考虑法向压力梯度的三维旋转边界层计算

本文以量级分析为基础,建立了一般曲线坐标系上的三维旋转边界层方程。对旋转在边界层中的影响进行分析之后,提出了一个能够处理壁面法向压力梯度不为零问题的压力梯度迭代方法。在传统的Box法的基础上发展了一套完整的求解三维旋转边界层的方法和程序,并对螺旋面、压气机转子叶面以及圆柱面上的旋转边界层进行了计算,与他人的计算和实验的对比分析表明,该方法和程序是正确的,可用于求解任意几何物面上的三维旋转边界层。     

Control of a Nonlinear System Using the Saturation Phenomenon

In this paper, a theoretical investigation of nonlinear vibrations of a 2 degrees of freedom system when subjected to saturation is studied. The method has been especially applied to a system that consists of a DC motor with a nonlinear controller and a harmonic forcing voltage. Approximate solutions are sought using the method of multiple scales. It is shown that the closed-loop system exhibits different response regimes. The nature and stability of these regimes are studied and the stability boundaries are obtained. The effects of the initial conditions on the response of the system have also been investigated. Furthermore, the second-order solution is presented and the corresponding results are compared with those of the first-order solution. It is shown that by increasing the amplitude of the excitation voltage, the higher-order term in the solution becomes significant and causes a drift in the response. In order to verify the obtained theoretical results, they are compared with the corresponding numerical results. Good agreement between the two sets of results is observed.     

Nonlinear Analysis of Store-Induced Limit Cycle Oscillations

Flight tests of modern high-performance fighter aircraft reveal the presence of limit cycle oscillation (LCO) responses for aircraft with certain external store configurations. Conventional linear aeroelastic analysis predicts flutter for conditions well beyond the operational envelope, yet these store-induced LCO responses occur at flight conditions within the flight envelope. Several nonlinear sources may be present, including aerodynamic effects such as flow separation and shock-boundary layer interaction and structural effects such as stiffening, damping, and system kinematics. No complete theory has been forwarded to accurately explain the mechanisms responsible. This research examines a two degree-of-freedom aeroelastic system which possesses kinematic nonlinearities and a strong nonlinearity in pitch stiffness. Nonlinear analysis techniques are used to gain insight into the characteristics of the behavior of the system. Numerical simulation is used to verify and validate the analysis. It is found that when system damping is low, the system clearly exhibits nonlinear interaction between aeroelastic modes. It is also shown that although certain applied forcing conditions may appear negligible, these same forces produce large amplitude LCOs under specific realizable circumstances.     

Model studies of the optical rotation,and theoretical determination of its sign for β‐pinene and trans‐pinane

We have carried out extensive studies on the basis set dependence of the calculated specific optical rotation (OR) in molecules at the level of the time–dependent Hartree–Fock and density functional approximations. To reach the limits of the basis set saturation, we have devised an artificial model, the asymmetrically deformed (chiral) methane (CM) molecule. This small system permits to use basis sets which are prohibitively large for real chiral molecules and yet shows all the important features of the basis set dependence of the OR values. The convergence of the OR has been studied with n‐aug‐cc‐pVXZ basis sets of Dunning up to the 6–ζ. In a parallel series of calculations, we have used the recently developed large polarized (LPolX) basis sets. The relatively small LPolX sets have been shown to be competitive to very large n‐aug‐cc‐pVXZ basis sets. The conclusions reached in calculations of OR in CM concerning the usefulness of LPolX basis sets have been further tested on (S)‐methyloxirane and (S)‐fluoro‐oxirane. The smallest set of the LPolX family (LPol–ds) has been found to yield OR values of similar quality as those obtained with much larger Dunning's aug‐cc‐pVQZ basis set. These results have encouraged us to carry out the OR calculations with LPol–ds basis sets for systems as large as β‐pinene and trans‐pinane. In both cases, our calculations have lead to the correct sign of the OR value in these molecules. This makes the relatively small LPol–ds basis sets likely to be useful in OR calculations for large molecules. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010     

Vibronic spectra, ab initio calculations, and structures of conformationally non-rigid molecules of oxalyl halides in the ground and lowest excited electronic states - Part VI: Oxalyl bromide (COBr)2 and summary

The structure of oxalyl bromide (COBr)₂ in the ground and four lowest excited electronic states was theoretically investigated using the CASPT2/cc-pVTZ-DK method. Structural information obtained allowed the reassignment of the and vibronic absorption spectra. Generalization of oxalyl halide structural data for the ground and excited electronic states is presented.     

The microwave spectrum of methyl chlorodifluoroacetate: Methyl internal rotation and chlorine nuclear electric quadrupole coupling

A chirped pulse microwave spectrometer has been used to record microwave spectra of the ³⁵Cl and ³⁷Cl isotopologues of methyl chlorodifluoroacetate, CClF₂C(O)OCH₃, between 8 GHz and 16 GHz. The target compound was spectroscopically examined as it participated in a supersonic expansion of argon. Only one conformer was observed. The rotational spectra were recorded with sufficient resolution to observe (i) splittings due to the internal rotation of the methyl group, and (ii) splittings from the coupling of the chlorine quadrupolar nucleus. A total of 785 transitions have had quantum numbers assigned. Analysis of the spectra observed has produced an experimental barrier to the methyl group internal rotation, V₃, of 370(2) cm⁻¹. It is noted that this barrier is a little lower than that determined for methyl acetate [V₃ = 425 cm⁻¹, J. Sheridan, W. Bossert and A. Bauder, J. Mol. Spectrosc., 80 (1980) 1-11], and this is rationalized through a comparison of molecular structures. Lastly, all components of both the ³⁵Cl and ³⁷Cl chlorine nuclear electric quadrupolar coupling tensor have been determined.     

Gas-phase Raman spectra of hot bands of fundamentals and combinations associated with the torsional vibration of s-trans-1,3-butadiene and its deuterated isotopologues

The gas-phase Raman spectra of 1,3-butadiene and its 2,3-d₂, 1,1,4,4-d₄, and d₆ isotopologues have been recorded with high sensitivity and resolution of 0.7 cm⁻¹. Hot band series of fundamentals and combinations involving the ν₁₃ torsional vibration of the s-trans rotamer have been observed for each of the isotopologues. Modes studied were ν₁₀ (CH wag), ν₁₂ (CH₂ twist), ν₁₀ + ν₁₂, ν₁₅ (CH₂ wag) + ν₁₆ (CH₂ twist), and ν₂₃ (CH₂ rock) + ν₂₄ (CCC deformation). The spacings of the quantum states of the torsional contribution were found to decrease with additional excitation of this mode (ν₁₃) in the upper vibrational states except for the ν₂₃ + ν₂₄ combination state.     

Analysis of the pure-rotational spectrum in the ν28 = 1 excited torsional state and torsional couplings of trans-ethyl methyl ether

The trans-ethyl methyl ether molecule has three low-lying torsional modes, that is, two inequivalent methyl internal rotations and an asymmetric skeletal torsion. The internal rotations of the CCH₃ and OCH₃ methyl rotors and the skeletal torsion correspond to the vibrational modes, ν₂₈, ν₂₉ and ν₃₀ respectively. In this study, the microwave absorption spectrum in the ν₂₈ = 1 CCH₃ torsional state was analyzed for the first time. Nine hundred fifty seven lines up to J = 48 and K = 4 were assigned, and the rotational, centrifugal distortion and internal rotational tunneling parameters were determined with the use of a tunneling matrix formalism. By combining the present results on the ν₂₈ = 1 torsional state with those for the ν₃₀ = 1 skeletal torsional state and the ν₂₉ = 1 OCH₃ torsional state, torsional couplings are estimated in order to understand quantitatively the inverted A/E sequence patterns observed for those three excited torsional states.     

Properties of a vector soliton laser passively mode-locked by a fiber-based semiconductor saturable absorber operating in transmission

We experimentally demonstrate a passively mode-locked fiber laser employing a fiber-based semiconductor saturable absorber (SSA) operating in transmission. Polarization rotation locked vector solitons are observed in the laser. Due to the intrinsic dynamic feature of the laser, period-doubling of these vector solitons has also been observed. Furthermore, extra spectral sidebands are formed on the optical spectrum, caused by the energy exchange between the two orthogonal polarization components of the vector solitons. By careful reduction of the pump power together with fine adjustment to the cavity birefringence, period-one state can further be obtained. Additionally, the phase noise properties of the vector soliton fiber laser have also been characterized experimentally and analytically.     

Full L-band coverage of multiwavelength erbium-doped fiber laser

We propose and experimentally demonstrate a dual-pass unbalanced in-line Sagnac interferometer as a novel comb filter for implements in erbium-doped fiber lasers to obtain wide wavebands of multiwavelength radiations with enhancements of signal-to-noise ratios (SNRs). The hybrid combinations of the comb filter with the two schemes of nonlinear polarization rotation and intensity dependent loss have successfully not only enlarged the lasing bandwidth up to 47 nm but also enhanced the SNR up to 40 dB. The simultaneous lasing-wavelengths of 117 channels are also obtained. The multiwavelength lasing spectra with free spectral range of 0.4 nm covering the full L-band are useful for the applications of DWDM and WDM-PON systems.