Arbitrary Curvilinear Coordinates Research Articles

The inviscid incompressible limit of Kelvin–Helmholtz instability for plasmas

Introduction: The Kelvin–Helmholtz Instability (KHI) is an interface instability that develops between two fluids or plasmas flowing with a common shear layer. KHI occurs in astrophysical jets, solar atmosphere, solar flows, cometary tails, planetary magnetospheres. Two applications of interest, encompassing both space and fusion applications, drive this study: KHI formation at the outer flanks of the Earth’s magnetosphere and KHI growth from non-uniform laser heating in magnetized direct-drive implosion experiments. Here, we study 2D KHI with or without a magnetic field parallel to the flow. We use both the GAMERA code, which solves the compressible Euler equations, and the STRATOSPEC code, which solves the Navier-Stokes equations under the Boussinesq approximation, coupled with the magnetic field dynamics. GAMERA is a global three-dimensional MHD code with high-order reconstruction in arbitrary nonorthogonal curvilinear coordinates, which is developed for a large range of astrophysical applications. STRATOSPEC is a three-dimensional pseudo-spectral code with an accuracy of infinite order (no numerical diffusion). Magnetized KHI is a canonical case for benchmarking hydrocode simulations with extended MHD options.Methods: An objective is to assess whether or not, and under which conditions, the incompressibility hypothesis allows to describe a dynamic compressible system. For comparing both codes, we reach the inviscid incompressible regime, by decreasing the Mach number in GAMERA, and viscosity and diffusion in STRATOSPEC. Here, we specifically investigate both single-mode and multi-mode initial perturbations, either with or without magnetic field parallel to the flow. The method relies on comparisons of the density fields, 1D profiles of physical quantities averaged along the flow direction, and scale-by-scale spectral densities. We also address the triggering, formation and damping of filamentary structures under varying Mach number or Atwood number, with or without a parallel magnetic field.Results: Comparisons show very satisfactory results between the two codes. The vortices dynamics is well reproduced, along with the breaking or damping of small-scale structures. We end with the extraction of growth rates of magnetized KHI from the compressible regime to the incompressible limit in the linear regime assessing the effects of compressibility under increasing magnetic field.Discussion: The observed differences between the two codes are explained either from diffusion or non-Boussinesq effects.

An Exactly Energy-conserving Electromagnetic Particle-in-cell Method in Curvilinear Coordinates

In this paper, we introduce and discuss an exactly energy-conserving particle-in-cell method for arbitrary curvilinear coordinates. The flexibility provided by curvilinear coordinates enables the study of plasmas in complex-shaped domains by aligning the grid to the given geometry or by focusing grid resolution on regions of interest without overresolving the surrounding, potentially uninteresting domain. We have achieved this through the introduction of the metric tensor, the Jacobian matrix, and contravariant operators combined with an energy-conserving fully implicit solver. We demonstrate the method’s capabilities using a Python implementation to study several one- and two-dimensional test cases: the electrostatic two-stream instability, the electromagnetic Weibel instability, and the geomagnetic environment modeling reconnection challenge. The test results confirm the capability of our new method to reproduce theoretical expectations (e.g., instability growth rates) and the corresponding results obtained with a Cartesian uniform grid when using curvilinear grids. Simultaneously, we show that the method conserves energy to machine precision in all cases.

Область правильной координации пространства нормальными коническими координатами

The field theory is widely represented in spherical and cylindrical coordinate systems, since the mathematical apparatus of these coordinate systems is well studied. Field sources with more complex structures require new approaches to their study. The purpose of this study is to determine the correct coordination of space by normal conic coordinates. This is necessary in subsequent studies, the task of which will be to simplify the expressions for the characteristics of the field by introducing a special coordination of space, which reflect the shape of the source and/or sink of the field. For example, a field with a rectilinear source is more convenient to refer to cylindrical coordinates, and a field with a point source - to spherical coordinates. Basically, the use of field theory in the study of physical processes by methods of applied geometry is limited to two classical curvilinear systems, although their presentation in arbitrary curvilinear coordinates is known. We will distinguish between global and local coordinate systems. The global system, as well as the coordinates of a point in this system, will be denoted by x, y, z. She is unchanging. The local system, as well as the coordinates of a point in this system, will be denoted by t, u, v. Local system variable. At each point in space belonging to the area of existence of the system, the local coordinate system is defined

The boundary integral equation for curved solid/liquid interfaces propagating into a binary liquid with convection

The boundary integral method is developed for unsteady solid/liquid interfaces propagating into undercooled binary liquids with convection. A single integrodifferential equation for the interface function is derived using the Green function technique. In the limiting cases, the obtained unsteady convective boundary integral equation transforms into a previously developed theory. This integral is simplified for the steady-state growth in arbitrary curvilinear coordinates when the solid/liquid interface is isothermal (isoconcentration). Finally, we evaluate the boundary integral for a binary melt with a forced flow and analyze how the melt undercooling depends on Péclet and Reynolds numbers.

Exact Solutions to the Three-Dimensional Navier–Stokes Equations Using the Extended Beltrami Method

Abstract In a 1966 publication, Chi-Yi Wang used the streamfunction in concert with the vorticity equations to develop a methodology for obtaining exact solutions to the incompressible Navier–Stokes equations, now known as the extended Beltrami method. In Wang's approach, the vorticity is represented by the sum of a linear function of the streamfunction and an assumed auxiliary function, such that the vorticity equation can be reduced to a quasi-linear partial differential equation, and exact solutions are obtainable for many choices of the auxiliary function. In the present work, a natural extension of Wang's formulation to three-dimensional flows in arbitrary orthogonal curvilinear coordinates has been derived, wherein two auxiliary functions are formed at the outset, with the caveat that the pressure and velocity components may vary in two spatial dimensions. As is the case with two-dimensional extended Beltrami flows, exact solutions are only obtainable when the forms of the auxiliary functions are “simple enough” to render the governing equations solvable. To demonstrate the solutions which may be obtained using the extended formulation, the well-known Kovasznay flow is generalized to a three-dimensional flow. A unique solution in plane polar coordinates is found. An extension to the solution to Burgers vortex has been derived and discussed in the context of existing literature. Finally, a new 3D swirling flow solution which is the angular analogue to Kovasznay flow has been developed.

A compact streamfunction-velocity scheme for the 2-D unsteady incompressible Navier-Stokes equations in arbitrary curvilinear coordinates

A streamfunction-velocity formulation-based compact difference method is suggested for solving the unsteady incompressible Navier-Stokes equations in the arbitrary curvilinear coordinates, in which the streamfunction and its first derivatives as the unknown variables are utilized. Numerical examples, involving the boundary layer problem, a constricted channel flow, driven polar cavity flow and trapezoidal cavity flow problem, are solved by the present method. Numerical results demonstrate the accuracy of the proposed scheme and exhibit the numerical capability to simulate the flow problems on geometries beyond rectangular. For driven polar cavity flow problem, the results show that the flow for Re = 5 000 is not steady but time-periodic, and the critical Reynold number (Rec) for the occurrence of a Hopf bifurcation is given.

A note on Navier-Stokes equations with nonorthogonal coordinates

There are many fluid flow problems involving geometries for which a nonorthogonal curvilinear coordinate system may be the most suitable. To the authors’ knowledge, the Navier–Stokes equations for an incompressible fluid formulated in terms of an arbitrary nonorthogonal curvilinear coordinate system have not been given explicitly in the literature in the simplified form obtained herein. The specific novelty in the equations derived here is the use of the general Laplacian in arbitrary nonorthogonal curvilinear coordinates and the simplification arising from a Ricci identity for Christoel symbols of the second kind for flat space. Evidently, however, the derived equations must be consistent with the various general forms given previously by others. The general equations derived here admit the well-known formulae for cylindrical and spherical polars, and for the purposes of illustration, the procedure is presented for spherical polar coordinates. Further, the procedure is illustrated for a nonorthogonal helical coordinate system. For a slow flow for which the inertial terms may be neglected, we give the harmonic equation for the pressure function, and the corresponding equation if the inertial effects are included. We also note the general stress boundary conditions for a free surface with surface tension. For completeness, the equations for a compressible flow are derived in an appendix. doi:10.1017/S144618111700058X

A NOTE ON NAVIER–STOKES EQUATIONS WITH NONORTHOGONAL COORDINATES

There are many fluid flow problems involving geometries for which a nonorthogonal curvilinear coordinate system may be the most suitable. To the authors’ knowledge, the Navier–Stokes equations for an incompressible fluid formulated in terms of an arbitrary nonorthogonal curvilinear coordinate system have not been given explicitly in the literature in the simplified form obtained herein. The specific novelty in the equations derived here is the use of the general Laplacian in arbitrary nonorthogonal curvilinear coordinates and the simplification arising from a Ricci identity for Christoffel symbols of the second kind for flat space. Evidently, however, the derived equations must be consistent with the various general forms given previously by others. The general equations derived here admit the well-known formulae for cylindrical and spherical polars, and for the purposes of illustration, the procedure is presented for spherical polar coordinates. Further, the procedure is illustrated for a nonorthogonal helical coordinate system. For a slow flow for which the inertial terms may be neglected, we give the harmonic equation for the pressure function, and the corresponding equation if the inertial effects are included. We also note the general stress boundary conditions for a free surface with surface tension. For completeness, the equations for a compressible flow are derived in an appendix.

Surface elastic properties in silicon nanoparticles

The elastic behavior of the external surface of a solid body plays a key role in nanomechanical phenomena. While bulk elasticity enjoys the benefits of a robust theoretical understanding, many surface elasticity features remain unexplored: some of them are here addressed by blending together continuum elasticity and atomistic simulations. A suitable readdressing of the surface elasticity theory allows to write the balance equations in arbitrary curvilinear coordinates and to investigate the dependence of the surface elastic parameters on the mean and Gaussian curvatures of the surface. In particular, we predict the radial strain induced by surface effects in spherical and cylindrical silicon nanoparticles and provide evidence that the surface parameters are nearly independent of curvatures and, therefore, of the surface conformation.

Covariant quantizations in plane and curved spaces

We present covariant quantization rules for nonsingular finite-dimensional classical theories with flat and curved configuration spaces. In the beginning, we construct a family of covariant quantizations in flat spaces and Cartesian coordinates. This family is parametrized by a function omega (theta ), theta in (1,0), which describes an ambiguity of the quantization. We generalize this construction presenting covariant quantizations of theories with flat configuration spaces but already with arbitrary curvilinear coordinates. Then we construct a so-called minimal family of covariant quantizations for theories with curved configuration spaces. This family of quantizations is parametrized by the same function omega (theta ). Finally, we describe a more wide family of covariant quantizations in curved spaces. This family is already parametrized by two functions, the previous one omega (theta ) and by an additional function varTheta (x,xi ). The above mentioned minimal family is a part at varTheta =1 of the wide family of quantizations. We study constructed quantizations in detail, proving their consistency and covariance. As a physical application, we consider a quantization of a non-relativistic particle moving in a curved space, discussing the problem of a quantum potential. Applying the covariant quantizations in flat spaces to an old problem of constructing quantum Hamiltonian in polar coordinates, we directly obtain a correct result.

A new analytical method for the evaluation of transverse displacements and stresses in plane problems of elasticity

Numerous solutions of the plane theory of elasticity, derived over the past hundred years, provide a very good approximation to the in-plane stress components. However, an accurate evaluation of the out-of-plane (or transverse) stresses is often impossible as the three-dimensional stress field may not follow either plane stress or plane strain assumptions, specifically, near the edges formed by notches, cracks and other stress concentrators. For relatively simple geometries and loading conditions, the 3D stress and displacement fields can be analytically evaluated using the first-order plate theory. This theory can be considered as an elementary extension of the plane theory of elasticity and it retains the simplicity of the two-dimensional formulation. However, the derivation of exact solutions within this theory is far more difficult. In this paper, we present the governing equations of the first-order plate theory in arbitrary curvilinear coordinates, and based on these equations we develop a simplified method for the evaluation of the out-of-plane displacements and stress components. Several examples are considered to demonstrate the effectiveness and the accuracy of the new method. Due to simplicity it can be applied to investigate many 3D elastic problems involving sharp wedges and plate geometries weakened by cracks, notches and other stress concentrators. All these problems currently represent a great challenge for analytical approaches.

Equilibrium equations and boundary conditions of strain gradient theory in arbitrary curvilinear coordinates

AbstractEquilibrium equations and boundary conditions of the strain gradient theory in arbitrary curvilinear coordinates have been obtained. Their special form for an axisymmetric plane strain problem is also given.

Canonical quantization of classical mechanics in curvilinear coordinates. Invariant quantization procedure

In the paper is presented an invariant quantization procedure of classical mechanics on the phase space over flat configuration space. Then, the passage to an operator representation of quantum mechanics in a Hilbert space over configuration space is derived. An explicit form of position and momentum operators as well as their appropriate ordering in arbitrary curvilinear coordinates is demonstrated. Finally, the extension of presented formalism onto non-flat case and related ambiguities of the process of quantization are discussed.

Generalization of the Second Order Vector Potential Formulation for Arbitrary Non-Orthogonal Curvilinear Coordinates Systems from the Covariant Form of Maxwell's Equations

A great number of semi-analytical models, notably the representation of electromagnetic fields by integral equations are based on the second order vector potential (SOVP) formalism which introduces two scalar potentials in order to obtain analytical expressions of the electromagnetic fields from the two potentials. However, the scalar decomposition is often known for canonical coordinate systems. This paper aims in introducing a specific SOVP formulation dedicated to arbitrary non-orthogonal curvilinear coordinates systems. The electromagnetic field representation which is derived in this paper constitutes the key stone for the development of semi-analytical models for solving some eddy currents moelling problems and electromagnetic radiation problems considering at least two homogeneous media separated by a rough interface. This SOVP formulation is derived from the tensor formalism and Maxwell’s equations written in a non-orthogonal coordinates system adapted to a surface characterized by a 2D arbitrary aperiodic profile.

$\mathfrak{D}$ -Differentiation in Hilbert Space and the Structure of Quantum Mechanics

An appropriate kind of curved Hilbert space is developed in such a manner that it admits operators of \(\mathcal{C}\) - and \(\mathfrak{D}\) -differentiation, which are the analogues of the familiar covariant and D-differentiation available in a manifold. These tools are then employed to shed light on the space-time structure of Quantum Mechanics, from the points of view of the Feynman ‘path integral’ and of canonical quantisation. (The latter contains, as a special case, quantisation in arbitrary curvilinear coordinates when space is flat.) The influence of curvature is emphasised throughout, with an illustration provided by the Aharonov-Bohm effect.

Large-eddy simulation of two-phase spray combustion for gas turbine combustors

In three-dimensional arbitrary curvilinear coordinates, an Eulerian–Lagrangian formulation is applied to large-eddy simulation (LES) of instantaneous gas–liquid two-phase turbulent combustion flows in gas turbine combustors. Three dimensional block-structured grids are generated by zone method and solving a system of elliptic partial differential equations. The k-equation sub-grid scale model is used to simulate the sub-grid eddy viscosity and the EBU combustion sub-grid scale model is employed to predict the chemical reaction rate. The gas-phase governing equations are solved with SIMPLE algorithm and hybrid scheme in non-staggered grid system. A stochastic separated flow formulation is used to track the droplet trajectories velocities, size and temperature history by Lagrangian equations of motion and thermal balance. Multi-zone coupling method is employed to transport data between interfaces. The influences of two different primary hole positions and three different fuel–air ratios on turbulent two-phase reacting flows are calculated. Predictions are in reasonable agreement with the measured velocity using PIV system and temperature, species concentration measurements at the exit. It is shown that the present approach may be used to study spray combustion flow fields for guiding the design of advanced gas turbine combustors.

Finite-difference method for incompressible Navier–Stokes equations in arbitrary orthogonal curvilinear coordinates

A finite-difference method for solving three-dimensional time-dependent incompressible Navier–Stokes equations in arbitrary curvilinear orthogonal coordinates is presented. The method is oriented on turbulent flow simulations and consists of a second-order central difference approximation in space and a third-order semi-implicit Runge–Kutta scheme for time advancement. Spatial discretization retains some important properties of the Navier–Stokes equations, including energy conservation by the nonlinear and pressure-gradient terms. Numerical tests cover Cartesian, cylindrical-polar, spherical, cylindrical elliptic and cylindrical bipolar coordinate systems. Both laminar and turbulent flows are considered demonstrating reasonable accuracy and stability of the method.

Volume preserving integrators for solenoidal fields on a grid

A novel method of integrating solenoidal flows given on a grid is developed. In three dimensions, the method involves splitting the flow into two or possibly three flows that are two dimensional and area preserving, and can therefore be integrated by Crank–Nicolson or a modified form of leapfrog, both of which exactly conserve area. The method involves an interpolation scheme which ensures the solenoidal nature of the field throughout the domain (on and off the grid) by means of tricubic splines. It is shown that the method is easily generalized to arbitrary curvilinear coordinates and also generalizes to higher dimensions. A method of integrating compressional three-dimensional flows given on a grid is outlined.

Comment on “Matrix representations for vector differential operators in general orthogonal coordinates” by Š. Višňovský

In a recent tutorial paper Vǐsňovský [1] presented the correct matrix representations of some well-known vector differential operators valid in arbitrary orthogonal curvilinear coordinates. In a real tour de force he showed, inter alia, that the Laplacian of the vector field can be obtained as either the divergence of the dyadic ∇A or via the matrix representation of the right-hand side of the differential vector identity ∇ · (∇A) = ∇(∇ ·A)−∇× (∇×A) , but also applying the Laplacian operator, ∇ · ∇ ≡ ∇2, to the vector field (compare, e. g., [2]). The key point in Vǐsňovský’s analysis is the equation giving the partial derivative of the unit vectors in arbitrary orthogonal curvilinear coordinates (Eq. (25 ) in [1]; in this Comment all notation in italics will be that of [1]):

Matrix Representations for Vector Differential Operators in General Orthogonal Coordinates

The restrictions in arbitrary orthogonal coordinates to a unified matrix presentation of vector operations using generalized differential matrix operators [Y. Chen et al.: IEEE Trans. on Educ., Vol. 41, (1998), p. 61] are pointed out. The corrected matrix representation for the operations ∇⋅(∇A) and B⋅(∇A) valid in arbitrary orthogonal curvilinear coordinates are obtained and shown to be consistent with the calculation using the dyadic technique. The presentation is accessible to undergraduate students.