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Nikolaos Nikiforakis - One of the best experts on this subject based on the ideXlab platform.

  • The evolution of the temperature field during Cavity Collapse in liquid nitromethane. Part I: inert case
    Shock Waves, 2019
    Co-Authors: Louisa Michael, Nikolaos Nikiforakis
    Abstract:

    This work is concerned with the effect of Cavity Collapse in non-ideal explosives as a means of controlling their sensitivity. The main objective is to understand the origin of localised temperature peaks (hot spots) which play a leading order role at the early stages of ignition. To this end, we perform two- and three-dimensional numerical simulations of shock-induced single gas-Cavity Collapse in liquid nitromethane. Ignition is the result of a complex interplay between fluid dynamics and exothermic chemical reaction. In order to understand the relative contribution between these two processes, we consider in this first part of the work the evolution of the physical system in the absence of chemical reactions. We employ a multi-phase mathematical formulation which can account for the large density difference across the gas–liquid material interface without generating spurious temperature peaks. The mathematical and physical models are validated against experimental, analytic, and numerical data. Previous inert studies have identified the impact of the upwind (relative to the direction of the incident shock wave) side of the Cavity wall to the downwind one as the main reason for the generation of a hot spot outside of the Cavity, something which is also observed in this work. However, it is also apparent that the topology of the temperature field is more complex than previously thought and additional hot spot locations exist, which arise from the generation of Mach stems rather than jet impact. To explain the generation mechanisms and topology of the hot spots, we carefully follow the complex wave patterns generated in the Collapse process and identify specifically the temperature elevation or reduction generated by each wave. This enables tracking each hot spot back to its origins. It is shown that the highest hot spot temperatures can be more than twice the post-incident shock temperature of the neat material and can thus lead to ignition. By comparing two-dimensional and three-dimensional simulation results in the context of the maximum temperature observed in the domain, it is apparent that three-dimensional calculations are necessary in order to avoid belated ignition times in reactive scenarios.

  • the evolution of the temperature field during Cavity Collapse in liquid nitromethane part i inert case
    arXiv: Computational Physics, 2017
    Co-Authors: Louisa Michael, Nikolaos Nikiforakis
    Abstract:

    We study the effect of Cavity Collapse in non-ideal explosives as a means of controlling their sensitivity. The aim is to understand the origin of localised temperature peaks (hot spots) which play a key role at the early stages of ignition. Thus we perform 2D and 3D numerical simulations of shock induced gas-Cavity Collapse in nitromethane. Ignition is the result of a complex interplay between fluid dynamics and exothermic chemical reaction. To understand the relative contribution between these two processes we consider in this first part of the work the evolution of the physical system in the absence of chemical reactions. We employ a multi-phase mathematical formulation which accounts for the large density difference across the gas-liquid interface without generating spurious temperature peaks. The mathematical and physical models are validated against experimental, analytic and numerical data. Previous studies identified the impact of the upwind side of the Cavity wall to the downwind one as the main reason for the generation of a hot-spot outside of the Cavity; this is also observed in this work. However, it is apparent that the topology of the temperature field is more complex than previously thought and additional hot spots locations exist, arising from the generation of Mach stems rather than jet impact. To explain the generation mechanisms and topology of the hot spots we follow the complex wave patterns generated and identify the temperature elevation or reduction generated by each wave. This allows to track each hot spot back to its origins. We show that the highest hot spot temperatures can be more than twice the post-incident shock temperature of the neat material and can thus lead to ignition. By comparing the maximum temperature observed in the domain in 2D and 3D simulations we show that 3D calculations are necessary to avoid belated ignition times in reactive scenarios.

  • a hybrid formulation for the numerical simulation of condensed phase explosives
    Journal of Computational Physics, 2016
    Co-Authors: Louisa Michael, Nikolaos Nikiforakis
    Abstract:

    We develop a new model for simulating the combustion of condensed-phase explosives, interacting with compliant inert materials.An associated numerical algorithm is also developed.Several Mie-Gruneisen type equations of state (EoS) and reaction rate laws have been implemented and validated.An extensive review of current formulations is presented.A very extensive validation and evaluation suite is carried out. In this article we present a new formulation and an associated numerical algorithm, for the simulation of combustion and transition to detonation of condensed-phase commercial- and military-grade explosives, which are confined by (or in general interacting with one or more) compliant inert materials. Examples include confined rate-stick problems and interaction of shock waves with gas cavities or solid particles in explosives. This formulation is based on an augmented Euler approach to account for the mixture of the explosive and its products, and a multi-phase diffuse interface approach to solve for the immiscible interaction between the mixture and the inert materials, so it is in essence a hybrid (augmented Euler and multi-phase) model. As such, it has many of the desirable features of the two approaches and, critically for our applications of interest, it provides the accurate recovery of temperature fields across all components. Moreover, it conveys a lot more physical information than augmented Euler, without the complexity of full multi-phase Baer-Nunziato-type models or the lack of robustness of augmented Euler models in the presence of more than two components. The model can sustain large density differences across material interfaces without the presence of spurious oscillations in velocity and pressure, and it can accommodate realistic equations of state and arbitrary (pressure- or temperature-based) reaction-rate laws. Under certain conditions, we show that the formulation reduces to well-known augmented Euler or multi-phase models, which have been extensively validated and used in practice. The full hybrid model and its reduced forms are validated against problems with exact (or independently-verified numerical) solutions and evaluated for robustness for rate-stick and shock-induced Cavity Collapse case-studies.

Louisa Michael - One of the best experts on this subject based on the ideXlab platform.

  • The evolution of the temperature field during Cavity Collapse in liquid nitromethane. Part I: inert case
    Shock Waves, 2019
    Co-Authors: Louisa Michael, Nikolaos Nikiforakis
    Abstract:

    This work is concerned with the effect of Cavity Collapse in non-ideal explosives as a means of controlling their sensitivity. The main objective is to understand the origin of localised temperature peaks (hot spots) which play a leading order role at the early stages of ignition. To this end, we perform two- and three-dimensional numerical simulations of shock-induced single gas-Cavity Collapse in liquid nitromethane. Ignition is the result of a complex interplay between fluid dynamics and exothermic chemical reaction. In order to understand the relative contribution between these two processes, we consider in this first part of the work the evolution of the physical system in the absence of chemical reactions. We employ a multi-phase mathematical formulation which can account for the large density difference across the gas–liquid material interface without generating spurious temperature peaks. The mathematical and physical models are validated against experimental, analytic, and numerical data. Previous inert studies have identified the impact of the upwind (relative to the direction of the incident shock wave) side of the Cavity wall to the downwind one as the main reason for the generation of a hot spot outside of the Cavity, something which is also observed in this work. However, it is also apparent that the topology of the temperature field is more complex than previously thought and additional hot spot locations exist, which arise from the generation of Mach stems rather than jet impact. To explain the generation mechanisms and topology of the hot spots, we carefully follow the complex wave patterns generated in the Collapse process and identify specifically the temperature elevation or reduction generated by each wave. This enables tracking each hot spot back to its origins. It is shown that the highest hot spot temperatures can be more than twice the post-incident shock temperature of the neat material and can thus lead to ignition. By comparing two-dimensional and three-dimensional simulation results in the context of the maximum temperature observed in the domain, it is apparent that three-dimensional calculations are necessary in order to avoid belated ignition times in reactive scenarios.

  • the evolution of the temperature field during Cavity Collapse in liquid nitromethane part i inert case
    arXiv: Computational Physics, 2017
    Co-Authors: Louisa Michael, Nikolaos Nikiforakis
    Abstract:

    We study the effect of Cavity Collapse in non-ideal explosives as a means of controlling their sensitivity. The aim is to understand the origin of localised temperature peaks (hot spots) which play a key role at the early stages of ignition. Thus we perform 2D and 3D numerical simulations of shock induced gas-Cavity Collapse in nitromethane. Ignition is the result of a complex interplay between fluid dynamics and exothermic chemical reaction. To understand the relative contribution between these two processes we consider in this first part of the work the evolution of the physical system in the absence of chemical reactions. We employ a multi-phase mathematical formulation which accounts for the large density difference across the gas-liquid interface without generating spurious temperature peaks. The mathematical and physical models are validated against experimental, analytic and numerical data. Previous studies identified the impact of the upwind side of the Cavity wall to the downwind one as the main reason for the generation of a hot-spot outside of the Cavity; this is also observed in this work. However, it is apparent that the topology of the temperature field is more complex than previously thought and additional hot spots locations exist, arising from the generation of Mach stems rather than jet impact. To explain the generation mechanisms and topology of the hot spots we follow the complex wave patterns generated and identify the temperature elevation or reduction generated by each wave. This allows to track each hot spot back to its origins. We show that the highest hot spot temperatures can be more than twice the post-incident shock temperature of the neat material and can thus lead to ignition. By comparing the maximum temperature observed in the domain in 2D and 3D simulations we show that 3D calculations are necessary to avoid belated ignition times in reactive scenarios.

  • a hybrid formulation for the numerical simulation of condensed phase explosives
    Journal of Computational Physics, 2016
    Co-Authors: Louisa Michael, Nikolaos Nikiforakis
    Abstract:

    We develop a new model for simulating the combustion of condensed-phase explosives, interacting with compliant inert materials.An associated numerical algorithm is also developed.Several Mie-Gruneisen type equations of state (EoS) and reaction rate laws have been implemented and validated.An extensive review of current formulations is presented.A very extensive validation and evaluation suite is carried out. In this article we present a new formulation and an associated numerical algorithm, for the simulation of combustion and transition to detonation of condensed-phase commercial- and military-grade explosives, which are confined by (or in general interacting with one or more) compliant inert materials. Examples include confined rate-stick problems and interaction of shock waves with gas cavities or solid particles in explosives. This formulation is based on an augmented Euler approach to account for the mixture of the explosive and its products, and a multi-phase diffuse interface approach to solve for the immiscible interaction between the mixture and the inert materials, so it is in essence a hybrid (augmented Euler and multi-phase) model. As such, it has many of the desirable features of the two approaches and, critically for our applications of interest, it provides the accurate recovery of temperature fields across all components. Moreover, it conveys a lot more physical information than augmented Euler, without the complexity of full multi-phase Baer-Nunziato-type models or the lack of robustness of augmented Euler models in the presence of more than two components. The model can sustain large density differences across material interfaces without the presence of spurious oscillations in velocity and pressure, and it can accommodate realistic equations of state and arbitrary (pressure- or temperature-based) reaction-rate laws. Under certain conditions, we show that the formulation reduces to well-known augmented Euler or multi-phase models, which have been extensively validated and used in practice. The full hybrid model and its reduced forms are validated against problems with exact (or independently-verified numerical) solutions and evaluated for robustness for rate-stick and shock-induced Cavity Collapse case-studies.

Omar K. Matar - One of the best experts on this subject based on the ideXlab platform.

  • Dynamics of a surfactant-laden bubble bursting through an interface
    Journal of Fluid Mechanics, 2021
    Co-Authors: Cristian R. Constante-amores, Lyes Kahouadji, Assen Batchvarov, Seungwon Shin, Jalel Chergui, Damir Juric, Omar K. Matar
    Abstract:

    We study the effect of surfactant on the dynamics of a bubble bursting through an interface. We perform fully three-dimensional direct numerical simulations using a hybrid interface-tracking/level-set method accounting for surfactant-induced Marangoni stresses, sorption kinetics, and diffusive effects. We select an initial bubble shape corresponding to a large Laplace number and a vanishingly small Bond number in order to neglect gravity, and isolate the effects of surfactant on the flow. Our results demonstrate that the presence of surfactant affects the dynamics of the system through Marangoni-induced flow, driving motion from high to low concentration regions, which is responsible for the onset of a recirculation zone close to the free surface. These Marangoni stresses rigidify the interface, delay the Cavity Collapse, and influence the jet breakup process.

Cristian R. Constante-amores - One of the best experts on this subject based on the ideXlab platform.

  • Dynamics of a surfactant-laden bubble bursting through an interface
    Journal of Fluid Mechanics, 2021
    Co-Authors: Cristian R. Constante-amores, Lyes Kahouadji, Assen Batchvarov, Seungwon Shin, Jalel Chergui, Damir Juric, Omar K. Matar
    Abstract:

    We study the effect of surfactant on the dynamics of a bubble bursting through an interface. We perform fully three-dimensional direct numerical simulations using a hybrid interface-tracking/level-set method accounting for surfactant-induced Marangoni stresses, sorption kinetics, and diffusive effects. We select an initial bubble shape corresponding to a large Laplace number and a vanishingly small Bond number in order to neglect gravity, and isolate the effects of surfactant on the flow. Our results demonstrate that the presence of surfactant affects the dynamics of the system through Marangoni-induced flow, driving motion from high to low concentration regions, which is responsible for the onset of a recirculation zone close to the free surface. These Marangoni stresses rigidify the interface, delay the Cavity Collapse, and influence the jet breakup process.

Xianqian Wu - One of the best experts on this subject based on the ideXlab platform.

  • unsteady characteristics of cloud cavitating flow near the free surface around an axisymmetric projectile
    International Journal of Multiphase Flow, 2016
    Co-Authors: Yiwei Wang, Chenguang Huang, Xiaocui Wu, Xianqian Wu
    Abstract:

    The effect of free surface on unsteady cloud cavitation is important for high-speed surface vehicles, However, previously published experimental and numerical works regarding this topic are limited. In this paper, a typical launching experiment is performed with the presence of free surface. A numerical approach is established by using large eddy simulation and volume-of-fluid methods. Firstly, unsteady evolutions of the Cavity and re-entry jet are obtained in both experimental and numerical results, which agree well with each other. Results indicate that the Cavity evolution on the upper side of projectile is remarkably different compared to the lower side under the free-surface effect. For instance, on the upper side, Cavity growth is slower, the velocity of the re-entry jet is higher, the Cavity sheds faster, and the position of shedding Cavity Collapse is closer to the main Cavity. Secondly, the effect of the free surface is studied by analyzing the constraint variation. Because the flow stream around the upper surface is thin, changing its direction under the effect of pressure difference inside and outside the Cavity is easy. Non-axisymmetric Collapse features generate a mass of strong vortexes on the cylindrical surface, and the non-uniform distribution of high pressure region is also one of the most important factors to induce lateral and vertical forces on the projectile. Finally, the heights of wave elevation in cases with and without cavitation are compared. The presence of cavitation leads to an increase in wave height, but the increment is about half the thickness of the Cavity. This finding indicates that the actual constraint effect is between the effects of the infinite water field and the fully free condition.