This thesis describes the study of the hydrodynamic boundary condition at the solid-liquid interface using the colloidal probe Atomic Force Microscope. Quantitative comparison between measured lubrication forces and theoretical lubrication forces show that the measured forces agree with theory when the no slip boundary condition is employed. We measure an effective slip length of 0 ± 2 nm at shear rates up to 250,000 sec-1. Our results are consistent with the Taylor lubrication equation without the need to invoke a slip length fitting parameter. Our results are also consistent with molecular dynamic simulations that predict no slip at the shear rates that are currently experimentally accessible.

This thesis describes the study of the hydrodynamic boundary condition at the solid-liquid interface using the colloidal probe Atomic Force Microscope. Quantitative comparison between measured lubrication forces and theoretical lubrication forces show that the measured forces agree with theory when the no slip boundary condition is employed. We measure an effective slip length of 0 ± 2 nm at shear rates up to 250,000 sec-1. Our results are consistent with the Taylor lubrication equation without the need to invoke a slip length fitting parameter. Our results are also consistent with molecular dynamic simulations that predict no slip at the shear rates that are currently experimentally accessible.

The MIUU meso-scale model has been used to evaluate a new lateral boundary condition. The new lateral boundary condition is a combination of two lateral boundary conditions used in regional models, the flow relaxation scheme and the tendency modification scheme. The impact of different lateral boundary formulations on meso-scale phenomena, such as convective boundary layers, nocturnal jets, sea breezes and mountain waves (Bora winds) has been studied. When, for instance, stably stratified air with a constant wind speed is advected through the lateral boundaries into a meso-scale model with a flat and homogenous land surface, the convective boundary layer is reduced in height and the nocturnal jet is reduced in magnitude up to a distance of 750 km from the inflow lateral boundary. This is the case, when the most common lateral boundary condition is used, namely the flow relaxation scheme, where the flow relaxation parameter is constant with height and a function of the horizontal grid points only. In the other tests a flow relaxation parameter is used that is very small up to a certain level above ground, increasing with height to a maximum value higher up, and being constant above this upper level. Then, the convective boundary layer and the nocturnal jet are fully developed already at 23 km from the inflow lateral boundary. When, for instance, islands are not represented in the large-scale model, due to the coarse grid resolution, but well represented in the meso-scale model, stably stratified air can be advected into the meso-scale model even during daytime. Then, artificial thermal circulations can arise at the lateral boundaries of the meso-scale model, and collide with a real sea-breeze circulation that develops at the coast-line. These artificial thermal circulations disappear only when the flow relaxation parameter is very small in the lowest levels. When mountain waves (Bora winds) are simulated in a relatively small model domain, the critical layer, i.e. the layer where the nonlinear large-amplitude mountain wave is generated and broken, is surprisingly displaced irrespective of the tested lateral boundary formulations. In many simulations large-scale fields have to be introduced into meso-scale models. If only the flow relaxation scheme is used, the flow relaxation parameter has to be “constant-in-height” and relatively large in order to introduce large-scale temperature and wind changes with the right time-scale at all levels. However, with the new lateral boundary condition, the flow relaxation parameter can be kept very small in the lowest kilometers above ground. A small value of the flow relaxation parameter means that the convective boundary layer and the nocturnal jet at the lateral boundaries are not affected by the boundary conditions, and furthermore, no artificial thermal circulations are created. At the same time, large-scale temperature and wind changes are correctly introduced at all heights during the prescribed time into the meso-scale model through the tendency modification scheme. / Den mesoskaliga MIUU modellen har använts för test av olika laterala gränsvillkor. Ett nytt lateralt gränsvillkor har konstruerats. Detta nya gränsvillkor är en kombination av två gränsvillkor, nämligen ’the flow relaxation scheme’ och ’the tendency modification scheme’. Inverkan av olika gänsvillkorsformuleringar på mesoskaliga fenomen som konvektiva gränsskikt, ’nocturnal’ jets, sjöbrisar och bergsvågor (Boravindar) har studerats. När stabilt skiktad luft med konstant vindhastighet advekteras in genom de laterala ränderna in till en mesoskalig modelldomän, som har en slät och homogen landyta, kommer det konvektiva gränsskiktets höjd och styrkan på ’nocturnal’ jeten att påverkas av gränsvillkoret. Randvillkoret kan påverka temperatur och hastighetsfältet upp till 750 km:s avstånd från inflödesranden. Detta sker när det vanligaste laterala gränsvillkoret används, nämligen, ’the flow relaxation scheme’. I detta schema är ’flow relaxation’-parametern konstant med höjden, dvs endast en funktion av de horisontella gridpunkterna. Sensivitetsstudier på värdet och formen av ’flow relaxation’-parametern har utförts. En ’flow relaxation’-parameter, som är mycket liten upp till en viss nivå och sedan ökar med höjden påverkar temperatur- och hastighetsfältet mycket mindre. Randvillkorets påverkan är då minimal redan på 23 km:s avstånd från inflödesranden och det konvektiva gränsskiktet och ’nocturnal’ jeten kan bli fullt utvecklade. Om till exempel öar, som är väl representerade i den mesoskaliga modellen, inte är representerade i den storskaliga modellen pga dess grova upplösning, kan stabilt skiktad luft advekteras in till den mesoskaliga modelldomänen till och med under dagtid. Det kan då uppstå en artificiell termisk cirkulation vid de laterala ränderna hos den mesoskaliga modellen. Denna artificiella termiska cirkulation kan sedan kollidera med en verklig sjöbriscirkulation. Detta kan förstöra den mesoskaliga modellösningen totalt. Denna artificiella termiska cirkulation försvinner endast då ’flow relaxation’-parametern är väldigt liten i de lägsta nivåerna. När bergsvågor (Boravindar) simuleras i en relativt liten modelldomän så är det kritiska skiktet, dvs det skikt där de icke-linjära vågorna med stor amplitud bryts och genereras, förflyttat jämfört med referensfallet där de laterala ränderna var långt borta från det studerade området. Detta sker förvånansvärt oberoenda av vilken lateral gränsvillkorsformulering som används. I många simuleringar ska storskaliga processer såsom fronter och geostrofiska vindänd-ringar införas till den mesoskaliga modellen. Om endast ’the flow relaxation scheme’ används måste ’flow relaxation’-parametern vara konstant med höjden och relativt stor. Detta för att storskaliga temperatur- och vindändringar skall kunna introduceras till den mesoskaliga modellen med rätt tidskonstant och på alla höjder. I det nya laterala gränsvillkoret behöver ’flow relaxation’-parametern inte vara lika stor och inte heller konstant med höjden. Temperatur- och vindändringar är ändå korrekt introducerade med exakt tidsskala i alla nivåer in till den mesoskaliga modellen. Detta sker genom användandet av det s.k. ’tendency modification’-schemat. Dessutom kan det konvektiva gränsskikt, ’nocturnal’ jeten och sjöbrisar utveklas korrekt i närheten av de laterala ränderna.

The no-slip boundary condition is the foundation of traditional lubrication theory. It says that fluid adjacent to a solid boundary has zero velocity relative to that solid surface. For most practical applications the no-slip boundary condition is a good model for predicting fluid behavior. However, recent experimental research has found that for special engineered surfaces the no-slip boundary condition is not applicable. Measured velocity profiles suggest that slip is occurring at the interface. In the present study, it is found that judicious application of slip to a bearings surface can lead to improved bearing performance. The focus of this thesis is to analyze the effect an engineered slip/no-slip surface could have on hydrodynamic bearing performance. A heterogeneous pattern is applied to the bearing surface in which slip occurs in certain regions and is absent in others. Analysis is performed numerically for both plane pad slider bearings and journal bearings. The performance parameters evaluated for the bearings are load carrying capacity, side leakage rate and friction force. Fluid slip is assumed to occur according to the Navier relation and the effect of a critical value for slip onset is considered.

La simulation d’écoulements incompressibles pose de nombreuses difficultés. Une première est la question de savoir comment traiter la contrainte d’incompressibilité et le couplage vitesse/pression afin d’obtenir une solution précise à moindre coût. Pour cela, nous nous intéressons en particulier à deux méthodes de time splitting : la correction de pression et la correction de vitesse. Une seconde difficulté porte sur des conditions limites de sortie. Nous nous intéressons ici à deux d’entre elles : la condition limite de traction et la condition limite d’Orlanski. Après avoir détaillé les difficultés pouvant apparaître lors de l’implémentation des méthodes de time-splitting, nous proposons une nouvelle implémentation de la condition limite de traction qui permet d’améliorer les ordres de convergence obtenus. Nous nous intéressons ensuite à la condition limite d’Orlanski qui nécessite une certaine vitesse d’advection C dans la direction normale à la limite dont nous proposons ici une nouvelle définition. Nos propositions sont confrontées à de multiples écoulements physiques afin de valider leurs comportements : l’écoulement en aval d’une marche descendante, l’écoulement au niveau d’une bifurcation,l’écoulement autour d’un obstacle et des écoulements de Poiseuille-Rayleigh-Bénard. / One of the understudied difficulties in the simulation of incompressible flows is how to treat the incompressibilityconstraint and the velocity/pressure coupling in order to obtain an accurate solution at low computationnalcost. In this context, we develop two methods: pressure-correction and velocity-correction. An anotherdifficulty is due to the boundary conditions. We study here two of them : the traction boundary condition andthe Orlanski boundary condition. After having developed the difficulties that appears when implementing timesplittingmethods, we propose a new way to enforce the traction boundary condition which improves the orderof convergence. Then we propose a new definition of the advective velocity C which is needed for the Orlanskiboundary condition. Our propositions are validated against multiple physical flows: flow over a backward facingstep, flow around a biffurcation, flow around an obstacle and several Poiseuille-Rayleigh-Bénard flows.

This thesis studies structural dynamic system identification in a frequency-based framework. The basic consideration stems from the fact that frequencies may generally be measured with higher accuracy than other pertinent modal data such as mode shapes; however only a limited number of frequencies may be measured in the conventional context of natural frequencies. Being able to measure extra frequencies is a key to the success of a frequency-based method. The main part of the thesis is therefore organised around the involvement of the so-called artificial boundary condition (ABC) frequencies to augment the frequency dataset for general structural damage identification. In essence, the ABC frequencies correspond to the natural frequencies of the system with additional pin supports, but may be extracted from specially configured incomplete frequency response function matrix of the original structure without the need of physically imposing the additional supports. In the first part of the research, a particular focus is placed on the actual extraction of these ABC frequencies from physical experiments through effective modal testing, data collection, data processing and analysis. The influences of key processes involved in a typical modal experimental procedure, including high-fidelity measurement of the (impact) excitation input, averaging, windowing, and an effective use of post-processing techniques, particularly the Singular Value Decomposition (SVD) technique, are scrutinised in relation to the extraction of the ABC frequencies. With appropriate implementation of testing and data processing procedures, results demonstrate that all one-pin and two-pin ABC frequencies from the first few modes can be extracted with good quality in a laboratory setting, and the accuracy of extracted ABC frequencies is comparable to natural frequencies of corresponding orders. A comprehensive study is then carried out to investigate the sensitivities of ABC frequencies to damages. Two-pin ABC frequency sensitivity is formulated by extending the expression of anti-resonance sensitivity. On this basis, the mode shape contribution is adopted as a criterion for the selection of more sensitive ABC frequencies to be employed in detailed parameter identification or finite element model updating procedures. The soundness of using ABC frequencies in structural parameter identification and the effectiveness of the above ABC frequency selection method are subsequently examined through case studies involving laboratory experiments and the corresponding FE model updating. Furthermore, a preliminary study is carried out to examine the possibility of formulating ABC frequency-based damage indicator, herein with an analogy to the mode shape curvature, for direct damage assessment. As an extended investigation in the general framework of frequency-based dynamic identification, in the last part of the thesis, a complex dynamic system, namely a railway bridge under moving loads & masses, is evaluated with regard to the various frequency characteristics involved. The variation of the natural frequencies of the bridge-moving mass system, as well as the presence of the apparent frequencies from the trainloads, are analysed in detail. Besides simplified theoretical analysis, a computational model is developed to simulate the combined bridge-moving vehicle/train system, where the vehicle mass is coupled with the bridge via surface contact. The model is verified by comparison with field measurement data and theoretical predictions. Parametric studies enable a clear identification of the correlation of the frequency contents between the response and the trainload, and provide new insight into the significance of the so-called driving and dominant frequencies. It is found that much of the dynamic response phenomena, including the resonance effect, may be explained from the view point of the frequency characteristics of the trainload pattern, which is governed primarily by the ratio between the carriage length and the bridge length. Finally, a resonance severity indicator (the Z-factor) is developed for the assessment of the resonance effect in the railway bridge response when the trainload moves at a resonance speed. Numerical results demonstrate that the proposed methods are effective for the determination of the critical speed and the resonance effects, including the situations where a significant carriage mass is incorporated.

This study is aimed at developing modeling methodologies for simulating the flow of air and aerosol particles through fibrous filter media made up of micro- or nano-fibers. The study also deals with modeling particle deposition (due to Brownian diffusion, interception, and inertial impaction) and particle cake formation, on or inside fibrous filters. By computing the air flow field and the trajectory of airborne particles in 3-D virtual geometries that resemble the internal microstructure of fibrous filter media, pressure drop and collection efficiency of micro- or nano-fiber filters are simulated and compared with the available experimental studies. It was demonstrated that the simulations conducted in 3-D disordered fibrous domains, unlike previously reported 2-D cell-model simulations, do not need any empirical correction factors to closely predict experimental observations. This study also reports on the importance of fibers’ cross-sectional shape for filters operating in slip (nano-fiber filters) and no-slip (micro-fiber filters) flow regimes. In particular, it was found that the more streamlined the fiber geometry, the lower the fiber drag caused by a nanofiber relative to that generated by its micron-sized counterpart. This work also presents a methodology for simulating pressure drop and collection efficiency of a filter medium during instantaneous particle loading using the Fluent CFD code, enhanced by using a series of in-house subroutines. These subroutines are developed to allow one to track particles of different sizes, and simulate the formation of 2-D and 3-D dendrite particle deposits in the presence of aerodynamic slip on the surface of the fibers. The deposition of particles on a fiber and the previously deposited particles is made possible by developing additional subroutines, which mark the cells located at the deposition sites and modify their properties to so that they resemble solid or porous particles. Our unsteady-state simulations, in qualitative agreement with the experimental observations reported in the literature, predict the rate of increase of pressure drop and collection efficiency of a filter medium as a function of the mass of the loaded particles.

Chemical and physical interactions play important roles in surface film formation and fluid slip at the fluid-solid interface. It has been shown that the fluid molecules at this solid interface behave differently than the molecules in the bulk. To investigate fluid film formation and the fluid’s transition between bulk and interfacial regions, a dynamic wetting technique is utilized. This technique allows the formation of variable thickness fluid films. When used in conjunction with vibrational spectroscopy and ellipsometry, direct analysis of variable thicknesses films, spanning the bulk to interfacial transition, can be obtained. Film thickness are predicted using the Landau-Levich model and the Lifshitz model, and comparisons generally agree with experimental results. According to hydrodynamic no slip boundary condition, fluid molecules near a solid surface can have no velocity with respect to the solid substrate. Recent theories state more specifically that, if a fluid comes in contact with an ultra-smooth surface (< 5-7 nm RMS roughness), the no slip boundary condition might be violated. We confirmed violation of the no slip boundary condition in two specific cases for fluid layers on SAM-modified substrates. To understand how the fluid/solid properties affect this condition, an acetophenone and bare silver surface was studied. Our results show that the structure and ordering of fluid molecules within these films are highly dependent on the film’s thickness and confinement. Temperature control wetting studies also corroborate with these results showing that as a frozen film of large thickness approaches the melting point, a molecular reorganization occurs creating a crystalline structure before the film melts into an isotropic bulk structure. Structure dependence on alkyl-chain length was then investigated using a series of trialkylamine fluids. Results show significant changes in the vibrational profile as a function of film thicknesses and rotational velocity as the alkyl-chains increase in length. These are ascribed to changes in primary carbon attached to the nitrogen as a function shearing and the rigidity of the molecule. These results reveal interactions taking place at the solid-liquid interface and have impacts on a broad spectrum of industrial, commercial, and research applications including lubrication and transportation vehicles.

In this thesis, we demonstrated the effectiveness and the accuracy of the FD-FD method for complex optical waveguide structures such as the micro ring resonator, micro disk resonator, tapered waveguides and waveguides terminated with tilted facets. We are able to achieve the goals by deriving the following modification/extension of the original FD-FD methods. In frequency domain, we can build an accurate frequency-domain modal absorbing boundary condition (ABC) for both the homogeneous region and for the layered background. This allows us to connect the analytical modal solutions with FD solutions and thus reduce the area of the FD domain. In addition, we adopt an effective index averaging method for representing equivalent material for grid cells containing more than one kind of materials. For the TM case, for each grid cell we need to compute effective indices for all four surrounding cells (left, right, up, and down). For the TE case, we need to compute just one effective index within each grid cell. Note that we employ two different averaging schemes for the TE and the TM cases. To solve the huge block tri-diagonal matrix equation (derived from the FD-FD approximation) we modified the Thomas method and we were able to obtain the solutions of linear equations involving more than a hundred thousand variables under a few minutes. We used our method to analyze optical micro-ring waveguides, micro-disk cavities, adiabatic tapered waveguides and waveguides terminated with tilted facets. The simulated results include the reflection coefficients, transmission coefficients and field distribution.

This thesis reports a new set of experimental data and presents an in-depth analysis of the flow physics of a jet stream produced by a large quarter-round corner-mounted displacement diffuser. The air velocity, temperature and turbulence intensity inside the displacement ventilation (DV) jet have been thoroughly analyzed and compared with the reported findings of previous studies and model predictions. Furthermore, thermal comfort has been analyzed using the measured data following the ASHRAE standard. This thesis also aims at establishing an accurate numerical approach for simulating the heat and fluid flow in a room ventilated by a DV system. The supply boundary condition has been thoroughly investigated, which includes tests of the conventional box and momentum modeling methods, and proposal of a more accurate modeling approach. In addition, the predictive accuracy of the standard k-ϵ, RNG k-ϵ, SST k-ω and RSM turbulence models has been examined against the experimental data.