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Ice affects many chemical reactions in nature, which greatly influences the atmosphere, climate, and life. However, the exact mechanism of ice in these chemical reactions remains elusive. For example, it is still an open question as to whether ice can act as a catalyst to greatly enhance the reactivity and selectivity, which is essential for the production of some natural compounds in our planet. Here, we discover that ice can lead to high efficiency and stereoselectivity of the [2 + 2] photodimerization of coumarin and its derivatives. The conversion of the [2 + 2] photodimerization of coumarins enhanced by ice is dozens of times higher than that in the unfrozen saturated solution, and the reaction displays a high syn-head-head stereoselectivity (95%) in comparison with those in the absence of the ice. Note that almost no reaction occurs in the crystal powder and melt of the coumarins, indicating that the role of ice in the photodimerization reaction is not simply due to the usual mechanisms found in the freezing concentration. We further reveal that the reaction rate is found to be proportional to the total area of the ice surface and follows Michaelis-Menten-like kinetics, indicating that the ice surface catalyzes the reaction. Molecular dynamics simulations demonstrate that ice surfaces can induce reactants to form a two-dimensional liquid-crystal-ordered layer with a suitable intermolecular distance and unique side-by-side packing, facilitating stereoselective photodimerization for syn-head-head dimers. These findings give evidence that ice-surface-induced molecular assembly may play an important role in atmospheric heterogeneous photoreaction processes.

Crosswinds will lead to large-scale flow separation in the nacelle inlets, which seriously affects the flight safety of the aircraft; there is an urgent need to develop flow control measures. As a plasma flow control method, the application of surface dielectric barrier discharge in the field of nacelle inlet separation control is of great significance for improving the intake quality. Based on the characteristic law of the baseline flow field, the flow control effect of the nacelle inlet separation flow field experiments with NS-DBD, and the influence of the actuation frequency on the flow control is discussed. A comparative experimental study of NS-DBD and AC-DBD is carried out. Finally, the flow control mechanisms for both are discussed. The results show that under the condition that the flow velocity of the wind tunnel is 35 m/s and the crosswind angle is 10°, the average total pressure loss coefficient and distortion index decrease by 29.62% and 44.14% by NS-DBD actuation. At the same time, exists an inherent optimal coupling frequency in NS-DBD, and the control effect of NS-DBD is better than that of AC-DBD. NS-DBD mainly through shock waves and induced vortices, while AC-DBD mainly through the induced generation of near-wall jets to reduce the inverse pressure gradient and improve nacelle flow separation.

Modern engineering has an increasing demand for engineering structures with various desired properties. Metamaterials are endowed with multiple functions by changing the type of structural unit cell or geometric parameters, and show many advantages over traditional materials. However, there exists a gap in the research of the compressive theory of tubular anti-tetrachiral structure. In this study, we investigated the deformation theory of tubular anti-tetrachiral structure subjected to quasi-static compression. We derived the expressions for the plateau stress and plateau length of the structure, and analyzed the impact of geometric parameters on the safety parameters (i.e., plateau stress and plateau length). We performed multi-objective optimization within a limited range of geometric parameters and obtained a set of parameters that increased the plateau stress and plateau length by 244.24% and 3.25%, respectively, when compared to the initial parameters. Our findings provide a foundation for the application of tubular anti-tetrachiral structure.

In this paper, an experimental study on the stability of hypersonic plate boundary layer is carried out using a spanwise plasma actuation array. The characteristics and evolution of different kinds of unstable waves in the hypersonic plate boundary layer are analyzed based on the results of linear stability theory and high-frequency pulsation sensors. The typical morphological characteristics of the boundary layer and the macro-control effect of the plasma actuation array are explored through a high-speed schlieren method. Finally, based on grayscale mode extraction and proper orthogonal decomposition (POD), the influences of three different actuation frequencies on the instability waves and characteristic structure of the boundary layer are studied, including the dominant frequency of the first mode wave, the half-frequency of the dominant frequency of the first mode wave, and the dominant frequency of the second mode wave, the change of characteristic structures under the regulation of plasma actuation is further discussed. The corresponding regulation rules and mechanisms are summarized. The results show that the plasma actuation array can advance the starting position of laminar discontinuities and that the induced coherent structure can excite instabilities at an earlier flow-direction position. The actuation can be used to control the stability of the boundary layer by acting on the first mode wave to break the original unstable wave spectrum characteristics. This verifies the ability of extensional array plasma actuation to regulate the stability of the hypersonic plate boundary layer and suggests it has great potential in the promotion of hypersonic boundary layer transition.

Introducing bi-material double-triangular elements into a standard re-entrant structure, a novel metamaterial is proposed in this work. The effect of geometrical parameters on negative Poisson’s ratio (NPR) and coefficient of thermal expansion (CTE) is studied theoretically and verified by finite element (FE) method. Results show that the proposed structure provides tunable Poisson’s ratios in wide ranges (from negative to positive) by adopting base materials with different elastic moduli. The horizontal Poisson’s ratio of the structure increases with the increment of the re-entrant angle, and decreases with the decrement of the elastic modulus ratio (i.e., the ratio of the elastic modulus of MAT-1 and MAT-2); while the vertical Poisson’s ratio decreases with the increment of the re-entrant angle, but is independent on the elastic modulus ratio. Moreover, comparing with various reported re-entrant structures, the proposed design enhances elastic modulus significantly owing to the introduced bi-material triangles. The structure also exhibits tailorable CTEs. The CTE of the structure decreases with the increment of the CTE ratios and the re-entrant angle.

Boundary layer transition has always been a frontier in the field of fluid mechanics, and hypersonic boundary layer stability experiments can help reveal the physical mechanisms behind such transitions. In particular, the regulation of unstable waves in boundary layers is critical for transition prediction and control. Plasma actuation is a popular flow control method that has made progress in moderating the stability of supersonic boundary layer. However, there have been few studies on regulating the stability of hypersonic boundary layers with plasma actuation. In this paper, wind tunnel experiments are carried out under Mach 6 flow to study the stability regulation of a hypersonic sharp cone boundary layer with nanosecond pulsed plasma actuation. First, the typical characteristic structure of the rope-like structure is captured by the high-speed schlieren method. Then, combining the sensor results and theoretical analysis, the rope-like structure and the dominant instability wave resolved by the schlieren power spectrum density method are determined to correspond to the second-mode wave. The characteristic unstable quasi-ordered structure of the boundary layer under actuation is then extracted, and the impact effect and modulation effect of this structure on the second-mode wave are analyzed. Finally, the mechanism by which actuation influences the boundary layer instability is studied using proper orthogonal decomposition. The results show that actuation can enhance boundary layer pulsation, and the coupling effect between the actuation and boundary layer can produce regular unstable quasi-ordered structures. The intrinsic mechanism works by impinging on and modulating the second-mode waves, and there are characteristic modes of the rope-like structure and the unstable structures distributed over the whole flow-direction range. This verifies the ability of plasma actuation to stimulate the instability of hypersonic cone boundary layers and provides technical support for the further development of transition control methods.

A novel optical fiber sensor based on weak coupling twin-core fiber (TCF) is proposed and experimentally demonstrated. The sensing structure consists of two single mode fibers (SMF) and fabricated by program controlled tapering the spliced region between the first SMF and a segment of TCF. During this period the light power was gradually transferred from the SMF into the cladding mode near the waist zone, after the waist zone, the optical power was gradually concentrated from the cladding mode into the cores mode, which could affect the extinction ratio of the interference peaks in the transmission spectrum. In order to obtain better interference spectrum, we adjusted the fusion structure of the tail fiber and the TCF, and the cross sections of the optical fibers are dislocated to a certain extent, so that the interference process of the beam changes. In the process of adjusting the structure, we observed the spectral changes in the spectrometer at the same time until the best interference spectrum appeared, and then we completed the fusion. The interference between different modes can be affected by changes in the external environment, like refractive index (RI) and strain, which also dictates the wavelength shift of the transmission spectrum. In the experiment, we have studied the sensing response of the optical fiber sensor to the RI and strain, and the sensing sensitivity is 131.1nm/RIU and 1.26x10-3 dB/μe respectively. All sensors fabricated in this paper show good linearity in terms of the spectral wavelength shift.

Understanding the effects of soluble impurities or suspended particles on ice growth is of significant importance from Earth science to materials engineering. Ions are common impurities with ice in a wide range of fields, but their effects on ice growth remain largely elusive. Here, we studied the ion-specific effects on single ice crystal growth in various electrolyte and polyelectrolyte solutions and found F- and NH4+ show remarkable abilities of inducing single ice crystals to form hexagonal shapes and reducing the growth rates of ice crystals. Molecular dynamics simulations reveal the accumulation of F- around the ice/solution interface that plays a key role in the shapes and growth rates of single ice crystals. The understanding of ion-specific effects on ice growth opens up more possibilities for improving related fields, e.g., freeze desalination and cryopreservation.

Swept shock wave/boundary layer interactions occur widely in the internal and external flows of supersonic and hypersonic aircraft, which can seriously affect aircraft stability and engine intake efficiency. Based on the conventional S–A turbulence model, this study investigates surface arc plasma actuation for regulating swept shock wave/boundary layer interactions at Mach 2.95 to explore the ability and the three-dimensional shock wave/boundary layer interactions control method of plasma actuation. First, the flow control effect is explored in terms of indirect control by applying actuation in the upstream boundary layer or in front of the separation line, and in terms of direct control by applying actuation in the separation region. These three methods all achieve clear control effects. Control results show that the first method is more effective in regulating the wall pressure and friction coefficient and can improve the friction and heat transfer of the wall in a wide range of flow direction and cone direction. The second method is more effective in regulating separated shock waves. The third aspect is more effective in regulating the reattachment region. The associated control mechanisms are then refined. The control effects of the first control method depend on the transmission of vortices, those of the second are based on the virtual surface generated by actuation, and those of the third rely on energy injection. Finally, the application scenarios of the different control methods are determined according to the flow control requirements of aircraft and the corresponding control mechanisms. This study provides a reference method for solving more complex three-dimensional shock boundary layer interaction problems.

Effective control of hypersonic transition is essential. In order to avoid affecting the structural profile of the aircraft, as well as reducing power consumption and electromagnetic interference, a low-frequency surface arc plasma disturbance experiment to promote hypersonic transition was carried out in the Φ0.25 m double-throat Ludwieg tube wind tunnel at Huazhong University of Science and Technology. Contacting printed circuit board sensors and non-contact focused laser differential interferometry testing technology were used in combination. Experimental results showed that the low-frequency surface arc plasma actuation had obvious stimulation effects on the second-mode unstable wave and could promote boundary layer transition by changing the spectral characteristics of the second-mode unstable wave. At the same time, the plasma actuation could promote energy exchange between the second-mode unstable wave and other unstable waves. Finally, the corresponding control mechanism is discussed.

A novel compact fiber optic sensor fabricated by twin-core fiber (TCF) and multimode fibers (MMF) was proposed and demonstrated. The interference light generated in structures can be used for sensing applications. Through a variety of experiments, we have studied the sensing application of structure, and obtained a good sensing sensitivity.

We proposed and experimentally demonstrated a miniature optical fiber temperature sensor consisting of a seven core fiber (SCF) and a few mode fiber (FMF). The device is fabricated by splicing a section of FMF with a segment of SCF to form a FMF-SCF based sensing structure, and during the FMF region, few modes can be excited and will propagate within the SCF. In experiment, the proposed device has good quality interferometric spectra, and the highest extinction ratio of 27 dB was achieved. When the temperature increases from room temperature to 110 °C, the temperature response properties of the sensor have been investigated, the wavelength sensitivity of about 91.8 pm/°C and the amplitude sensitivity of about 1.57 × 10−2 a.u./°C are obtained, respectively. Due to its easy and controllable fabrication, the sensor can be a suitable candidate in temperature sensing applications.

A fiber optic sensor capable of measuring refractive index (RI), strain, and temperature simultaneously is proposed and demonstrated. The sensor is formed by the integration of a four-core fiber (FCF) with a fiber Bragg grating (FBG). When the FCF is kept straight, a pronounced interference pattern appears in the transmission spectrum. Compared with previously reported optical fiber modal interferometers, higher fringe visibility can be obtained in our scheme. The maximum fringe visibility of the interference resonance dips exceeds 21 dB. By monitoring the wavelength shifts of three dips, simultaneous measurement of RI, strain, and temperature can be achieved. For 10 pm wavelength resolution, the resolution of the sensor is 0.0004 RIU, 11.06 μe, and 0.17 °C in RI, strain, and temperature, respectively. The proposed sensor has the advantages of easy to construct, higher fringe visibility, and the capability to measure RI, strain, and temperature simultaneously, which are desirable features in RI measurement.

It has been a long-standing and intriguing issue to develop robust methods to identify metastable states and interstate transitions from simulations or experimental data to understand the functional conformational changes of proteins. It is usually hard to define the complicated boundaries of the states in the conformational space using most of the existing methods, and they often lead to parameter-sensitive results. Here, we present a new approach, visualized Trajectory Map (vTM), to identify the metastable states and the rare interstate transitions, by considering both the conformational similarity and the temporal successiveness of conformations. The vTM is able to give a nonambiguous description of slow dynamics. The case study of a β-hairpin peptide shows that the vTM can reveal the states and transitions from all-atom MD trajectory data even when a single observable (i.e, one-dimensional reaction coordinate) is used. We also use the vTM to refine the folding/unfolding mechanism of HP35 in explicit water by analyzing a 125 μs all-atom MD trajectory and obtain folding/unfolding rates of about 1/μs, which are in good agreement with the experimental values.

A novel refractive index (RI) sensor based on middle-tapered multicore fiber (TMCF) is proposed and experimentally demonstrated. The sensing structure consists of two singlemode fibers (SMF) and simply spliced a section tapered four-core fiber between them. The light injected from the SMF into the multicore fiber (MCF) will excite multiple cladding mode, and interference between these modes can be affected by the surrounding refractive index (SRI), which also dictates the wavelength shift of the transmission spectrum. Our experimental investigations achieved a sensitivity around 171.2 nm/RIU for a refractive index range from 1.3448 to 1.3774. All sensors fabricated in this paper show good linearity in terms of the spectral wavelength shift versus changes in RI.

Efficiently and accurately analyzing high-dimensional time series, such as the molecular dynamics (MD) trajectory of biomolecules, is a long-standing and intriguing task. Two different but related techniques, i.e., dimension reduction methods and clustering algorithms, have been developed and applied widely in this field. Here we show that the combination of these techniques enables further improvement of the analyses, especially with very complicated data. Specifically, we present an approach that combines the trajectory mapping (TM) method, which constructs slow collective variables of a time series, with density peak clustering (DPC) [A. Rodriguez and A. Laio, Science 344, 1492 (2014)SCIEAS0036-807510.1126/science.1242072], which identifies similar data points to form clusters in a static data set. We illustrate the application of the TMDPC approach with hundreds of microseconds of all-atomic MD trajectories of two proteins, the villin headpiece and protein G. The results show that TMDPC is a powerful tool for achieving the metastable states and slow dynamics of these high-dimensional time series due to the efficient consideration of the time successiveness and the geometric distances between data points.

Using all-atomic molecular dynamics (MD) simulations, we show that the structure of interfacial water (IW) induced by substrates characterizes the ability of a substrate to nucleate ice. We probe the shape and structure of ice nuclei and the corresponding supercooling temperatures to measure the ability of IW with various hydrogen polarities for ice nucleation, and find that the hydrogen polarization of IW even with the ice-like oxygen lattice increases the contact angle of the ice nucleus on IW, thus lifting the free energy barrier of heterogeneous ice nucleation. The results show that not only the oxygen lattice order but the hydrogen disorder of IW on substrates are required to effectively facilitate the freezing of top water.

Most current enhanced sampling (ES) algorithms attempt to bias a potential energy surface based on preset slow collective variables to improve simulation efficiency. However, due to difficulty in obtaining slow variables in complex molecular systems, approximate slow variables are usually applied in ES, which often fail to achieve the expected high efficiency and sufficient accuracy when reconstructing equilibrium properties. In this paper, we demonstrate that the trajectory mapping (TM) technique has the potential to provide the required slow variables for ES.We illustrate the application of a typical ES algorithm (metadynamics) based on the slow variables constructed from the TM in a hairpin peptide system. In this system, both the equilibrium properties and slow dynamics are accurately obtained within approximately two to three orders of magnitude shorter simulation time than in regular molecular dynamics simulation.

A novel fiber-optic sensor using fiber ring cavity laser based on multipath Mach–Zehnder interferometer (m-MZI) is proposed and experimentally demonstrated. The m-MZI consists of a segment of four-core fiber (FCF) spliced between two single-mode fibers. The m-MZI is inserted in the fiber ring laser cavity, which acts as a bandpass filter and the sensing component simultaneously. Compared with previously reported MZI, higher phase sensitivity can be obtained in the proposed m-MZI due to the multipath interference configuration embedded in one fiber. The interference fringe of the FCF would shift with the variation of monitoring parameters and then caused the lasing wavelength changes. So it is available to measure these parameters by simply monitoring the variation of lasing wavelength. Experimental results show that the sensitivities of the sensor in strain, RI, and curvature are 2.21 pm/ $\mu \varepsilon $ , 113.27 nm/RIU, and 2.55 nm/ $\text{m}^{-1}$ , respectively. Thus, the proposed sensor is easy to conduct and monitor, in addition, it also has the advantages of high resolution and high signal-to-noise ratio.

A novel, simple and compact optical fiber sensor based on multipath Mach-Zehnder interferometer (m-MZI) is proposed and experimentally demonstrated. The device consists of a segment of four-core fiber (FCF) spliced between two single-mode fibers (SMFs). When the m-MZI is kept straight, an obvious interference pattern appears in the transmission spectrum. Compared with previously reported optical fiber modal interferometers, higher phase sensitivity can be obtained in our scheme due to the multipath interference configuration embedded in one fiber. The maximum fringe visibility of the interference resonance dips exceeds 23 dB. The interference fringe of the FCF would shift with the variation of strain, temperature, refractive index (RI) and curvature. So it is possible to measure these parameters by simply monitoring the wavelength shifts. Experimental results show that the sensitivities of the sensor in strain, temperature, RI and curvature are 1.78 pm/μe, 209 pm/°C, 91.39 nm/RIU and 20.18 nm/m↙1, respectively. To the best of our knowledge, the multi-purpose sensing applications of the sensor have been experimentally investigated for the first time. Moreover, the proposed sensor has the advantages of high fringe visibility and simple fabrication process.

A fiber optic sensor capable of simultaneous measurement of liquid level and temperature is proposed and demonstrated. The sensor is formed by the integration of an asymmetrical fiber Mach–Zehnder interferometer (aFMZI) with a fiber Bragg grating (FBG). The aFMZI was realized by concatenating a fiber taper and a lateral-shifted junction. By using the temperature sensing property of FBG, the liquid level sensor with dynamic temperature compensation can be achieved. For 10 pm wavelength resolution, a resolution of 0.15 cm in liquid level and 1.01 °C in temperature can be achieved. The prototype has the advantages of low fabrication cost and temperature compensated, which are desirable features in liquid level measurement.

An image processing method has been developed to obtain multicore fiber geometric parameters. According to the characteristics of multicore fiber, we using MATLAB to processing the sectional view of the multicore fiber (MCF), and the algorithm mainly concludes the following steps: filter out image noise, edge detection, use an appropriate threshold for boundary extraction and an improved curve-fitting algorithm for reconstruction the cross section, then we get the relative geometric parameters of the MCF in pixels. We also compares different edge detection operator and analyzes each detection results, which can provide a meaningful reference for edge detection.

Including constant, linear function and exponential function, three good guess functions of forward signal power are developed to deal with the difficulty of setting suitable estimates in the multipoint pumping Yb 3+ -doped double-cladding fiber lasers (YDCFLs) model with boundary conditions using MATLAB BVP solvers. Simulation results show that three good guess functions can get rapidly the exact values of YDCFLs after only one iteration times. Among three good guess functions, conversion efficiency critical guesses of linear functions are more stable than those of constant and exponential functions at different fiber lengths. Furthermore, the more pump points, the smaller fluctuations of conversion efficiency critical guesses in linear functions at different fiber lengths. Conversion efficiency critical guesses of linear functions keep small fluctuations with the increases of pump powers at different points pump.

It is a central issue to find the slow dynamic modes of biological macromolecules via analyzing the large-scale data of molecular dynamics simulation (MD). While the MD data are high-dimensional time-successive series involving all-atomic details and sub-picosecond time resolution, a few collective variables which characterizing the motions in longer than nanoseconds are needed to be chosen for an intuitive understanding of the dynamics of the system. The trajectory map (TM) was presented in our previous works to provide an efficient method to find the low-dimensional slow dynamic collective-motion modes from high-dimensional time series. In this paper, we present a more straight understanding about the principle of TM via the slow-mode linear space of the conformational probability distribution functions of MD trajectories and more clearly discuss the relation between the TM and the current other similar methods in finding slow modes.

An optical fiber temperature and strain fiber sensor based on the few-mode fiber (FMF) and the fiber Bragg grating (FBG) is proposed and demonstrated. The sensor unit is fabricated by a length of FMF offset splicing with one section of single-mode fiber (SMF). The temperature and strain are measured simultaneously because of the different sensitivities of the spectrum dips. The interference with the highest extinction ratio of 23 dB can be achieved. The temperature sensitivities of -34.3 pm ∕ ° C and 10.7 pm ∕ ° C and strain sensitivities of -2 pm ∕ μ e and 0.67 pm ∕ μ e are achieved, respectively. The proposed sensor may exhibit the great potential in fields of the dual-parameter measurement due to its compact structure, simple configuration.

We propose an optical fiber strain sensor consisting of a spliced twin-core fiber (TCF). The sensor structure has two fiber tapers, which are weak taper and T-shaped taper. The taper waist of T-shaped taper is about half-size of the weak taper, but the length of the T-shaped taper is also about half- length of the weak taper. In the sensing structure, core mode and cladding modes of the TCF are excited simultaneously. Two tapered regions around the splicing interfaces can play the roles of splitting and combining the modes in the fiber. In experiment, the proposed device has good quality interferometric spectra, and the highest extinction ratio of 20 dB is achieved. When the stress increases from 0 μe to 841.5 μe, different wavelength sensitivities of 3.31 pm/μe and 6.11 pm/μe are achieved. Different intensity sensitivities of 7.5 × 10−3 dB/μe and 9.9 × 10−3 dB/μe are also achieved, respectively. The sensor has shown good test stability and low temperature sensitivity, which is good for precise strain measurement in practical applications.

A strain sensor implemented by cascading a Sagnac interferometer (SI) and a fiber modal interferometer (FMI) was proposed and experimentally demonstrated. The SI serving as the fixed part comprises a length of polarization maintaining fiber (PMF), and the FMI serving as the sliding part comprises single mode fiber (SMF) and a length of few mode fiber (FMF) with core-offset structure. By closing the free spectral ranges (FSRs) of the SI and FMI, Vernier-effect exists and makes contribution to magnify the sensitivity. Through monitoring the FSR of the spectral envelope, the strain sensing is realized. A high sensitivity of 65.71 pm/μe which is more than 20 times higher than that of the single FMI is obtained experimentally. Moreover, thanks to the polarization insensitivity of FMF, the proposed sensor is immune to environmental disturbance and suitable for physical sensing applications.

An optical fiber magnetic field sensor based on the no-core fiber (NCF) and magnetic fluid (MF) is proposed and demonstrated. By using the temperature sensing property of fiber Bragg grating (FBG), the magnetic field sensor with dynamic temperature compensation can be achieved. The transmittance of the NCF that is sealed in a capillary with MF is highly sensitive to the surrounding magnetic field. Experimental results show that the sensor has the magnetic field sensitivity of 9.2 pm/mT ranging from 0 to 9 mT and the temperature sensitivity of 11.9 pm/°C ranging from 15 to 45 °C. The prototype might be attractive due to its low fabrication cost and simple configuration, which are desirable features in magnetic field measurement.

An adaptive shooting method for solving 4-point side-pumping high power Yb3+-doped double-clad fiber lasers (YDCFLs) is developed. The adaptive shooting method combines the simple identification process and shooting method procedure. Simulation results show that the adaptive shooting method can identify automatically and easily eight different cases in the 4-point side-pumping YDCFLs with different pump schemes. The initial estimate values of pump powers as independent variables are given approximate expressions and the signal powers are set random functions to speed the adaptive shooting method. The adaptive shooting method can succeed rapidly to get the exact results after average less than eight iteration steps updating initial guess values.

Excellent initial guess functions for shooting method, providing for setting suitable initial estimates, are developed to solve multipoint pumping high power Yb3+-doped fiber lasers (YDFLs) model with boundary conditions. Simulation results show that shooting method with excellent initial guess functions can get rapidly the exact values of YDFLs after less than five iteration times. In excellent initial guess functions for signal power and pump power of 3-point pumping or 4-point pumping YDFLs, the guess value of conversion efficiency is greater than or equal to the critical guess value of conversion efficiency. The critical guess value of conversion efficiency is less than 0.21 for all the fiber length in three-point pumping or four-point pumping YDFLs. Comparing to number sequence transition method based on MATLAB BVP solvers of bvp4c(NSTM-bvp4c) and shooting method with answer ranges definition, the simple shooting method with excellent initial guess functions is more reliable and faster for multipoint pumping YDFLs.

An optical fiber sensor based on a Mach-Zehnder interferometer with hybrid structure optical fiber for simultaneous measurement of refractive index (RI), strain, and temperature is proposed and demonstrated. The proposed structure is a hybrid structure based on a non-core fiber combined with few-mode fiber. The possibility of simultaneously measuring RI, strain, and temperature relies on the different sensitivity responses of three resonance peaks in the transmission spectrum. Thus, simultaneous measurement of RI, strain, and temperature is realized by calculating the wavelength shift of the three resonance peaks. The experimental results show that the sensitivities of RI are 22.9 pm/RIU, 24.6 pm/RIU, and 97 pm/RIU when RI changes from 1.3707 to 1.39809, respectively. The sensitivities of strain are $ - {3.5}\,\, \text{pm}/ \unicode{x00B5}\unicode{x03B5}$-3.5pm/µε, $ - {1.9}\,\, \text{pm}/ \unicode{x00B5} \unicode{x03B5}$-1.9pm/µε, and $ - {4.1}\,\, \text{pm}/ \unicode{x00B5} \unicode{x03B5}$-4.1pm/µε in the range from 0 to 1400 pm/µ$\unicode{x03B5}$ε. The sensitivities of temperature ranging from 35°C to 55°C are 162 pm/°C, 194 pm/°C, and 162 pm/°C, respectively. The proposed sensor, with advantages of simple configuration, compact structure, and high sensitivity, exhibits great potential in fields of multi-parameter measurement.

We employ multiple order parameters to analyze the local structure of liquid water obtained from all-atom simulations, and accordingly identify three types of molecules in water. In addition to the well-known low-density-liquid and high-density-liquid molecules, the newly identified third type possesses an ultra-high density and over-coordinated H-bonds. The existence of this third type decreases the probability of transition of high-density-liquid molecules to low-density-liquid molecules and increases the probability of the reverse one.

The abrupt event monitoring is a challenging and critical issue in water environment systems. There are two main different abrupt events in the monitoring system, namely, the emergency water pollution accident and the abrupt sensor fault. The two different abrupt events have similar data characteristics, and few methods can be used to recognize the events. In this paper, a novel abrupt event monitoring approach based on kernel principal component analysis (KPCA) and support vector machines is proposed, which is combined with the physical redundancy method. The trust mechanism is introduced into the proposed approach to reduce the interference of external noise and improve the performance of quick response for the abrupt events. A spare data area is set up to store the data for the KPCA modeling. The data in the spare data area are updated continuously, and the KPCA model is updated subsequently to improve the adaptivity of the KPCA model for the abrupt event monitoring. The experimental results show that the proposed approach is capable of detecting and recognizing the two different abrupt events efficiently.

Pyrophosphate ion (PPi) release during transcription elongation is a signature step in each nucleotide addition cycle. The kinetics and energetics of the process as well as how it proceeds with substantial conformational changes of the polymerase complex determine the mechano-chemical coupling mechanism of the transcription elongation. Here we investigated detailed dynamics of the PPi release process in a single-subunit RNA polymerase (RNAP) from bacteriophage T7, implementing all-atom molecular dynamics (MD) simulations. We obtained a jump-from-cavity kinetic model of the PPi release utilizing extensive nanosecond MD simulations. We found that the PPi release in T7 RNAP is initiated by the PPi dissociation from two catalytic aspartic acids, followed by a comparatively slow jump-from-cavity activation process. Combining with a number of microsecond long MD simulations, we also found that the activation process is hindered by charged residue associations as well as by local steric and hydrogen bond interactions. On the other hand, the activation is greatly assisted by a highly flexible lysine residue Lys472 that swings its side chain to pull PPi out. The mechanism can apply in general to single subunit RNA and DNA polymerases with similar molecular structures and conserved key residues. Remarkably, the flexible lysine or arginine residue appears to be a universal module that assists the PPi release even in multi-subunit RNAPs with charge facilitated hopping mechanisms. We also noticed that the PPi release is not tightly coupled to opening motions of an O-helix on the fingers domain of T7 RNAP according to the microsecond MD simulations. Our study thus supports the Brownian ratchet scenario of the mechano-chemical coupling in the transcription elongation of the single-subunit polymerase., Author Summary RNA polymerases (RNAPs) are usually recognized to work as Brownian ratchet machines, with mechanical movements and chemical reactions loosely coupled during transcription elongation. Nevertheless, structural studies on the single-subunit T7 RNAP had suggested an alternative power stroke scenario, which requires the PPi product release reaction tightly couples with or drives the polymerase translocation. To resolve the conflicting views in the mechano-chemical coupling scenario, we conducted atomistic molecular dynamics (MD) simulations to investigate the PPi release mechanism of T7 RNAP. Using a large number of nanosecond short simulations, we constructed the Markov state model (MSM) and found that the PPi release undergoes a jump-from-cavity activation process, unlikely to further support the translocation. The MSM implementations to multi-subunit RNAPs previously demonstrated instead the charge facilitated hopping mechanisms of the PPi release. To further explore essential slow motions during the PPi release, we also performed microsecond long MD simulations. The PPi release does not appear to be tightly coupled to the rotational opening of an O-helix that is directly tied to the polymerase translocation. Hence, the study again disfavors the power stroke scenario. Remarkably, we discovered a key residue Lys472 that is able to assist PPi to jump out of the cavity. The mechanism can be general to a group of polymerases sharing conserved structural features with T7 RNAP, while the lysine or arginine module to assist the PPi release can be universal to both the single and multi-subunit RNAPs, even though the overall structural features and the PPi release mechanisms are significantly different.