403 research outputs found
3D-electrical resistivity tomography monitoring of salt transport in homogeneous and layered soil samples
No full textMonitoring transport of dissolved substances in soil deposits is particularly relevant where safety is concerned, as in the case of geo-environmental barriers. Geophysical methods are very appealing, since they cover a wide domain, localising possible preferential flow paths and providing reliable links between geophysical quantities and hydrological variables. This paper describes a 3D laboratory application of Electrical Resistivity Tomography (ERT) used to monitor solute transport processes. Dissolution and transport tests on both homogeneous and heterogeneous samples were conducted in an instrumented oedometer cell. ERT was used to create maps of electrical conductivity of the monitored domain at different time intervals and to estimate concentration variations within the interstitial fluid. Comparisons with finite element simulations of the transport processes were performed to check the consistency of the results. Tests confirmed that the technique can monitor salt transport, infer the hydro-chemical behaviour of heterogeneous geomaterials and evaluate the performances of clay barrier
Implicit Large-Eddy Simulation of Wind Turbine Wakes and Turbine-Wake Interactions using the Vorticity Transport Equations
No full textRecent development of a multi-dimensional upwind-based finite volume scheme for the vorticity transport equations (Foti and Duraisamy, Comp & Fluids 167, 17-32) has enabled simulation of vortical coherent structures on relatively coarse grids while maintaining key flow invariants. In this work, we assess the ability of the scheme to simulate high Reynolds number wind turbine wakes. The wake of a wind turbine is composed of several large coherent structures, which dominate the unsteady flow field. The wake of the MEXICO wind turbine blades is discretized on an adaptively refined mesh through actuator line model parameterization. The accurate statistics of the near wake compare well with experimental measurements while employing a relatively coarse grid with a resolution of 32 cells per rotor diameter. Despite the coarse grid resolution, pertinent vortical structures are captured and preserved throughout the wake and through changes in grid refinement. The reduced computational costs of the present scheme allows multiple turbines to be simulated and capture the complex vortical structures present in turbine-turbine wake interactions
Flame Stabilization Modes in Lean Premixed Swirl Stabilized Combustion
No full textFlame holding and its propagation characteristics in a lean premixed swirl stabilized combustor is studied using large eddy simulations (LES) with a goal to understand the different stabilization modes seen in an experimental combustor, which combines swirl inflow with a central bluff body to create multiple stabilization locations. Experimentally observed transition from a flame stabilized along the inner shear layer to a flame that exists both in the inner and the outer shear layer are numerically simulated by just an increase in the equivalence ratio, as done in the experiment. Comparison with experimental data shows excellent agreement and confirms the ability of LES to capture these different flame stabilization processes without adhoc model adjustments. Despite the change in heat release, the large scale flow features such as the vortex bubble breakdown and recirculation behind the dump expansion remain similar for each case. This indicates that the flame in the outer shear layer does not affect the flow substantially. Simulation results are also used to understand the difference in the unsteady flame structure under these operating conditions. © 2012 by D. Foti and S. Menon
Deformation of landfill from measurements of shear wave velocity and damping - Discussion
No full textThe author presents a method for using in-situ seismic test results to estimate the short-term deformations of geomaterials. The effort to introduce rational algorithms for the evaluation of settlements of shallow foundations on the basis of measured physical quantities is noteworthy, considering that current geotechnical design, especially on small-scale projects, is often based on empirical correlations between
settlements and penetration test results that rarely reflect the actual site conditions. In this context the development of simple procedures based on the results of relatively inexpensive in-situ tests including seismic tests is of paramount importance.
Nevertheless it is important to account properly for the physics of wave propagation when inferring material parameters from seismic test results. In this respect, the writers would like to make some observations related to the methods used to evaluate the shear wave velocity and material
damping ratio of the solid waste from surface wave measurements. This discussion is focused on and restricted to the aspects of the paper related to the use of surface wave methods to estimate the waste properties
Coherent vorticity dynamics and dissipation in a utility-scale wind turbine wake with uniform inflow
No full textThe vorticity dynamics and its relationship to dissipation in the wake of a utility-scale wind turbine are investigated through large-eddy simulation. The vorticity dynamics is assessed through the enstrophy, which is related to the turbulent dissipation. The averaged enstrophy and turbulent dissipation are shown to be quantitatively similar in the wake. Using temporal phase averaging, the vorticity fluctuations are decomposed into coherent and random fluctuations with respect to the frequency of the tip vortices. The enstrophy in the tip vortices is dominated by coherent fluctuations, while the coherent fluctuations of root vortices are immediately saturated by the random vorticity fluctuations of the unstable hub vortex. The coherent strain rate has significant differences compared to the coherent enstrophy within one diameter downwind of blade tip, but the random enstrophy and strain rate are relatively similar. Differences in coherent enstrophy and strain rate decrease further from the rotor
Microbial ecology of halo-alkaliphilic sulfur bacteria
No full textThe research of this thesis focussed on the investigation of the microbial diversity in soda lakes, giving a special attention to the micro-organisms involved in the sulphur cycle. The present PhD was part of a bigger project aiming to develop a biological process for the removal of hydrogen sulphide under halo-alkaliphilic conditions. In this new process, sulphide is mainly oxidized to elemental sulphur by sulphur-oxidizing bacteria (SOB). However, a small percentage of unwanted sulphate is still produced, which is reduced back to sulphide by sulphate-reducing bacteria (SRB). Since this process operates under halo-alkaliphilic conditions, which are naturally occurring in soda lakes, a better understanding of the sulphur microbial diversity in such environments was required. Soda lakes are extreme environments, characterized by an extremely high pH (up to 11.5) and moderately to extremely high salinities (up to 500 g/L). They are a specific type of saline lakes and are spread all over the world, generally confined to arid and semi-arid regions. We especially researched soda lakes from the Kulunda Steppe, which are located in the south-east of Siberia (Russia). These lakes are still less investigated, due to their difficulty to be reached. So-called halo-alkaliphilic organisms inhabit these lakes, which raise the interest of the bioindustry for their capability to stand such harsh conditions. In this research we focussed on the halo-alkaliphilic sulphur bacteria. Special attention was given to the genetic diversity and biogeography of a specific group of halo-alkaliphilic chemolithoautotrophic SOB, i.e. Thioalkalivibrio, and to the diversity, activity and dynamics of the SRB communities in different soda lakes of the Kulunda Steppe, also in relation to the increase of the salt concentration.Applied Science
Quantification and reduction of uncertainty of model predictions of wind turbines and plants via high-fidelity simulations
No full textUniversity of Minnesota Ph.D. dissertation. December 2016. Major: Mechanical Engineering. Advisors: Fotis Sotiropoulos, Lian Shen. 1 computer file (PDF); xvii, 250 pages.With increasing energy demands renewable energy sources are continuing to receive attention and investment to become a larger source for electricity production. Today, wind generated power through wind turbines creates 4% of the electricity in the United States. The wind energy share of the electricity market is expected to grow rapidly as the United States Department of Energy goal is to reach 20% wind generated electricity by 2030. Computational models for wind plants can be used to predict wind plant performance and optimize the turbine placement and controls. However, uncertainties associated with such models, due to, among others, the computationally expedient simplifications need to be carefully assessed, quantified and reduced. A numerical investigation of model wind turbines employing large-eddy simulation and the curvilinear immersed boundary method to resolve the geometrical details of the turbine is undertaken revealing that the unstable hub vortex interacts with the turbine tip shear layer. Using a spatio-temporal filtering technique, wake meandering, a large scale displacement of the wake, is reconstructed into three-dimensional helical meander profiles. Statistics of the amplitudes and wavelengths corresponding to the intensity and streamwise elongation of the periodic wake meandering indicate complex coherent structures. Similar simulations are performed using the computationally expedient wind turbine actuator surface models with and without a nacelle model to parameterize the turbine. All simulations are validated against substantial experimental measurements. The simulations with the nacelle model are able to accurately capture the geometry dependent near wake and the dynamics in the far wake. The simulations without the nacelle model predict a stable, columnar hub vortex which does not interact with the turbine tip shear layer. Moreover, the amplitude of the meandering profiles is shown to be larger in the immersed boundary method simulations and simulations with a nacelle model compared to the simulation without the nacelle model proving that the nacelle and unstable hub vortex augment the meandering intensity in wind turbines. Due to the exceptional performance of the computationally efficient actuator surface with nacelle model, several turbine designs are simulated with diameters ranging from the laboratory scale (0.1 meters) to the utility scale (96 meters). Despite significant geometrical differences, a characteristic velocity based on the turbine thrust collapses the profile of both the wake turbulence kinetic energy and the amplitude of wake meandering based on the meandering profile for all turbine sizes. This result suggests that the turbulence levels and wake meandering intensity are explicitly linked. The wavelengths of wake meandering are properly scaled by the diameter of the turbine. In agreement with numerous measurements, the wake meandering and hub vortex Strouhal number based on the incoming hub height velocity and diameter is found to be approximately 0.3 and 0.7, respectively, for all turbines. Dynamic mode decomposition of the velocity field indicates that the modes related to these frequencies contain a majority of the energy in the meandering wake and confirms that an unstable hub vortex is a necessary requirement for simulating wind turbine wakes. The Horn Rev offshore wind plant is investigated showing conclusive evidence that the nacelle and hub vortex are important in large arrays of wind turbines. The consistency across scales and wind plant rows of the stochastic distributions of the wake meandering amplitudes and wavelengths allows for the development of a reduced-order kinematic wake model with statistic-based wake meandering inputs. Finally, uncertainties in the model parameters or model inadequacy are investigated using a framework of non-intrusive polynomial chaos. The feasibility of using a kinematic wake model is determined by investigating the parameter uncertainty of surface roughness and induction factor. The parameter uncertainty of the nacelle model is considered in a series of large-eddy simulations. The aleatoric uncertainty of the surface friction on the model and the epistemic nacelle geometry uncertainty propagate downstream in the inner wake and have implications on the uncertainty of the turbulence levels in the entire far wake.Foti, Daniel. (2016). Quantification and reduction of uncertainty of model predictions of wind turbines and plants via high-fidelity simulations. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/185179
Assessing Reward Functioning Across Distinct Symptom Dimensions of Internalizing Psychopathology
No full textRecent work has linked deficits in reward processing as a potential endophenotype for major depressive disorder. Neural and behavioral evidence demonstrated that reduced reward sensitivity is more characteristic of depression than other internalizing psychopathologies such as anxiety disorders. However, emerging evidence indicates the presence of reward processing abnormalities across different anxiety disorders, including generalized and social anxiety disorders. Differences across studies may in part be due to the heterogeneity within anxiety disorders. We sought to explore differences in patterns of reward sensitivity across distinct internalizing symptoms dimensions on two laboratory paradigms: social and monetary reward. ERPs were recorded to isolate distinct neural indicators of anticipatory and consummatory reward processing. The results showed that depressive and anhedonic symptomatology were unique predictors of attenuated neural indicators of anticipatory reward sensitivity to uncertain social outcomes (i.e., Stimulus Preceding Negativity) and the early evaluation of social rewards (i.e., Reward Positivity). We also found that neural sensitivity of monetary and social reward was predicted by multiple interrelationships of distinct internalizing symptom dimensions. The results are discussed in the context of previous work on reward processing and psychiatric conditions. The findings underlines the significance of attending to the heterogeneity of reward and symptom dimensions
Cognitive Effort-based Decision-making & Task Preferences
No full textIndividual differences in cognitive effort-based decision-making can reveal the variety of decision strategies used in action valuations. For example, factors such as how challenging an action is or how much reward can be gained are often considered when weighing how valuable an action is. Experiment 1 considers task preferences offered at different demand levels (i.e., the degree of challenge) to determine whether decision-making strategies are related primarily to 1) demand levels, 2) individual capability, or 3) task components. Results suggest that participants’ decisions were primarily driven by task options rather than their performance. Experiment 2 then compares task preferences in different incentive-related conditions. While the majority of decisions were in the predicted direction (favoring lower demand levels and higher monetary amounts), there were individual differences that suggested valuations of both task options as well as incentive conditions. The results of these experiments suggest individuals use various decision strategies involving factors that may have been overlooked in past research. These findings challenge the assumption that task preferences are primarily related to how challenging an action is and instead suggest that preferences may be highly susceptible to experimental design factors as well as factors intrinsic to the individual
Identifying the Pathophysiology of Depression and its Permeability Across the Lifespan
No full textMajor depressive disorder (MDD) and risk for its development are characterized by reduced reactivity and flexibility to environmental demands. Frontal alpha asymmetry (FAA), heart rate variability (HRV), and salivary cortisol reactivity are each well-established indicators of regulation across neural, autonomic, and hypothalamic-pituitary-adrenal (HPA) physiological systems, respectively. Growing literature suggests that each of these processes is dysregulated in individuals with a history of MDD. However, patterns of dysregulation across these physiological systems and relative MDD risk are unknown. Moreover, these physiological regulatory patterns may extent beyond markers of MDD risk in adulthood to also capture the transmission of risk for MDD from parent to offspring. The following series of five studies investigated the pathophysiology of MDD and the permeability of risk across the lifespan. First, the pattern of dysregulation across physiological indices—representing neural, autonomic, and HPA functioning—in adults was examined with regard to depressive symptoms. Second, the associations amongst infant FAA, HRV, and cortisol reactivity and maternal depressive symptoms were assessed as potential early markers of depression risk. Third, mother-infant associations across physiological indices were investigated to assess direct intergenerational transmission of depression risk. Studies 4 and 5 further investigated pathophysiological functioning in mothers and infants within the context of comorbid anxiety and current depressive symptomatology versus lifetime MDD illness. Mothers and their 12-month-old infants (n = 35 dyads) completed restingstate and stressor tasks to assess regulatory patterns across neural, autonomic, and HPA systems, associations with MDD, and intergenerational transmission. In adults, results suggest that lifetime history of MDD is significantly associated with blunted cortisol reactivity; FAA and highfrequency HRV also demonstrated the same direction of associations. In infants, results demonstrated that maternal depressive symptoms, particularly current symptoms, relate to blunted physiological regulation in infants specifically for FAA and HRV indices. For mothers and infants, there was support for the direct intergenerational transmission of FAA and HRV indices. These intergenerational associations did not fully account for intergenerational risk of depression, as maternal physiological regulation and maternal depression were found to each significantly predict infant regulation as simultaneous predictors. Accounting for comorbid anxiety and examining current symptoms versus lifetime illness were essential to investigating associations amongst physiological functioning and depression. These patterns in conjunction with the literature suggest a developmental model to MDD pathophysiology that encompasses multiple theoretical frameworks. Future research is necessary to clarify regulatory patterns across physiological systems within individuals and across time with regard to MDD risk, onset, and course
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