Convective heat transfer to CO2 flowing upward in electrically heated vertical tubes at supercritical pressures was studied for wall heat fluxes q within ranges that included values corresponding to the onset of heat transfer deterioration (HTD). The inlet pressure was P = 8.35 MPa, the mass flux was in the range 200 kg/m2s ≤ G ≤ 1500 kg/m2s, and the inlet temperature was in the range 0 ◦C ≤ Tin ≤ 35 ◦C. Wall temperature measurements were collected in three tubular test sections, having inner diameters of D = 4.6, 8, and 22 mm. The abilities of three different HTD identification methods to separate the entire data set into deteriorated and normal heat transfer modes were tested. Two types of buoyancy parameters were tested as HTD detection methods, and correction factors for changes in mass flux were devised. The minimum heat flux at HTD onset was found to follow a power law of mass flux with the same exponent for all three sections and the same proportionality coefficient for the two smaller sections but a smaller one for the larger test section. For heat flux values that were larger than this minimum, HTD was found to occur only within a limited range of Tin, whose width increased with increasing heat flux. The heat transfer coefficient for normal heat transfer was expressed as an exponential function of the diameter.

With the current development of the computer industry, CFD simulations have become the widespread standard in the industry, forming a baseline tool for numerous designs and safety procedures. This extensive dependence on the CFD codes rather than experiments raises the issue of the reliability of the results obtained from these codes. This thesis is intended to study the dependence of the CFD results on the grid types, numerical schemes and turbulence models. Additionally, comparisons between a general purpose commercial code STAR-CCM+ and a specialized code FDS are presented towards the end of this thesis. To study the numerical errors introduced by the grids and schemes, a laminar flow induced by natural convection inside a square cavity was considered first. Using Richardson’s extrapolation, a grid independent solution was calculated and compared with the results obtained from different grid types and schemes for Rayleigh (Ra) numbers , and . Comparison plots showed a higher dependence of the accuracy of the results on the cell shapes along with the order of the scheme and the cell size. Additionally, with the same cavity a grid dependence study for the and model has been done at .To test the reliability of the Quasi-DNS performed by an Unstructured Finite Volume (FV) CFD code, Turbulent Kinetic Energy (TKE) budgets should be calculated. User subroutines were developed to calculate the budgets of the TKE and to verify the user subroutines, prior to coaxial cylinder test case, a Q-DNS of the channel flow at has been performed using different grid configurations and numerical schemes. Results obtained from the Q-DNS of the channel flow on the polyhedral cells with the bounded central differencing scheme were found to be in good agreement with the reference DNS data. After the validation test case, a Q-DNS of the buoyancy driven turbulent flow inside a horizontal annular cavity at a high Rayleigh number, Ra = 1.18x109 with outer to inner cylinder ratio of 4.85 was carried out using a commercial code. Comparisons of Q-DNS results with low-Re URANS models, and model, showed that the latter models are able to capture the general flow features but fail to predict the large unsteadiness and high turbulence levels in the plume. However, local heat transfer rates along the inner and outer cylinder walls are on average of acceptable accuracy for engineering purposes. Finally, a full scale industrial test case of a fire in a compartment has been simulated. Both URANS ( model) and LES (Smagorinsky model) approaches are applied to model the turbulence with and without incorporating the combustion modelling. A comparison of the CFD results with the experimental data showed that for building fire simulations, accuracy of the results is more sensitive to the correlations used in the combustion modelling rather than the type of the turbulence model.

Jellyfish have inhabited this planet for millions of years and are the oldest known metazoans that swim using muscles. They are found in freshwater sources and in oceans all over the world. Over millions of years of evolution, they have adapted to survive in a given environment. They are considered as one of the most energy efficient swimmers. Owing to these characteristics, jellyfish has attracted a lot of attention for developing energy efficient unmanned undersea vehicles (UUVs). The goal of this thesis is to provide understanding of the different physical mechanisms that jellyfish employs to achieve efficient swimming by using analytical and computational models. The models were validated by using the experimental data from literature. Based upon these models refinements and changes to engineering vehicles was proposed that could lead to significant enhancement in propulsion efficiency. In addition to the propulsion, the thesis addresses the practical aspects of deploying a jellyfish-inspired robotic vehicle by providing insights into buoyancy control and energy generation. The thesis is structured in a manner such that propulsive and structural models inspired from the natural animal were systematically combined with the practical aspects related to ionic diffusion driven buoyancy control system and thermal -- magnetic energy harvesting system. Jellyfish morphology, swimming mechanism and muscle architecture were critically reviewed to accurately describe the natural behavior and material properties. We provide full understanding of mesoglea, which plays most significant role towards swimming performance, in terms of composition, mechanical properties and nonlinear dynamics. Different jellyfish species exhibit different microstructure of mesoglea and thus there is a wide variety of soft materials. Mechanical properties of collagen fibers that form the main constituent toward imparting elasticity to mesoglea were reviewed and analyzed. The thesis discusses the theoretical models describing the role of structure of mesoglea towards its mechanical properties and explains the variation occurring in stiffness under given experimental environment. Muscle architecture found in jellyfish, nerve nets and its interconnection with the muscles were investigated to develop comprehensive understanding of jellyfish propulsion and its reaction to external stimuli. Different muscle arrangements were studied including radial, coronal muscle, and coronal-muscles-with-breaks in-between them as observed in Cyanea capillata. We modeled these muscle arrangements through finite element modeling (FEM) to determine their deformation and stroke characteristics and their overall role in bell contraction. We found that location and arrangement of coronal muscle rings plays an important role in determining their efficient utilization. Once the understanding of natural jellyfish was achieved, we translated the findings onto artificial jellyfish vehicle designed using Bio-inspired Shape Memory Alloy Composite (BISMAC) actuators. Detailed structural modeling was conducted to demonstrate deformation similar to that of jellyfish bell. FEM model incorporated hyperelastic behavior of artificial mesoglea (Ecoflex-0010 RTV, room temperature vulcanizing silicone with shore hardness (0010)), experimentally measured SMA temperature transformation, gravity and buoyancy forces. The model uses the actual control cycle that was optimized for driving the artificial jellyfish vehicle "robojelly". Using a comparative analysis approach, fundamental understanding of the jellyfish bell deformation, thrust generation, and mechanical efficiency were provided. Meeting energy needs of artificial vehicle is of prime importance for the UUVs. Some jellyfish species are known to use photosynthesis process indirectly by growing algae on their exumbrella and thereby utilizing the sunlight to generate energy. Inspired by this concept, an extensive model was developed for harvesting solar energy in underwater environment from the jellyfish bell structure. Three different species were modeled for solar energy harvesting, namely A.aurita, C.capillata and Mastigia sp., using the amorphous silicon solar cell and taking into account effect of fineness ratio, bell diameter, turbidity, depth in water and incidence angle. The models shows that in shallow water with low turbidity a large diameter vehicle may actually generate enough energy as required for meeting the demand of low duty cycle propulsion. In future, when the solar energy harvesting technology based upon artificial photosynthesis, referred to as "dye-sensitized solar cells", matures the model presented here can be easily extended to determine its performance in underwater conditions. In order to supplement the energy demand, a novel concept of thermal -- magnetic energy harvesting was developed and extensively modeled. The proposed harvester design allows capturing of even small temperature differences which are difficult for the thermoelectrics.  A systematic step-by-step model of thermo-magnetic energy harvester was presented and validated against the experimental data available in literature. The multi-physics model incorporates heat transfer, magnetostatic forces, mechanical vibrations, interface contact behavior, and piezoelectric based energy converter. We estimated natural frequency of the harvester, operating temperature regimes, and electromechanical efficiency as a function of dimensional and physical variables. The model provided limit cycle operation regimes which can be tuned using physical variables to meet the specific environment. Buoyancy control is used in aquatic animals in order to maintain their vertical trajectory and travel in water column with minimum energy expense. Some crustaceans employ selective ion replacement of heavy or lighter ions in their dorsal carapace. A model of a buoyancy chamber was developed to achieve similar buoyancy control using electro-osmosis. The model captures all the essential ionic transport and electrochemistry to provide practical operating cycle for the buoyancy engine in the ocean environment. / Ph. D.

<p>Presently, it is possible to use virtual learning environments for simulating abstract and/or complex scientific concepts. Multimodal Virtual Learning Environments use multiple sensory stimuli, including haptic feedback, in the representation of concepts. Past research</p> <p>on the utilization of haptics for learning has shown inconsistent results when gains in conceptual knowledge had been assessed. This research focused on two abstract phenomena</p> <p>Electricity and Magnetism and Buoyancy. These abstract concepts were experienced by students using either visual, visuo-haptic, or hands-on learning activities. Embodied</p> <p>Cognition Theory was used as a for the implementation of the learning environments. Both phenomena were assessed using qualitative and quantitative data analysis techniques.</p> <p>Results suggested that haptic, visual, and physical modalities affected positively the acquisition of conceptual knowledge of both concepts.</p>

Nowadays underwater gliders have become to play a vital role in ocean exploration and allow to obtain the valuable information about underwater environment. The traditional approach to the development of such vehicles requires a thorough design of each subsystem and conducting a number of expensive full scale tests for validation the accuracy of connections between these subsystems. However, present requirements to cost-effective development of underwater vehicles need the development of a reliable sampling and testing platform that allows the conducting a preliminary design of components and systems (hardware and software) of the vehicle, its simulation and finally testing and verification of missions. This paper describes the development of the HIL test bench for underwater applications. Paper discuses some advantages of HIL methodology provides a brief overview of buoyancy variation systems. In this paper we focused on hydraulic part of the developed test bench and its architecture, environment and tools. Some obtained results of several buoyancy variation systems testing are described in this paper. These results have allowed us to estimate the most efficient design of the buoyancy variation system. The main contribution of this work is to present a powerful tool for engineers to find hidden errors in underwater gliders development process and to improve the integration between glider’s subsystems by gaining insights into their operation and dynamics.

ABSTRACT Modeling Diffusion and Buoyancy-Driven Convection with Application to Geological CO2 Storage Rebecca Allen Geological CO2 storage is an engineering feat that has been undertaken around the world for more than two decades, thus accurate modeling of flow and transport behavior is of practical importance. Diffusive and convective transport are relevant processes for buoyancy-driven convection of CO2 into underlying fluid, a scenario that has received the attention of numerous modeling studies. While most studies focus on Darcy-scale modeling of this scenario, relatively little work exists at the pore-scale. In this work, properties evaluated at the pore-scale are used to investigate the transport behavior modeled at the Darcy-scale. We compute permeability and two different forms of tortuosity, namely hydraulic and diffusive. By generating various pore ge- ometries, we find hydraulic and diffusive tortuosity can be quantitatively different in the same pore geometry by up to a factor of ten. As such, we emphasize that these tortuosities should not be used interchangeably. We find pore geometries that are characterized by anisotropic permeability can also exhibit anisotropic diffusive tortuosity. This finding has important implications for buoyancy-driven convection modeling; when representing the geological formation with an anisotropic permeabil- ity, it is more realistic to also account for an anisotropic diffusivity. By implementing a non-dimensional model that includes both a vertically and horizontally orientated 5 Rayleigh number, we interpret our findings according to the combined effect of the anisotropy from permeability and diffusive tortuosity. In particular, we observe the Rayleigh ratio may either dampen or enhance the diffusing front, and our simulation data is used to express the time of convective onset as a function of the Rayleigh ratio. Also, we implement a lattice Boltzmann model for thermal convective flows, which we treat as an analog for CO2 storage modeling. Our model contains the multiple- relaxation-time scheme and moment-based boundary conditions to avoid the numer- ical slip error that is associated with standard bounce-back. The model’s accuracy and robustness is demonstrated by an excellent agreement between our results and benchmark data for thermal flows ranging from Ra = 103 to 108. Our thermal model captures analogous flow behavior to that of CO2 through fluid-filled porous media, including the transition from diffusive transport to initiation and development of convective fingering.

In der Reaktorsicherheitsforschung sind auftriebsgetriebene Strömungen von Relevanz für Störfall-szenarien mit Verdünnung der Borkonzentration und für thermische Schockbelastungen des Reak-tordruckbehälters. In der numerischen Simulation der Strömungen werden neben der Berücksichtigung der Auftriebskräfte Quell- und Korrekturterme in die Bilanzgleichungen für die turbulente Energie und die turbulente Dissipation eingeführt. Es wurden erweiterte Modelle entwickelt, in die zusätzliche Gleichungen für die Turbulenzgrößen turbulenter Massenstrom und Dichtevarianz eingehen. Die Modelle wurden in den CFD-Code ANSYS-CFX implementiert. Die Validierung der Modelle erfolgte an einem speziellen Versuchsaufbau (VeMix-Versuchsanlage), mit Einspeisung von Fluid höherer Dichte in eine Vorlage. Als Kriterien für die Validierung wurde der Umschlag zwischen impulsdominiertem Strömungsregime mit vertikalem Jet oder ein vertikales Absinken bei Dominanz von Dichteeffekten herangezogen sowie lokale Konzentrationsmessungen mit Hilfe eines speziell entwickelten Leitfähigkeits-Gittersensors. Eine Verbesserung der Simulation dichtedominierter Vermischungsprozesse mit den erweiterten Turbulenzmodellen konnte allerdings nicht nachgewiesen werden, da die Unterschiede zwischen den Rechnungen mit verschiedenen Turbulenzmodellen zu gering sind. Andererseits konnte jedoch die Simulation der Stratifikation von Fluiden unterschiedlicher Dichte im kalten Strang einer Reaktoranlage deutlich verbessert werden. Anhand der Nachrechnung von Ver-suchen am geometrisch ähnlichen Reaktor-Strömungsmodell ROCOM wurde gezeigt, dass diese Stratifikation von bedeutendem Einfluss auf die Vermischung und somit letztendlich auch auf die Temperatur- bzw. Borkonzentrationsverteilung innerhalb des Reaktordruckbehälters ist. Sie lässt sich nur korrekt simulieren, wenn ausreichend große Abschnitte des kalten Stranges mit modelliert werden. Somit konnte doch eine bessere Vorhersagegenauigkeit der Simulation der Vermischung erreicht werden. In reactor safety research, buoyancy driven flows are of relevance for boron dilution accidents or pressurised thermal shock scenarios. Concerning the numerical simulation of these flows, besides of the consideration of buoyancy forces, source and correction terms are introduced into the balance equations for the turbulent energy and its dissipation rate. Within the project, extended turbulence models have been developed by introducing additional balance equations for the turbulent quantities turbulent mass flow and density variance. The models have been implemented into the computati-onal fluid dynamics code ANSYS-CFX. The validation of the models was performed against tests at a special experimental set-up, the VeMix facility, were fluid of higher density was injected into a vertical test section filled with lighter fluid. As validation criteria the switching-over between a momentum controlled mixing pattern with a horizontal jet and buoyancy driven mixing with vertical sinking down of the heavier fluid was used. Additionally, measurement data gained from an especially developed conductivity wire mesh sensor were used. However, an improvement of the modelling of buoyancy driven mixing by use of the extended models could not be shown, because the differences between calculations with the different models were not relevant. On the other hand, the modelling of the stratification of fluids with different density in the cold leg of a reactor primary circuit could be significantly improved. It has been shown on calculations of experi-ments at the ROCOM mixing test facility, a scaled model of a real reactor plant, that this stratification is relevant as a boundary condition for the mixing process inside the reactor pressure vessel. It can be correctly simulated only if sufficient large parts of the cold legs are included in the modelling. On this way, an improvement of the accuracy of the prediction of mixing processes was achieved.

Some passive houses are provided a warm supply flow inlet coming from a mechanical ventilation system, creating issues of shortcut ventilation as a result of an originated density layer stratification, since the supplied warm air is confined to the ceiling level. In this way, there exist several complaints of thermal discomfort and poor indoor environmental quality in passive houses. Thus, a method of periodic variation of the ventilation supply frequency is an option to increase mixing, aimed at de-stratifying the room conditions. A small-scale water model is employed in order to systematically explore the influence of the created standing waves from the supply input frequency and its interaction with stratification characteristics in the studied volume, hence operating with water as a working fluid and a paddle as an oscillating mechanical input. Measurements at three different input frequencies and at three input paddle locations have been performed, gathering vertical temperature gradients and visualization data from them. Thus, ventilation efficiency of passive houses is set to improve, consequently increasing their public acceptability, via reaching buoyancy resonance, i.e., matching the input frequency with the internal Brunt-Väisäla frequency of the fluid. Consequently, the shortcut ventilation issues may be mitigated and the public acceptability of passive houses increased, achieving a higher thermal comfort and economic and energy demand savings, while enhancing sustainable and renewable heating alternatives such as the heat recovery from the outgoing exhaust flow.

ROBEX is a unique research project combining Airbus Defence and Space’s robotics expertise with deep-sea exploration technology to discover more about the most extreme environments environments known to man. As a part of this project, a deep-sea glider called MOTH, is under development with the objective to determine whether gliders can be used as a platform for bathymetric and electromagnetic soundings of the seafloor as well as for new water column research. This master’s thesis aims to design and implement the MOTH glider’s control system. The glider will have an independent emergency system, a power unit, an on-board computer (OBC), actuators, navigation sensors and scientific measurement instruments which can be swopped between missions and are connected via remote terminal units. The selected OBC is a Linux embedded Axotec GX-6300 with RS232 and CAN bus interfaces, as selected in the electrical architecture, and the chosen operative system is Linux Debian. The glider communicates with GNS/Iridium antenna and also has an ethernet cable link for ground station operations and a future option of an acoustic transceiver. To control actuation, the glider is equipped with a rudder, a left and a right wing flap, a moveable mass and a buoyancy tank. It travels in sawtooth patterns and is therefore always descending, ascending or transitioning during operation and at times ascending all the way to the surface to transmit and receive data via satellite communication. A model based feedback controller for longitudinal control has been programmed based on the equations of motion described in this report. The modelled longitudinal trajectory is as desired until a transition point is reached, the model is, presumable because of the uncertainty of the model parameters, unstable as the actuators are unable to correct the pitch angle. An AHRS navigation sensor emulator and an OBC emulator have been programmed to simulate the communication between these two and the emulated system is well operating both as a continuous stream and for polling data. The emulator and the pitch controller, when updated parameter values are available, will be used for simulation and verification tests in the laboratory environment. The ROBEX alliance will, if the objectives with the MOTH glider are met, continue to design gliders with the aim to increase the maximum duration time and speed in order to reach greater depths of the oceans.

A Rayleigh-scatter lidar operated from Utah State University (41.7°N, 111.8°W) for a period spanning 11 years ― 1993 through 2004. Of the 900 nights observed, data on 150 extended to 90 km or above. They were the ones used in these studies related to atmospheric gravity waves (AGWs) between 45 and 90 km. This is the first study of AGWs with an extensive data set that spans the whole mesosphere. Using the temperature and temperature gradient profiles, we produced a climatology of the Brunt-Väisälä (buoyancy) angular frequency squared, N2 (rad/s)2. The minimum and maximum values of N2 vary between 2.2×10-4 (rad/s)2 and 9.0×10-4 (rad/s)2. The corresponding buoyancy periods vary between 7.0 and 3.5 minutes. While for long averages the atmosphere above Logan, Utah, is convectively stable, all-night and hourly profiles showed periods of convective instability (i.e., negative N2). The N2 values were often significantly different from values derived from the NRL-MSISe00 model atmosphere because of the effects of inversion layers and semiannual variability in the lidar data. Relative density fluctuation profiles with 3-km altitude resolution and 1-hour temporal resolution showed the presence of monochromatic gravity waves on almost every night throughout the mesosphere. The prevalent values of vertical wavelength and vertical phase velocity were 12-16 km and 0.5-0.6 m/s, respectively. However, the latter has the significant seasonal variation. Using these two observed parameters, buoyancy periods, and the AGW dispersion relation, we derived the ranges of horizontal wavelength, phase velocity, and source distance. The prevalent values were 550-950 km, 32-35 m/s, and 2500-3500 km, respectively. The potential energy per unit mass Ep showed great night-to-night variability, up to a factor of 20, at all heights. Ep grew at approximately the adiabatic rate below 55-65 km and above 75-80 km. Step function decreases in Ep imply that the AGWs in between gave up considerable energy to the background atmosphere. In addition, Ep varies seasonally. Below 70 km, it has a semiannual variation with a maximum in winter and minima in the equinoxes. At the highest altitudes it has an annual variation with a maximum in winter and a minimum in summer.