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Square-wave cathodic current electrodeposition was used to produce for the first time nanocrystalline zinc electrodeposits from both zinc chloride and zinc sulfate-based electrolytes. The influence of pulse electrodeposition parameters, namely, pulse on-time, pulse off-time, and peak current density on the grain size, surface morphology, and preferred orientation of zinc deposits was determined. Furthermore, the effect of polyacrylamide (PAA) and thiourea additions was also investigated. The microstructure and surface morphology of the zinc electrodeposits were studied by scanning electron microscopy (SEM), field emission scanning electron microscopy (FESEM), and atomic force microscopy (AFM). X-ray diffraction was used to determine the preferred orientation of these deposits. In zinc chloride-based bath, nanocrystalline zinc deposits with average grain sizes ranging from 50 to 95 nm were produced. The optimized concentrations of PAA and thiourea in the bath that yield the finest grain sizes were 0.7 and 0.05 g/L, respectively. Increasing current on-time in the range of 0.1 to 7 ms resulted in grain refinement which was attributed to increased overpotential. Increasing the current off-time in the range of 9 to 50 ms was found to yield grain growth which was explained by the decrease of the overpotential and by the fact that longer off-times allow zinc adatoms to migrate over the crystal surface and enhance the grain-growth process. Grain refinement was also observed by increasing peak current density, as expected, and 50-nm zinc grains were obtained at a peak current density of 1000 mA/cm2. In zinc sulfate-based electrolyte, the grain size of zinc deposits decreased gradually with increasing current on-time at constant current off-time and peak current density. An increase in the current off-time at constant current on-time and peak current density resulted in grain growth. A progressive decrease of the grain size was observed with increasing peak current density at constant current on-time and off-time. Nanocrystalline zinc with an average grain size of 38 nm was obtained at current on-time of 7 ms, current off-time of 9 ms, and at peak current density of 1200 mA/cm2. The crystallographic orientations developed were correlated to the change in the cathodic overpotential, the angle between the preferred oriented plane and basal (0002) plane, and the pulse electrodeposition parameters. The hardness of nanocrystalline zinc deposits increases from 5 to 8 times higher than that of pure polycrystalline zinc (0.29 GPa). This high hardness was correlated with the presence of additives, strong prismatic texture (11 0) , and the internal lattice strain. Calorimetric investigations of the as-electrodeposited samples using DSC show two exothermic peaks. The first peak, with an onset temperature of 377 K and peak temperature of 429 K, was attributed to the release of internal lattice strain. Abnormal grain growth was observed by the AFM and the second peak from the DSC scan, which begins at 576 K with a peak temperature of 608 K. The abnormal grain growth may be linked with the segregated sulfur at grain boundaries and interfaces. Potentiodynamic and alternating current impedance testing of nanocrystalline zinc deposits show that the corrosion current density of nanocrystalline zinc was about 60% lower than that of electrogalvanized (EG) steel, 90 mA/cm2 and 229 mA/cm2, respectively. The surface morphology of corroded nanocrystalline zinc was characterized by discrete etch pits; however, uniform corrosion was obtained after potentiodynamic polarization of EG steel. The passive film formed on the nanocrystalline zinc surface seems to be a dominating factor for the corrosion behavior observed.

Wakes of lifting bodies contain vortex sheets that roll up into strong streamwise vortices. The long time behavior of such vortices depends on the turbulence in the wake and the stability characteristics of the vortices themselves. In the near wake of a rectangular wing the flow field consists of a spiraling wake that winds around a pair of vortex cores. The study of the turbulence structure and life of wing tip vortices is of great importance to air traffic control in congested airports. In this dissertation a computer code is developed for the temporal as well as spatial simulations of trailing vortices. A sixth-order compact finite-difference method is used in the cross plane. The streamwise derivatives are represented either by a Fourier series for temporal simulations (periodic flow) or by a sixth-order compact scheme for spatial simulations. The time marching scheme is a third-order Runge-Kutta method. The code is used to study the nonlinear development of temporal helical instability waves in a trailing vortex. Contours of a passive scalar are used to study the entrainment process that redistributes angular and axial momenta between the core and its surroundings. Such a process leads to quenching of the instability waves in the vortex core. The code is also used to predict the spatial development of mean flow in the wake of a rectangular wing. New treatment of the outflow boundary condition on the pressure is formulated so that a strong streamwise vortex can exit the computational domain without distortion. Temporal large-eddy simulation (LES) is performed to study the development of large scale structures in the wake and their interaction with the tip vortex. A modified MacCormack scheme developed by Gottlieb and Turkel(1976) has been used to solve the LES equations. A model of the initial conditions in the near wake of a rectangular wing is devised to investigate mechanisms of turbulence production in the spiral wake around the core of a tip vortex. The model consists of a streamwise vortex sheet whose strength is found from Prandtl lifting line theory. A Gaussian streamwise velocity profile is superimposed on the field of the vortex sheet. This profile represents the spanwise vorticity. The integrated spanwise vorticity of this profile is zero. A novel feature of this study is that the mean flow contains both streamwise and spanwise vorticity. The model is then used to initialize the flow field for temporal LES of the instabilities of the spanwise vorticity during roll up. The results show that the sinuous mode prevails in the spiral wake around the core. The strength and streamwise length scale of the instability vary along the span because of the continuous variation of the wake thickness due to stretching by the tip vortex. The large scale structures produced by the instability of the spiral wake cause the formation of undulations on the core - consistent with the hypothesis of Devenport et al. (1996).