Explore the vast range of titles available.

A modified fifth order Z-type (nonlinear) weights, which consist of a linear term and a nonlinear term, in the weighted essentially non-oscillatory (WENO) polynomial reconstruction procedure for the WENO-Z finite difference scheme in solving hyperbolic conservation laws is proposed. The nonlinear term is modified by a modifier function that is based on the linear combination of the local smoothness indicators. The WENO scheme with the modified Z-type weights (WENO-D) scheme and its improved version (WENO-A) scheme are proposed. They are analyzed for the maximum error and the order of accuracy for approximating the derivative of a smooth function with high order critical points, where the first few consecutive derivatives vanish. The analysis and numerical experiments show that, they achieve the optimal (fifth) order of accuracy regardless of the order of critical point with an arbitrary small sensitivity parameter , aka, satisfy the Cp-property. Furthermore, with an optimal variable sensitivity parameter, they have a quicker convergence and a significant error reduction over the WENO-Z scheme. They also achieve an improved balance between the linear term, which resolves a smooth function with the fifth order upwind central scheme, and the modified nonlinear term, which detects potential high gradients and discontinuities in a non-smooth function. The performance of the WENO schemes, in terms of resolution, essentially non-oscillatory shock capturing and efficiency, are compared by solving several one- and two-dimensional benchmark shocked flows. The results show that they perform overall as well as, if not slightly better than, the WENO-Z scheme.

The properties of modern WENO schemes are examined as applied to large eddy simulation (LES). The WENO-ZM5 scheme with modified smoothness indicators (“upwind”) and the WEN-O‑SYMBOO6 scheme on a symmetric stencil (“symmetric WENO scheme”) are chosen. The schemes are compared on one-dimensional test problems (advection, Hopf, and Burgers equations) with both smooth and discontinuous solutions. The decay of isotropic turbulence is modeled within LES and the results are discussed. The solutions produced by the new schemes are compared with those based on the central difference scheme, the classical WENO5 scheme, and a hybrid scheme. The level of dissipation of the schemes is compared by analyzing their energy spectra. A similar comparison is made between the LES computations of the temporal evolution of a mixing layer, where the profiles of mean velocity and Reynolds stresses are considered in addition to the energy spectrum.

Fast sweeping methods are a class of efficient iterative methods developed in the literature to solve steady-state solutions of hyperbolic partial differential equations (PDEs). In Zhang et al. (J Sci Comput 29:25–56, 2006) and Xiong et al. (J Sci Comput 45:514–536, 2010), high order accuracy fast sweeping schemes based on classical weighted essentially non-oscillatory (WENO) local solvers were developed for solving static Hamilton–Jacobi equations. However, since high order classical WENO methods (e.g., fifth order and above) often suffer from difficulties in their convergence to steady-state solutions, iteration residues of high order fast sweeping schemes with these local solvers may hang at a level far above round-off errors even after many iterations. This issue makes it difficult to determine the convergence criterion for the high order fast sweeping methods and challenging to apply the methods to complex problems. Motivated by the recent work on absolutely convergent fast sweeping method for steady-state solutions of hyperbolic conservation laws in Li et al. (J Comput Phys 443:110516, 2021), in this paper we develop high order fast sweeping methods with multi-resolution WENO local solvers for solving Eikonal equations, an important class of static Hamilton–Jacobi equations. Based on such kind of multi-resolution WENO local solvers with unequal-sized sub-stencils, iteration residues of the designed high order fast sweeping methods can settle down to round-off errors and achieve the absolute convergence. In order to obtain high order accuracy for problems with singular source-point, we apply the factored Eikonal approach developed in the literature and solve the resulting factored Eikonal equations by the new high order WENO fast sweeping methods. Extensive numerical experiments are performed to show the accuracy, computational efficiency, and advantages of the new high order fast sweeping schemes for solving static Hamilton–Jacobi equations.

This paper develops a framework for finite volume radial basis function (RBF) approximation of a function u on a stencil of mesh cells in multiple dimensions. The theory of existence of the approximation is given. In one dimension, as the cell diameters tend to zero, numerical evidence is given to show that the RBF approximation converges to u to the same order as a polynomial approximation when the RBF is infinitely differentiable. Specific multiquadric RBFs on stencils of 2 and 3 mesh cells are proven to have this convergence property. A two-level RBF based weighted essentially non-oscillatory (WENO) reconstruction with adaptive order (RBF-WENO-AO) is developed. WENO-AO reconstructions use arbitrary linear weights, and so they can be developed easily for RBF approximations, even on nonuniform meshes in multiple dimensions. Following the classical polynomial based WENO, a smoothness indicator is defined for the reconstruction. For one dimension, the convergence theory is given regarding the cases when u is smooth and when u has a discontinuity. These reconstructions are applied to develop finite volume schemes for hyperbolic conservation laws on nonuniform meshes over multiple space dimensions. The focus is on reconstructions based on multiquadric RBFs that are third order when the solution is smooth and second order otherwise, i.e., RBF-WENO-AO(3,2). Numerical examples show that the scheme maintains proper accuracy and achieves the essentially non-oscillatory property when solving hyperbolic conservation laws.

A new type of finite difference weighted essentially non-oscillatory (WENO) schemes for hyperbolic conservation laws was designed in Zhu and Qiu (J Comput Phys 318:110–121, 2016 ), in this continuing paper, we extend such methods to finite volume version in multi-dimensions. There are two major advantages of the new WENO schemes superior to the classical finite volume WENO schemes (Shu, in: Quarteroni (ed) Advanced Numerical Approximation of Nonlinear Hyperbolic Equations, Lecture Notes in Mathematics, CIME subseries, Springer, Berlin, 1998 ), the first is the associated linear weights can be any positive numbers with only requirement that their summation equals one, and the second is their simplicity and easy extension to multi-dimensions in engineering applications. The new WENO reconstruction is a convex combination of a fourth degree polynomial with two linear polynomials defined on unequal size spatial stencils in a traditional WENO fashion. These new fifth order WENO schemes use the same number of cell average information as the classical fifth order WENO schemes Shu ( 1998 ), could get less absolute numerical errors than the classical same order WENO schemes, and compress nonphysical oscillations nearby strong shocks or contact discontinuities. Some benchmark tests are performed to illustrate the capability of these schemes.

First this paper analyzes the reason for the accuracy losing of the third-order weighted essentially non-oscillatory (WENO) scheme. It is shown that one reason is that the local smoothness indicators of the third-order WENO scheme cannot correctly treat the smooth three-point stencil containing a non-nodal critical point, here, ‘non-nodal’ means the critical point is not a grid point. And then a discontinuity-detecting method for a four-point stencil is proposed and applied to develop the high order accurate hybrid-WENO scheme by combining the third-order WENO scheme and a third-order upstream scheme. This four-point stencil is actually the stencil used for constructing the third-order WENO scheme (positive and negative numerical fluxes), hence the resulting hybrid-WENO scheme proposed by this paper does not introduce new grid point. Numerical examples show that the detecting method and the hybrid scheme are robust for problems with shocks, and the hybrid scheme obtains real third-order convergence rate for smooth solutions containing critical points.

In the context of preserving stationary states, e.g. lake at rest and moving equilibria, a new formulation of the shallow water system, called flux globalization has been introduced by Cheng et al. (J Sci Comput 80(1):538–554, 2019). This approach consists in including the integral of the source term in the global flux and reconstructing the new global flux rather than the conservative variables. The resulting scheme is able to preserve a large family of smooth and discontinuous steady state moving equilibria. In this work, we focus on an arbitrary high order WENO finite volume (FV) generalization of the global flux approach. The most delicate aspect of the algorithm is the appropriate definition of the source flux (integral of the source term) and the quadrature strategy used to match it with the WENO reconstruction of the hyperbolic flux. When this construction is correctly done, one can show that the resulting WENO FV scheme admits exact discrete steady states characterized by constant global fluxes. We also show that, by an appropriate quadrature strategy for the source, we can embed exactly some particular steady states, e.g. the lake at rest for the shallow water equations. It can be shown that an exact approximation of global fluxes leads to a scheme with better convergence properties and improved solutions. The novel method has been tested and validated on classical cases: subcritical, supercritical and transcritical flows.

A proppant transport simulator coupled with multi-planar 3D (multi-PL3D) fracture propagation has been developed to examine the proppant distribution among multiple hydraulic fractures during multi-cluster fracturing in a horizontal well. The multi-PL3D fracture model considers wellbore friction, multi-fracture stress interaction, fluid leak-off, and multi-scale propagation regimes. The proppant transport is described by the two-phase (slurry/proppant) flow equations that consider proppant settling, jamming and flow regime transition. A high-resolution weighted essentially non-oscillatory (WENO) finite difference (FD) scheme is adopted to solve the nonlinear proppant transport equations. An efficient time-stepping scheme is developed to solve the solid/fluid coupling equations and moving boundaries for the multi-PL3D model. The proppant transport model and multi-PL3D model are both validated against previously published results. Using the model, we examine the proppant distributions under different injection schedules, proppant sizes, proppant density, and fluid viscosity. Results show that proppant distribution among multiple fractures is different as the flow rate and fracture width distribution vary due to multi-fracture stress interaction. The proppant in the middle cluster settles remarkably as the flow rate is lowest among the multiple clusters. The proppant is usually jammed at the pinch point, where the fracture width reduces sharply. Proppant adding schedule has a significant effect on the proppant distribution. A constant-concentration results in a proppant stack at the fracture front. In contrast, an increasing concentration favors the prop of the near-wellbore fracture. The proppant distribution area ratio (defined as the proppant distribution area divided by the fracture area) is only 20% for 20/40 mesh proppant, while the ratio is 45% for 100 mesh proppant. Slick water can increase the fracture area but not favor promoting the proppant distribution area ratio. The results can be helpful for proppant design for multi-cluster fracturing in a horizontal well.

Weighted essentially non-oscillatory (WENO) schemes are a class of high-order shock capturing schemes which have been designed and applied to solve many fluid dynamics problems to study the detailed flow structures and their evolutions. However, like many other high-order shock capturing schemes, WENO schemes also suffer from the problem that it can not easily converge to a steady state solution if there is a strong shock wave. This is a long-standing difficulty for high-order shock capturing schemes. In recent years, this non-convergence problem has been studied extensively for WENO schemes. Numerical tests show that the key reason of the non-convergence to steady state is the slight post shock oscillations, which are at the small local truncation error level but prevent the residue to settle down to machine zero. Several strategies have been proposed to reduce these slight post shock oscillations, including the design of new smoothness indicators for the fifth-order WENO scheme, the development of a high-order weighted interpolation in the procedure of the local characteristic projection for WENO schemes of higher order of accuracy, and the design of a new type of WENO schemes. With these strategies, the convergence to steady states is improved significantly. Moreover, the strategies are applicable to other types of weighted schemes. In this paper, we give a brief review on the topic of convergence to steady state solutions for WENO schemes applied to Euler equations.

The dissipation and dispersion (spectral) properties of the nonlinear fifth order classical weighted essentially non-oscillatory finite difference scheme (WENO-JS5) and its improved version (WENO-Z5) using the approximate dispersion relation (ADR) (Pirozzoli in J Comput Phys 219:489–497, 2006 ) and the nonlinear spectral analysis (NSA) (Fauconnier et al. in J Comput Phys 228(6):1830–1861, 2009 ) are studied. Unlike the previous studies, the influences of the sensitivity parameter in the definition of the WENO nonlinear weights are also included for completeness. The fifth order upwinded central linear scheme (UW5) serves as the reference and benchmark for the purpose of comparison. The spectral properties of the WENO differentiation operator is well predicted theoretically by the ADR and validated numerically by the simulations of the WENO schemes in solving the scalar linear advection equation. In a long time simulation with an initial broadband wave, the WENO schemes generate spurious high modes with amplitude and spread of wavenumbers depend on the value of the sensitivity parameter. The NSA is applied to investigate the statistical nonlinear behavior, due to the nonlinear stencils adaptation of the WENO schemes, with a large set of initial conditions consisting of synthetic scalar fields with a prescribed energy spectrum and random phases. The statistics indicate that there is a small probability of an existence of a mild anti-dissipation in the low wavenumber range regardless of the size of the sensitivity parameter. Numerical examples demonstrate that the WENO-Z5 scheme is not only less dissipative and dispersive but also less sensitive to random phases than the WENO-JS5 scheme. Furthermore, a sensitivity parameter adaptive technique, in which its value depends on the local smoothness of the solution at a given spatial location and time, is introduced for solving a linear advection problem with a discontinuous initial condition. The preliminary result shows that the solution computed by the sensitivity parameter adaptive WENO-Z5 scheme agrees well with those computed by the WENO-Z5 scheme and the UW5 scheme in regions containing discontinuities and smooth solutions, respectively.