Turbulent flows affected by buoyancy lie at the basis of many applications, both within engineering and the atmospheric sciences. A prominent example of such an application is the atmospheric boundary layer, the lowest layer of the atmosphere, in which many physical processes are heavily influenced by both stably stratified and convective turbulent transport. Modelling these turbulent flows correctly, especially in the presence of stable stratification, has proven to be a great challenge and forms an important problem in the context of climate models. In this thesis, we address this issue considering an advanced class of turbulence models, the so-called explicit algebraic models.In the presence of buoyancy forces, a mutual coupling between the Reynolds stresses and the turbulent heat flux exists, which makes it difficult to derive a fully explicit turbulence model. A method to overcome this problem is presented based on earlier studies for cases without buoyancy. Fully explicit and robust models are derived for turbulence in two-dimensional mean flows with buoyancy and shown to give good predictions compared with various data from direct numerical simulations (DNS), most notably in the case of stably stratified turbulent channel flow. Special attention is given to the problem of determining the production-to-dissipation ratio of turbulent kinetic energy, for which the exact equation cannot be solved analytically. A robust approximative method is presented to calculate this quantity, which is important for obtaining a consistent formulation of the model.The turbulence model derived in this way is applied to the atmospheric boundary layer in the form of two idealized test cases. First, we consider a purely stably stratified boundary layer in the context of the well-known GABLS1 study. The model is shown to give good predictions in this case compared to data from large-eddy simulation (LES). The second test case represents a full diurnal cycle containing both stable stratification and convective motions. In this case, the current model yields interesting dynamical features that cannot be captured by simpler models. These results are meant as a first step towards a more thorough investigation of the pros and cons of explicit algebraic models in the context of the atmospheric boundary layer, for which additional LES data are required. / <p>QC 20150522</p>

The principal objective of this investigation is to develop a nonlinear continuum based finite element formulation to examine dynamic response of flexible riser structures with large displacement and large rotation. Updated Lagrangian incremental approach together with the 2nd Piola-Kirchhoff stress tensor and the Green-Lagrange strain tensor is employed to derive the nonlinear finite element formulation. The 2nd Piola-Kirchhoff stress and the Green-Lagrange strain tensors are energy conjugates. These two Lagrangian tensors are not affected by rigid body rotations. Thus, they are used to describe the equilibrium equation of the body independent of rigid rotations. While the current configuration in Updated Lagrangian incremental approach is unknown, the resulting equation becomes strongly nonlinear and has to be modified to a linearized form. The main contribution of this work is to obtain a modified linearization method during development of incremental Updated Lagrangian formulation for large displacement and large rotation analysis of riser structures. For this purpose, the Green-Lagrange strain and the 2nd Piola-Kirchhoff stress tensors are decomposed into two second-order six termed functions of through-thethickness parameters. This decomposition makes it possible to explicitly account for the nonlinearities in the direction along the riser thickness, as well. It is noted that using this linearization scheme avoids inaccuracies normally associated with other linearization schemes. The effects of buoyancy force, riser-seabed interaction as well as steady-state current loading are considered in the finite element solution for riser structure response. An efficient riser problem fluid-solid interaction Algorithm is also developed to maintain the quality of the mesh in the vicinity of the riser surface during riser and fluid mesh movements. To avoid distortions in the fluid mesh two different approaches are proposed to modify fluid mesh movement governing elasticity equation matrices values; 1) taking the element volume into account 2) taking both element volume and distance between riser centre and element centre into account. The formulation has been implemented in a nonlinear finite element code and the results are compared with those obtained from other schemes reported in the literature.

Evidence based performance of novel ventilation systems in existing low energy buildings is invaluable as it provides data on the system operation in a real dynamic environment. This thesis presents the outcomes from research involving a number of experimental field studies of a single sided ventilation system installed in a single cell office space as part of a building retrofit pilot project in Cork, Ireland. The solution consists of a purpose provided, multi configuration opening, comprising a narrow slotted architectural louvre component split across a low level manual opening section and a high level automated opening section. A review of published research found that little experimental data exists on the performance of such systems and air flow rate correlations developed for plain openings are currently used by designers to make predictions about their performance. Three experimental campaigns were designed and carried out. First, in order to quantify performance of the system, long term and short term monitoring of the internal thermal and air flow environment at the experimental building was completed. Second, ventilation rate measurements in existing and retrofit spaces were completed using a tracer gas concentration decay technique. Thirdly, air flow through the single sided slot louvre opening was investigated. In addition, the annual cooling potential of the multi-configuration system was investigated computationally. Results show there was a significant difference between both thermal environments with the retrofit space consistently displaying lower air temperatures over the cooling season and throughout all Air Change Rate measurement periods. Lower levels of vertical thermal stratification and diurnal temperature variation were also observed. On average, across a wide range of boundary conditions, lower ventilation rates were observed for the slotted louvre system with a narrower spread of values when compared with the existing building. The dominant driving force was either buoyancy or wind depending on the opening configuration adopted in the slotted louvre system. The slot louvre was found to be wind dominant for lower opening heights when compared with a plain opening of the same dimensions. Existing single sided correlations were found to perform better when predicting airflow rates through a plain opening when compared with the slot louvre system and a new dimensionless exchange rate parameter is proposed for predicting wind driven airflow through the slot louvre. Simulations indicate that 80% of annual occupied hours required an enhanced ventilative cooling airflow rate to achieve internal thermal comfort. Using a combination of configurations the system was able to provide the required cooling airflow rate for 93% of the occupied hours.

A ventilação nas edificações tem por objetivo principal criar um ambiente interno confortável e saudável, preservando a qualidade do ar interior. Um dos principais sistemas de renovação do ar, e praticamente a única opção economicamente viável, utilizado no projeto de ambientes industriais é a ventilação natural. Esta pode ser promovida por dois mecanismos: o denominado efeito chaminé e o efeito dos ventos, porém também podem ocorrer pelos dois simultaneamente. O objetivo deste trabalho é pesquisar a viabilidade do emprego de modelos reduzidos no projeto da ventilação natural em pavilhões industriais, considerando unicamente o efeito chaminé. Para atingir esse objetivo foi ensaiado um modelo, com escala geométrica 1/100, de um pavilhão industrial real situado na cidade de Passo Fundo – RS, que será utilizado como uma fábrica para injeção de alumínio em carcaças e rotores elétricos. Nos ensaios foram medidas velocidades na abertura de saída, com anemômetro de fio quente, e temperaturas com termopares, tanto dentro como fora do modelo. Os resultados alcançados permitem avaliar quais são as leis de semelhança relevantes neste tipo de ensaio com modelo reduzido, as limitações do procedimento experimental e, ainda, determinar valores de vazões de ar nas aberturas de saída e compará-los com vazões obtidas através da utilização de alguns modelos teóricos. / The ventilation in the constructions has for main objective to create a comfortable and healthful internal environment, preserving the quality of the interior air. One of the main systems of renewal of air, and practically the only economically viable option, used in the industrial environment project is the natural ventilation. This can be promoted by two mechanisms: the called buoyancy and the effect of the winds, however also can occur for the two simultaneously. The objective of this work is to search the viability of the job of models reduced in the project of the natural ventilation in industrial pavilions, considering solely the buoyancy. To reach this objective a model was assayed, with geometric scale 1/100, of a situated real industrial pavilion in the city of Passo Fundo – RS, that will be used as a plant for injection of aluminum in carcasses and electric rotors. In the assays speeds in the exit opening had been measured, with anemometer of hot wire, and temperatures with thermocouples, in such a way inside as it are of the model. The reached results allow to evaluate which are the excellent laws of similarity in this type of assay with reduced model, the limitations of the experimental procedure and still to determine values of air outflows in the exit openings and to compare them with outflows gotten through the use of some theoretical models.

The Apollo Command Module had a tendency to flip over upon impact with the ocean surface after returning from space (9/19 times). In an effort to better characterize the inversion process for future water landing vehicles, experimental results for a simplified buoyant cylindrical puck impacting the water surface are presented. This study focuses on the dependence of inversion upon vertical velocity, horizontal velocity, and the pitch angle of the puck relative to the free surface. High-speed images reveal an asymmetric cavity that forms upon water impact. The asymmetric cavity then collapses, applying a moment, which can be sufficient to invert the puck after impact. Increasing the vertical velocity increases the likelihood of inversion. The puck never flipped over below a vertical velocity of 3.75 m/s. Increasing the horizontal velocity also slightly increases the likelihood of inversion. However, the largest effect of increasing horizontal velocity is to shift the range of impact angles for which the puck will invert to lower angles. The buoyant cylindrical puck used in this study requires a higher Froude number (4.34) to invert than previous geometries which have been studied.

Compared with the free atmosphere, the aquatic environment is oxygen poor. As a result many secondarily aquatic organisms have adaptations that allow them to continue to use the atmosphere, directly or indirectly, to supply their oxygen requirements. This thesis examines how diving insects use bubbles of air collected at the surface of the water as oxygen reserves, gills and flotation devices, and how an aquatic angiosperm channels convective flows of air from its emergent leaves to its submerged organs. 1. Backswimmers (Anisops spp.) begin a dive positively buoyant, but rapidly enter a protracted period of near neutral buoyancy. A bubble of air held on the insect’s abdomen shrinks as respiration consumes its oxygen, while at the same time highly soluble carbon dioxide dissolves into the surrounding water. The reduced air volume confers neutral buoyancy. In response to low oxygen partial pressure (PO2) in the bubble, oxygen is released from large haemoglobin cells in the abdomen. The haemoglobin’s sensitivity to falling PO2 maintains the oxygen tension between 5.1 and 2.0 kPa. This stabilises the volume and buoyancy of the bubble. During a dive the haemoglobin and air-store supply 0.25 and 0.26 μL of oxygen, respectively. 2. The oxygen affinity of backswimmer haemoglobin determines the stability of the neutrally buoyant phase as well as its ability to satisfy the insect’s respiration. An oxygen equilibrium curve (OEC) determined in vivo has a highly sigmoid shape and an oxygen affinity of 3.9 kPa. In comparison with OEC made in vitro, the in vivo measurements show increased cooperativity and oxygen affinity, consistent with the presence of cationic effectors. Models strongly support the accuracy of the in vivo OEC method. 3. It has long been assumed that a bubble of air held over the spiracles of an insect enables the uptake of oxygen from the surrounding water and thus acts as a ‘gas gill’. Oxygen diffuses into a bubble of air when its PO2 is lower than the surrounding water, but a coincident higher nitrogen partial pressure causes it to dissolve. Several models have been produced to describe the gas exchange process, but all are based on untested assumptions of gill parameters. Measurements of gas gill volume and PO2 made on water bugs (Agraptocorixa eurynome) demonstrate that both drop quickly at the beginning of a dive, but PO2 reaches a stable level while the gas gill continues to dissolve. The importance of ventilation in maintaining an acceptable rate of oxygen consumption is also shown. 4. Interconnected gas spaces within the leaves, stems and rhizomes are a common feature of many emergent aquatic plants. Pressurised air from the leaves and culms of these plants ventilate these lacunae, flowing back to the atmosphere through efflux points. Unlike most aquatic plants, which have simple interconnected pith spaces, sacred lotus, Nelumbo nucifera, possess discrete gas canals which only interconnect where a leaf grows from the rhizome. Silicone casts and pneumatic tests of the gas canals reveal a complex repeating pattern of interconnections which channel air from specific regions of the leaf blade to the rhizome and out through efflux points on adjacent leaves. 5. Lotus, Nelumbo nucifera, possess in the centre of their leaves a specialised efflux organ which connects the gas canals in the leaves and stems with the atmosphere through the apertures of large stomata. Measurements made on excised lotus leaves and in situ reveal that the large stomata act as exhaust valves, opening and closing in a diurnal pattern to regulate the flow of pressurised gas from the leaf lamina and gas canals. This behaviour is shown to regulate gas flow rate and direction. The aquatic environment offers similar respiratory challenges to both plants and insects. While the oxygen uptake and transport mechanisms evolved by these groups are markedly different, they all function according to the same physical laws. Diving insects are separated from the atmosphere while underwater, forcing them to rely on oxygen either carried with them from the surface or extracted from the surrounding water. Emergent aquatic plants have permanent access to atmospheric oxygen, but must transport it long distances from their aerial leaves and stems to their roots and rhizomes. This thesis examines the uptake and storage of oxygen by diving insects and the gas transport system of the sacred lotus. / Thesis(Ph.D.) -- University of Adelaide, School of Earth and Environmental Sciences, 2008

The self-similar evolution of a turbulent Rayleigh-Taylor (R-T) mix is investigated through experiments and numerical simulations. The experiments consisted of velocity and density measurements using thermocouples and Particle Image Velocimetry techniques. A novel experimental technique, termed PIV-S, to simultaneously measure both velocity and density fields was developed. These measurements provided data for turbulent correlations, power spectra, and energy balance analyses. The self-similarity of the flow is demonstrated through velocity profiles that collapse when normalized by an appropriate similarity variable and power spectra that evolve in a shape-preserving form. In the self-similar regime, vertical r.m.s. velocities dominate over the horizontal r.m.s. velocities with a ratio of 2:1. This anisotropy, also observed in the velocity spectra, extends to the Taylor scales. Buoyancy forcing does not alter the structure of the density spectra, which are seen to have an inertial range with a -5/3 slope. A scaling analysis was performed to explain this behavior. Centerline velocity fluctuations drive the growth of the flow, and can hence be used to deduce the growth constant. The question of universality of this flow was addressed through 3D numerical simulations with carefully designed initial conditions. With long wavelengths present in the initial conditions, the growth constant was found to depend logarithmically on the initial amplitudes. In the opposite limit, where long wavelengths are generated purely by the nonlinear interaction of shorter wavelengths, the growth constant assumed a universal lower bound value of

<p>Nuclear reactors release small amounts of radioactivity during their normal operations. The most common method of calculating the dose to the public that results from such releases uses Gaussian Plume models. We are investigating these methods using CAP88-PC, a computer code developed for the Environmental Protection Agency (EPA) in the USA that calculates the concentration of radionuclides released from a stack using Pasquill stability classification. A buoyant or momentum driven part is also included. The uptake of the released radionuclide by plants, animals and humans, directly and indirectly, is then calculated to obtain the doses to the public. This method is well established but is known to suffer from many approximations and does not give answers that are accurate to be better than 50% in many cases. More accurate, though much more computer-intensive methods have been developed to calculate the movement of gases&nbsp / using fluid dynamic models. Such a model, using the code FLUENT can model complex terrains and will also be investigated in this work. This work is a preliminary study to compare the results of the traditional Gaussian plume model and a fluid dynamic model for a simplified case. The results indicate that Computational Fluid Dynamics calculations give qualitatively similar results with the possibility of including much more effects than the simple Gaussian plume model.</p>

There are two coupled primary objectives for this study of buoyancy-driven turbulence. The first objective is to create a new diagnostic for collection of measurements to capture the physics of Rayleigh-Taylor (RT) mixing. The second objective is to use the new diagnostic to specifically elucidate the physics of large Atwood number, ( )( )2 1 2 1 / ρ ρ ρ ρ + − = t A , RT mixing. Both of these objectives have been satisfied through the development of a new hot-wire diagnostic to study buoyancy-driven turbulence in a statistically steady gas channel of helium and air ( 6 . 0 03 . 0 ≤ ≤ t A ). The capability of the diagnostic to simultaneously and instantaneously measure turbulent velocity and density fluctuations allows for a unique investigation into the dynamics of Rayleigh-Taylor mixing layers at large At, through measurements of turbulence and mixing statistics. The new hot-wire diagnostic uses temperature as a fluid marker for helium and air, which is possible due to the Lewis number ~ 1 (Le = ratio of thermal diffusivity to mass diffusivity) for helium and air, and the new diagnostic has been validated in an At = 0.03 mixing layer. The energy density spectrum of v′ ′ ρ , measured experimentally for the first time in RT mixing, is found to closely follow the energy distribution of v′ , up to the Reynolds numbers investigated ( ( ) mix t h gA h υ 6 2 Re 2 / 3 = ~ 1450). Large At experiments, with At = 0.6, have also been achieved for the first time in a miscible RT mixing layer. An asymmetric penetration of the bubbles (rising fluid) and spikes (falling fluid) has been observed, resulting in measured self similar growth parameters αb = 0.060 and αs = 0.088 for the bubbles and spikes, respectively. The first experimental measurements of turbulent velocity and density fluctuations for the large At case, show a strong similarity to lower At behaviors when normalized. However conditional statistics, which separate the bubble (light fluid) and spike (heavy fluid) dynamics, has highlighted differences in v′ ′ ρ and rms v′ in the bubbles and spikes. Larger values of v′ ′ ρ and rms v′ were found in the downward falling spikes, which is consistent with the larger growth rates and momentum of the spikes compared to the bubbles. These conditional statistics are a first in RT driven turbulence.

A computer program is developed for the dynamic analysis of a spar platform coupled with mooring lines and risers in waves, winds, and currents. The new multi-contact analysis program is developed for the nonlinear multi-contact coupling between vertical risers and guide frames inside of the spar moon-pool. The program extends capability of the current coupled dynamic analysis program, WINPOST, by adding the capability of analyzing riser effects caused by the contact forces and moments from buoyancy-cans inside the spar moon-pool on the global spar motions. The gap between the buoyancy can and riser guide frames are modeled using three different types of nonlinear gap springs. The new riser model also considers the Coulomb damping between the buoyancy-cans and riser guide frames, and it also calculates the impact force on risers for use in fatigue analysis. The spar platform generally uses vertical risers with dry trees. However, as the water depth increases, the size of the buoyancy-can increases, and it makes installation more difficult. The pneumatic riser support system does not use buoyancy-cans and is an alternative solution to the buoyancy-can approach. The dynamic characteristics of pneumatic riser support system are studied by using the newly developed numerical analysis program. The damped Mathieu instability diagram for the damped Mathieu's equation is developed. Due to spar heave and pitch coupling, Mathieu's instability may become excited in long period waves. In the numerical analysis program, pitch and roll hydrostatic stiffness are recalculated for heave motion in every time step to check Mathieu's instability for the spar platform. Simplified vortex-induced vibration effects on the spar platform are considered in newly developed numerical analysis program, and the results are systematically compared with those of the original program WINPOST. The results in this paper show that the buoyancy-can effect significantly reduces the spar pitch motion, and the Coulomb damping effect also significantly reduces the spar heave motion. The buoyancy-can effect also plays an important role in Mathieu instability. The results also show that a pneumatic riser support system increases the spar heave motion and payload.