一维守恒的Lagrangian ADER格式

基于欧拉框架下ADER格式,构造一维守恒只有一个时间步的、高精度中心型拉格朗日ADER(LADER)格式.构造r阶LADER格式包括:从欧拉方程出发推导拉格朗日框架下积分形式的方程、采用WENO方法高精度重构节点处守恒量和从1阶到r-1阶的空间导数、求拉氏框架下这些变量的Godunov值,并计算1阶到r-1阶的时间全导数,最后高精度离散积分形式的流通量函数.对光滑流场的模拟表明,LADER格式达到设计的精度;对含强间断的流场模拟表明,数值解在间断附近基本无振荡.     

Fully HOC Scheme for Mixed Convection Flow in a Lid-Driven Cavity Filled with a Nanofluid

A fully higher-order compact (HOC) finite difference scheme on the 9-point two-dimensional (2D) stencil is formulated for solving the steady-state laminar mixed convection flow in a lid-driven inclined square enclosure filled with water-$Al_2O_3$ nanofluid. Two cases are considered depending on the direction of temperature gradient imposed (Case I, top and bottom; Case II, left and right). The developed equations are given in terms of the stream function-vorticity formulation and are non-dimensionalized and then solved numerically by a fourth-order accurate compact finite difference method. Unlike other compact solution procedure in literature for this physical configuration, the present method is fully compact and fully higher-order accurate. The fluid flow, heat transfer and heat transport characteristics were illustrated by streamlines, isotherms and averaged Nusselt number. Comparisons with previously published work are performed and found to be in excellent agreement. A parametric study is conducted and a set of graphical results is presented and discussed to elucidate that significant heat transfer enhancement can be obtained due to the presence of nanoparticles and that this is accentuated by inclination of the enclosure at moderate and large Richardson numbers.     

空间相机CCD像面拼接重叠像元数分析与计算

针对一类具有分段仿射形式的混杂系统模型的控制方法问题,提出了一种高效显式模型预测控制算法。该算法通过将最优控制问题转化为多参数规划问题,离线求得具有分段仿射形式的显式控制器;在线过程,应用一种新的搜索算法,它能够快速准确的对系统状态点进行定位,确定其所属的控制器分区,再根据该分区所对应的子控制率,进行简单的线性运算,即可得到系统的输入。该控制方法避免了反复的在线优化计算,大大减少了计算量,并且,在线计算的速度更快,控制的实时性更好。将该算法应用到具有典型混杂特性的两容水箱系统中,仿真结果表明：水箱的液位从初始液位能够快速平稳的达到期望的液位,且与其它的控制算法相比较,该算法更加高效。     

TIME-REVERSAL SYMMETRY AND TIME-DEPENDENT PHYSICS

In this letter I study the concept of time-reversal invariance in both classical and quantum physics in the absence of time-translation invariance (explicit time dependence/external time-dependent fields). Accordingly I generalize the concept of time-reversed process when the time-origin of the process has physical significance. The cases of classical physics, standard quantum mechanics and time-neutral quantum mechanics with and without explicit time dependence are discussed.     

分辨率优化的混合WENO格式

为提高有限差分格式的分辨率,利用傅里叶分析对WENO格式进行色散及耗散优化,并给出优化的线性权重.用优化后的WENO格式与保单调格式（MP）进行加权混合,得到新的加权混合WENO格式（H-WENO）.通过一维激波管问题、Shu-Osher问题及二维双Mach反射问题及R-T不稳定性问题对格式进行数值测试.结果显示,新格式具有强健的激波捕捉能力和对小尺度波结构的高分辨率,与原WENO格式相比改进明显.     

New formulas for normalizing photon-added（-subtracted） two-mode squeezed thermal states

For the first time,we derive the compact forms of normalization factors for photon-added(-subtracted) two-mode squeezed thermal states by using the P-representation and the integration within an ordered product of operators(IWOP) technique.It is found that these two factors are related to the Jacobi polynomials.In addition,some new relationships for Jacobi polynomials are presented.     

Two-color laser wavelength effect on intense terahertz generation in air

Near-IR femtosecond lasers have been proposed to produce high-field terahertz radiation in the air via the laser-plasma interaction, but the physical mechanism still needs to be further explored. In this work, we theoretically investigate the effect of the two-color laser wavelength on the terahertz generation in the air based on a transient photocurrent model.We show that the long wavelength laser excitation can greatly enhance the terahertz amplitude for a given total laser intensity. Furthermore, we utilize a local current model to illustrate the enhancement mechanism. Our analysis shows that the terahertz amplitude is determined by the superposition of contributions from individual ionization events, and for the long wavelength laser excitation, the electron production concentrates in a few ionization events and acquires the larger drift velocities, which results in the stronger terahertz radiation generation. These results will be very helpful for understanding the terahertz generation process and optimizing the terahertz output.     

Symmetry Reduction of the (2+1)-Dimensional Modified Dispersive Water-Wave System

Using the standard truncated Painlevé expansion, the residual symmetry of the (2+1)-dimensional modified dispersive water-wave system is localized in the properly prolonged system with the Lie point symmetry vector. Some different transformation invariances are derived by utilizing the obtained symmetries. The symmetries of the system are also derived through the Clarkson-Kruskal direct method, and several types of explicit reduction solutions relate to the trigonometric or the hyperbolic functions are obtained. Finally, some special solitons are depicted from one of the solutions.     

轴对称射流气动声场的数值模拟

构造了Kirchhoff积分和CFD计算相结合的算法,采用高精度差分格式对不同喷口马赫数的轴对称射流进行数值模拟,并以此作为近场声源,运用Kirchhoff积分方法研究远场气动噪声.数值结果表明,远场噪声具有方向性,噪声声压在离开对称轴20°处达到最大值.随着传播距离增大,噪声方向性逐渐减弱.     

TDI线扫相机光学通道建模与仿真

卫星相机建模与仿真研究不论在民用还是军事领域都具有重要的理论研究和实际应用价值。根据TDI线扫相机的工作原理建立了一个TDI线扫相机光学通道的数学模型,该模型包括从景物到TDI CCD离散图像的几何投影关系和能量传递关系两部分,对于确定的成像系统,给定景物参数和光学系统参数,就可以确定对应图像传感器上的离散图像。为了验证模型,对一幅卫星图像进行了计算机仿真。建立的模型可以用来研究当成像条件改变时对图像的影响,进一步还可以用来发展虚拟卫星相机系统。