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The accurate simulation of compressible flows requires the appropriate modeling of the reflection of acoustic waves at the boundaries. In the present study we discuss time domain impedance boundary conditions (TDIBC). The formulation proposed allows to impose a desired reflection coefficient at the inflow and outflow boundaries. Our formulation is an extension of the well known Navier–Stokes characteristic boundary conditions. The frequency dependent reflections at the boundaries are implemented with a state-space model in the time domain. We provide a comprehensive discussion on how such state-space models can be constructed and interpreted. This discussion shows that the state-space description allows a robust and flexible implementation. It allows to consider complex reflection coefficients and account for non-constant CFD time steps in a straight forward manner. Furthermore, we prove analytically and demonstrate numerically that the formulation proposed is consistent, i.e. the formulation ensures that the flow simulation exhibits the reflection coefficient imposed accurately, as long as the waves impinging on the boundary are plane, and it prohibits drift of the mean flow variables. Finally, the boundary conditions are tested successfully for laminar and turbulent flows. [ABSTRACT FROM AUTHOR]

This paper addresses issues in high-fidelity numerical simulations of transcritical turbulent flows at supercritical pressure. The proposed strategy builds on a tabulated look-up table method based on REFPROP database for an accurate estimation of non-linear behaviors of thermodynamic and fluid transport properties at the transcritical conditions. Based on the look-up table method we propose a numerical method that satisfies high-order spatial accuracy, spurious-oscillation-free property, and capability of capturing the abrupt variation in thermodynamic properties across the transcritical contact surface. The method introduces artificial mass diffusivity to the continuity and momentum equations in a physically-consistent manner in order to capture the steep transcritical thermodynamic variations robustly while maintaining spurious-oscillation-free property in the velocity field. The pressure evolution equation is derived from the full compressible Navier–Stokes equations and solved instead of solving the total energy equation to achieve the spurious pressure oscillation free property with an arbitrary equation of state including the present look-up table method. Flow problems with and without physical diffusion are employed for the numerical tests to validate the robustness, accuracy, and consistency of the proposed approach. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, we present constructions of first-, second-, and third-order schemes for diffusion by the method introduced in Nishikawa (2007) [10]. In this method, numerical schemes for diffusion are constructed by advection schemes via an equivalent hyperbolic system. This paper demonstrates that the method enables straightforward constructions of diffusion schemes for finite-volume methods on unstructured grids. In particular, it is demonstrated that a robust first-order upwind scheme leads to a robust first-order diffusion scheme, and a high-order advection scheme leads to a high-order diffusion scheme. It is shown that first-, second-, and third-order schemes are capable of producing first-, second-, and third-order accurate solution gradients, respectively, on irregular grids. Accuracy, Fourier stability, and the energy stability of the developed schemes are discussed. A new hyperbolic diffusion system having virtually no source terms is also introduced to simplify the construction of the third-order scheme. Numerical results are presented for regular and irregular triangular grids to demonstrate not only the superior accuracy but also the accelerated steady convergence over a traditional method. [Copyright &y& Elsevier]

Abstract: We present a new approach to the simulation of viscous fingering instabilities in incompressible, miscible displacement flows in porous media. In the past, high resolution computational simulations of viscous fingering instabilities have always been performed using high-order finite difference or Fourier-spectral methods which do not posses the flexibility to compute very complex subsurface geometries. Our approach, instead, by means of a fully-coupled nonlinear implementation of the discontinuous Galerkin method, possesses a fundamental differentiating feature, in that it maintains high-order accuracy on fully unstructured meshes. In addition, the proposed method shows very low sensitivity to mesh orientation, in contrast with classical finite volume approximation used in porous media flow simulations. The robustness and accuracy of the method are demonstrated in a number of challenging computational problems. [Copyright &y& Elsevier]

Abstract: In this work we describe the development of a robust simulator of the streamer phenomenon. A very common problem of computational plasma physics is the stability of the numerical scheme when the quasi-neutral regime is approached. Actually the stability condition may imply very tight time step constraints. We prove, both numerically and analytically, the good behavior of our approach when the system approaches the quasi-neutral regime. In particular, the semi-implicit method we employ requires mild time step restrictions to guarantee the stability. Some other properties such as the positivity of the scheme are investigated and a detailed review of the numerical implementation is included. [Copyright &y& Elsevier]