a foundation for the solution to the practical problem encountered when a parabolic cross-section bluff body was used as a mixing blade. / Thesis / Master of Engineering (ME)