Rotational Constants Research Articles

Multi-Objective Optimal Design of μ–Controller for Active Magnetic Bearing in High-Speed Motor

In this paper, a control strategy based on the inverse system decoupling method and μ-synthesis is proposed to control vibration in a rigid rotor system with active magnetic bearings that are built into high-speed motors. First, the decoupling method is used to decouple the four-degrees-of-freedom state equation of the electromagnetic bearing rigid rotor system; the strongly coupled and nonlinear rotor system is thus decoupled into four independent subsystems, and the eigenvalues of the subsystems are then configured. The uncertain parametric perturbation method is used to model the subsystem, and the multi-objective ant colony algorithm is then used to optimize the sensitivity function and the pole positions to obtain the optimal μ-controller. The closed-loop system thus has the fastest possible response, the strongest internal stability, and the best disturbance rejection capability. Then, the unbalanced force compensation algorithm is used to compensate for the high-frequency eccentric vibration; this algorithm can attenuate the unbalanced eccentric vibration of the rotor to the greatest extent and improve the robust stability of the rotor system. Finally, simulations and experiments show that the proposed control strategy can allow the rotor to be suspended stably and suppress its low-frequency and high-frequency vibrations effectively, providing excellent internal and external stability.

Nuclear Spin Relaxation in Cold Atom-Molecule Collisions.

We explore the quantum dynamics of nuclear spin relaxation in cold collisions of 1Σ+ molecules with structureless atoms in an external magnetic field. To this end, we develop a rigorous coupled-channel methodology, which accounts for rotational and nuclear spin degrees of freedom of 1Σ+ molecules and their interaction with an external magnetic field as well as anisotropic atom-molecule interactions. We apply the methodology to study the collisional relaxation of the nuclear spin sublevels of 13CO molecules immersed in a cold buffer gas of 4He atoms. We find that nuclear spin relaxation in the ground rotational manifold (N = 0) of 13CO occurs extremely slowly due to the absence of direct couplings between the nuclear spin sublevels. The rates of collisional transitions between the rotationally excited (N = 1) nuclear spin states of 13CO are generally much higher due to the direct nuclear spin-rotation coupling between the states. These transitions obey selection rules, which depend on the values of space-fixed projections of rotational and nuclear spin angular momenta (MN and MI) for the initial and final molecular states. For some initial states, we also observe a strong magnetic field dependence, which can be understood by using the first Born approximation. We use our calculated nuclear spin relaxation rates to investigate the thermalization of a single nuclear spin state of 13CO(N = 0) immersed in a cold buffer gas of 4He. The calculated nuclear spin relaxation times (T1 ≃ 1 s at T = 1 K at a He density of 10-14 cm-3) display a steep temperature dependence decreasing rapidly at elevated temperatures due to the increased population of rotationally excited states, which undergo nuclear spin relaxation at a much faster rate. Thus, long relaxation times of N = 0 nuclear spin states in cold collisions with buffer gas atoms can be maintained only at sufficiently low temperatures (kBT ≪ 2Be), where Be is the rotational constant.

TECHNOLOGICAL ASSURANCE OF RELIABILITY OF HIGH-SPEED MIXING DEVICES BASED ON ADAPTIVE LEVITATION SUPPORTS

Rotating machines are an important and crucial component of numerous mechanical systems in modern industry, vehicles and a number of other applications. Excessive vibrations on rotating equipment due to numerous faults can lead to machine failures and lead to accidents. Thus, there is a need to understand the dynamic nature and identify faults for safe, uninterrupted and efficient operation of machines. This paper proposes a new approach to assess misalignment in a test using the same concept as in an imbalance test when balancing the rotor of high-speed machines in the oil and gas industry. The misalignment of the active magnetic bearing with the rotor was investigated. To accomplish this methodology, a four-degree-of-freedom dynamic model of an unbalanced and misaligned rigid rotor and two offset disks supported by two active magnetic bearings was mathematically developed. Disks with displacement lead to a gyroscopic effect at high rotor speeds. The equations of motion of the rotor-bearing system were derived and solved in order to obtain a displacement of the rotor in the time domain and control the current responses in the positions of the magnetic bearing. The algorithm for identifying the estimation of the unbalance and misalignment parameters of the active magnetic bearing was carried out by using the fast Fourier transform method. The effectiveness of this method was estimated at rotor speeds from 100 to 350 rad/s. In the process of determining the efficiency, random modeling errors of 1, 2 and 5 % were also introduced to the mass of the rotor and the moment of inertia of the disk.

Flight dynamics modeling and analysis for a Mars helicopter

Flight dynamics modeling for the Mars helicopter faces great challenges. Aerodynamic modeling of coaxial rotor with high confidence and high computational efficiency is a major difficulty for the field. This paper builds an aerodynamic model of coaxial rotor in the extremely thin Martian atmosphere using the viscous vortex particle method. The aerodynamic forces and flow characteristics of rigid coaxial rotor are computed and analyzed. Meanwhile, a high fidelity aerodynamic surrogate model is built to improve the computational efficiency of the flight dynamics model. Results in this paper reveal that rigid coaxial rotor can bring the Mars helicopter sufficient controllability but result in obvious instability and control couplings in forward flight. This highlights the great differences in flight dynamics characteristics compared with conventional helicopters on Earth.

Spectroscopic constants and transition properties on the singlet states of van der Waals molecules Cd-RG(RG = He,Ne,Ar,Kr,Xe,Rn)

By using CCSD(T) and EOM-CCSD methods together with quasirelativistic energy-consistent small-core pseudopotentials and large atom-centered basis sets as well as bond functions, the potential energy curves (PECs) for the ground state and several low-lying singlet excited states of the van der Waals molecular systems between cadmium atom interacting with RG atoms (RG = He, Ne, Ar, Kr, Xe and Rn) are obtained. The dispersion coefficients of ground state for Cd-RG have been computed. And the spectroscopic constants of the bound states of Cd-RG molecules have been calculated, the spectroscopic constants of ground state and C1Π state of Cd-RG complexes are in reasonable agreement with the available theoretical and experimental results; the vertical excitation energies of singlet low-lying excited states are also obtained. And the transition dipole moments (TDMs) of (3)1Σ+-X1Σ+ and (4)1Σ+-X1Σ+ transitions are discussed, which would be helpful to understand the transition properties of Cd-RG complex. Furthermore, frequencies of vibrational transitions, rotational constants, Franck-Condon factors (FCFs) of C1Π-X1Σ+ transition and oscillator strengths of Cd-RG have also been acquired. The present work would be of value to understand on the electronically excited states of Cd-RG molecule, especially the electronic structure and spectroscopic characteristics of the singlet excited states.

Eliminating molecular-alignment dephasing with a phase-modulated femtosecond laser pulse

Alignment dephasing caused by centrifugal distortion is a significant effect accompanying laser-induced field-free molecular alignment. Centrifugal distortion is the manifestation of the intrinsic nonrigidity of molecules, which is especially prominent when molecules are excited to high angular velocities. In this work, we show that this type of dephasing can be almost completely eliminated by modulating the phase of the femtosecond laser pulse. Nearly perfect molecular alignment can be achieved as if the molecules were ideal rigid rotors. The dephasing effect can be cancelled in various time windows by tuning the modulation strength parameter. The dephasing cancellation mechanism is explained by considering the relative phases of the eigenstates forming the rotational wave packet. This work is of great significance to the experiment and dynamics studies of molecular alignment.

Long-lived room-temperature phosphorescent complex of B, N, P co-doped carbon dots and silica for afterglow imaging

Room temperature phosphorescent (RTP) materials based on carbon dots (CDs) have attracted abundant attention in bioimaging owing to their long lifetimes and wide Stokes shift, which can reduce the autofluorescence and light scattering in tissue. Although solid-state RTP CDs have been widely reported at present, long-lived RTP CDs in aqueous solutions still remain a challenge because they are usually quenched by oxygen in water. This work develops a heteroatom doping strategy to synthesize long-lived matrix-free phosphorescent B, N, P co-doped CDs (B, N, P-CDs) with cross-linked structure and rich hydrogen bonding network. B, N, P-CDs have an ultralong phosphorescence lifetime of 1.89 s. Subsequently, the complex of B, N, P co-doped CDs and silica (B, N, P-CDs@SiO2) was further obtained by coating the surfaces of B, N, P-CDs with silica by sol-gel method to further confine their molecular vibrations and rotations, which shows an excellent RTP property in aqueous solution with a phosphorescent lifetime of 1.97 s. In addition, the complex displays good biocompatibility, low cytotoxicity, and ability of phosphorescent labeling cells and organism. All these results extend a potential application of the B, N, P-CDs@SiO2 complex as phosphorescent bioimaging probe.

A novel effect of dynamic pressure on a nonequilibrium flow of a rarefied polyatomic gas through a diverging nozzle

Nonequilibrium flows of a rarefied polyatomic gas through a diverging nozzle under the effect of dynamic pressure are studied. Even in a diverging nozzle, there arises a possibility of occurring choking phenomena if a gas at the inlet of the nozzle is in a nonequilibrium state. And the controllability of gas flows in a nozzle by manipulating the temperature of molecular internal modes such as molecular rotation and vibration is discussed.

Microwave measurements and calculations for the glyoxylic acid – Formic acid hydrogen-bonded complex

The gas-phase doubly hydrogen-bonded glyoxylic acid - formic acid hydrogen-bonded complex was obtained by mixing a heated sample of glyoxylic acid with room-temperature formic acid in argon. High-level DFT and MP2 calculations with various basis sets were performed and the structures and rotational constants were determined for the lowest energy dimers of glyoxylic acid - formic acid. The microwave spectrum was measured in the 6-12 GHz frequency range using two Flygare-Balle type pulsed beam Fourier transform microwave (FTMW) spectrometers. The rotational constants were determined to have the following values: A = 5533.911(3), B = 923.3883 (5), and C = 792.1132(6) MHz using 18 transitions. B3LYP/ cc-pVQZ calculations for the lowest energy dimer yielded calculated rotational constants of A = 5551.2, B = 922.9 and C = 791.3 MHz.

Microwave spectrum and structure of a glyoxylic acid – formic acid complex☆

The microwave spectrum of a second doubly hydrogen-bonded glyoxylic acid - formic acid hydrogen-bonded complex was measured in the 6–12 GHz frequency range using two Flygare-Balle type pulsed beam Fourier transform microwave (FTMW) spectrometers. The rotational constants were determined to have the following values: A = 4113.081(4), B = 1026.9957(5), and C = 822.9584(6) MHz. The doubly hydrogen bonded structures and rotational constants were calculated for the lowest energy dimers of glyoxylic acid - formic acid using DFT and MP2 calculations with various basis sets. M11/ def2QZVPP calculations for the third lowest energy dimer yielded rotational constants of A = 4248.7, C = 1031.4, and C = 829.9 MHz, in good agreement with experimental values.

Observation of the electronic band system 23Σg--a3Πu of C2 in the vacuum ultraviolet region.

A systematic spectroscopic study of the dicarbon molecule C2 has important applications in various research fields, such as astrochemistry and combustion. In the short vacuum ultraviolet (VUV) wavelength region, recent theoretical calculations have predicted many absorption band systems of C2, but only few of them have been verified experimentally yet. In this work, we employed a tunable VUV laser radiation source based on the two-photon resonance-enhanced four-wave mixing method and a time-of-flight mass spectrometer to investigate the absorption bands of C2 in the VUV range of 64 000-66 000cm-1. The electronic transition 23Σg-(v')-a3Πu(v″) of C2 has been observed and identified experimentally for the first time. The term value Te for the 23Σg- state is determined to be 66 389.9 ± 0.5cm-1 above the ground state X1Σg+, and the vibrational and rotational constants are also determined. The experimentally measured spectroscopic parameters in this study are in excellent agreement with the theoretical results based on high-level abinitio calculations.

Numerical simulations of the wake and deformations of a flexible rotor blade in a turbulent flow

We present, for the first time, the mean deflection evolution of a flexible rotor blade using a coupled model based on Navier–Stokes equations, for the fluid flow, and linear elasticity equations for the blade deformation. Three turbulence models are tested to reach Reynolds numbers as high as 8 104. The absolute tip speed ratios are in the range [0,25]. The numerical results are validated by comparisons with available tip displacements from experiments. For the parameter ranges, above mentioned, the elastic behavior of the flexible rotor is characterized, and the vorticity field is compared with results obtained for a rigid rotor. The effects of the pitch, the tip speed ratio (or frequency), and its sign on the blade deformation are reported. Typically, the blade deforms in the downstream direction, and it is shown that this deformation is a non-monotonic function of the rotation frequency and the pitch angle. Furthermore, it is found that, for particular values of the frequency and pitch angle, the blade is subject to deformations in the upstream direction. It is shown also that the flexible rotor could develop a vortex ring state, but not the rigid one, under the same conditions. It is found that there is a supercritical frequency associated with the apparition of this vortex ring state and this frequency occurs for negative pitches only, for the considered blade. The vorticity field revealed, as well, that the tip vortex changes sign with that of the blade deflection. Finally, we present the effect of the pitch and frequency on the twist angle of the blade and characterize its evolution along the span.

Spectral extension of two glycolic acid conformers into the submillimeter regime

The rotational spectrum of glycolic acid has been extended to frequencies from 318 to 1000 GHz. Analysis of this spectrum builds upon previous experimental and theoretical studies performed on glycolic acid conformers. Extensive new assignments were made for the ground vibrational state of the SSC conformer of glycolic acid in addition to the two lowest vibrational states. Assignments were also expanded for the AAT conformer of glycolic acid. This work improves the precision of some rotational constants and nearly all of the centrifugal distortion constants for each conformer and vibrational state. The spectral predictions based on these refined parameters will facilitate the search for glycolic acid in the interstellar medium at frequencies covered by submillimeter observatories.

The chirped pulse, Fourier transform microwave spectrum of 1-chloromethyl-1-fluorosilacyclopentane

The microwave spectrum of 1-chloromethyl-1-fluorosilacyclopentane has been recorded for the first time using the chirped pulse, Fourier transform microwave technique. Quantum chemical calculations show the two lowest energy conformers as being a twist-trans and a gauche form with the gauche form previously being shown as having two separate conformations, a gauche+ (lower in energy) and gauche- (higher in energy) form. Analysis of the spectrum provided the observation of the twist-trans conformer only, with 253 and 85 transitions being assigned to the 35Cl and 37Cl isotopologues, respectively. R-branch, a- and b-type transitions were observed. The spectrum was fit to a Watson S-reduced Hamiltonian and consisted of rotational constants, quartic centrifugal distortion constants, and nuclear quadrupole coupling constants, including the determination of the off-diagonal nuclear quadrupole coupling constant, χab. Interpretation of the structure was provided using second moments and is found to have a similar ring structure to other known silacyclopentanes. Analysis of the χzz has been carried out and compared to other similar molecules. An investigation of the known quantum chemical energies of the gauche conformer reveals that the reported B3LYP energies do not align with the observed microwave results.

Phase structures and interaction of BEDT-TTF molecules

Structural phases in crystal BEDT-TTF under the influence of doping molecules were studied. The system demonstrates the transition from a parallel aligned molecular phase (β-phase) to a new equilibrium state, where the molecules are rotated at a certain angle (α- or θ-phase). The precursor of this transition is the electrostatic and quadrupole interaction between BEDT-TTF and doping molecules. The modified potential of the Girifalco type is introduced, it depends on both intermolecular distance and the angle of the molecular rotation. It is found that the equilibrium distance between molecules in a stack increases and the deflection angle arises with an increase of the charge of doping molecules, which leads to new equilibrium states.

Spontaneous Hybrid Nano‐Domain Behavior of the Organic–Inorganic Hybrid Perovskites

AbstractIn hybrid perovskites, the organic molecules and inorganic frameworks exhibit distinct static and dynamic characteristics. Their coupling will lead to fascinating phenomena, such as large polarons, dynamic Rashba–Dresselhaus effects, etc. In this paper, deep potential molecular dynamics (DPMD) is employed, a large‐scale MD simulation scheme with DFT accuracy, to study hybrid perovskites formamidinium lead iodide (FAPbI3) and methylamonium lead iodide (MAPbI3). A spontaneous hybrid nano‐domain behavior, namely multiple molecular rotation nano‐domains embedded into a single [PbI6]4− octahedra rotation domain, is first discovered at low temperatures. The behavior originates from the interplay between the long range order of molecular rotation and local lattice deformation, and clarifies the puzzling structural features of FAPbI3 at low temperatures. The work provides new insights into the structural characteristics and stability of hybrid perovskite, as well as new ideas for the structural characterization of organic–inorganic coupled systems.

Dopant-Free Pyrrolopyrrole-Based (PPr) Polymeric Hole-Transporting Materials for Efficient Tin-Based Perovskite Solar Cells with Stability Over 6000h.

A new set of pyrrolopyrrole-based (PPr) polymers incorporated with thioalkylated/alkylated bithiophene (SBT/BT) is synthesized and explored as hole-transporting materials (HTMs) for Sn-based perovskite solar cells (TPSCs). Three bithiophenyl spacers bearing the thioalkylated hexyl (SBT-6), thioalkylated tetradecyl (SBT-14), and tetradecyl (BT-14) chains are utilized to examine the effect of the alkyl chain lengths. Among them, the TPSCs are fabricated using PPr-SBT-14 as HTMs through a two-step approach by attaining a power conversion efficiency (PCE) of 7.6% with a remarkable long-term stability beyond 6000 h, which has not been reported elsewhere for a non-PEDOT:PSS-based TPSC. The PPr-SBT-14 device is stable under light irradiation for 5 h in air (50% relative humidity) at the maximum power point (MPP). The highly planar structure, strong intramolecular S(alkyl)···S(thiophene) interactions, and extended π-conjugation of SBT enable the PPr-SBT-14 device to outperform the standard poly(3-hexylthiophene,-2,5-diyl (P3HT) and other devices. The longer thio-tetradecyl chain in SBT-14 restricts molecular rotation and strongly affects the molecular conformation, solubility, and film wettability over other polymers. Thus, the present study makes a promising dopant-free polymeric HTM model for the future design of highly efficient and stable TPSCs.

Computational studies of HCCCCS isomers

The recent identification of interstellar molecules HCCCHCCC and HCCCCS in TMC-1 has prompted an ab initio investigation of these isomers. The isomers of HCCCHCCC have been studied previously but not so much HCCCCS. In this paper we carry out high-level CCSD(T) calculations to characterise the lowest eleven isomers of HCCCCS. We also apply complete basis set extrapolation including core correlation and relativistic effects to further investigate the key isomers and provide high-precision rotational constants which will hopefully aid astronomers in their assignments. Source: This image has been adapted from http://www.eso.org/public/images/eso1209d/ licensed under CC BY-SA 3.0.

The gas-phase structure determination of α-pinene oxide: An endo-cyclic epoxide of atmospheric interest.

The gas-phase rotational spectra of α-pinene oxide have been recorded using a chirped-pulse Fourier transform microwave spectrometer in the 6-18GHz frequency range. The parent species and all heavy atom isotopologues (13C and 18O) have been observed in their natural abundance. The experimental rotational constants of all isotopic species observed have been determined and used to obtain the substitution (rs) and the effective (r0) structures of the most stable conformer of α-pinene oxide. Calculations using the density functional theories B3LYP, M06-2X, and MN15-L and the abinitio method MP2 level of theory were carried out to check their performance against experimental results. The structure of the heavy atom's skeleton of α-pinene oxide has been compared to that of α-pinene and has shown that epoxidation does not overly affect the structure of the bicycle, validating its robustness. Furthermore, the structural features have been compared to those of other bicyclic molecules, such as nopinone and β-pinene.

The Microwave Rotational Electric Resonance (RER) Spectrum of Benzothiazole.

The microwave spectra of benzothiazole were measured in the frequency range 2-26.5 GHz using a pulsed molecular jet Fourier transform microwave spectrometer. Hyperfine splittings arising from the quadrupole coupling of the 14N nucleus were fully resolved and analyzed simultaneously with the rotational frequencies. In total, 194 and 92 hyperfine components of the main species and the 34S isotopologue, respectively, were measured and fitted to measurement accuracy using a semi-rigid rotor model supplemented by a Hamiltonian accounting for the 14N nuclear quadrupole coupling effect. Highly accurate rotational constants, centrifugal distortion constants, and 14N nuclear quadrupole coupling constants were deduced. A large number of method and basis set combinations were used to optimize the molecular geometry of benzothiazole, and the calculated rotational constants were compared with the experimentally determined constants in the course of a benchmarking effort. The similar value of the χcc quadrupole coupling constant when compared to other thiazole derivatives indicates only very small changes of the electronic environment at the nitrogen nucleus in these compounds. The small negative inertial defect of -0.056 uÅ2 hints that low-frequency out-of-plane vibrations are present in benzothiazole, similar to the observation for some other planar aromatic molecules.