Image processing techniques for the characterization of explosively driven dispersions

PresentationDispersions driven by explosions are challenging to characterize mainly due to the extreme test conditions, the different time and spatial scales of the flow, and the variation of intensity due to the combustion. An intensity based optical method to characterize the dispersion driven by an explosion is proposed. The velocity and intensity maps of the dispersion are accessed through the post- processing of the images of the dispersion. These images can be obtained either from a global visualization (using a light source, such as in the image given in Figure 1, or the combustion light itself) or from a transversal visualization (using a laser sheet illuminating inside the cloud, such as in the image given in Figure 2). The developed method is organized into three steps. First, the contour of the cloud is detected via a dynamic grey-scale threshold criterion. The dispersion contours allow the computation of the velocity of the expansion as long as the plume presents a regular edge. Then, Large-Scale Particle Image Velocimetry technique is applied to obtain the velocity map of the dispersion. Additionally, information about the combustion phenomenon can also be accessed via an intensity-based analysis. The method has been initially verified using a numerical test case. It has been thereafter applied on different experimental measurements presenting challenging features such as variations of light intensity, time scales, and spatial scales

Investigation of the Pollutant Dispersion Driven by a Condensed-Phase Explosion in an Urban Environment

Understanding and predicting the consequences of dangerous phenomena linked to the industrial activity is critical to ensure the highest safety level possible. Explosions represent a high risk of fatalities and economic loss and even though the phenomenon is studied in the literature, accidents still occur.The explosion in air of a heterogeneous charge (i.e. an explosive charge surrounded or mixed with a gaseous, solid, or liquid pollutant) has various consequences. While the presence of buildings around the explosion will reduce considerably the impact of most of them, it will affect in more complex ways the propagation of the pressure released, also called bast wave, and the pollutant dispersion. Simplified models to characterize the blast propagation and the dispersion following an explosion are necessary tools for industries to access a first estimation of the risks related to their activities. However, the existing models do not take into account the effect of the urban environment.The global objective of the project is to improve the understanding of the blast propagation and the dispersion inside an urban environment by generating a quantitative database. In parallel to the project, simplified models are being developed by the Commissariat à l’Energie Atomique et aux Energies Alternatives (CEA) Gramat (center from the Direction des Applications Militaires (DAM)) and will be validated by the generated experimental database. To achieve this goal, experimental techniques are developed and applied to a controlled reduced-scale urban environment, and a CFD approach is used to facilitate the understanding of the blast experimental results. The challenges associated with the creation of a new experimental system led to innovative solutions in response to technical issues. The main originality of the project is the investigation of explosively driven dispersion under a controlled atmospheric boundary layer, which represents a novelty in the area from the author's knowledge. Existing experimental techniques have been extended and validated for the large dynamic ranges involved in the explosive dispersion. The research is separated into two parts: the investigation of the blast, and the investigation of the dispersion driven by an explosion, both inside an urban environment in a 1:200 reduced scale.In the first part, the blast propagation has been first investigated in free field to characterize the energy, the geometry, and the repeatability of each studied explosives. Then the blast has been studied inside four selected typical urban configurations. To help the understanding of the blast path, a numerical model has been developed in OpenFoam to simulate the propagation and a good overlap between the experimental and numerical results has been observed. The second part of the research focuses on the investigation of the explosively driven dispersion. Micro-sized talc particles have been added around the explosives to simulate the pollutant dispersion. Large-Scale Particle Image Velocimetry (LS-PIV) and Mie-Scattering techniques have been first investigated and validated on a supersonic jet. They have been thereafter applied to the explosively driven dispersion. Three atmospheric conditions, two masses of talc, and two diameters of powder have been investigated, both in free field and inside a T-junction.The experimental techniques used to characterize the explosion of a heterogeneous charge show promising results. They are powerful tools to investigate complex large-scale and large dynamic range flows.Doctorat en Sciences de l'ingénieur et technologieinfo:eu-repo/semantics/nonPublishe

Experimental investigation of blast wave propagation in an urban environment

PresentationLab-scale experimental investigations on blast wave propagation in a complex environment are proposed in this paper. Studies of blast propagation are described in the literature, but only a few studies at lab-scale were found while this scale option represents an economic and safe approach. Five experimental configurations, built with wood boxes on a 2.8 m wood table, are tested in a 1:200 reduced scale using three types of explosives. Several characteristics of the explosives are given: the geometry of the explosion, the repeatability, and the TNT equivalent. An overview of impacts of a complex environment on the blast wave characteristics is proposed. The urban configurations investigated are the straight street, the T-junction, the cross junction, and the channeling. Investigations on reduced-scale effects on blast measurement and characteristics are detailed

PIV à grande échelle appliquée sur la dispersion par explosif

International audienceBien que la dispersion atmosphérique soit largement étudiée dans la littérature,peu d’études s’intéressent à la dispersion par explosif, qui représente une source dynamique à haute vitesse et de courte durée. Ce savoir est essentiel pour développer et valider des outils permettant de prédire la dispersion après une explosion dans un milieu complexe. Le principal objectif de cette recherche est donc d’étudier la dispersion d’un polluant causée par une charge explosive. Une explosion en phase condensée générant la dispersion de particules solides est expérimentale-ment simulée par l’utilisation d’un détonateur RP80-EBW et d’une poudre de talc dont la granulométrie moyenne est de 7.8 μm de diamètre. La dispersion se fait en champ libre dans une soufflerie subsonique où une couche atmosphérique urbaine contrôlée est reproduite. La dispersion est étudiée à l’aide d’une caméra rapide travaillant à une fréquence allant de 2000 Hz à 10869 Hz. L’évolution temporelle des champs de vitesse lors de la dispersion est obtenue par vélocimétrie par imagerie de particules à grande échelle (LS-PIV). Deux types d’éclairage sont utilisés : le premier éclairage est obtenu à l’aide d’une lampe placée au-dessus de la caméra rapide, donnant une vision globale de la dispersion, le second est effectué à l’aide d’une tranche laser en régime continu, générant une vision 2D.Les deux types de visualisation sont comparés ; leurs similitudes et différences sont présentés. Deux méthodes sont utilisées pour l’analyse LS-PIV. Les points forts et les points faibles de ces deux méthodes sont soulignés. Enfin, les propriétés nécessaires qu’une image doit posséder pour obtenir une analyse par corrélation croisée de qualité sont comparées entre l’analyse PIV classique et l’analyse LS-PIV.La dispersion en champ libre est étudiée pour différentes vitesses de vent (de 0 m/sà 5 m/s). L’évolution temporelle des champs de vitesse permet d’étudier l’effet du vent sur la dispersion