On $L^2$-Stability Analysis of Time-Domain Acoustic Scattering Problems with Exact Nonreflecting Boundary Conditions

This paper is devoted to stability analysis of the acoustic wave equation exterior to a bounded scatterer, where the unbounded computational domain is truncated by the exact time-domain circular/spherical nonreflecting boundary condition (NRBC). Different from the usual energy method, we adopt an argument that leads to $L^2$-a priori estimates with minimum regularity requirement for the initial data and source term. This needs some delicate analysis of the involved NRBC. These results play an essential role in the error analysis of the interior solvers (e.g., finite-element/spectral- element/spectral methods) for the reduced scattering problems. We also apply the technique to analyze a time-domain waveguide problem.     

Radiation and Wall Boundary Conditions for Computational Aeroacoustics: A Review

A review of unsteady computational boundary conditions for computational aeroacoustics (CAA) problems is presented. This review is meant to serve as a general overview of previous work on solid wall, radiation and outflow boundary conditions that have been proposed and used in CAA calculations. Both the physical nature of the boundary condition problem as well as the numerical considerations affecting their implementation are discussed.     

Influence of Axisymmetric Assumptions on Large Eddy Simulations of a Confined Jet and a Swirl Flow

Axisymmetric geometries can be found in many practical flow applications. In the attempt to predict these flows numerically, RANS flow solvers can decrease the computational efforts dramatically by taking this axisymmetry into account and by computing only a pie-segment of the flow. However, the extension of the concept of axisymmetric flows to LES computations is not straightforward, since the boundary conditions on the axis of symmetry are altering the instantaneous flow field. In this study, the influence of the introduction of an axis of symmetry to LES computations is assessed by computations of a flow with and without swirl over an axisymmetric expansion. The LES computations are performed on a full three-dimensional and a 90° segment of the geometry. The results are compared and the influence of the axis put into relation with the gain in computational costs.     

Entrainment Boundary Conditions for Free Shear Flows

In this paper the performance of different free stream boundary conditions has been investigated. By performing two dimensional free jet calculations, a test case for which an analytical solution is available, it has been shown that boundary conditions in which the pressure is prescribed perform significantly better than boundary conditions in which the velocity is prescribed. Furthermore, it has been shown that pressure boundary conditions allow for much smaller computational domains than velocity (free-slip) boundary conditions.     

Generalized characteristic interface conditions for high-order multi-block computation

In practical fluid computation with structured grids around complex geometries, singular points with metric discontinuity can frequently be found. Generally, the grid singularities may cause numerical oscillations when some high-order finite difference scheme is applied. Recently, an excellent theory has been proposed which solves the above singular problem by block decomposition along the singular surface and by imposition of the characteristic interface conditions (CIC) on the block interface. However, the original theory has constraints on the mathematical treatment of the block interface, and therefore prevents numerical flexibility from a practical point of view. In this article, in order to extend the functions of the original CIC, we propose the generalized characteristic interface conditions (GCIC). Proper numerical test analysis is conducted to validate the performance of the GCIC, and as a practical application, multi-block computation is performed with the GCIC applied to complex geometry.     

强一致收敛下的初值敏感性与等度连续性

首先,举例指出了《Nonlinear Anglgsis》文中定理3.2的条件下并不能使函数序列的初值敏感性遗传至极限函数,并证明了若函数序列的敏感常数的上极限为某一正数,则在强一致收敛下,函数序列的极限函数也具有初值敏感性.其次,证明了在强一致收敛下,序列系统的等度连续性和一致几乎周期性能被极限系统所继承.     

贮存条件对纤维素/大豆蛋白膜结构与性能的影响

通过流延成膜法制备一系列纤维素/大豆分离蛋白复合膜材料,分别以10%醋酸溶液、5%醋酸和5%乙醇的混合溶液、75%乙醇溶液等3种水溶液及蒸馏水为贮存体系,通过力学性能测试、扫描电镜观察、紫外测试等方法研究了不同贮存条件对纤维素/大豆分离蛋白复合膜结构与性能的影响.结果表明,10%醋酸水溶液贮存纤维素/大豆分离蛋白复合膜30d能够较好地维持其力学性能和微观结构,可作为该复合膜较为适合的贮存体系.     

不等式约束优化问题的低阶精确罚函数的光滑化算法

对不等式约束优化问题提出了一个低阶精确罚函数的光滑化算法. 首先给出了光滑罚问题、非光滑罚问题及原问题的目标函数值之间的误差估计,进而在弱的假
设之下证明了光滑罚问题的全局最优解是原问题的近似全局最优解. 最后给出了一个基于光滑罚函数的求解原问题的算法,证明了算法的收敛性,并给出数值算例说明算法的可行性.