arXiv subjects

Nikolaos Nikiforakis

Publications and source records attributed to Nikolaos Nikiforakis.

At least 19 recordsLinked to original sources

arXiv2022

Thermal Simulation of Millimetre Wave Ablation of Geological Materials

This work is concerned with the numerical simulation of ablation of geological materials using a millimetre wave source. To this end, a new mathematical model is developed for a thermal approach to the problem, allowing for large scale simulations, whilst being able to include the strong temperature dependence of material parameters to ensure accurate modelling of power input into the rock. The model presented is implemented within an adaptive meshing framework, such that resolution can be placed where needed, for example at the borehole wall, to further improve the computational efficiency of large scale simulations. This approach allows for both the heating of the rock, and the removal of evaporated material, allowing rate of penetration and the shape of the resulting borehole to be quantified. The model is validated against experimental results, which indicates that the approach can accurately predict temperatures, and temperature gradients within the rock. The validated model is then exercised to obtain initial results demonstrating its capabilities for simulating the millimetre wave drilling process. The effects of the conditions at the surface of the rock are investigated, highlighting the importance of understanding the physical processes which occur between the wave guide and the rock. Additionally, the absorptivity of the rock, and the impact this has on the evaporation behaviour is considered. Simulations are carried out both for isotropic rock, and also for a multi-strata configuration. It is found that strata between similar rock types, such as granite and basalt, absorptive properties pose little problem for uniform drilling. However, larger variations in material parameters are shown to have strong implications on the evaporation behaviour of the wellbore, and hence the resulting structure.

physics.geo-ph↗

arXiv2022

A multi-physics method for fracture and fragmentation at high strain-rates

This work outlines a diffuse interface method for the study of fracture and fragmentation in ductile metals at high strain-rates in Eulerian finite volume simulations. The work is based on an existing diffuse interface method capable of simulating a broad range of different multi-physics applications, including multi-material interaction, damage and void opening. The work at hand extends this method with a technique to model realistic material inhomogeneities, and examines the performance of the method on a selection of challenging problems. Material inhomogeneities are included by evolving a scalar field that perturbs a material's plastic yield stress. This perturbation results in non-uniform fragments with a measurable statistical distribution, allowing for underlying defects in a material to be modelled. As the underlying numerical scheme is three dimensional, parallelisable and multi-physics-capable, the scheme can be tested on a range of strenuous problems. These problems especially include a three-dimensional explosively driven fracture study, with an explicitly resolved condensed phase explosive. The new scheme compares well with both experiment and previous numerical studies.

physics.comp-ph↗

arXiv2022

The Development of a Multi-Physics Approach for Modelling the Response of Aerospace Fastener Assemblies to Lightning Attachment

This work is concerned with the development of a numerical modelling approach for studying the time-accurate response of aerospace fasteners subjected to high electrical current loading from a simulated lightning strike. The electromagnetic, thermal and elastoplastic response of individual fastener components is captured by this method allowing a critical analysis of fastener design and material layering. Under high electrical current loading, ionisation of gas filled cavities in the fastener assembly can lead to viable current paths across internal voids. This ionisation can lead to localised pockets of high pressure plasma through the Joule heating effect. The multi-physics approach developed in this paper extends an existing methodology that allows a two-way dynamic non-linear coupling of the plasma arc, the titanium aerospace fastener components, the surrounding aircraft panels, the internal supporting structure and internal plasma-filled cavities. Results from this model are compared with experimental measurements of a titanium fastener holding together carbon composite panels separated by thin dielectric layers. The current distribution measurements are shown to be accurately reproduced. A parameter study is used to assess the internal cavity modelling strategy and to quantify the relation between the internal cavity plasma pressure, the electrical current distribution and changes in the internal cavity geometry.

physics.comp-ph↗

arXiv2022

A unified diffuse interface method for the interaction of rigid bodies with elastoplastic solids and multi-phase mixtures

This work outlines a new multi-physics-compatible immersed rigid body method for Eulerian finite-volume simulations. To achieve this, rigid bodies are represented as a diffuse scalar field and an interface seeding method is employed to mediate the interface boundary conditions. The method is based on an existing multi-material diffuse interface method that is capable of handling an arbitrary mixture of fluids and elastoplastic solids. The underlying method is general and can be extended to a range of different applications including high-strain rate deformation in elastoplastic solids and reactive fluid mixtures. As such, the new method presented here is thoroughly tested through a variety of problems, including fluid-rigid body interaction, elastoplastic-rigid body interaction, and detonation-structure interaction. Comparison is drawn between both experimental work and previous numerical results, with excellent agreement in both cases. The new method is straightforward to implement, highly local, and parallelisable. This allows the method to be employed in three dimensions with multiple levels of adaptive mesh refinement using complex immersed geometries. The rigid body field can be static or dynamic, with the THINC interface reconstruction method being used to keep the interface sharp in the dynamic case.

physics.comp-ph↗

arXiv2021

Reacting condensed phase explosives in direct contact

In this article we present a new formulation and an associated algorithm for the simultaneous numerical simulation of multiple condensed phase explosives in direct contact with each other, which may also be confined by (or interacting with one or more) compliant inert materials. Examples include composite rate-stick problems and interaction of shock waves with chemically-active particles in condensed-phase explosives. There are several formulations which address the compliant or structural response of confiners and particles due to detonations, but the direct interaction of explosives remains a challenge for most formulations and algorithms. The proposed formulation addresses this problem by extending the conservation laws and mixture rules of an existing hybrid formulation to model the interaction of multiple explosive mixtures. An algorithm for the solution of the resulting system of partial differential equations is presented, which includes a new robust method for the retrieval of the densities of the constituents of each explosive mixture. The algorithm is implemented in a hierarchical adaptive mesh refinement framework and validated against results from problems with known solutions. It is evaluated for robustness for rate-stick and shock-induced flows in particle-laden explosives case-studies. It is shown that the method can simulate the interaction of detonation waves produced by military grade and commercial explosives in direct contact, each with its own distinct equation of state and reaction rate law. The ability of the new model to simulate reactive particles which are explicitly resolved in a heterogeneous explosive is demonstrated by a case-study of a shock wave interacting with a high explosive bead embedded in liquid nitromethane.

physics.comp-ph↗

arXiv2021

A unified multi-phase and multi-material formulation for combustion modelling

The motivation of this work is to produce an integrated formulation for material response due to detonation wave loading. Here, we focus on elastoplastic structural response. In particular, we are interested to capture miscible and immiscible behaviour within condensed-phase explosives arising from the co-existence of a reactive carrier mixture of miscible materials, and several material interfaces due to the presence of immiscible impurities such as particles or cavities. The dynamic and thermodynamic evolution of the explosive is communicated to one or more inert confiners through their shared interfaces, which may undergo severe topological change. We also wish to consider elastic and plastic structural response of the confiners, rather than make a hydrodynamic assumption for their behaviour. Previous work by these authors has met these requirements by means of the simultaneous solution of appropriate systems of equations for the behaviour of the condensed-phase explosive and the elastoplastic behaviour of the confiners. In the present work, we employ a single system of partial differential equations (PDEs) proposed by Peshkov and Romenski, which is able to account for different states of matter by means of generalising the concept of distortion tensors beyond solids. We amalgamate that formulation with a single system of PDEs which meets the requirement of co-existing miscible and immiscible explosive mixtures. We present the mathematical derivation and construct appropriate algorithms for its solution. The resulting model is validated against exact solutions for several use-cases, including mechanically- and thermally-induced, inviscid and viscous detonations. Results indicate that the model can accurately simulate a very broad range of problems involving the nonlinear interaction between reactive and inert materials within a single framework.

physics.comp-ph↗

arXiv2021

A Flux-enriched Godunov Method for Multi-material Problems with Interface Slide and Void Opening

This work outlines a new three-dimensional diffuse interface finite volume method for the simulation of multiple solid and fluid components featuring large deformations, sliding and void opening. This is achieved by extending an existing reduced-equation diffuse interface method by means of a number of novel flux-modifiers and interface seeding routines that enable the application of different material boundary conditions. The method allows for slip boundary conditions across solid interfaces, material-void interaction, and interface separation. The method is designed to be straightforward to implement, inexpensive and highly parallelisable. This makes it suitable for use in large, multi-dimensional simulations that feature many complex materials and physical processes interacting over multiple levels of adaptive mesh refinement. Furthermore, the new method allows for the generation of new interfaces in a conservative fashion and therefore naturally facilitates the simulation of high-strain rate fracture. Hence, the governing model is augmented to include ductile damage to allow for validation of the method against demanding physical experiments. The method is shown to give excellent agreement with both experiment and existing Eulerian interface tracking algorithms that employ sharp interface methods.

physics.comp-ph↗

arXiv2021

A Diffuse Interface Model of Reactive-fluids and Solid-dynamics

This article presents a multi-physics methodology for the numerical simulation of physical systems that involve the non-linear interaction of multi-phase reactive fluids and elastoplastic solids, inducing high strain-rates and high deformations. Each state of matter is governed by a single system of non-linear, inhomogeneous partial differential equations, which are solved simultaneously on the same computational grid, and do not require special treatment of immersed boundaries. To this end, the governing equations for solid and reactive multiphase fluid mechanics are written in the same mathematical form and are discretised on a regular Cartesian mesh. All phase and material boundaries are treated as diffuse interfaces. An interface-steepening technique is employed at material boundaries to keep interfaces sharp whilst maintaining the conservation properties of the system. These algorithms are implemented in a highly-parallelised hierarchical adaptive mesh refinement platform, and are verified and validated using numerical and experimental benchmarks. Results indicate very good agreement with experiment and an improvement of numerical performance compared to certain existing Eulerian methods, without loss of conservation.

physics.comp-ph↗

arXiv2021

Ripple : Simplified Large-Scale Computation on Heterogeneous Architectures with Polymorphic Data Layout

GPUs are now used for a wide range of problems within HPC. However, making efficient use of the computational power available with multiple GPUs is challenging. The main challenges in achieving good performance are memory layout, affecting memory bandwidth, effective use of the memory spaces with a GPU, inter-GPU communication, and synchronization. We address these problems with the Ripple library, which provides a unified view of the computational space across multiple dimensions and multiple GPUs, allows polymorphic data layout, and provides a simple graph interface to describe an algorithm from which inter-GPU data transfers can be optimally scheduled. We describe the abstractions provided by Ripple to allow complex computations to be described simply, and to execute efficiently across many GPUs with minimal overhead. We show performance results for a number of examples, from particle motion to finite-volume methods and the eikonal equation, as well as showing good strong and weak scaling results across multiple GPUs.

cs.DC↗

arXiv2021

An improved equation of state for air plasma simulations

This work is concerned with the development of a novel, accurate equation of state for describing partially ionised air plasma in local thermodynamic equilibrium. One key application for this new equation of state is the simulation of lightning strike on aircraft. Due to the complexities of species ionisation and interaction, although phenomenological curve fitting of thermodynamic properties is possible, these curves are intractable for practical numerical simulation. The large difference in size of the parameters (many orders of magnitude) and complexity of the equations means they are not straightforward to invert for conversion between thermodynamic variables. The approach of this paper is to take an accurate 19-species phenomenological model, and use this to generate a tabulated data set. Coupled with a suitable interpolation procedure this offers an accurate and computationally efficient technique for simulating partially ionised air plasma. The equation of state is implemented within a multiphysics methodology which can solve for two-way coupling between a plasma arc and an elastoplastic material substrate. The implementation is validated against experimental results, both for a single material plasma, and an arc coupled to a substrate. It is demonstrated that accurate, oscillation-free thermodynamic profiles can be obtained, with good results even close to material surfaces.

physics.comp-ph↗

arXiv2020

Effect of spatial distribution of mesoscale heterogeneities on the shock-to-detonation transition in liquid nitromethane

The sensitizing effect of cavities in the form of microbubbles on the shock initiation of a homogeneous liquid explosive is studied computationally. While the presence of voids in an explosive has long been known to induce so-called hot spots that greatly accelerate the global reaction rate, the ability to computationally resolve the details of the interaction of the shock front with heterogeneities existing on the scale of the detonation reaction zone has only recently become feasible. In this study, the influence of the spatial distribution of air-filled cavities has been examined, enabled by the use of graphic processing unit (GPU) accelerated computations that can resolve shock initiation and detonation propagation through an explosive while fully resolving features at the mesoscale. Different spatial distributions of cavities are examined in two-dimensional simulations, including regular arrays of cavities, slightly perturbed arrays, random arrays (with varying minimum spacing being imposed on the cavities), and randomly distributed clusters of cavities. The presence of the cavities is able to reduce the time required to initiate detonation---for a given input shock strength---by greater than 50%, in agreement with previous experimental results. Randomly distributing the cavities results in a 15-20% decrease in detonation initiation time in comparison to a regular array of cavities. Clustering the cavities---as would occur in the case of agglomeration---results in an additional 10% decrease in detonation initiation time in comparison to random arrays. The effect of clustering is shown not to be a result of the clusters forming an effectively larger cavity, but rather due to interactions between clusters upon shock loading occurring on the microscale.

physics.flu-dyn↗

arXiv2019

An embedded boundary approach for efficient simulations of viscoplastic fluids in three dimensions

We present a methodology for simulating three-dimensional flow of incompressible viscoplastic fluids modelled by generalised Newtonian rheological equations. It is implemented in a highly efficient framework for massively parallelisable computations on block-structured grids. In this context, geometric features are handled by the embedded boundary approach, which requires specialised treatment only in cells intersecting or adjacent to the boundary. This constitutes the first published implementation of an embedded boundary algorithm for simulating flow of viscoplastic fluids on structured grids. The underlying algorithm employs a two-stage Runge-Kutta method for temporal discretisation, in which viscous terms are treated semi-implicitly and projection methods are utilised to enforce the incompressibility constraint. We augment the embedded boundary algorithm to deal with the variable apparent viscosity of the fluids. Since the viscosity depends strongly on the strain rate tensor, special care has been taken to approximate the components of the velocity gradients robustly near boundary cells, both for viscous wall fluxes in cut cells and for updates of apparent viscosity in cells adjacent to them. After performing convergence analysis and validating the code against standard test cases, we present the first ever fully three-dimensional simulations of creeping flow of Bingham plastics around translating objects. Our results shed new light on the flow fields around these objects.

physics.flu-dyn↗

arXiv2019

Multi-physics simulations of lightning strike on elastoplastic substrates

This work is concerned with the numerical simulation of elastoplastic, electromagnetic and thermal response of aerospace materials due to their interaction with a plasma arc under lightning strike conditions. Current approaches treat the interaction between these two states of matter either in a decoupled manner or through one-way coupled 'co-simulation'. In this paper a methodology for multiphysics simulations of two-way interaction between lightning and elastoplastic materials is presented, which can inherently capture the non-linear feedback between these two states of matter. This is achieved by simultaneously solving the magnetohydrodynamic and the elastoplastic systems of equations on the same computational mesh, evolving the magnetic and electric fields dynamically. The resulting model allows for the topological evolution and movement of the arc attachment point coupled to the structural response and Joule heating of the substrate. The dynamic communication between the elastoplastic material and the plasma is facilitated by means of Riemann problem-based ghost fluid methods. This two-way coupling, to the best of the authors' knowledge, has not been previously demonstrated. The proposed model is first validated against experimental laboratory studies, demonstrating that the growth of the plasma arc can be accurately reproduced, dependent on the electrical conductivity of the substrate. It is then subsequently evaluated in a setting where the dynamically-evolved properties within the substrate feed back into the plasma arc attachment. Results are presented for multi-layered substrates of different materials, and for a substrate with temperature-dependent electrical conductivity. It is demonstrated that these conditions generate distinct behaviour due to the interaction between the plasma arc and the substrate.

physics.comp-ph↗

arXiv2019

A multi-physics methodology for four-states of matter

We propose a numerical methodology for the simultaneous numerical simulation of four states of matter; gas, liquid, elastoplastic solids and plasma. The distinct, interacting physical processes are described by a combination of compressible, inert and reactive forms of the Euler equations, multiphase equations, elastoplastic equations and resistive MHD equations. Combinations of systems of equations are usually solved by coupling finite element for solid modelling and CFD models for fluid modelling or including material effects through boundary conditions rather than full material discretisation. Our simultaneous solution methodology lies on the recasting of all the equations in the same, hyperbolic form allowing their solution on the same grid with the same finite-volume numerical schemes. We use a combination of sharp and diffuse interface methods to track or capture material interfaces, depending on the application. The communication between the distinct systems of equations (i.e., materials separated by sharp interfaces) 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. To demonstrate the applicability of the new methodology we consider a case study where we investigate the possibility of ignition of a combustible gas that lies over a liquid in a metal container that is struck by a plasma-arc akin to a lightning strike. We study the effect on the ignition of the metal container material and conductivity, of the presence of a dielectric coating, of insensitive combustible gases and sealed and pre-damaged metal surfaces.

physics.comp-ph↗

arXiv2019

Meso-resolved simulations of shock-to-detonation transition in nitromethane with air-filled cavities

Two-dimensional, meso-resolved numerical simulations are performed to investigate the complete shock-to-detonation transition (SDT) process in a mixture of liquid nitromethane (NM) and air-filled, circular cavities. The shock-induced initiation behaviors resulting from the cases with neat NM, NM with an array of regularly spaced cavities, and NM with randomly distributed cavities are examined. For the case with randomly distributed cavities, hundreds of cavities are explicitly resolved in the simulations using a diffuse-interface approach to treat two immiscible fluids and GPU-enabled parallel computing. Without invoking any empirically calibrated, phenomenological models, the reaction rate in the simulations is governed by Arrhenius kinetics. For the cases with neat NM, the resulting SDT process features a superdetonation that evolves from a thermal explosion after a delay following the passage of the incident shock wave and eventually catches up with the leading shock front. For the cases wherein mesoscale heterogeneities are explicitly considered, a gradual SDT process is captured. These two distinct initiation behaviors for neat NM and heterogeneous NM mixtures agree with experimental findings. Via examining the global reaction rate of the mixture, a unique time scale characterizing the SDT process, i.e., the overtake time, is measured for each simulation. For an input shock pressure less than approximately $9.4~\mathrm{GPa}$, the overtake time resulting from a heterogeneous mixture is shorter than that for neat NM. This sensitizing effect is more pronounced for lower input shock pressures. A random distribution of cavities is found to be more effective in enhancing the SDT process than a regular array of cavities. Statistical analysis on the meso-resolved simulation data provides more insights into the mechanism of energy release underlying the SDT process.

physics.comp-ph↗

arXiv2018

Highly parallelisable simulations of time-dependent viscoplastic fluid flow simulations with structured adaptive mesh refinement

We present the extension of an efficient and highly parallelisable framework for incompressible fluid flow simulations to viscoplastic fluids. The system is governed by incompressible conservation of mass, the Cauchy momentum equation and a generalised Newtonian constitutive law. In order to simulate a wide range of viscoplastic fluids, we employ the Herschel-Bulkley model for yield-stress fluids with nonlinear stress-strain dependency above the yield limit. We utilise Papanastasiou regularisation in our algorithm to deal with the singularity in apparent viscosity. The resulting system of partial differential equations is solved using the IAMR code (Incompressible Adaptive Mesh Refinement), which uses second-order Godunov methodology for the advective terms and semi-implicit diffusion in the context of an approximate projection method to solve on adaptively refined meshes. By augmenting the IAMR code with the ability to simulate regularised Herschel-Bulkley fluids, we obtain efficient numerical software for time-dependent viscoplastic flow in three dimensions, which can be used to investigate systems not considered previously due to computational expense. We validate results from simulations using this new capability against previously published data for Bingham plastics and power-law fluids in the two-dimensional lid-driven cavity. In doing so, we expand the range of Bingham and Reynolds numbers which have been considered in the benchmark tests. Moreover, extensions to time-dependent flow of Herschel-Bulkley fluids and three spatial dimensions offer new insights into the flow of viscoplastic fluids in this test case, and we provide missing benchmark results for these extensions.

physics.flu-dyn↗

arXiv2018

On the viscoplastic squeeze flow between two identical infinite circular cylinders

Direct numerical simulations of closely interacting infinite circular cylinders in a Bingham fluid are presented, and results compared to asymptotic solutions based on lubrication theory in the gap. Unlike for a Newtonian fluid, the macroscopic flow outside of the gap between the cylinders is shown to have a large effect on the pressure profile within the gap and the resulting lubrication force on the cylinders. The presented results indicate that the asymptotic lubrication solution can be used to predict the lubrication pressure only if the surrounding viscoplastic matrix is yielded by a macroscopic flow. This has implications for the use of sub-grid-scale lubrication models in simulations of non-colloidal particulate suspensions in viscoplastic fluids.

physics.flu-dyn↗

arXiv2017

A complete equation of state for non-ideal condensed phase explosives

The objective of this work is to improve the robustness and accuracy of numerical simulations of both ideal and non-ideal explosives by introducing temperature dependence in mechanical equations of state for reactants and products. To this end, we modify existing mechanical equations of state to appropriately approximate the temperature in the reaction zone. Mechanical equations of state of Mie-Grüneisen form are developed with extensions, which allow the temperature to be evaluated appropriately, and the temperature equilibrium condition to be applied robustly. Furthermore the snow plow model is used to capture the effect of porosity on the reactants equation of state. We apply the methodology to predict the velocity of compliantly confined detonation waves. Once reaction rates are calibrated for unconfined detonation velocities, simulations of confined rate sticks and slabs are performed, and the experimental detonation velocities are matched without further parameter alteration, demonstrating the predictive capability of our simulations. We apply the same methodology to both ideal (PBX9502, a high explosive with principal ingredient TATB) and non-ideal (EM120D, an ANE or ammonium nitrate based emulsion) explosives.

physics.flu-dyn↗