Coherent structures and wavepackets in subsonic transitional turbulent jets

A large eddy simulation (LES) is performed for two subsonic jets with a Reynolds number of , which have different core temperatures, i.e., the cold and hot jet. The far-field overall sound pressure levels (OASPL) and noise spectra are well validated against previous experimental results. It is found that the OASPL is raised by heating at shallow angles. The most energetic coherent structures are extracted with specified frequencies using the filter based on the frequency domain variant of the snapshot method of proper orthogonal decomposition (POD). The modes have high coherence of near-field pressure for both jets, while the coherence of modes is enhanced greatly by heating. Based on the coherent structures, spatial wavepackets are educed and the characteristics of growth, saturation and decay are analyzed and compared between the two jets in detail. The results show that heating would enhance the linear growth rate for high frequency components, and nonlinear growth rates for low frequency components in general, which are responsible for higher OASPL in the hot jet. The far-field sound generated by wavepackets is computed using the Kirchhoff extrapolation, which matches well with that of LES at shallow angles. This indicates that the wavepackets associated with coherent structures are dominant sound sources in forced transitional turbulent jets. Additionally, the present POD method is proven to be a robust tool to extract the salient features of the wavepackets in turbulent flows.

On Minimum Wavepackets for Generalized Conjugate Space

The forms of minimum wavepackets (MWPs) corresponding to the geueralized momentum space and coordinates spaca are derived by using the generalized Hermitian expression of the momentum operator in curvilinear coordinates. The several MWPs are discussed according to different upper and lower limits of the usual coordinate componets, and the relevant conclusions are drawn in this paper.

Laser Controlling Wavepacket Trains of a Paul Trapped Ion

我们学习了量，单身的保罗的古典运动套住离子与一个时间周期的激光领域交往。由使用测试功能方法，我们构造量动力学的 nexact 答案与是的期望轨道描述概括挤压的协调状态相应的古典的。准确概率密度的时空进化显示出一些波浪包火车。它是由调整激光紧张和频率，我们能控制波浪包火车的中心运动的表明的经分解。Wealso 讨论象波浪包，和稳定性的回声损失的精力，宽度和高度的期望价值那样的另外的物理性质。

A pseudoclassical theory for the wavepacket dynamics of the kicked rotor model

In this study, we propose a generalized pseudoclassical theory for the kicked rotor model in an attempt to discern the footprints of the classical dynamics in the deep quantum regime. Compared with the previous pseudoclassical theory that applies only in the neighborhoods of the lowest two quantum resonances, the proposed theory is applicable in the neighborhoods of all quantum resonances in principle by considering the quantum effect of the free rotation at a quantum resonance. In particular, it is confirmed by simulations that the quantum wavepacket dynamics can be successfully forecasted based on the generalized pseudoclassical dynamics, offering an intriguing example where it is feasible to bridge the dynamics in the deep quantum regime to the classical dynamics. The application of the generalized pseudoclassical theory to the PT-symmetric kicked rotor is also discussed.

两态模型中的振动态布居转移

以氢化锂极性双原子分子为例,运用含时量子波包法数值求解两态模型,从理论方面研究了基电子态X^(1)Σ^(+)通过跃迁偶极矩到激发电子态A^(1)Σ^(+),两态模型中的振动态布居转移过程.基电子态上以及激发电子态上各自的振动态通过激光脉冲与永久偶极矩作用完成布居转移过程也考虑在内.在紫外光和红外光区域内,使用短的线性偏振激光脉冲实现了高效的布居转移.经过选择激光脉冲的峰值强度、中心频率以及持续时间等参数,在激光脉冲关闭后,最终的布居转移几率都超过了90%.

The improved Hagedorn wavepacket method for semiclassical Schrödinger equation

The Hagedorn wavepacket method is an important numerical method for solving the semiclassical time-dependent Schrödinger equation.In this paper,a new semi-discretization in space is obtained by wavepacket operator.In a sense,such semi-discretization is equivalent to the Hagedorn wavepacket method,but this discretization is more intuitive to show the advantages of wavepacket methods.Moreover,we apply the multi-time-step method and the Magnus-expansion to obtain the improved algorithms in time-stepping computation.The improved algorithms are of the Gauss–Hermite spec-tral accuracy to approximate the analytical solution of the semiclassical Schrödinger equation.And for the given accuracy,the larger time stepsize can be used for the higher oscillation in the semiclassical Schrödinger equation.The superiority is shown by the error estimation and numerical experiments.

Characteristics of nonlinear evolution of wavepackets in boundary layers

The nonlinear evolution of a finite-amplitude disturbance in a 3-D supersonic boundary layer over a cone was investigated recently by Liu et al.using direct numerical simulation(DNS).It was found that certain small-scale 3-D disturbances amplified rapidly.These disturbances exhibit the characteristics of second modes,and the most amplified components have a welldefined spanwise wavelength,indicating a clear selectivity of the amplification.In the case of a cone,the three-dimensionality of the base flow and the disturbances themselves may be responsible for the rapid amplification.In order to ascertain which of these two effects are essential,in this study we carried out DNS of the nonlinear evolution of a spanwise localized disturbance(wavepacket) in a flat-plate boundary layer.A similar amplification of small-scale disturbances was observed,suggesting that the direct reason for the rapid amplification is the three-dimensionality of the disturbances rather than the three-dimensional nature of the base flow,even though the latter does alter the spanwise distribution of the disturbance.The rapid growth of 3-D waves may be attributed to the secondary instability mechanism.Further simulations were performed for a wavepacket of first modes in a supersonic boundary layer and of Tollmien-Schlichting(T-S) waves in an incompressible boundary layer.The results show that the amplifying components are in the band centered at zero spanwise wavenumber rather than at a finite spanwise wavenumber.It is therefore concluded that the rapid growth of 3-D disturbances in a band centered at a preferred large spanwise wavenumber is the main characteristic of nonlinear evolution of second mode disturbances in supersonic boundary layers.

强激光场驱动的原子里德堡态激发及电离(特邀)

强场飞秒激光作用于原子时,原子中的电子除了发生多光子电离、隧穿电离等过程外,还有很大的概率处于高激发态。这种高激发态的原子在超强超短激光场中非常稳定,并且与强场中的其它许多现象密切相关,例如,中性粒子加速、多光子拉比振荡、近阈值谐波辐射等,因此近十几年来是强场超快物理领域的研究热点之一。在这些研究中,强激光场中里德堡原子的产生机制、激光对里德堡态的调控、里德堡态强场电离及稳定性等,是关注的主要问题。本文将概述强激光驱动的里德堡态的产生机制,包括多光子共振激发,受挫隧穿电离等。重点介绍强激光场驱动的里德堡态原子激发过程中的多种干涉现象。这些干涉现象提供了里德堡原子强场激发的动态过程信息。同时,还将介绍激发态原子在强激光场中的电离过程,特别是圆偏光驱动的里德堡原子电离的圆二色性。

一种改进的合成孔径聚焦算法研究

在超声混凝土检测时,合成孔径聚焦技术(SAFT)是一种有效的方法。为了进一步提高混凝土材料深度探测分辨力,该文提出了一种改进的合成孔径成像算法。此法将SAFT原理与波包原子分解算法相结合,能更有效聚焦,提高混凝土探测成像的分辨力,并可减少成像算法的运算量。利用仿真实验对这种改进的合成孔径算法进行了验证。结果表明,和传统SAFT算法比较,此法的成像目标定位能力增强,能获得更好的成像效果。

高亚声速湍流喷流气动噪声数值分析

为适应航空噪声管制规定要求,发动机喷流噪声控制成为目前气动声学研究中的重要课题,预测分析喷流噪声辐射并揭示其产生机理将为噪声控制奠定基础.采用高精度并行LES(large eddy simulation)方法计算分析马赫数0.9高亚声速喷流的湍流演化和气动噪声现象.首先,仔细验证喷流LES湍流场计算保真性,并分析流场中不同尺度涡结构的演化形态.其次,利用可穿透面FW-H(Ffowcs Williams and Hawkings)方法外推喷流近场声源数据获得精确声辐射远场,进而分析声场主导声模态特性.最后,通过分析声源机制、分离声模态等方法研究势流核末端大尺度拟序涡运动演化形成的低波数波包在噪声主导声模态产生中的重要作用.数值结果表明LES结合可穿透面FW-H方法可精确预测高亚声速喷流的流场及声场特征,且数值分析揭示涡环对并形成的大尺度拟序结构在喷流中心线上沿径向融合,产生了在远场低方位角占优的主导声模态,并构成强指向性声场,噪声峰值方位角约为30?.

波包分解技术在合成孔径聚焦成像中的应用

在各种无损检测方法中,超声回波法是探测钢筋混凝土结构最有潜力的方法之一.为了增加探测范围,一般使用比较低的超声频率进行探测,但是在对混凝土内部成像时,各种干扰信号的出现造成了低信噪比.合成孔径聚焦技术(SAFT)能够对反射信号进行有效聚焦,提高图像分辨力.由于激励波形频率低,时宽大,反射波包在聚焦过程中会产生很大的变形,影响了探测分辨力.因此引入了波包分解技术,能够将反射波包整体进行聚焦运算,克服了波包变形问题,能够更有效聚焦,提高混凝土探测成像的分辨力.此外,由于参与SAFT运算的点大量减少,可以提高成像运算速度.利用仿真实验对这种方法进行了验证,结果表明,这一方法的成像目标定位能力大大增强,能够获得更好的成像效果.

波包动力学-飞秒激光操纵波包过程

本文介绍了波包动力学的基本理论和数值模拟方法，尤其是在飞秒激光与分子相互作用领域中的应用，探讨了飞秒激光引起的波包过程（包括波包的制备、波包的干涉），以及利用调频脉冲、脉冲串和优化脉冲形状控制波包动力学的方案。

双原子分子多光子电离强场效应的含时波包动力学研究

强激光非线性条件下分子的多光子电离过程呈现出较为明显的强场效应.由于这种效应不能用传统的量子微扰论来处理,“缀饰态”模型方法提供了物理图象清晰的处理光与物质相互作用的方案.本文基于含时波包动力学的基本理论,将激光场看作经典场,利用“缀饰态”模型研究了强场下双原子分子(NO、RbI等)的多光子电离过程.研究表明,激光场的强度、泵浦-探测脉冲延迟时间等对多光子电离光电子能谱的形状有着重要的影响,而这种影响是由光诱导势引起的.另外,在研究具有两个连续态的RbI体系时,自电离现象的发生也与势能面的交叉密切相关,并受外场强度的影响.本文计算模拟外场中分子的光电子能谱时所得到的强场效应对理解和实现原子分子过程的激光操控具有重要的意义.

Instability waves and low-frequency noise radiation in the subsonic chevron jet

Spatial instability frequency noise radiation at waves associated with low- shallow polar angles in the chevron jet are investigated and are compared to the round counterpart. The Reynolds-averaged Navier-Stokes equations are solved to obtain the mean flow fields, which serve as the baseflow for linear stability analysis. The chevron jet has more complicated instability waves than the round jet, where three types of instability modes are identified in the vicinity of the nozzle, corresponding to radial shear, azimuthal shear, and their integrated effect of the baseflow, respectively. The most unstable frequency of all chevron modes and round modes in both jets decrease as the axial location moves downstream. Besides, the azimuthal shear effect related modes are more unstable than radial shear effect related modes at low frequencies. Compared to a round jet, a chevron jet reduces the growth rate of the most unstable modes at down- stream locations. Moreover, linearized Euler equations are employed to obtain the beam pattern of pressure generated by spatially evolving instability waves at a dominant low frequency St = 0.3, and the acoustic efficiencies of these linear wavepackets are evaluated for both jets. It is found that the acoustic efficiency of linear wavepacket is able to be reduced greatly in the chevron jet, compared to the round jet.

中层大气受迫 Rossby 波包的非线性相互作用Ⅰ.理论分析

本文研究了中层大气受迫Ｒｏｓｓｂｙ波包非线性三波相互作用问题．理论分析得出，如果仅有最大波长或最小波长的波包受外源作用，系统仅能维持受迫波包的运动，平衡状态下外源能量不向另两个波包传递；仅有中等波长波包受外源作用时，随着外源强度的增加，系统的平衡态会发生分岔，并且在分岔点以后，中等波长波包的波幅不再增大，但大气高度愈高该振幅饱和值愈大，外源增大的能量全部通过非线性相互作用馈送给其他两个波包，从而外源可以维持三个波包的长期等幅运动．本文还给出了系统中有两个受迫Ｒｏｓｓｂｙ波包时系统的平衡态和稳定性条件，这时无论三个波包的尺度关系如何，外源能量均能在三个波包之间传递，因此外源也可以维持三个波包的运动．进一步的结果和波包的演化过程将通过数值计算给出．

Laser controlled quantum motion of two Paul trapped ions

We investigate the quantum motion of two ions stored in a Paul trap and interacting with a time-periodic laser field. In the pseudopotential approximation and large detuning condition, we find that the relative motion is independent of the laser field, but the exact centre-of-mass motion is closely related to the laser field. By adjusting the laser intensity and frequency, we can well control the quantum motion of the centre-of-mass. We illustrate some physical properties described by the centre-of-mass states, such as the squeezed coherent property, the widths and heights of the wavepackets of probability density, the classical-quantum correspondence, the resonance ladders of expectation energy and the transition probabilities between time-dependent quantum levels.

Dynamics of electron in a surface quantum well

This paper studies the quantum dynamics of electrons in a surface quantum well in the time domain with auto- correlation of wave packet. The evolution of the wave packet for different manifold eigenstates with finite and infinite lifetimes is investigated analytically. It is found that the quantum coherence and evolution of the surface electronic wave packet can be controlled by the laser central energy and electric field. The results show that the finite lifetime of excited states expedites the dephasing of the coherent electronic wave packet significantly. The correspondence between classical and quantum mechanica is shown explicitly in the system.

基于核主元分析和高斯混合模型的轴承转子状态评估

针对轴承转子运行状态评估问题,提出了基于核主元分析与高斯混合模型的新方法。采用小波包变换得到信号在各频带上的能量谱,然后通过核主元分析提取特征空间的主成分;并对其进行高斯混合模型建模。通过EM算法进行参数估计,由高斯混合模型的重合度对轴承转子的运行状态进行评估。通过仿真的轴承转子振动数据的验证发现,核主元分析能够使信号在特征空间的能量更加集中,在此基础上计算的高斯混合模型的重合度,能够更好地表征轴承转子的运行状态。

HOBr/DOBr态到态光解动力学的量子波包研究

本文基于高水平的多参考组态相互作用计算,报导了HOBr的最低三个单重态的三维势能面.并且对该分子的光解离过程用实波包传播的方法进行了量子动力学研究,不仅计算结果得到了吸收光谱,而且还产生了光解产物的内态和角度分布.结果与在紫外区域测得的HOBr的总吸收截面定量吻合,并很好的重现了实验上在266 nm处观察到的振动冷和转动热的光解OH/OD产物.此外,预测了OH/OD的反冲各向异性参数接近平行跃迁的极限值,表明在266 nm处从基态(1^1A')到21A'态的面内跃迁是一个快速的解离过程.这与实验上从测量转动取向得出的结论一致.然而,想要实现与实验上的定量一致,还需要考虑自旋和电子角动量的影响.

相对论带电粒子与电磁波包之间的相互作用

研究了相对论带电粒子与电磁波包之间的相互作用 ,给出了带电粒子运动方程的严格解 ,得出了粒子的位置、速度和能量的显示表达式。本文结果对基础等离子体物理、自由电子激光。