This thesis examines the link between airborne flux estimates of CO$ sb2$, sensible heat, and water vapor, and surface parameters retrieved by remote sensing. Chapter 1 analyses the relationship between surface temperature and vegetation indices, obtained from the Advanced Very High Resolution Radiometer on board of NOAA-9 and -10 satellites, and fluxes of sensible heat, latent heat, and CO$ sb2$, estimated from aircraft. Linear relationships between CO$ sb2$ and the Normalized Difference Vegetation Index (NDVI) or the Simple Ratio vegetation index (SR) are found on a daily basis, but a highly nonlinear relationship appears for the seasonal variation. Latent Heat fluxes showed the poorest correlations with surface parameters. A seasonal linear relationship appeared between sensible heat and NDVI. Local extreme flux values due to the intermittency of boundary layer dynamics largely contribute to lower the correlations; such variations are the reason for the difficulties in relating fluxes obtained from single overpasses and over short distances to fixed points at the surface. This problem is further examined in Chapter 2, in which conditional sampling of airborne flux estimates is used to characterize the turbulent structures that are carrying flux, and their link to the surface. The analysis confirms that few extreme events may carry a significant fraction of the flux. Missing or hitting one of these structures may translate into very large oscillations on the flux estimate that are often not directly coupled to surface characteristics. A much clearer surface 'signature' emerges when measurements are taken within the surface layer, since the reorganization of turbulent structures that takes place with increasing height will result in a merging of the signature that came from different sources at the surface. This helps to explain some of the poor correlations obtained in Chapter 1 and reinforces the need for a better understanding of the distributions of these tu

Thesis: M.S., Massachusetts Institute of Technology, Department of Meteorology, 1980 / Bibliography: leaves 39-41. / by Aribilola Samuel Omolayo. / M.S. / M.S. Massachusetts Institute of Technology, Department of Meteorology

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No description available.

The interaction of water vapor with the surface and bulk of poly(imidesiloxane) copolymers has been characterized in an attempt to determine the important factors in the copolymer's resistance to water ingression. The multi-block copolymers were synthesized from benzophenone tetracarboxylic dianhydride, bisaniline P and pre-formed amine-terminated poly(dimethylsiloxane) oligomers, with phthalic anhydride as an end-capping agent. Similar copolymers had been previously shown to have reduced water sorption, increased surface hydrophobicity, and increased adhesive durability in hot/wet environments. Inverse gas chromatography was used to conduct surface energetics studies on copolymers of different siloxane concentration and a polyirnide homopolymer. Free energies of specific interaction of water vapor, ΔG_sp°, with the polymer surfaces were found to decrease with the incorporation of siloxane into the polyirnide. The dispersive components of the solid surface free energy of the siloxane- containing copolymers were equal within error to that of pure poly(dimethylsiloxane), indicating a PDMS-rich, hydrophobic surface. The ΔG_sp° of the copolymers were not significantly different, suggesting that the copolymer surfaces were very similar. This indicated a minimum weight percent of siloxane incorporation required to maximize the copolymer's surface water resistance. The minimum amount for the studied system was at most ten percent. Diffusion coefficients of water vapor in the polyimide and copolymers were determined from gravimetric sorption experiments. Higher levels of siloxane incorporation caused a definite increase in the diffusion coefficient, indicating a decreased resistance to water ingression. The increase in diffusion was found to be influenced by siloxane block length and was interpreted in terms of morphological and free volume theories. The diffusion coefficient of a 10 weight percent PDMS copolymer, however, was found to be the same within error as the polyimide diffusion oefficient. The incorporation of siloxane into polyimides has been shown to increase water resistance due to the hydrophobicity of the siloxane-rich surface. However, high siloxane contents also increased the rate of water ingression in the bulk of the polymer. Increased water resistance of the surface may be achieved at lower siloxane concentrations without increasing diffusive (or decreasing mechanical) properties to undesirable levels. / Ph. D.

Thesis (PhD)--Stellenbosch University, 2011. / ENGLISH ABSTRACT: In this study, the use of surfactants, plate-like clays, organophilic molecules and side-chain crystallinity was investigated for their impact on the transport mechanisms of water vapour through polymer films. A model polymer latex, poly(styrene-co-butyl acrylate), was prepared using miniemulsion polymerization. Three different types of surfactants, sodium dodecyl benzene sulfonate (SDBS, an anionic surfactant), octyl phenol ethoxylate (OPE, a nonionic surfactant) and dodecyl ammonium- 3-butenoate (DA3B, a reactive surfactant) were used to stabilize the latex. Films were prepared from the resultant latices and their water vapour sorption behaviour determined across a water vapour partial pressure range of 0.1 to 0.9. Sigmoidal kinetic behaviour was seen for all three films, with the DA3B stabilized film exhibiting high diffusion coefficients compared to films stabilized with SDBS and OPE. The thermodynamic behaviour of the films differed and was dependent on the reactivity of the surfactant. SDBS and OPE stabilized films exhibited Flory- Huggins behaviour, while the DA3B stabilized film followed Henry’s Law. Despite significant differences in terms of these properties, the permeability coefficients were similar for the three films across the water vapour partial pressure range evaluated. The impact of sodium montmorillonite (Na-MMT) clay and an organophilic modifier, 2- acrylamido-2-methylpropanesulfonic acid (AMPS), on the water vapour sorption properties of poly(styrene-co-butyl acrylate) was evaluated. These polymer clay nanocomposites (PCNs) were synthesized using miniemulsion polymerization. The resultant latex films were characterized and used for water vapour sorption analyses. It was shown that complete exfoliation of the Na-MMT was necessary to minimize the equilibrium water vapour uptake. Even when Na-MMT was completely exfoliated, the amount of water vapour sorbed by the PCN was high and this was attributed to the hydrophilic nature of the clay. Using a least squares regression fit, good correlation was obtained between the experimental isotherms and the sorption behaviour predicted by the Dual Mode Sorption model which was originally developed for polymers in their glassy state. The impact of side chain crystallinity on the water vapour sorption properties of poly(methyl methacrylate-co-octadecyl acrylate) was evaluated. These random copolymers containing increasing amounts of octadecyl acrylate, and therefore increasing degrees of crystallinity, were synthesized using solvent polymerization. Although it could be expected that side chain crystallinity would be the main contributing factor resulting in a reduction in the diffusion coefficient, it was shown that the methyl group on the á- carbon of the vinyl group in the methacrylate reduced the diffusion to a greater extent through the increased stiffness of the polymer backbone. This was also reflected in poly(methyl methacrylate-co-octadecyl acrylate) having a greater activation energy for diffusion compared to polyoctadecyl acrylate. / AFRIKAANSE OPSOMMING: In hierdie studie is die gebruik van sepe, plaatagtige kleie, organofiliese molekules en sykettingkristalliniteit ondersoek ten opsigte van die impak op transportmeganismes van waterdamp deur polimeerfilms. ‘n Model polimeerlateks, polistireen-ko-butielakrilaat, is voorberei deur miniemulsiepolimerisasie. Drie verskillende tipes sepe, natriumdodekielbenseensulfonaat (NDBS, ‘n anioniese seep), oktielfenoletoksilaat (OFE, ‘n nie-ioniese seep) en dodekielammonium-3-butenoaat (DA3B, ‘n reaktiewe seep) is gebruik om die lateks te stabiliseer. Films is van die resultante lateks voorberei en hul waterdampsorpsie –eienskappe oor die parsiële waterdampdrukreeks van 0.1 tot 0.9 bepaal. Sigmodale kinetiese gedrag is vir al drie films waargeneem, met die DA3B gestabiliseerde film wat hoër diffusiekoëffisiënte toon in vergelyking met die films wat met NDBS en OFE gestabiliseer is. Die termodinamiese gedrag van die films het verskil en was afhanklik van die reaktiwiteit van die seep. NDBS en OFE gestabiliseerde films het Flory-Huggins gedrag getoon, terwyl die DA3B gestabiliseerde film Henry se Wet gevolg het. Ten spyte van die beduidende verskille ten opsigte van hierdie eienskappe was die permitiewe koëffisiënte soortgelyk vir die drie films regoor die parsiële waterdampdrukreeks wat vir die evaluasie gebruik is. Die impak van natriummontmorilloniet (Na-MMT) klei en ‘n organofiliese modifiseerder, 2- akrielamido-2-metielpropaansulfoonsuur (AMPS), op die waterdampsorpsie-eienskappe van polistireen-ko-butielakrilaat is geevalueer. Hierdie polimeer-klei-nanosaamgesteldemateriale (PKNe) is gesintetiseer deur van miniemulsiepolimerisasie gebruik te maak. Die resultante lateksfilms is gekarakteriseer en gebruik vir waterdampsorpsie analises. Daar is getoon dat algehele afskilfering van die Na-MMT nodig was om die ewewigswaterdampopname te minimaliseer. Selfs wanneer Na-MMT algeheel afgeskilfer was, was die hoeveelheid waterdamp gesorbeer deur die PKN hoog en kan dit toegeskryf word aan die hidrofiliese karakter van die klei. Deur ‘n kleinste-kwadrate-regressie passing te doen, is ‘n goeie korrelasie verkry tussen die eksperimentele isoterme en die sorpsie gedrag voorspel deur die Dubbelmodussorpsiemodel wat oorspronklik ontwikkel is vir polimere in hul glasagtige toestand. Die impak van sykettingkristalliniteit op die waterdampsorpsie-eienskappe van poli(metielmetakrilaat-ko-oktadekielakrilaat) is ondersoek. Hierdie ewekansige kopolimere wat toenemende hoeveelhede oktadekielakrilaat, en dus toenemende grade van kristalliniteit bevat, is gesintetiseer deur van oplossingspolimerisasie gebruik te maak. Alhoewel dit te wagte was dat sykettingkristalliniteit die hoofbydraende faktor is in die redusering van die diffusiekoeffisiente, is daar getoon dat die metielgroep aan die α-koolstof van die vinielgroep in die metakrilaat die diffusie tot 'n groter mate gereduseer het deur toenemende styfheid van die polimeerrugraat. Dit is ook gereflekteer deur poli(metielmetakrilaat-ko-oktadekielakrilaat) wat 'n groter aktiveringsenergie vir diffusie het in vergelyking met polioktadekielakrilaat.

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No description available.

This thesis consists of analysis of three different data sets: (i) Aircraft-based eddy correlation data collected above irrigated and non-irrigated agricultural land in Southern California during the California Ozone Deposition Experiment (CODE) summer 1991; (ii) micrometeorological tower data, collected over grape and cotton canopies as part of CODE; (iii) aircraft-based eddy correlation flux data above two grid sites in the Canadian boreal forest during the Boreal Ecosystem-Atmosphere Study (BOREAS), spring and summer of 1994 and 1996. / Results from the CODE aircraft data document composition and size of the dominant structures, which transport heat and gases (H2O, CO 2 and ozone) over water stressed and non-water stressed surfaces, and the relative frequency with which structures carrying only a single scalar, or given combinations of scalars, were encountered along the flight paths. Interpretation of results provides further evidence for the existence of a second (nonphysiological) sink for ozone. The relative preponderance of structures that carry moisture, carbon dioxide and ozone simultaneously, particularly in the gradient-up mode, reflects the importance of vegetation as co-located source/sink for these scalars. The detrending procedures described in this study may help to define a more effective separation between local and mesoscale events in biosphere-atmosphere interaction. / Results from the CODE tower data indicates a single vegetated ozone sink for the grape site, but a vegetated as well as a non-vegetated sink for the cotton site. For both sites, structures simultaneously transporting significant flux contributions of CO2, H2O, heat and ozone dominate during unstable conditions. During stable conditions, unmixed single flux structures dominated over cotton but not over grape. The results of this study contribute empirical evidence about the relationship between ozone uptake and the physical and physiological state of vegetation, as well as the limitations placed on eddy scales in simulation models. / Results from the BOREAS aircraft data shows a decoupling between the surface and the atmosphere, where the patterns of vegetation, greenness and surface temperature may be quite dissimilar to those of the fluxes of sensible heat, latent heat and---to a lesser degree---CO2. Reasons for this lie in the extraordinary boundary layer conditions, high vapour pressure deficit, moist soil and hot canopies, and the response of the vegetation to these conditions. Analysis of the coherent structure compositions to some extent permits the characterization of the different sources and sinks. Overall, this study shows the importance of understanding the various interacting components of soil, vegetation and atmosphere when attempting to design process-based models for predictions in 'micrometeorologiacally' complex ecosystems.

<div> <p>The distribution of local water vapor in the surrounding air has been shown to be the driving mechanism for several phase change phenomena during dropwise condensation and condensation frosting. This thesis uses reduced-order modeling approaches, which account for the effects of the vapor distribution to predict the droplet growth dynamics during dropwise condensation in systems of many droplets. High-fidelity modeling techniques are used to further probe and quantify the heat and mass transport mechanisms that govern the local interactions between a freezing droplet and its surrounding ambient, including neighboring droplets. The relative significance of these transport mechanisms in the propagation of frost are investigated. A reduced-order analytical method is first developed to calculate the condensation rate of each individual droplet within a group of droplets on a surface by resolving the vapor concentration field in the surrounding air. A point sink superposition method is used to account for the interaction between all droplets without requiring solution of the diffusion equation for a full three-dimensional domain. For a simplified scenario containing two neighboring condensing droplets, the rates of growth are studied as a function of the inter-droplet distance and the relative droplet size. Interactions between the pair of droplets are discussed in terms of changes in the vapor concentration field in the air domain around the droplets. For representative systems of condensing droplets on a surface, the total condensation rates predicted by the reduced-order model match numerical simulations to within 15%. The results show that assuming droplets grow as an equivalent film or in a completely isolated manner can severely overpredict condensation rates.</p> <p>The point superposition model is then used to predict the condensation rates measured during condensation experiments. The results indicate that it is critical to consider a large number of interacting droplets to accurately predict the condensation behavior. Even though the intensity of the interaction between droplets decreases sharply with their separation distance, droplets located relatively far away from a given droplet must be considered to accurately predict the condensation rate, due to the large aggregate effect of all such far away droplets. By considering an appropriate number of interacting droplets in a system, the point sink superposition method is able to predict experimental condensation rates to within 5%. The model was also capable of predicting the time-varying condensation rates of individual droplets tracked over time. These results confirm that diffusion-based models that neglect the interactions of droplets located far away, or approximate droplet growth as an equivalent film, overpredict condensation rates.</p> <p>In dropwise condensation from humid air, a full description of the interactions between droplets can be determined by solving the vapor concentration field while neglecting heat transfer across the droplets. In contrast, the latent heat released during condensation freezing processes cause droplet-to-ambient as well as droplet-to-droplet interactions via coupled heat and mas transfer processes that are not well understood, and their relative significance has not been quantified. As a first step in understanding these mechanisms, high-fidelity modeling of the solidification process, along with high-resolution infrared (IR) thermography measurements of the surface of a freezing droplet, are used to quantify the pathways for latent heat dissipation to the ambient surroundings of a droplet. The IR measurements are used to show that the crystallization dynamics are related to the size of the droplet, as the freezing front moves slower in larger droplets. Numerical simulations of the solidification process are performed using the IR temperature data at the contact line of the droplet as a boundary condition. These simulations, which have good agreement with experimentally measured freezing times, reveal that the heat transferred to the substrate through the base contact area of the droplet is best described by a time-dependent temperature boundary condition, contrary to the constant values of base temperature and rates of heat transfer assumed in previous numerical simulations reported in the literature. In further contrast to the highly simplified descriptions of the interaction between a droplet and its surrounding used in previous models, the model developed in the current work accounts for heat conduction, convection, and evaporative cooling at the droplet-air interface. The simulation results indicate that only a small fraction of heat is lost through the droplet-air interface via conduction and evaporative cooling. The heat transfer rate to the substrate of the droplet is shown to be at least one order of magnitude greater than the heat transferred to the ambient air.</p> <p>Subsequently, the droplet-to-droplet interactions via heat and mass exchange between a freezing droplet and a neighboring droplet, for which asymmetries are observed in the final shape of the frozen droplet, are investigated. Side-view infrared (IR) thermography measurements of the surface temperature for a pair of freezing droplets, along with three-dimensional numerical simulations of the solidification process, are used to quantify the intensity and nature of these interactions. Two droplet-to-droplet interaction mechanisms causing asymmetric freezing are identified: (1) non-uniform evaporative cooling on the surface of the freezing droplet caused by vapor starvation in the air between the droplets; and (2) a non-uniform thermal resistance at the contact area of the freezing droplet caused by the heat conduction within the neighboring droplet. The combined experimental and numerical results show that the size of the freezing droplet relative to its neighbor can significantly impact the intensity of the interaction between the droplets and, therefore, the degree of asymmetry. A small droplet freezing in the presence of a large droplet, which blocks vapor from freely diffusing to the surface of the small droplet, causes substantial asymmetry in the solidification process. The droplet-to-droplet interactions investigated in thesis provide insights into the role of heat dissipation in the evaporation of neighboring droplets and ice bridging, and open new avenues for extending this understanding to a system-level description for the propagation of frost.</p> </div> <br>