The bifurcation structure of viscous steady axisymmetric vortex breakdown with open lateral boundaries

International audienceThe effect of small viscosity on the behavior of the incompressible axisymmetric flow with open lateral and outlet boundaries near the critical swirling number has been studied by numerical simulations and asymptotic analysis. This work extends the theoretical studies of Wang and Rusak and numerical results of Beran and Culik to the case of flow with open lateral and outlet boundaries. In the inviscid limit the columnar flow state constitutes a solution that is known to become unstable at a particular swirl parameter. An asymptotic expansion shows that for small perturbations about this inviscid state an exchange of stability gives rise to a double saddle node bifurcation. The solution of the Euler equations breaks into two branches of the Navier-Stokes equations with a gap between the branches in which no near-columnar flow can exist. Around this region, two steady-state solutions exist for the same boundary conditions, one close to the columnar state and the other corresponding to either an accelerated or a decelerated state. This bifurcation structure is verified by numerical simulations, where the Navier-Stokes solutions are computed using branch continuation techniques based on the recursive projection method. For relatively small Reynolds numbers the numerically computed bifurcation curve does not exhibit any characteristic fold, and thus no hysteresis behavior. In this case, only a single equilibrium solution is found to exist, which changes monotonically from the quasicolumnar state to the breakdown solution. For large Reynolds numbers, however, the numerically determined bifurcation diagram confirms the fold structure characterized by the disappearance of the nearly columnar state via a saddle node bifurcation. Using the minimum axial velocity on the axis as a measure of the flow state we show that the agreement between theory and numerics is asymptotically good. © 2009 American Institute of Physics

Stability and Control of Spiral Vortex Breakdown

The physical origin of spiral vortex breakdown is investigated using the direct and adjoint Navier-Stokes equations linearized around axisymmetric vortex breakdown. The wave modes region, defined as the overlap region between adjoint and direct global mode, allows us to determine whether the wake of the recirculation region or the recirculation region itself causes the spiral vortex breakdown

The dynamics of vortex breakdown

Swirling flows with jet-like axial velocities are studied by means of direct numerical simulations for various control parameters. The numerical code solves the low-Mach number approximation of the Navier-Stokes equations in cylindrical coordinates which allows the incorporation of compressible and variable-density effects without the adverse effect of acoustic waves on the numerical time-step. The governing parameters, such as swirl and coflow, are varied, and their effect on the breakdown type and breakdown location is studied. The nonlinear direct numerical simulation traces the steady states of the swirling flow, and the structure of the bifurcation is described as a function of the swirl parameter, the Reynolds number and the coflow parameter. In particular, unstable branches are computed and their physical relevance is discussed. The analysis gives new insight into the prevalence of coherent states and their controllability

Statistics, lessons learned and recommendations from analysis of HIAD 2.0 database

The manuscript firstly describes the data collection and validation process for the European Hydrogen Incidents and Accidents Database (HIAD 2.0), a public repository tool collecting systematic data on hydrogen-related incidents and near-misses. This is followed by an overview of HIAD 2.0, which currently contains 706 events. Subsequently, the approaches and procedures followed by the authors to derive lessons learned and formulate recommendations from the events are described. The lessons learned have been divided into four categories including system design; system manufacturing, installation and modification; human factors and emergency response. An overarching lesson learned is that minor events which occurred simultaneously could still result in serious consequences, echoing James Reason's Swiss Cheese theory. Recommendations were formulated in relation to the established safety principles adapted for hydrogen by the European Hydrogen Safety Panel, considering operational modes, industrial sectors, and human factors. This workprovide an important contribution to the safety of systems involving hydrogen, benefitting technical safety engineers, emergency responders and emergency services. The lesson learned and the discussion derived from the statistics can also be used in training and risk assessment studies, being of equal importance to promote and assist the development of sound safety culture in organisations

Bifurcations d'un écoulement tournant

This thesis presents a numerical and analytical study of the stability of incompressible swirling jets. The entrainment of fluid by the jet is modeled numerically by assuming open lateral and outlet boundaries, while the inlet flow is modeled as a Grabowski profile. The effect of a small viscosity near the critical swirl number is studied by means of axisymmetric numerical simulations and asymptotic analysis. A numerical arc-length continuation algorithm based on the recursive projection method (RPM) was implemented in order to identify steady state solutions, study their stability and follow them in parameter space. Continuation versus the swirl parameter reveals the existence of a saddle node bifurcation at high Reynolds number. Asymptotic analysis confirms these numerical results. The bifurcation diagram for a swirling jet with a recirculation bubble is studied in the axisymmetric case. It is shown that the steady solution undergoes a supercritical Hopf bifurcation. The global three-dimensional stability of the flow with a recirculation region is investigated numerically using an Arnoldi method. The axisymmetric vortex breakdown state is shown to be unstable to three-dimensional helical perturbations. Finally, the effect of an external pressure gradient on the bifurcation diagram is also investigated numerically. For a Reynolds number Re=1000, the predicted columnar state exists in the case of a favorable pressure gradient at high swirl parameter, but disappears when the pressure gradient reduced back to zero. This suggests a control strategy in order to delay the appearance of vortex breakdown.Cette thèse presente une étude numérique et analytique de la stabilité d'un écoulement incompressible de type jet tournant. L'entraînement du fluide externe par le jet est modélisé numériquement par l'hypothèse de frontières latérales et de sortie ouvertes, les conditions d'entrée correspondant à un profile de Grabowski. L'effet d'une faible viscosité dans le voisinage du nombre de swirl critique est étudié via une analyse asymptotique couplée à des simulations numériques axisymétriques. Un algorithme de continuation basé sur une méthode de projection récursive (RPM) a été implémenté pour capturer les états stationnaires et suivre ces branches de solutions dans l'espace de paramètres ainsi que leur stabilité. La continuation des solutions stationnaires vis-à-vis du paramètre de swirl montre l'existence d'une bifurcation pour les nombres de Reynolds assez grands. L'analyse asymptotique confirme ces résultats numériques. Le diagramme de bifurcation d'un jet tournant possédant une région de recirculation est déterminé dans le cas axisymétrique. Il est montré que l'état stationnaire subit une bifurcation de Hopf supercritique. Enfin, la stabilité globale tridimensionnelle d'un jet tournant avec une région de recirculation est étudiée numériquement par une méthode d'Arnoldi. L'état éclaté axisymétrique apparaît instable vis-à-vis de perturbations tridimensionnelles hélicoïdales. L'effet d'un gradient d'une pression extérieur sur le diagramme de bifurcation est étudié numériquement. Pour un nombre de Reynolds Re=1000, la branche colonnaire (solutions sans recirculation) existe dans le cas d'un gradient de pression favorable pour les grnads paramètres de swirl, mais disparaît quand le gradient de pression est zero. Ce résultat ouvre des perspectives pour une stratégie de contrôle pour retarder l'apparition de l'éclatement tourbillonnaire

Bifurcations in a swirling flow=Bifurcations d'un écoulement tournant

PALAISEAU-Polytechnique (914772301) / SudocSudocFranceF

Potential for Hydrogen DDT with Ambient Vaporizers

PresentationThe ignition of a hydrogen-air mixture that has engulfed a typical set of ambient vaporizers (i.e., an array of finned tubes) may result in a deflagration-to-detonation transition (DDT). Simplified curve-based vapor cloud explosion (VCE) blast load prediction methods, such as the Baker- Strehlow-Tang (BST) method, would predict a DDT given that typical ambient vaporizers would be rated as medium or high congestion and hydrogen is a high reactivity fuel (i.e., high laminar burning velocity). Computational fluid dynamic (CFD) analysis of a single vaporizer of typical construction was carried out using the FLACS code to evaluate the potential for a DDT with a vaporizer engulfed by a hydrogen-air mixture at the worst-case concentration. This analysis showed that while significant flame acceleration occurs within the vaporizer, as expected, a DDT is not predicted. However, the analysis did indicate that a DDT may occur for two or more closely spaced vaporizers. This is relevant since multiple vaporizers are frequently present at industrial installations and are typically placed closely together to limit the required area. Spacing adjacent vaporizers further apart could preclude a DDT. However, specification of the spacing to preclude a DDT would require refined CFD analysis and/or testing, neither of which has been performed at this time. This paper also discusses the application of simplified VCE blast load methods to ambient vaporizers engulfed in a flammable hydrogen-air cloud in order to illustrate the impact of a DDT

Worst Case Scenario for Delayed Explosion of Hydrogen Jets at a High Pressure: Ignition Position

International audienceDelayed explosion of free field hydrogen releases at a high pressure is subject of multiple investigation performed by various authors in the past years. These studied considered various parameters such as pressures, flow rates etc., and their influence on the resulting overpressure. However, the influence of the ignition position on the maximum overpressure was not fully explored. Current investigation addressed by computational fluid dynamics (CFD) simulations and experimental measurement fills this gap. This work demonstrates that the ignition positions corresponding to 55%-65% of H2/air mixture give the maximum overpressure. This observation initially observed numerically and afterword confirmed experimentally. A simple model is also suggested