arXiv subjects

Nikolaos Nikiforakis

Publications and source records attributed to Nikolaos Nikiforakis.

23 records · Page 2Linked to original sources

arXiv2017

Detonation propagation in annular arcs of condensed phase explosives

We present a numerical study of detonation propagation in unconfined explosive charges shaped as an annular arc (rib). Steady detonation in a straight charge propagates at constant speed but when it enters an annular section, it goes through a transition phase and eventually reaches a new steady state of constant angular velocity. This study examines the speed of the detonation wave along the annular charge during the transition phase and at steady state, as well as its dependence on the dimensions of the annulus. The system is modeled using a recently proposed diffuse-interface formulation which allows for the representation of a two-phase explosive and of an additional inert material. The explosive considered is the polymer-bonded TATB-based LX-17 and is modeled using two JWL equations of state and the Ignition and Growth reaction rate law. Results show that steady state speeds are in good agreement with experiment. In the transition phase, the evolution of outer detonation speed deviates from the exponential bounded growth function suggested by previous studies. We propose a new description of the transition phase which consists of two regimes. The first is caused by local effects at the outer edge of the annulus and leads to a dependence of outer detonation speed on angular position along the arc. The second regime is induced by effects originating from the inner edge of the annular charge and leads to the deceleration of the outer detonation until steady state is reached. The study concludes with a parametric study where the dependence of the steady state and the transition phase on the dimensions of the annulus is investigated.

physics.comp-ph↗

arXiv2017

The evolution of the temperature field during cavity collapse in liquid nitromethane. Part II: Reactive case

We study effect of cavity collapse in non-ideal explosives as a means of controlling their sensitivity. The main aim is to understand the origin of localised temperature peaks (hot spots) that play a leading order role at early ignition stages. Thus, we perform 2D and 3D numerical simulations of shock induced single gas-cavity collapse in nitromethane. Ignition is the result of a complex interplay between fluid dynamics and exothermic chemical reaction. In part I of this work we focused on the hydrodynamic effects in the collapse process by switching off the reaction terms in the mathematical model. Here, we reinstate the reactive terms and study the collapse of the cavity in the presence of chemical reactions. We use a multi-phase formulation which overcomes current challenges of cavity collapse modelling in reactive media to obtain oscillation-free temperature fields across material interfaces to allow the use of a temperature-based reaction rate law. The mathematical and physical models are validated against experimental and analytic data. We identify which of the previously-determined (in part I of this work) high-temperature regions lead to ignition and comment on their reactive strength and reaction growth rate. We quantify the sensitisation of nitromethane by the collapse of the cavity by comparing ignition times of neat and single-cavity material; the ignition occurs in less than half the ignition time of the neat material. We compare 2D and 3D simulations to examine the change in topology, temperature and reactive strength of the hot spots by the third dimension. It is apparent that belated ignition times can be avoided by the use of 3D simulations. The effect of the chemical reactions on the topology and strength of the hot spots in the timescales considered is studied by comparing inert and reactive simulations and examine maximum temperature fields and their growth rates.

physics.comp-ph↗

arXiv2017

The evolution of the temperature field during cavity collapse in liquid nitromethane. Part I: Inert case

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.

physics.comp-ph↗

arXiv2017

A multi-physics methodology for the simulation of the two-way interaction of reactive flow and elastoplastic structural response

We propose a numerical methodology for the numerical simulation of distinct, interacting physical processes described by a combination of compressible, inert and reactive forms of the Euler equations, multiphase equations and elastoplastic equations. These systems of equations are usually solved by coupling finite element and CFD models. Here we solve them simultaneously, by recasting all the equations in the same, hyperbolic form and solving them on the same grid with the same finite-volume numerical schemes. The proposed compressible, multiphase, hydrodynamic formulation can employ a hierarchy of five reactive and non-reactive flow models, which allows simple to more involved applications to be directly described by the appropriate selection. The communication between the hydrodynamic and elastoplastic systems is facilitated by means of mixed-material Riemann solvers at the boundaries of the systems, which represent physical material boundaries. To this end we derive approximate mixed Riemann solvers for each pair of the above models based on characteristic equations. The components for reactive flow and elastoplastic solid modelling are validated separately before presenting validation for the full, coupled systems. Multi-dimensional use cases demonstrate the suitability of the reactive flow-solid interaction methodology in the context of impact-driven initiation of reactive flow and structural response due to violent reaction in automotive (e.g. car crash) or defence (e.g. explosive reactive armour) applications. Several types of explosives (C4, Deetasheet, nitromethane, gaseous fuel) in gaseous, liquid and solid state are considered.

physics.comp-ph↗

arXiv2017

Propagation of gaseous detonation waves in a spatially inhomogeneous reactive medium

Detonation propagation in a compressible medium wherein the energy release has been made spatially inhomogeneous is examined via numerical simulation. The inhomogeneity is introduced via step functions in the reaction progress variable, with the local value of energy release correspondingly increased so as to maintain the same average energy density in the medium, and thus a constant Chapman Jouguet (CJ) detonation velocity. A one-step Arrhenius rate governs the rate of energy release in the reactive zones. The resulting dynamics of a detonation propagating in such systems with one-dimensional layers and two-dimensional squares are simulated using a Godunov-type finite-volume scheme. The resulting wave dynamics are analyzed by computing the average wave velocity and one-dimensional averaged wave structure. In the case of sufficiently inhomogeneous media wherein the spacing between reactive zones is greater than the inherent reaction zone length, average wave speeds significantly greater than the corresponding CJ speed of the homogenized medium are obtained. If the shock transit time between reactive zones is less than the reaction time scale, then the classical CJ detonation velocity is recovered. The spatio-temporal averaged structure of the waves in these systems is analyzed via a Favre averaging technique, with terms associated with the thermal and mechanical fluctuations being explicitly computed. The analysis of the averaged wave structure identifies the super-CJ detonations as weak detonations owing to the existence of mechanical non-equilibrium at the effective sonic point embedded within the wave structure. The correspondence of the super-CJ behavior identified in this study with real detonation phenomena that may be observed in experiments is discussed.

physics.flu-dyn↗