非定常尾迹控制叶栅分离研究

本文采用大涡模拟对某大转角叶栅的非定常分离流动及其在非定常尾迹作用下的分离控制机理进行了数值分析。主要捕捉了两个特征频率:分离泡不稳定频率f_shear和尾缘脱落涡频率f_shed,研究了不同的激励频率、尾迹移动方向、随机脉动等激励特征控制流动分离的效果。结果显示:特定外部频率强化了分离剪切层中的K-H展向涡结构,f_shed能同时影响分离区域和尾涡区域,f_shear只能作用于分离区域;尾迹从吸力面向压力面移动时,分离结构表现出对来流周期性更明显的响应;进口随机脉动对破坏K-H展向涡结构非常有效。     

合成射流控制高负荷扩压叶栅分离数值研究

采用数值模拟的方法,研究了合成射流对高负荷扩压叶栅分离流动的控制效果,分析了关键的控制参数激励频率、幅值和位置对控制效果的影响。结果表明,合成射流能够有效削弱高负荷扩压叶栅内的大尺度分离结构。激励频率与原流场的主分离涡脱落频率相近时控制效果占优。激励幅值存在较为明显的阈值,当激励幅值大于该阈值时控制效果较为显著。激励位置位于主分离涡起始位置附近时,控制效果较好。     

非定常激励抑制轴流叶栅分离的数值分析

本文对大转角压气机叶栅大攻角工况在来流尾迹非定常激励作用下的时空结构进行了数值分析.合理的非定常激励明显提升了叶栅的时均性能,与激励频率对应的涡结构得到强化,其它频率的杂涡被卷吸,无序的吸力面分离流结构变得有序.文中分析了周期性来流尾迹控制分离结构的作用机理,波涡共振促进涡的卷起和配对;湍动能强化动量交换.基于此,比较了两种来流尾迹模式下分离结构对来流周期性的响应问题.另外,针对管道风扇引起的远场噪声,在来流尾迹非定常激励作用下对其降低的可能性进行了设想.     

低雷诺数下周期性尾迹／层流分离泡相互作用的大涡模拟

本文采用大涡模拟(LES)方法,对低雷诺数条件下周期性尾迹/层流分离泡的相互作用进行了数值模拟.计算域入口尾迹采用独立计算的平面尾迹流给出.对周期性尾迹作用下的层流分离泡进行了统计特性及瞬态流场分析,并与相同条件下的无尾迹流动进行比较.计算结果表明:在尾迹作用下,时均层流分离泡长度明显缩短,分离泡变小;相位平均的分离点/再附点随时间不断变化;从瞬态结果容易看出,尾迹诱导的流向涡结构是导致分离泡提前转捩的主要原因.     

纳秒脉冲表面介质阻挡等离子体激励唯象学仿真

结合NS-DBD实验数据和理论分析, 建立NS-DBD单区非均匀唯象学模型, 旨在通过合理的模型进行流动控制仿真, 揭示流动控制机理. 在平板无来流时, 运用单区非均匀唯象学模型, 通过引入涡量输运方程, 求解涡量方程各项, 分析展向涡形成机理. 展向涡主要是由压力升诱导激励区压力梯度和密度梯度的不正交性产生的, 其次是激励区附近流场的对流引起的涡量转移. 圆柱上的激励仿真得到与实验一致的压缩波结构和冲击波位置, 验证了模型合理性. NACA 0015翼型大迎角分离控制的仿真表明, 激励诱导展向涡促使主流和分离流相互作用, 使分离点移向下游; 脉冲激励频率通过诱导展向涡的数量对流动分离产生不同的作用效果, 本文最佳的无量纲激励频率为6.     

导叶展弦比对部分进气涡轮性能影响的数值研究

本文针对导叶展弦比对部分进气涡轮性能的影响进行了数值研究。结果表明:转子前缘靠近导叶吸力面边界处流动发生分离形成分离涡,使得损失增加;展弦比增加能增加进气区转子前缘靠近导叶吸力面处主流速度,进而降低分离涡强度并降低部分进气效应对进气区中心区域流动的影响,改善进气扇区中心区域流动,从而减少损失提升效率。与此相反,叶高、稠度不变时展弦比增加使得叶片数增加,叶型损失增加,且进气度越大,叶型损失增加量越大。当进气度小于50%时,进气扇区较小,展弦比增加诱发的分离涡强度降低的作用效果明显强于叶片数增加的影响,二者的综合作用使得涡轮效率得到提升;当进气度大于50%以后,分离涡对进气扇区中心区域的影响降低,分离涡强度的降低仅能改善进气区边界靠近导叶吸力面处的流动状态,但此时叶片数增加导致的叶型损失会显著增加,二者的综合作用导致涡轮效率提升的幅度降低。     

基于合成射流控制技术的高负荷压气机叶栅流动特性研究

数值研究了合成射流控制高负荷压气机叶栅非定常流动分离的效果及机理。结果表明,激励频率、幅值是影响合成射流控制效果的重要参数,当激励频率为最优值、幅值大于某一阈值时,叶栅时均总压损失系数最大降低40.3%,静压升系数提高12.1%。合成射流的动量注入显著增大了激励缝下游流场的湍流剪切应力,使得剪切层内的速度脉动增强,在宏观上表现为近壁区域与主流流体间的动量交换得到加强,这是非定常流动分离得到抑制的主要原因。     

脉动流方柱绕流特性研究

本文数值模拟雷诺数Re=100的条件下,脉动流振幅和频率分别为0.2≤A≤0.8和0≤f_P≤20 Hz时方柱绕流特性.通过数值计算得到方柱绕流的升、阻力系数,涡脱频率及尾涡特性,且稳定流下计算结果与文献结果一致。研究结果表明脉动流是一种有效的主动流动控制方法;在脉动流频率f_P为1.2~2.6自然涡脱频率f_(sn)时,旋涡脱落频率存在"锁定"现象;在锁定范围内,尾涡形成区域变短,时均阻力系数显著增大,且在脉动流频率等于两倍频率时,时均阻力系数达到峰值;随着振幅的增大,频率"锁定"范围增大。     

压缩拐角激波/湍流边界层干扰特性分析

用大涡模拟方法对Mach数3.0下的压缩拐角激波/湍流边界层干扰问题进行数值研究.对拐角上游平板区域边界层转捩及湍流进行模拟,设定平板区域长度,使得转捩过程于平板区域发生并充分完成,从而在拐角处产生激波/湍流边界层相互干扰,研究激波/湍流边界层的作用机理.研究表明:流场能够在非定常扰动激励下迅速转捩,并于平板区发展为完全湍流;湍流边界层与激波相互作用过程中,拐角附近分离区较层流情况明显减小;展向不同区域分离区大小差异较大,局部区域分离现象消失.     

纳秒脉冲等离子体激励控制低雷诺数压气机叶栅激波/附面层干扰探索研究

为了揭示等离子体激励调控低雷诺数压气机叶栅激波/附面层干扰的机理,本文选取典型超音速压气机预压缩叶型,利用大涡模拟研究了纳秒脉冲等离子体激励对低雷诺数下超音速压气机叶型附面层流动的调控作用。首先对低雷诺数工况下超音速压气机叶型流动特性和叶栅通道激波系结构进行了研究,以此设计了两种等离子体激励布局。研究发现,位于叶片吸力面和压力面附面层分离点前的等离子体激励均可通过诱导产生畸变团,触发分离剪切层的K-H不稳定并进一步形成展向大涡结构,促进主流与分离区低能流体之间的掺混从而抑制流动分离。同时叶栅通道激波系结构发生改变,分离区形态与通道激波位置相互关联耦合,附面层黏性损失和激波损失占比变化不尽相同。     

ZnSe在外电场下的基态性质和激发特性研究

以LANL2DZ为基组, 采用Hartree-Fock(HF)方法研究了不同外电场(-0.025–0.040 a.u.)对ZnSe分子的基态几何结构、电荷分布、能量、电偶极矩、最高占据轨道(HOMO)能级、最低空轨道(LUMO)能级、能隙、红外光谱特性的影响; 继而采用含时的TD-HF方法研究了ZnSe分子在外电场下前9 个激发态的吸收谱、激发能、振子强度等激发特性. 研究结果表明: 当电场从-0.025 a.u.变化到0.04 a.u. 时, 键长先减小后增加; 分子偶极矩先由正减小到0, 然后又反向增加; 体系总能量一直减小; 谐振频率先增加后减小, 红外光谱强度先减小后又增加. ZnSe分子的LUMO能级一直增加, HOMO能级先增加后又减小, 变化趋势较小, 而能隙一直增大. 外电场对ZnSe分子的激发特性影响较大, 当电场从-0.025 a.u.变化到0.04 a.u.时, 激发能增加, 相应的激发波长减小; 对应的振子强度也受到很大影响, 原来振子强度最强的激发态变得很弱, 而原来振子强度很弱的激发态变得最强. 因此, 可以通过改变电场来控制ZnSe的激发特性.     

General solution of a system of equations for acoustic wave propagation in a partially ionized gas in an external electric field

In this paper a system of equations describing the propagation of acoustic waves in multicomponent partially ionized plasma in an external electric field is solved generally, when these waves are generated by an external neutral sound wave in plasma. The equations are solved by means of Laplace transformation. It has been found that a sound wave generates a group of waves with different frequencies and wave vectors. For the sake of comparison, the solution without an external field and the linear solution in the form of waves having the same frequencies as the frequency of the excitation wave are given.     

Wavelength‐dependent amplitude of Teflon Raman lines

Raman and other spectroscopy systems operating in the deep ultraviolet (DUV) spectral region below 250 nm lack scattering standards. Such standards are particularly important for experiments that use multiwavelength excitation and/or when results are compared across different experiment platforms. Teflon has been used as an external standard in the visible and near‐IR spectral regions and has been suggested for use in the ultraviolet (UV). Comparison of the relative amplitudes of prominent Teflon Raman lines indicates a significant effect on line ratios when the excitation wavelength is below 250 nm. This dependence on excitation wavelength has been commented on previously and attributed to pre‐resonance effects, but no detailed examination had been undertaken to date. We present the results of a study of Teflon Raman line ratios obtained from closely spaced excitation wavelengths in the DUV from 210 to 320 nm. The 731 cm⁻¹ line is identified as well suited for a standard. Electronic transition energies associated with resonance of principal Teflon Raman lines are obtained. Copyright © 2010 John Wiley & Sons, Ltd.     

Flow-induced oscillations of a cantilevered pipe conveying fluid with base excitation

It is known that a plain cantilevered pipe conveying fluid loses its stability by a Hopf bifurcation, leading to either planar or non-planar flutter for flow velocities beyond the critical flow velocity for Hopf bifurcation. If an external mass is attached to the end of the pipe (an end-mass), the resulting dynamics become much richer, showing 2D and 3D quasiperiodic and chaotic oscillations at high flow velocities. In this paper, a cantilevered pipe, with and without an end-mass, subjected to a small-amplitude periodic base excitation is considered. A set of three-dimensional nonlinear equations is used to analyze the pipe?s response at various flow velocities and with different amplitudes and frequencies of base excitation. The nonlinear equations are discretized using the Galerkin technique and the resulting set of equations is solved using Houbolt?s finite difference method. It is shown that for a plain pipe (with no end-mass), non-planar post-instability oscillations can be reduced to planar periodic oscillations for a range of base excitation frequencies and amplitudes. For a pipe with an end-mass, similarly to a plain pipe, three-dimensional period oscillations can be reduced to planar ones. At flow velocities beyond the critical flow velocity for torus instability, the three-dimensional quasiperiodic oscillations can be reduced to two-dimensional quasiperiodic or periodic oscillations, depending on the frequency of base excitation. In all these cases, a low-amplitude base excitation results in reducing the three-dimensional oscillations of the pipe to purely two-dimensional oscillations, over a range of excitation frequencies. These numerical results are in agreement with the previous experimental work.     

Structural and acoustic responses of a fluid-loaded cylindrical hull with structural discontinuities

This paper studies the low frequency vibrational behaviour and radiated sound of a submarine hull under axial excitation. The submarine is modelled as a fluid-loaded cylindrical shell with internal bulkheads and ring-stiffeners and closed at each end by circular plates. A smeared approach is used to model the ring stiffeners. The external pressure acting on the hull due to the fluid loading is calculated using an infinite model and is shown to be a good approximation at low frequencies. The radiated sound pressure is obtained by considering the finite cylindrical hull to be extended by two semi-infinite rigid baffles. The sound pressure is then only due to the radial displacement of the cylindrical shell, without taking into account the scattering at the finite ends. The main aim of this paper is to observe the influence of the various complicating effects such as the bulkheads, ring-stiffeners and fluid loading on the structural and acoustic responses of the finite cylindrical shell. Results from the analytical models presented in this paper are compared to the computational results from finite element and boundary element models.     

Properties of a Si₂N molecule under an external electric field

In the present work,we adopt the ccsd/6-31g(d) method to optimize the ground state structure and calculate the vibrational frequency of the Si2N molecule.The calculated frequencies accord satisfactorily with the experimental values,which helps confirm the ground state structure of the molecule.In order to find how the external electric field affects the Si2N molecule,we use the density functional method B3P86/6-31g(d) to optimize the ground state structure and the time-dependent density functional theory TDDFT/6-31g(d) to study the absorption spectra,the excitation energies,the oscillator strengths,and the dipole moments of the Si2N molecule under different external electric fields.It is found that the absorption spectra,the excitation energies,the oscillator strengths,and the dipole moments of the Si2N molecule are affected by the external electric field.One of the valuable results is that the absorption spectra of the yellow and the blue-violet light of the Si2N molecule each have a red shift under the electric field.The luminescence mechanism in the visible light region of the Si2N molecule is also investigated and compared with the experimental data.     

Excitation of radial collective modes in a quantum dot: Beyond linear response

The recent results on the linear breathing mode of the excitation spectrum of a quantum dot obtained by McDonald et. al [Phys. Rev. Lett. 111 , 256801 (2013)] are extended to the nonlinear regime. To accomplish this and analyze the results the response of five different models of two interacting electrons in a quantum dot to an external short lived radial excitation that is strong enough to excite the system well beyond the linear response regime is compared. The models considered describe the Coulomb interaction between the electrons in different ways ranging from mean‐field approaches to configuration interaction (CI) models, where the two‐electron Hamiltonian is diagonalized in a large truncated Fock space. The radially symmetric excitation is selected in order to severely put to test the different approaches to describe the interaction and correlations of an electron system in a nonequilibrium state. As can be expected for the case of only two electrons none of the mean‐field models can in full details reproduce the results obtained by the CI model. Nonetheless, some linear and nonlinear characteristics are reproduced reasonably well. All the models show activation of an increasing number of collective modes as the strength of the excitation is increased. By varying slightly the confinement potential of the dot it was observed how sensitive the properties of the excitation spectrum are to the Coulomb interaction and its correlation effects. In order to approach closer the question of nonlinearity one of the mean‐field models has been solved directly in a nonlinear fashion without resorting to iterations.     

Sloshing in partially filled liquid containers—Numerical and experimental study for 2-D problems

This paper deals with the numerical and experimental studies of sloshing of liquid in partially filled prismatic containers subjected to external excitation. Meshless local Petrov-Galerkin (MLPG) method is used for computing the nonlinear sloshing response of liquid in a two-dimensional rigid prismatic tank. At every instant of time, velocity potential is computed at each node and the nodal positions are updated. A local symmetric weak form (LSWF) for nonlinear sloshing of liquid is developed, and a truly meshless method, based on LSWF and moving least squares (MLS) approximation, is presented for the solution of Laplace equation with the requisite boundary conditions. An experimental set-up is also designed to study the behavior of liquid sloshing in partially filled prismatic tank. The resulting slosh heights for various excitation frequencies and amplitudes are compared with the data obtained from the numerical studies. The numerical results are close to that obtained experimentally and little variations in the data are due to ineptness of the experimental set-up and the input parameters.     

Non-linear gas oscillations in a pipe

The results of an experimental study of non-linear longitudinal gas oscillations in a closed tube are presented. Forced oscillations greater than in other experiments to date are obtained. A brief representation of the theory holding for excitation frequencies near and equal to the natural frequencies of a gas column is given. The experimental amplitude and the periodic shock wave form are compared with the calculated values. Oscillations whose excitation frequencies are twice as small as the first natural frequency are also studied.