T. Raja Sekhar

Similarity solutions for three dimensional Euler equations using Lie group analysis

A similarity analysis of the nonlinear three dimensional unsteady Euler equations of gas dynamics is presented using Lie group of transformations with commuting infinitesimal operators. Symmetry groups admitted by the governing system of partial differential equations (PDEs) are obtained, and the complete Lie algebra of infinitesimal symmetries is established. The symmetry generators are used for constructing similarity variables which lead to a reduced system of equations with one independent variable less at each step and eventually to a system of ordinary differential equations (ODEs); in some cases, it is possible to solve these equations exactly.

Solution to the Riemann problem in a one-dimensional magnetogasdynamic flow

The Riemann problem for a quasilinear hyperbolic system of equations governing the one-dimensional unsteady flow of an inviscid and perfectly conducting compressible fluid, subjected to a transverse magnetic field, is solved approximately. This class of equations includes as a special case the Euler equations of gasdynamics. It has been observed that in contrast to the gasdynamic case, the pressure varies across the contact discontinuity. The iterative procedure is used to find the densities between the left acoustic wave and the right contact discontinuity and between the right contact discontinuity and the right acoustic wave, respectively. All other quantities follow directly throughout the (x, t)-plane, except within rarefaction waves, where an extra iterative procedure is used along with a Gaussian quadrature rule to find particle velocity; indeed, the determination of the particle velocity involves numerical integration when the magneto-acoustic wave is a rarefaction wave. Lastly, we discuss numerical examples and study the solution influenced by the magnetic field.

Evolution of weak discontinuities in shallow water equations

In this paper, we determine the critical time, when a weak discontinuity in the shallow water equations culminates into a bore. Invariance group properties of the governing system of partial differential equations (PDEs), admitting Lie group of point transformations with commuting infinitesimal operators, are presented. Some appropriate canonical variables are characterized that transform equations at hand to an equivalent form, which admits non-constant solutions. The propagation of weak discontinuities is studied in the medium characterized by the particular solution of the governing system.

Application of Lie groups to compressible model of two-phase flows

This paper presents some exact solutions for the drift-flux model of two-phase flows using Lie group analysis. The analysis involves an isentropic no-slip conservation of mass for each phase and the conservation of momentum for the mixture. The present analysis employs a complete Lie algebra of infinitesimal symmetries. Subsequent to these theoretical analysis a symmetry group is established. The symmetry generators are used for constructing similarity variables which reduce the model equations to a system of ordinary differential equations (ODEs). In particular, a general framework is discussed for solving the model equations analytically. As a consequence of this, new classes of exact group-invariant solutions are developed. This provides new insights into the fundamental properties of weak discontinuities and helps one to understand better on existence of solutions.

Group classification for isothermal drift flux model of two phase flows

In this paper, a full symmetry group classification for isothermal multiphase drift flux model is presented. All invariant functions are developed for the Lie algebra, which play a vital role in construction of optimal systems. Further, with the help of one dimensional optimal classification group, invariant solutions are obtained which describe the asymptotic behavior of general solution.

A Robust and accurate Riemann solver for a compressible two-phase flow model

In this paper we analyze the Riemann problem for the widely used drift-flux two-phase flow model. This analysis introduces the complete information that is attained in the representation of solutions to the Riemann problem. It turns out that the Riemann waves have rarefactions, a contact discontinuity and shocks. Within this respect, an exact Riemann solver is developed to accurately resolve and represent the complete wave structure of the gas-liquid two-phase flows. To verify the solver, a series of test problems selected from the literature are presented including validation against independent numerical simulations where the solution of the Riemann problem is fully numerical. In this framework the governing equations are discretized by finite volume techniques facilitating the application Godunov methods of centred-type. It is shown that both analytical and numerical results demonstrate the broad applicability and robustness of the new exact Riemann solver.

Lie group analysis and propagation of weak discontinuity in one-dimensional ideal isentropic magnetogasdynamics

The aim of this paper is to carry out symmetry group analysis to obtain important classes of exact solutions from the given system of nonlinear partial differential equations (PDEs). Lie group analysis is employed to derive some exact solutions of one dimensional unsteady flow of an ideal isentropic, inviscid and perfectly conducting compressible fluid, subject to a transverse magnetic field for the magnetogasdynamics system. By using Lie group theory, the full one-parameter infinitesimal transformations group leaving the equations of motion invariant is derived. The symmetry generators are used for constructing similarity variables which leads the system of PDEs to a reduced system of ordinary differential equations; in some cases, it is possible to solve these equations exactly. Further, using the exact solution, we discuss the evolutionary behavior of weak discontinuity.

On the Riemann Problem Simulation for the Drift-Flux Equations of Two-Phase Flows

This work presents computational simulations and analytical techniques for solving the drift-flux two-phase flow model. The model equations are formulated to describe the exact solution of the Riemann problem. The solution is constructed by solving the conservation of mass for each phase and the mixture conservation momentum equation of the two phases under isothermal conditions. Particular attention is given to address the expressions for jump relationships and the Riemann invariants. The performance of the developed Riemann solver is assessed with respect to different test cases selected from the literature. Comparisons with Godunov methods of centred-type are provided to demonstrate the use of the proposed exact and computational framework. Excellent agreement is observed between analytical results and numerical predictions.

Exact solutions to magnetogasdynamic equations in Lagrangian coordinates

The Lie group of point transformations, which leave the equations for a simplified model of one dimensional ideal gas in magnetogasdynamics invariant, are used to obtain some exact solutions for the governing system of hyperbolic partial differential equations (PDEs). Similarity variables which reduces the governing system of PDEs into system of ordinary differential equations (ODEs) are determined through the transformations. The resulting ODEs are solved analytically to obtain some exact solutions that exhibits space-time dependence. Further, we study the propagation of weak discontinuity through a state characterized by one of the solutions.

Collision of characteristic shock with weak discontinuity in non-ideal magnetogasdynamics

Abstract In this paper, we consider a quasilinear hyperbolic system of partial differential equations (PDEs) governing unsteady planar or radially symmetric motion of an inviscid, perfectly conducting and non-ideal gas in which the effect of magnetic field is significant. A particular exact solution to the governing system, which exhibits space–time dependence, is derived using Lie group symmetry analysis. The evolutionary behavior of a weak discontinuity across the solution curve is discussed. Further, the evolution of a characteristic shock and the corresponding interaction with the weak discontinuity are studied. The amplitudes of the reflected wave, the transmitted wave and the jump in the shock acceleration influenced by the incident wave after interaction are evaluated. Finally, the influence of van der Waals excluded volume in the behavior of the weak discontinuity is completely characterized.