A New Stable Splitting for the Isentropic Euler Equations

In this work, we propose a new way of splitting the flux function of the isentropic compressible Euler equations at low Mach number into stiff and non-stiff parts. Following the IMEX methodology, the latter ones are treated explicitly, while the first ones are treated implicitly. The splitting is based on the incompressible limit solution, which we call reference solution. An analysis concerning the asymptotic consistency and numerical results demonstrate the advantages of this splitting.     

Nonlinear Upwind-Biased Free-Stream-Preserving Schemes for Compressible Euler Equations

The focus of the present research is on the analysis of local energy stability of high-order (including split-form) summation-by-parts methods, with e.g. two-point entropy-conserving fluxes, approximating non-linear conservation laws. Our main finding is that local energy stability, i.e., the numerical growth rate does not exceed the growth rate of the continuous problem, is not guaranteed even when the scheme is non-linearly stable and that this may have adverse implications for simulation results. We show that entropy-conserving two-point fluxes are inherently locally energy unstable, as they can be dissipative or anti-dissipative. Unfortunately, these fluxes are at the core of many commonly used high-order entropy-stable extensions, including split-form summation-by-parts discontinuous Galerkin spectral element methods (or spectral collocation methods). For the non-linear Burgers equation, we further demonstrate numerically that such schemes cause exponential growth of errors during the simulation. Furthermore, we encounter a similar abnormal behaviour for the compressible Euler equations, for a smooth exact solution of a density wave. Finally, for the same case, we demonstrate numerically that other commonly known split-forms, such as the Kennedy and Gruber splitting, are also locally energy unstable.

     

Positivity-Preserving Analysis of Explicit and Implicit Lax–Friedrichs Schemes for Compressible Euler Equations

This paper presents the positivity analysis of the explicit and implicit Lax–Friedrichs (LxF) schemes for the compressible Euler equations. The theoretical proof is closely based on the decomposition of fluid variables and their corresponding fluxes into the pseudo-particles representation. For both explicit and implicit 1st-order LxF schemes, from any initial realizable state the density and the internal energy could keep non-negative values under the CFL condition with Courant number 1.     

High Order Positivity- and Bound-Preserving Hybrid Compact-WENO Finite Difference Scheme for the Compressible Euler Equations

Based on the same hybridization framework of Don et al. (SIAM J Sci Comput 38:A691–A711 2016), an improved hybrid scheme employing the nonlinear 5th-order characteristic-wise WENO-Z5 finite difference scheme for capturing high gradients and discontinuities in an essentially non-oscillatory manner and the linear 5th-order conservative compact upwind (CUW5) scheme for resolving the fine scale structures in the smooth regions of the solution in an efficient and accurate manner is developed. By replacing the 6th-order non-dissipative compact central scheme (CCD6) with the CUW5 scheme, which has a build-in dissipation, there is no need to employ an extra high order smoothing procedure to mitigate any numerical oscillations that might appear in an hybrid scheme. The high order multi-resolution algorithm of Harten is employed to detect the smoothness of the solution. To handle the problems with extreme conditions, such as high pressure and density ratios and near vacuum states, and detonation diffraction problems, we design a positivity- and bound-preserving limiter by extending the one developed in Hu et al. (J Comput Phys 242, 2013) for solving the high Mach number jet flows, detonation diffraction problems and detonation passing multiple obstacles problems. Extensive one- and two-dimensional shocked flow problems demonstrate that the new hybrid scheme is less dispersive and less dissipative, and allows a potential speedup up to a factor of more than one and half times faster than the WENO-Z5 scheme.     

WENO Scheme with Subcell Resolution for Computing Nonconservative Euler Equations with Applications to One-Dimensional Compressible Two-Medium Flows

High order path-conservative schemes have been developed for solving nonconservative hyperbolic systems in (Parés, SIAM J.?Numer. Anal. 44:300?C321, 2006; Castro et al., Math. Comput. 75:1103?C1134, 2006; J.?Sci. Comput. 39:67?C114, 2009). Recently, it has been observed in (Abgrall and Karni, J.?Comput. Phys. 229:2759?C2763, 2010) that this approach may have some computational issues and shortcomings. In this paper, a modification to the high order path-conservative scheme in (Castro et al., Math. Comput. 75:1103?C1134, 2006) is proposed to improve its computational performance and to overcome some of the shortcomings. This modification is based on the high order finite volume WENO scheme with subcell resolution and it uses an exact Riemann solver to catch the right paths at the discontinuities. An application to one-dimensional compressible two-medium flows of nonconservative or primitive Euler equations is carried out to show the effectiveness of this new approach.     

Stable Interface Conditions for Discontinuous Galerkin Approximations of Navier-Stokes Equations

A study of boundary and interface conditions for Discontinuous Galerkin approximations of fluid flow equations is undertaken in this paper. While the interface flux for the inviscid case is usually computed by approximate Riemann solvers, most discretizations of the Navier-Stokes equations use an average of the viscous fluxes from neighboring elements. The paper presents a methodology for constructing a set of stable boundary/interface conditions that can be thought of as “viscous” Riemann solvers and are compatible with the inviscid limit.     

Well-Balanced Nodal Discontinuous Galerkin Method for Euler Equations with Gravity

We present a well-balanced nodal discontinuous Galerkin (DG) scheme for compressible Euler equations with gravity. The DG scheme makes use of discontinuous Lagrange basis functions supported at Gauss–Lobatto–Legendre (GLL) nodes together with GLL quadrature using the same nodes. The well-balanced property is achieved by a specific form of source term discretization that depends on the nature of the hydrostatic solution, together with the GLL nodes for quadrature of the source term. The scheme is able to preserve isothermal and polytropic stationary solutions upto machine precision on any mesh composed of quadrilateral cells and for any gravitational potential. It is applied on several examples to demonstrate its well-balanced property and the improved resolution of small perturbations around the stationary solution.     

A Modified Implicit Euler Algorithm for Solving Vehicle Dynamic Equations

Vehicle modelling is usually done by Multibody Systems. Very often the overall model consists of several subsystems, like the vehicle framework, the drive train and the steering system. Due to the tire forces and torques and due to small but essential compliances in the axle/wheel suspension systems the resulting differential equations are stiff. To improve the model quality dynamic models for some components like damper, and rubber elements are used. Again these models contain stiff parts. If the implicit Euler Algorithm is adopted to the specific problems in vehicle dynamics a very effective numerical solution can be achieved. Applied to vehicle dynamic equations the algorithm produces good and stable results even for integration step sizes in the magnitude of milliseconds. As it gets along with a minimum number of operations a very good run time performance is guaranteed. Hence, even with very sophisticated vehicle models real time applications are possible. Due to its robustness the presented algorithm is very well suited for co-simulations. The modifications in the implicit Euler Algorithm also make it possible to use a simple model for describing the dry friction in the damper and in the brake disks. A quarter car vehicle model with a longitudinal and a vertical compliancy in the wheel suspension and a dynamic damper model including dry friction is used to explain the algorithm and to show its benefits.     

Static Two-Grid Mixed Finite-Element Approximations to the Navier-Stokes Equations

A two-grid scheme based on mixed finite-element approximations to the incompressible Navier-Stokes equations is introduced and analyzed. In the first level the standard mixed finite-element approximation over a coarse mesh is computed. In the second level the approximation is post processed by solving a discrete Oseen-type problem on a finer mesh. The two-level method is optimal in the sense that, when a suitable value of the coarse mesh diameter is chosen, it has the rate of convergence of the standard mixed finite-element method over the fine mesh. Alternatively, it can be seen as a post processed method in which the rate of convergence is increased by one unit with respect to the coarse mesh. The analysis takes into account the loss of regularity at initial time of the solution of the Navier-Stokes equations in absence of nonlocal compatibility conditions. Some numerical experiments are shown.     

Numerical Methods for Euler Equations with Self-similar and Quasi Self-similar Solutions

Some problems of Euler equations have self-similar solutions which can be solved by more accurate method. The current paper proposes two new numerical methods for Euler equations with self-similar and quasi self-similar solutions respectively, which can use existing difference schemes for conservation laws and do not need to redesign specified schemes. Numerical experiments are implemented on one dimensional shock tube problems, two dimensional Riemann problems, shock reflection from a solid wedge, and shock refraction at a gaseous interface. For self-similar equations, one-dimensional results are almost equal to the exact solutions, and two-dimensional results also exhibit considerable high resolution. For quasi self-similar equations, the method can solve solutions that are not but close to self-similar, i.e. quasi self-similar, and this method can also achieve very high resolution when computing time is long enough. Numerical simulations to self-similar and quasi self-similar Euler equations have important implications on the study of self-similar problems, development of high resolution schemes, even the research for exact solutions of Euler equations.     

Comparison of Some Entropy Conservative Numerical Fluxes for the Euler Equations

Entropy conservation and stability of numerical methods in gas dynamics have received much interest. Entropy conservative numerical fluxes can be used as ingredients in two kinds of schemes: firstly, as building blocks in the subcell flux differencing form of Fisher and Carpenter (Technical Report NASA/TM-2013-217971, NASA, 2013; J Comput Phys 252:518–557, 2013) and secondly (enhanced by dissipation) as numerical surface fluxes in finite volume like schemes. The purpose of this article is threefold. Firstly, the flux differencing theory is extended, guaranteeing high-order for general symmetric and consistent numerical fluxes and investigating entropy stability in a generalised framework of summation-by-parts operators applicable to multiple dimensions and simplex elements. Secondly, a general procedure to construct affordable entropy conservative fluxes is described explicitly and used to derive several new fluxes. Finally, robustness properties of entropy stable numerical fluxes are investigated and positivity preservation is proven for several entropy conservative fluxes enhanced with local Lax–Friedrichs type dissipation operators. All these theoretical investigations are supplemented with numerical experiments.     

A Conservative Isothermal Wall Boundary Condition for the Compressible Navier–Stokes Equations

We present a conservative isothermal wall boundary condition treatment for the compressible Navier-Stokes equations. The treatment is based on a manipulation of the Osher solver to predict the pressure and density at the wall, while specifying a zero boundary flux and a fixed temperature. With other solvers, a non-zero mass flux occurs through a wall boundary, which is significant at low resolutions in closed geometries. A simulation of a lid driven cavity flow with a multidomain spectral method illustrates the effect of the new boundary condition treatment.     

A Priori Convergence Theory for Reduced-Basis Approximations of Single-Parameter Elliptic Partial Differential Equations

We consider Lagrangian reduced-basis methods for single-parameter symmetric coercive elliptic partial differential equations. We show that, for a logarithmic-(quasi-)uniform distribution of sample points, the reduced–basis approximation converges exponentially to the exact solution uniformly in parameter space. Furthermore, the convergence rate depends only weakly on the continuity-coercivity ratio of the operator: thus very low-dimensional approximations yield accurate solutions even for very wide parametric ranges. Numerical tests (reported elsewhere) corroborate the theoretical predictions.     

Generalized Hermite Approximations and Spectral Method for Partial Differential Equations in Multiple Dimensions

In this paper, we consider a spectral method based on generalized Hermite functions in multiple dimensions. We first introduce three normed spaces and prove their equivalence, which enables us to develop and to analyze generalized Hermite approximations efficiently. We then establish some basic results on generalized Hermite orthogonal approximations in multiple dimensions, which play important roles in the relevant spectral methods. As examples, we consider an elliptic equation with a harmonic potential and a class of nonlinear wave equations. The spectral schemes are proposed, and the convergence is proved. Numerical results demonstrate the spectral accuracy of this approach.     

A Characteristics Based Genuinely Multidimensional Discrete Kinetic Scheme for the Euler Equations

In this paper we present the results of a kinetic relaxation scheme for an arbitrary hyperbolic system of conservation laws in two space dimensions. We propose a new discrete velocity Boltzmann equation, which is an improvement over the previous models in terms of the isotropic coverage of the multidimensional domain by the foot of the characteristic. The discrete kinetic equation is solved by a splitting method consisting of a convection step and a collision step. The convection step involves only the solution of linear transport equations whereas the collision step instantaneously relaxes the distribution function to a local Maxwellian. An anti-diffusive Chapman-Enskog distribution is used to derive a second order accurate method. Finally some numerical results are presented which confirm the robustness and correct multidimensional behaviour of the proposed scheme.