Modeling Atmospheric Pressure Plasma Jets: Plasma Dynamics, Interaction with Dielectric Surfaces, Liquid Layers, and Cells.

Atmospheric pressure plasma jets (APPJ) have many beneficial effects in their use in surface treatment and, in particular, plasma medicine. One of these benefits is the controlled production of reactive oxygen and nitrogen species (RONS) in the active discharge through the molecular gases added to the primary noble gas in the input mixture, and through the interaction of reactive species in the plasma effluent with the ambient air. As the effluent reaches the treated surface, benefits in addition to the reactive species are the ion flux and electrical field effects to the surface. Encouraging results have been obtained in plasma medicine, surface sterilization, deactivation of bacteria, and surface functionalization. The surfaces being treated by APPJs range from plastics to tissue to various liquids. In the case of APPJs in biological applications, the plasma-produced charged and neutral species in the plume of the jet often interact with a thin layer of liquid covering the tissue being treated. The plasma-produced reactivity must then penetrate through the liquid layer to reach the tissue beneath. Many of these aspects of APPJs as used in plasma medicine are computationally investigated in this thesis. Initially, the plasma discharge dynamics and production of RONS by multiple pulses of an APPJ into humid air was studied. Then, the effect that the permittivity of the material being treated has on the dynamics of the plasma discharge from the APPJ was studied. Next, a reactive water layer acting as the thin layer of liquid often found on wounds was used as the target of the APPJ. The subsequent formation of aqueous species from the interaction of the plasma jet touching and not-touching the water layer was investigated. Lastly, cellular structure was included in the underlying tissue beneath the liquid layer of varying thicknesses and the effect of the electric field produced by three different voltages of APPJ on the cells was investigated.PhDMechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/113342/1/norbergs_1.pd

Coronal rain in magnetic arcades: Rebound shocks, Limit cycles, and Shear flows

We extend our earlier multidimensional, magnetohydrodynamic simulations of coronal rain occurring in magnetic arcades with higher resolution, grid-adaptive computations covering a much longer ($>6$ hour) timespan. We quantify how in-situ forming blob-like condensations grow along and across field lines and show that rain showers can occur in limit cycles, here demonstrated for the first time in 2.5D setups. We discuss dynamical, multi-dimensional aspects of the rebound shocks generated by the siphon inflows and quantify the thermodynamics of a prominence-corona-transition-region like structure surrounding the blobs. We point out the correlation between condensation rates and the cross-sectional size of loop systems where catastrophic cooling takes place. We also study the variations of the typical number density, kinetic energy and temperature while blobs descend, impact and sink into the transition region. In addition, we explain the mechanisms leading to concurrent upflows while the blobs descend. As a result, there are plenty of shear flows generated with relative velocity difference around 80 km s$^{-1}$ in our simulations. These shear flows are siphon flows set up by multiple blob dynamics and they in turn affect the deformation of the falling blobs. In particular, we show how shear flows can break apart blobs into smaller fragments, within minutes

25 Years of Self-Organized Criticality: Numerical Detection Methods

The detection and characterization of self-organized criticality (SOC), in both real and simulated data, has undergone many significant revisions over the past 25 years. The explosive advances in the many numerical methods available for detecting, discriminating, and ultimately testing, SOC have played a critical role in developing our understanding of how systems experience and exhibit SOC. In this article, methods of detecting SOC are reviewed; from correlations to complexity to critical quantities. A description of the basic autocorrelation method leads into a detailed analysis of application-oriented methods developed in the last 25 years. In the second half of this manuscript space-based, time-based and spatial-temporal methods are reviewed and the prevalence of power laws in nature is described, with an emphasis on event detection and characterization. The search for numerical methods to clearly and unambiguously detect SOC in data often leads us outside the comfort zone of our own disciplines - the answers to these questions are often obtained by studying the advances made in other fields of study. In addition, numerical detection methods often provide the optimum link between simulations and experiments in scientific research. We seek to explore this boundary where the rubber meets the road, to review this expanding field of research of numerical detection of SOC systems over the past 25 years, and to iterate forwards so as to provide some foresight and guidance into developing breakthroughs in this subject over the next quarter of a century.Comment: Space Science Review series on SO

View-Invariant Object Category Learning, Recognition, and Search: How Spatial and Object Attention Are Coordinated Using Surface-Based Attentional Shrouds

Air Force Office of Scientific Research (F49620-01-1-0397); National Science Foundation (SBE-0354378); Office of Naval Research (N00014-01-1-0624

Quasi-Modal Encounters Of The Third Kind: The Filling-In Of Visual Detail

Although Pessoa et al. imply that many aspects of the filling-in debate may be displaced by a regard for active vision, they remain loyal to naive neural reductionist explanations of certain pieces of psychophysical evidence. Alternative interpretations are provided for two specific examples and a new category of filling-in (of visual detail) is proposed

A NUMERICAL INVESTIGATION REALIZING THE FLOW STRUCTURE AND HEAT TRANSFER PERFORMANCE OF THE POTENTIAL IMPINGING JETS

The demand for improvement in the performance of gas turbines has led to the consideration of flows at increasingly high temperatures, but this introduces challenges in terms of maintaining their structural integrity and preventing overheating. To respond to these challenges, gas turbine manufacturers have turned to internal cooling, and jet impingement provides an effective solution for cooling the leading edge of the blades of gas turbines. In this study, the author numerically simulated the cooling performance of the leading edge of the blades of a gas turbine under constant heat flux by using five configurations of jet impingement: a steady jet, a sweeping jet, a swirling jet, a Chevroned Steady jet, and a Chevroned Sweeping jet. Fluidic oscillators are known for their sweeping behavior and expansive coverage of the cooling surface while swirling jet owing to spiral geometry add tangential velocity component to the fluid which combines with the axial velocity component that generates enhanced momentum transfer area. On other hand by chevron attachment at exit of the nozzle are known to excite the jet downstream by forming coherent vortical structures that increase turbulence and, thus, promote the rates of mixing and heat transfer. These potential jets are compared at stationary and rotatory conditions (3000, 10000, 15000 rpm’s) and results showed that at the stationary condition Chevroned Sweeping jet outperformed the steady jet configurations owing to oscillating jet impingement and a higher intensity of turbulence that increased the entrainment of jet flow. Under the configuration involving a Chevroned Sweeping jet, the target surface recorded an average Nusselt number that was 19.23% higher than that with a steady jet without chevrons, along with more uniform distributions of the temperature and the Nusselt number due to oscillations of the sweeping jet and higher turbulence at the exit of the nozzle with chevrons. While for rotation case sweeping jet performed the best as chevroned nozzles due to higher disturbance generated high recirculation regions leading to hotspots formation while swirling jet performed worse of all as swirling strength was negatively impacted due to rotatory motion. It can be concluded that the addition of chevrons and swirling angle improved heat transfer rate for sweeping and steady jet. However, upon rotation sweeping jet predominantly captures the best performance amongst all the jets

Towards cellular hydrodynamics: collective migration in artificial microstructures

The collective migration of cells governs many biological processes, including embryonic development, wound healing and cancer progression. Observed phenomena are not simply the sum of the individual motion of many isolated cells, but rather emerge as a consequence of their interactions. The movements in epithelial cell sheets display rich phenomenology, such as the occurrence of vortices spanning several cell diameters and the transition from fluid-like behavior at low densities to glass-like behavior at high densities. In this thesis, collective invasion of epithelial cell sheets into microchannels was studied on a phenomenological level within the scope of theoretical approaches to active fluids. In a first project, the motion profile of a cell layer in straight channels was investigated using single cell tracking and particle image velocimetry (PIV) on timelapse microscopy data. A defined plug-flow like velocity profile was observed across the channels. The cell density profile is well-described by the Fisher-Kolmogorov reaction-diffusion equation, which includes active migration and the contribution of proliferation. This study revealed a change in the short scale noise behavior in the presence of this global invasion into a channel. For a closer look at the system’s proliferation component, the effect of an underlying global migration direction on the orientation of the cells’ division axes was examined. We found strong alignment of the axes’ orientation with the imposed movement direction. Specifically, the strongest correlations were observed between the orientation of the cells’ division axes and the local strain rate tensor’s main axis. This is in agreement with the notion that stresses in the migrating cell sheet orient the cell divisions. Expanding the assay of invasion into straight channels, we introduced a constriction, which the cell sheet needs to pass through in order to progress. A plateau of low velocities was observed in the region ahead of the constriction, which was attributed to an increase in local cell density accompanied by jamming. These results were compared to an active isotropic-nematic mixture model. The suitability of this model to describe this assay could be ruled out, however, as it showed qualitatively very different behavior than the experiments. Finally, the frequency of topological nearest-neighbor T1 transitions within a cell sheet was investigated in minimal model systems. In order to study the smallest possible fundamental unit for such transitions, groups of four cells were confined to cloverleaf patterns, which could be shown to inhibit the onset of collective rotation states. Results showed that T1 transitions occurred more frequently for groups of cells with a lower average length of the cell-cell junction that shrinks in the process of this transition. These results are consistent with the notion that the energy barrier which needs to be overcome by the cells in order to perform this transition, scales with the original length of the shrinking junction. Taken together, the results of this thesis contribute to a better understanding of the flow fields for collective cell migration processes in confined geometries. In addition to the insights the phenomenological observations in this work could provide directly, they will also continue to prove useful as a standard for validating detailed theoretical models