arXiv subjects

Nikos Nikiforakis

Publications and source records attributed to Nikos Nikiforakis.

6 recordsLinked to original sources

arXiv2022

Numerical modelling of imposed magnetohydrodynamic effects in hypersonic flows

Weakly ionised plasmas, formed in high enthalpy hypersonic flows, can be actively manipulated via imposed magnetic fields - a concept termed magnetohydrodynamic (MHD) flow control. Imposed MHD effects, within flows which exhibit multiple shock interactions, are consequential for emerging aerospace technologies: including aerodynamic control via magnetic actuation. However, numerical modelling of this flow type remains challenging due to the sensitivity of feature formation and the real gas modelling of weakly ionised, electrically conductive, air plasma. In this work, numerical simulation capabilities have been developed for the study of MHD affected, hypersonic flows, around 2D axisymmetric non-simple geometries. The validated numerical methodology, combined with an advanced 19 species equation of state for air plasma, achieves quantitative agreement between simulation and experiment for a Mach 5.6 double cone geometry with applied magnetic field. Numerical studies are conducted for varied conical surface angle and magnetic field configuration. This paper demonstrates how, for hypersonic flows with complex shock interactions, the MHD affected flow is not only augmented in terms of shock position, but may exhibit topological adaptations in the fundamental flow structure. A classification system is introduced for the emergent flow topologies. The applied numerical studies examine the mechanisms by which the magnetic field configuration influences the MHD augmented shock structure, leading to: (1) differences in magnitude of MHD enhancement effect, and (2) structural adaptations of the flow topology. Most critically, classes of conditions are identified which produce topological equivalence between the magnetic interaction effects and a generalised mechanical control surface.

physics.comp-ph↗

arXiv2019

A unified Eulerian framework for multimaterial continuum mechanics

A framework for simulating the interactions between multiple different continua is presented. Each constituent material is governed by the same set of equations, differing only in terms of their equations of state and strain dissipation functions. The interfaces between any combination of fluids, solids, and vacuum are handled by a new Riemann Ghost Fluid Method, which is agnostic to the type of material on either side (depending only on the desired boundary conditions). The Godunov-Peshkov-Romenski (GPR) model is used for modeling the continua (having recently been used to solve a range of problems involving Newtonian and non-Newtonian fluids, and elastic and elastoplastic solids), and this study represents a novel approach for handling multimaterial problems under this model. The resulting framework is simple, yet capable of accurately reproducing a wide range of different physical scenarios. It is demonstrated here to accurately reproduce analytical results for known Riemann problems, and to produce expected results in other cases, including some featuring heat conduction across interfaces, and impact-induced deformation and detonation of combustible materials. The framework thus has the potential to streamline development of simulation software for scenarios involving multiple materials and phases of matter, by reducing the number of different systems of equations that require solvers, and cutting down on the amount of theoretical work required to deal with the interfaces between materials.

physics.comp-ph↗

arXiv2019

Dynamical continuum simulation of condensed matter from first-principles

Macroscale continuum mechanics simulations rely on material properties stemming from the microscale, which are normally described using phenomenological equations of state (EOS). A method is proposed for the automatic generation of first-principles unconstrained EOSs using a Gaussian process on a set of ab initio molecular dynamics simulations, thereby closing the continuum equations. We illustrate it on a hyperelasticity simulation of bulk silicon using density-functional theory (DFT), following the dynamics of shock waves after a cylindrical region is instantaneously heated.

physics.comp-ph↗

arXiv2018

A Numerical Scheme for Non-Newtonian Fluids and Plastic Solids under the GPR Model

A method for modelling non-Newtonian fluids (dilatants and pseudoplastics) by a power law under the Godunov-Peshkov-Romenski model is presented, along with a new numerical scheme for solving this system. The scheme is also modified to solve the corresponding system for power-law elastoplastic solids. The scheme is based on a temporal operator splitting, with the homogeneous system solved using a finite volume method based on a WENO reconstruction, and the temporal ODEs solved using an analytical approximate solution. The method is found to perform favorably against problems with known exact solutions, and numerical solutions published in the open literature. It is simple to implement, and to the best of the authors' knowledge it is currently the only method for solving this modified version of the GPR model.

physics.comp-ph↗

arXiv2018

A dimensionally split Cartesian cut cell method for the compressible Navier-Stokes equations

We present a dimensionally split method for computing solutions to the compressible Navier-Stokes equations on Cartesian cut cell meshes. The method is globally second order accurate in the L1 norm, fully conservative, and allows the use of time steps determined by the regular grid spacing. We provide a description of the three-dimensional implementation of the method and evaluate its numerical performance by computing solutions to a number of test problems ranging from the nearly incompressible to the highly compressible flow regimes. All the computed results show good agreement with reference results from theory, experiment and previous numerical studies. To the best of our knowledge, this is the first presentation of a dimensionally split cut cell method for the compressible Navier-Stokes equations in the literature.

physics.comp-ph↗

arXiv2018

A dimensionally split Cartesian cut cell method for hyperbolic conservation laws

We present a dimensionally split method for solving hyperbolic conservation laws on Cartesian cut cell meshes. The approach combines local geometric and wave speed information to determine a novel stabilised cut cell flux, and we provide a full description of its three-dimensional implementation in the dimensionally split framework of Klein et al. [1]. The convergence and stability of the method are proved for the one-dimensional linear advection equation, while its multi-dimensional numerical performance is investigated through the computation of solutions to a number of test problems for the linear advection and Euler equations. When compared to the cut cell flux of Klein et al., it was found that the new flux alleviates the problem of oscillatory boundary solutions produced by the former at higher Courant numbers, and also enables the computation of more accurate solutions near stagnation points. Being dimensionally split, the method is simple to implement and extends readily to multiple dimensions.

physics.comp-ph↗