We report capacitive measurements of the hysteresis and avalanche properties of $\sp4$He in Nuclepore. Hysteresis was measured on global loops and subloops for Nuclepore with 30 nm and 200 nm diameter pores. Subloops exhibit the property of return point memory, but subloops between the same chemical potential end points were not congruent. The failure of congruence showed that the Preisach model, in which the pores do not interact, does not describe this system. Modification of the Preisach model to allow for pore intersection within the membrane resulted in qualitative agreement with the data. Using a model for hystersis by Mason which included pore-pore interactions, we calculated the pore size distribution for Nuclepore. Close examination of the primary desorption curve resulted in the observation of large groups of pores draining together. This was the first observation of avalanches in the draining of fluid from a porous material. The properties of the avalanches were documented for 200 nm and 30 nm pore diameter Nuclepore. Using two capacitors on the same Nuclepore substrate, we found that avalanche events were spatially extended involving a low density of pores. With a composite porous/nonporous sample we showed that pore coupling occurred via the superfluid $\sp4$He film.

Abstract not available

We have measured the viscosity of a dilute mixture of 3He in 4He at temperatures from 5 to 100 mK and in magnetic fields of 0.35, 2.01 and 8.76 T using a vibrating wire viscometer. Subject to our choice of calibration function, the data at 0.35 T is in good agreement with theory. Due to an interaction of our viscometer with the magnetic field the motion of the wire at 2.01 and 8.76 T is not understood; however, the 2.01 T data can be brought into agreement with the 0.35 T data by adjusting a geometrical constant. An increase in the low temperature viscosity between 2.01 and 8.76 T is in qualitative agreement with theory (using an alternate choice of calibration function a decrease in viscosity between 2.01 and 8.76 T is observed).

There have been countless research investigations taking place on Marion Island (MI), both ecological and geological, which have reached conclusions that must necessarily neglect the impacts of wind on the systems under study. Since only the dominant wind direction of the general atmospheric wind is known from weather and satellite data, not much can be said about local wind conditions at ground level. Therefore, a baseline Computational Fluid Dynamics (CFD) model has been developed for simulating wind patterns over Marion and Prince Edward Islands, a South African territory lying in the subantarctic Indian Ocean. A review of the current state of the art of Computational Wind Engineering (CWE) revealed that large-scale Atmospheric Boundary Layer (ABL) simulations have been successfully performed before with varying degrees of success. With ANSYS Fluent chosen as the numerical solver, the Reynolds-Averaged Navier-Stokes (RANS) equations were set up to simulate a total of 16 wind flow headings approaching MI from each of the cardinal compass directions. The standard k-epsilon turbulence closure scheme with modified constants was used to numerically approximate the atmospheric turbulence. A strategy was devised for generating a reusable mesh system to simulate multiple climatic conditions and wind directions around MI. In conjunction with the computational simulations, a wind measurement campaign was executed to install 17 wind data logging stations at key locations around MI. Raw data output from the stations were cleaned and converted into an easily accessible MySQL database format using the Python scripting language. The Marion Island Recorded Experimental Dataset (MIRED) database contains all wind measurements gathered over the span of two years. The decision was taken to focus on validating only three of the 16 cardinal wind directions against the measured wind data; North-Westerly, Westerly and South-Westerly winds. An initial interrogation of the simulation results showed that island-to-island wake interactions could not be ignored as the turbulent stream from MI could definitely be intercepted by its neighbour under the right conditions, and vice versa. An underestimation of the true strength of the Coriolis effect led to larger wind deflection in the simulations than originally expected, thus resulting in the wind flow at surface levels having an entirely different heading to what was intended. The westerly and south-westerly wind validation cases did not seem too badly affected by the lapse in judgement but the north-westerly case suffered strong losses in accuracy. Significant effort was put into quantifying the error present in the simulations. After a full validation exercise, it was finally resolved to apply a conservative uncertainty factor of 35 % when using these simulations to predict actual wind speed conditions. Similarly, the predicted wind direction can only be trusted within the bounds of a 35 degree prediction uncertainty. Under these circumstances, the baseline CFD model was successfully validated against the measured wind data and can thus be used in further research. In terms of post-processing, all the wind direction simulations have been combined into a single wind velocity map, generated by weighting each of the simulations by the frequency of wind prevalence measured in the corresponding wind sector. A second turbulence intensity combined map has been provided using similar techniques. These maps, as well as the individual wind maps showing all 16 cardinal wind directions, are believed to be helpful to many future biological studies on MI as well as any possible forays into wind energy generation on the island. Despite the encountered deficiencies, this project offers significant value to academia by providing a reliable method of predicting fine-scale wind patterns in a location previously devoid of any accurate data. Furthermore, it has highlighted where future CFD attempts can be improved in order to produce a compelling approximation of the realistic atmospheric phenomena occurring in the Marion Island territory. While error cannot be avoided when modelling such complex systems, it has been well quantified and discussed here so that any further research may make informed judgements in future studies. / Dissertation (MEng (Mechanical Engineering))--University of Pretoria, 2021. / South African National Antarctic Programme (SANAP) grant number 110726 / National Research Foundation (NRF) / Mechanical and Aeronautical Engineering / MEng (Mechanical Engineering) / Unrestricted

The Weddell Sea is a primary bottom water formation region, where newly formed bottom water is exported to all the ocean basins. This water mass is partly supplied from the upper layers of the Antarctic Circumpolar Current (ACC), as well as from sources within the Antarctic Slope Current (ASC). In addition to the mean meridional overturning circulation (MOC) in the Weddell Gyre, observations support the idea that the water mass exchange between the Weddell Gyre and the ACC is also due to two other mechanisms, meso-scale eddies and large-scale fluctuations in forcing. The Ocean model and Lagrangian method are used to study the water mass exchange by these mechanisms. For the exchange caused by large-scale fluctuations, only the chaotic transport caused by the seasonal cycle (CTSC) is studied because of computational limitations. By the Lagrangian method, the three different mechanisms for water mass exchange are separately found. Based on the model, considering the Weddell Gyre as a whole system, the water mass exchange by the total velocity field is 10.9 Sv, of which 7.2 Sv is due to the mean velocity field, 1.3 Sv due to CTSC and 2.4 Sv due to eddies. In total, one third of the mass exchange is caused by the time-dependent part of the velocity field. According to the model, the transport of the ASC along 30°E is estimated as 4.2 Sv from the Antarctic coast northward to the 3000 m isobath, but only 1.0 Sv is from outside of the Weddell Gyre east of 94°E. The other 3.2 Sv transport is due to gyre circulation along the eastern boundary crossing 30°E, which then flows back along the Antarctic coast. Additionally, this work also shows the strength of the Lagrangian method in calculating water mass exchange. / A Dissertation submitted to the Geophysical Fluid Dynamics Institude in partial fulfillment of the requirements for the degree of Doctor of Philosophy. / Fall Semester, 2012. / October 26, 2012. / Includes bibliographical references. / Kevin Speer, Professor Directing Thesis; Eric Chassignet, University Representative; William Dewar, Committee Member; Doron Nof, Committee Member.

This study identifies the major contributing factors in estimating whitecap coverage globally from satellite observations. Power law functions for estimating both the active and total whitecap coverage from in situ observations are derived using U10 (in situ winds) and U10EN (satellite-reported equivalent neutral winds). Whitecap coverage estimates using U10EN and a power law reproduced in situ whitecap estimates better than using U10 and a power law, but, when compared with the satellite-based whitecap observations from the Whitecap Database, neither can adequately estimate the whitecap. New power law coefficients using U10EN, available in the Whitecap Database, are presented and estimates are compared to the active and total whitecap coverage from the Whitecap Database. An additional 17 parameters, independent of U10EN, are tested to determine their roles in influencing whitecap formation and duration using a modified power law; the most influential parameters are identified. The most influential parameters for the active whitecap coverage include SST, orbital velocity, air temperature, fetch, and the cross SST gradient wind; the most influential parameters for the total whitecap coverage include SST, orbital velocity, air temperature, air-sea temperature differential, and the cross SST gradient wind. Sampling and performance ranking techniques to manage large datasets are presented along with function fitting techniques for a modified power law. The improved understanding of whitecap coverage aids in estimating whitecap coverage globally using satellite products and in determining the global effects of whitecaps on air-sea processes and remote sensing of the surface. / A Dissertation submitted to the Geophysical Fluid Dynamics Institute in partial fulfillment of the requirements for the degree of Doctor of Philosophy. / Summer Semester, 2013. / April 26, 2013. / equivalent neutral wind, In Situ, Satellite, u10, Whitecap, wind / Includes bibliographical references. / Mark A. Bourassa, Professor Directing Dissertation; Guosheng Liu, University Representative; Doron Nof, Committee Member; Allan J. Clarke, Committee Member.

This dissertation explores wildfire dynamics. Chapters 2 and 3 are peer reviewed journal articles that present an understanding of three-dimensionality in grass fires and how it affects forward rate of spread (ROS) of the fire. In Chapter 2 the numerical model HIGRAD/FIRETEC was used to give arguments supporting that modeling wildfire in a two-dimensional vertical and stream-wise plane does not represent all of the physics that are required to determine a meaningful forward ROS. Chapter 2 inspired the work that makes up chapter 3. In chapter 3, HIGRAD/FIRETEC was again used, to determine the effect that ignition line length has on forward ROS. In both chapters, finger shaped structures were present in the combusting fuels, upstream of the fire front. The fingers correlated with counter-rotating vortex pairs in the gas-phase above them. It was also shown that increasing ignition line length does indeed increase forward ROS, an expected result supported by previous investigations. Results were presented that suggest physical reasons why a spreading grass fire develops flanks that move forward slower than the front of the fire. Chapter 4 describes the gas phase in the planetary boundary layer (PBL), where fires and other phenomena occur. A muti-component gas phase model was derived that represents individual ideal gas species. The mass dependent nature of this model allows the individual species to have dynamic effects on the flow field. The multi-component model was then coupled to HIGRAD to explore three PBL scenarios. The purpose of the first case was to numerically spin-up a moist unstable PBL. The second case used the mixture model to look at a hypothetical scenario representative of the Las Conchas wildfire. In the second case, an idealized column of a gaseous mixture containing heat, dry air, water vapor, and fullerene was initialized over the topography where the Las Conchas fire occurred. The gas column represented an idealized fire plume. As predicted, the column collapsed under its own weight. However, parts of the column rose to higher elevations too. The third case was to use the results from the first case to model fugitive methane in an unstable PBL. In this case, a small amount of methane was fluxed into a grid cell on the bottom boundary of the spun-up, moist PBL from the first case. The evolution of the methane plume was and continues to be studied. Some preliminary results are presented where methane concentration is compared to data collected from a field experiment. Chapter 5 is the conclusion. In this chapter, the results from chapters 2, 3, and 4 are summarized. An analytical fire spread model is proposed that ties chapters 2 and 3 to chapter 4. This spread model is a two-layer, multi-phase set of governing equations. It is assumed that the gas phase is a thin layer relative to the horizontal. This thin layer contains laminar, diffusion flames. The vertical distance that the flames remain laminar and diffusive determines the thickness of the layer. In other words, the vertical distance from the ground to the transition point to turbulent flames, determines the thickness of the layer. / A Dissertation submitted to the Geophysical Fluid Dynamics Institute in partial fulfillment of the requirements for the degree of Doctor of Philosophy. / Spring Semester, 2014. / April 11, 2014. / Flame Spread, Fluid Dynamics, Gaseous Species, Rate of Spread, Vorticity, Wildfire / Includes bibliographical references. / Doron Nof, Professor Directing Dissertation; Robert Hart, University Representative; Kevin Speer, Committee Member; Brian Ewald, Committee Member; Allan Clarke, Committee Member; Ruby Krishnamurti, Committee Member; Rodman Linn, Committee Member; Carol Anne Clayson, Committee Member.

Ch 1: This study provides the first long term data set on coastal oxygen dynamics in the northeastern Gulf of Mexico (NEGOM), located about 600 km east of the large hypoxic zone that extends west of the Mississippi delta. Oxygen concentrations and environmental parameters affecting the production and respiration of dissolved oxygen were measured over a 4-year period in the water column, and over a 2-year period in the benthic boundary layer (0.5 m above seafloor), along a 29 km long transect line following a depth gradient from 5 m to 18 m in the Florida Big Bend region of the West Florida Shelf. We show decreasing rates of oxygen production and consumption from the shallowest station to the deepest station that we attribute to the coastal energy gradient, i.e. higher light intensities, higher turbulent mixing rates and greater nutrient supply (organic-rich terrestrial runoff close to shore). Vertical profiles showed that oxygen production and consumption were highest in the benthic boundary layer (0.5 m above the seabed) from shore down to a water depth of 5 m; farther offshore, highest production and consumption values were generally measured higher in the water column. Water column photosynthetic oxygen production at an irradiance of 200 μE m-2 s-1 ranged from 0.5 to 4.5 mmol O2 m-3 h-1, and dark consumption rates ranged from 0.2 to 2.6 mmol O2 m-3 h-1. The water column was well mixed except during relatively short calm periods in summer and winter when a pycnocline developed. Oxygen below the pycnocline in summer dropped to 74% air saturation but never to levels that could have influenced aerobic life. Oxygen supersaturation as high as 118% air saturation was recorded during August 2007, reflecting high primary productivity of the upper water column. High wind events influenced dissolved oxygen down to the bottom at all stations. Our study shows the influence of the shore on the oxygen dynamics in the NEGOM Shelf and emphasizes the role of the benthic boundary layer for production and consumption processes in this shallow region. Ch 2: The West Florida Shelf is shallow and light can penetrate to its bottom facilitating photosynthesis at the sediment surface. This paper presents the first long term study of the activity of the benthic primary producers in the northeastern Gulf of Mexico. Photosynthetic rates of microphytobenthos colonizing permeable sandy sediments were quantified along a 29 km long transect starting at 5 m water depth and ending at 18 m water depth 29 km off the Northeast Gulf of Mexico coast during the 3-year study. The sediment of this region is classified as sand and slightly gravelly sand, being highly permeable. Chlorophyll concentrations and photosynthesis rates of the microphytobenthos showed relatively high rates of productivity for sediment from 20 m water depth compared to rates reported in studies from similar environments. In spring, the decrease of net production with depth amounted to approximately 0.11 mmol m-2 h-1 per 10 m water depth increase and consumption decreased by about 0.34 mmol m-2 h-1 per 10 m depth increase. In winter, these trends were reduced to one fourth to one third of the spring values reflecting a proportionally stronger influence of the coastal effect during the season with strongest growth. Our findings support the hypothesis that permeable sands can be photosynthetically highly active despite low standing stocks of algal cells and low nutrient concentrations due to their biocatalytical filtration function that effectively can provide limiting nutrients to photoautotrophs. The observed decrease in productivity with distance from shore indicates that benthic primary production in this region continues to approximately 30 m depth and thereby affects a large fraction of the shallow shelf in the northeastern Gulf of Mexico. Ch 3: With measurements conducted with instruments installed on the seafloor at 3 stations (5, 10, 18 m depth) along a transect in the Northeastern Gulf of Mexico, and one deeper station (40 m depth) nearby, we show that oxygen concentrations in the benthic boundary layer (50 cm above sediment) oscillate on a daily time scale and that these oscillations decrease with water depth to reach zero near 42 m. These oscillations, caused by primary production and respiration processes were affected by microphytobenthos activities. Maximum oxygen concentrations at all Stations were reached close to sunset with saturation values up to 108 % oxygen air saturation. The buildup of oxygen concentrations during the day was followed by an approximately equal drop of oxygen after sunset continuing throughout the night. Minimum oxygen concentrations were reached near sunrise but never fell below an oxygen saturation value of 77 % oxygen air saturation. Because these oscillations were present at water depth < 40 m, this process may affect the majority of the West Florida Shelf. As oxygen is a primary electron acceptor, the observed oscillations in the benthic boundary layer may influence coupled nitrification/denitrification, organic matter degradation, food web dynamics and they may act as biological timer and trigger function. / A Dissertation submitted to the Program in Geophysical Fluid Dynamics in partial fulfillment of the requirements for the degree of Doctor of Philosophy. / Fall Semester, 2013. / November 18, 2013. / Chlorophyll, Continental Shelf, Gulf of Mexico, Microphytobenthos, Oxygen, Permeable Sediment / Includes bibliographical references. / Markus Huettel, Professor Directing Dissertation; Janie Wulff, University Representative; Kevin Speer, Committee Member; David Thistle, Committee Member; Allan Clarke, Committee Member.

This dissertation work is aimed at improving the capability and confidence in a modernized and improved version of Los Alamos National Laboratory's coupled atmospheric and wild- land fire dynamics model, Higrad-Firetec. Higrad is the hydrodynamics component of this large eddy simulation model that solves the three dimensional, fully compressible Navier- Stokes equations, incorporating a dynamic eddy viscosity formulation through a two-scale turbulence closure scheme. Firetec is the vegetation, drag forcing, and combustion physics portion that is integrated with Higrad. The modern version of Higrad-Firetec incorporates multiple numerical methodologies and high performance computing aspects which combine to yield a unique tool capable of augmenting theoretical and observational investigations in order to better understand the multi-scale, multi-phase, and multi-physics, phenomena in- volved in coupled atmospheric and environmental dynamics. More specifically, the current work includes extended functionality and validation efforts targeting component processes in coupled atmospheric and wildland fire scenarios. Since observational data of sufficient quality and resolution to validate the fully coupled atmosphere-wildfire scenario simply does not exist, we instead seek to validate components of the full prohibitively convoluted pro- cess. This manuscript provides first, an introduction and background into the application space of Higrad-Firetec. Second we document the model formulation, solution procedure, and a simple scalar transport verification exercise. Third, we perform a validate model results against observational data for time averaged flow field metrics in and above four idealized forest canopies. Fourth, we carry out a validation effort for the non-buoyant jet in a crossflow scenario (to which an analogy can be made for atmosphere-wildfire interactions) comparing model results to laboratory data of both steady-in-time and unsteady-in- time metrics. Finally, an extension of model multi-phase physics is implemented, allowing for the representation of multiple collocated fuels as separately evolving constituents lead- ing to differences resulting rate of spread and total burned area. In combination these efforts demonstrate improved capability, increased validation of component functionality, and unique applicability the Higrad-Firetec modeling framework. As a result this work provides a substantially more robust foundation for future new, more widely acceptable investigations into the complexities of coupled atmospheric and wildland fire behavior. / A Dissertation submitted to the Program in Geophysical Fluid Dynamics in partial fulfillment of the requirements for the degree of Doctor of Philosophy. / Fall Semester, 2013. / November 12, 2013. / Atmospheric, Collocated Fuels, Fuel Heterogeneity, Numerical Modeling, Wildfire / Includes bibliographical references. / Doron Nof, Professor Directing Thesis; Eric Chicken, University Representative; Rodman R. Linn, Committee Member; Ming Ye, Committee Member; Ruby Krishnamurti, Committee Member.

Velocity and temperature profiles in mercury are reported for vertical flow in a pipe at Reynolds numbers of approximately 3.3 x 10⁴ and 5.4 x 10⁴ with variation in heat flux. At a Reynolds number ≈ 3.3 x 10⁴ profiles are reported for thermal calming lengths of approximately 17, 36, 61 and 84. The velocity profiles, temperature profiles and Nusselt numbers are shown to be strongly influenced by free convection effects. Correlations are presented whereby the amount of distortion of the velocity profile from the isothermal profile under given conditions may be estimated, and the value of the Nusselt number under given conditions may be predicted.