Joris Degroote

Dc excited glow discharges in atmospheric pressure air in pin-to-water electrode systems

Electrical and optical emission properties of non-equilibrium atmospheric air discharges between a metal pin and a tap water anode/cathode are presented. With a water anode the discharges are of the glow type as is derived from short-exposure time plasma imaging and electrical characteristics. Additionally, the validity of extrapolated scaling laws of low pressure glow discharge supports these findings.In the case of a water cathode the plasma is filamentary in nature at the water surface. In the case of a water anode, the plasma is diffuse down to 10 ns. The timescales on which the filaments are visible in the near water cathode region and estimates of the electrical field in the cathode layer are consistent with the assumption that these filaments occur due to the electrical instability of the water surface.Spatially resolved rotational temperature measurements and dependence of the rotational temperature on current are discussed in detail. The rotational temperatures of OH and N2 in the positive column of the plasma are identical and equal to 3250 ± 250 K. A 2500 K temperature drop in the near anode region clearly shows that the water anode acts as an effective heat sink for the discharge. This indicates that apart from the electrical stabilization of the discharge by the water electrode due to its distributed resistance, a water anode also thermally stabilizes the discharge. The rotational temperature of nitrogen near the metal anode is typically two times smaller.

Characteristics of atmospheric pressure air discharges with a liquid cathode and a metal anode

Electrical and optical emission properties of a burning plasma between a liquid cathode and a metal anode are presented in this paper. The plasma has constricted contact points at the liquid cathode and is clearly filamentary in nature near the water surface.The cathode voltage drop depends on conductivity rather than pH and is significantly different for distilled water and electrolyte solutions. An acidification of the liquid due to the plasma is always observed.The rotational temperature of OH and N2 in the bulk of the plasma is, respectively, in the range 3200?3750?K and 2500?2750?K. The rotational temperature of nitrogen near the metal anode is typically two times smaller. Electron densities near the cathode measured by Stark broadening of H? are in the range (5.5?8.0) ? 1014?cm?3, the atomic excitation temperatures in the range 5750?7250?K. Differences in electrical and optical emission properties between the cases when distilled water and electrolyte solutions are used as cathode are discussed in detail.

Variability of Computational Fluid Dynamics Solutions for Pressure and Flow in a Giant Aneurysm: The ASME 2012 Summer Bioengineering Conference CFD Challenge

Stimulated by a recent controversy regarding pressure drops predicted in a giant aneurysm with a proximal stenosis, the present study sought to assess variability in the prediction of pressures and flow by a wide variety of research groups. In phase I, lumen geometry, flow rates, and fluid properties were specified, leaving each research group to choose their solver, discretization, and solution strategies. Variability was assessed by having each group interpolate their results onto a standardized mesh and centerline. For phase II, a physical model of the geometry was constructed, from which pressure and flow rates were measured. Groups repeated their simulations using a geometry reconstructed from a micro-computed tomography (CT) scan of the physical model with the measured flow rates and fluid properties. Phase I results from 25 groups demonstrated remarkable consistency in the pressure patterns, with the majority predicting peak systolic pressure drops within 8% of each other. Aneurysm sac flow patterns were more variable with only a few groups reporting peak systolic flow instabilities owing to their use of high temporal resolutions. Variability for phase II was comparable, and the median predicted pressure drops were within a few millimeters of mercury of the measured values but only after accounting for submillimeter errors in the reconstruction of the life-sized flow model from micro-CT. In summary, pressure can be predicted with consistency by CFD across a wide range of solvers and solution strategies, but this may not hold true for specific flow patterns or derived quantities. Future challenges are needed and should focus on hemodynamic quantities thought to be of clinical interest.

Water surface deformation in strong electrical fields and its influence on electrical breakdown in a metal pin-water electrode system

Electrical breakdown and water surface deformation in a metal pin?water electrode system with dc applied voltages is studied for small inter-electrode distances (2?12?mm). The radius of curvature of the metal pin is 0.5?cm to exclude corona before breakdown at these small inter-electrode spacings. Calculations of the water surface deformation as a function of the applied voltage and initial inter-electrode spacing are compared with measurements of the water elevation. For distances smaller than 7?mm the calculated stability limit of the water surface corresponds with the experimentally obtained breakdown voltage. It is proved with fast CCD images and calculations of the electrical field distribution that the water surface instability triggers the electrical breakdown in this case. The images show that at breakdown the water surface has a Taylor cone-like shape. At inter-electrode distance of 7?mm and larger the breakdown voltage is well below the water stability limit and the conductive channel at breakdown is formed between the pin electrode and the static water surface. Both cases are discussed and compared.

A computational method to assess the in vivo stresses and unloaded configuration of patient-specific blood vessels

In the modelling process of cardiovascular diseases, one often comes across the numerical simulation of the blood vessel wall. When the vessel geometry is patient-specific and is obtained in vivo via medical imaging, the stress distribution throughout the vessel wall is unknown. However, simulating the full physiological pressure load inside the blood vessel without incorporating the in vivo stresses will result in an inaccurate stress distribution and an incorrect deformation of the vessel wall. In this work a computational method is formulated to restore the zero-pressure geometry of patient-specific blood vessels, and to recover the in vivo stress field of the loaded structures at the moment of imaging. The proposed backward displacement method is able to solve the inverse problem iteratively using fixed point iterations. As only an update of the mesh is required, the formulation of this method allows for a straightforward implementation in combination with existing structural solvers, even if the structural solver is a black box.

DC-excited discharges in vapour bubbles in capillaries

DC-excited discharges in vapour bubbles in capillaries are studied. A bubble is generated in a capillary filled with a NaCl solution due to Joule heating. The fluid columns on either side of the bubble serve as electrodes for the electrical discharges inside the bubble. The electrical breakdown (corresponding to a corona-to-spark transition) of quasi-static vapour bubbles is discussed. The breakdown electrical field decreases with bubble length. For larger bubbles, the reduced electrical field is smaller than the electrical field where electron attachment equals ionization, indicating that the discharge is a surface discharge. Linear translation of bubbles in the cathode direction, coinciding with intense discharges inside the bubbles, is observed and can be explained by asymmetric heating due to the plasma. The optical emission spectrum of a vapour bubble discharge consists of excited hydroxyl, hydrogen and sodium emission. A delay in the range of 0.1 s is observed between the emission of hydroxyl and sodium. The sodium emission is most intense on the anode side of the bubble where orange anode spots are visible.

Influence of the water surface on the glow-to-spark transition in a metal-pin-to-water electrode system

The glow-to-spark transition in a pin-to-water anode electrode system is investigated and compared with a pin-to-metal-plate system with 20 ns time resolution by fast imaging and corresponding current and voltage measurements. A contraction of the anode spot and cathode spot of the glow is observed in the pin–metal-plate system leading to a narrow filamentary spark, while the anode spot on the water anode electrode remains diffuse. In the latter case only a significant radial constriction of the glow in the bulk and near the metal cathode of the discharge gap is observed which causes a broadening of the spark near the water anode. Constriction of this broadened spark channel occurs several 100 ns after spark ignition. Additionally, the influence of the conductivity of the liquid electrode on the glow-to-spark transition is investigated.

The Quasi-Newton Least Squares Method: A New and Fast Secant Method Analyzed for Linear Systems

We present a new quasi-Newton method that can solve systems of equations of which no information is known explicitly and which requires no special structure of the system matrix, like positive definiteness or sparseness. The method builds an approximate Jacobian based on input-output combinations of a black box system, uses a rank-one update of this Jacobian after each iteration, and satisfies the secant equation. While it has originally been developed for nonlinear equations we analyze its properties and performance when applied to linear systems. Analytically, the method is shown to be convergent in

Haemodynamic impact of stent–vessel (mal)apposition following carotid artery stenting: mind the gaps!

n+1

DC Electrical Breakdown in a Metal Pin–Water Electrode System

iterations (