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A channel with one backward-facing step and one forward-facing step is a typical configuration in engineering applications. In the channel, good heat transfer performance is often required, and the enhancement is usually achieved by employing different passive control methods, such as modification of geometric configuration or application of nanofluid. However, the other control method, i.e., active flow control (AFC), which is likely more effective, has been rarely applied in such a scenario. This study aims to bridge this gap by exploring how a rigid plate affects the heat transfer of the channel. The plate either is stationary or actively rotates, corresponding to passive flow control or AFC. The influences of the horizontal position of the plate (S) and its orientation angle (θ) on the heat transfer performance are studied when the plate is stationary to provide a baseline. Compared to the baseline, the effects of S, θ, and the rotation frequency (fr) are revealed when the plate undergoes a sinusoidal rotation. Such a thermo-fluid dynamic problem is numerically simulated by the immersed-boundary lattice Boltzmann method. The results show that the plate can improve the heat transfer performance no matter whether it rotates or not, compared to the case without a plate. The rotating plate outperforms the stationary one when θ and fr are properly chosen at each S. Substantial improvement can be achieved when vortex synchronization or resonance occurs in the channel, i.e., when the natural vortex shedding frequency is close or equal to fr. [ABSTRACT FROM AUTHOR]

The development of lithium-niobate-on-insulator (LNOI) technology has sparked the advancement of numerous on-chip photonic devices for generating and manipulating light. However, the optical absorption of thin-film LNOI is very weak, especially in the near-infrared field, due to their extremely thin absorption layer and intrinsic band structure characteristics. Here, we propose a comprehensive framework to tackle these obstacles by means of ion implantation of Ag ions, which facilitates the encapsulation of plasmonic silver (Ag) nanoparticles within LNOI thin films. Through the incorporation of Ag nanoparticles, we have observed a tailored profile of optical nonlinear absorptions in LNOI thin films, transitioning from reverse saturable absorption to saturable absorption as well as optical-induced absorption to photobleaching behaviors. Moreover, the plasmonic effect elicited by the embedded Ag nanoparticles amplifies the third-order nonlinear coefficient by an order of magnitude. This work opens an avenue toward the development of high-performance on-chip photonic devices harnessing third-order nonlinearities of LNOI. [ABSTRACT FROM AUTHOR]

High-entropy oxide (HEO) nanoparticles have been regarded as a promising catalytic material system for oxygen evolution reaction in recent years. However, their traditional physical and chemical synthesis remains challenges due to the limitation of fabricating controllable small size HEO nanoparticles. Herein, a general and novel method of low-energy-recoil ion implantation and subsequent annealing is successfully developed to synthesize high-entropy oxide nanoparticles catalysts. By controlling the fluence of irradiation Ar+ ions, the size and the load of HEO nanoparticles can be accurately controlled. The obtained (FeCoNiCrAl)O HEO nanoparticles exhibit an overpotential of 295 mV at 10 mA cm−2, a small Tafel slope of 39 mV dec−1 and good stability in 1 M KOH, which is much better than the properties of binary and medium entropy oxide counterparts prepared by the same method, showing the good application prospect of low-energy-recoil ion implantation in the preparation of complicated multi-element-metal oxide nanoparticles. [ABSTRACT FROM AUTHOR]

A waveguide Bragg grating (WBG) can facilitate an adaptable method for glucose monitoring according to the optical properties of polymer materials. We propose the design and fabrication of a WBG for glucose monitoring. By exploiting glucose oxidase as the upper cladding, polydimethylsiloxane was used as the substrate, and polymethyl methacrylate was used as the core layer. We investigated the effects of the diffraction order, waveguide structure, and grating period on the reflected spectrum of the WBG. Finally, process reproducibility after long-term storage and the capability to eliminate background solution interference (to achieve more specific glucose detection) were evaluated. The experimental results showed that when the glucose concentration was in the range of [0, 3.6] mg/ml, as the glucose concentration increased, the wavelength decreased approximately linearly, with a sensitivity of approximately 242.9 pm/(mg/ml) in the range of 0–2.7 mg/ml, while maintaining good selectivity and stability. The WBG for glucose monitoring has the advantages of a large measurement range and high sensitivity. This approach facilitates the application potential of such polymer material-based WBG photonic sensors in wearable technology and realizes the measurement of human blood glucose. [ABSTRACT FROM AUTHOR]

The flow around two tandem circular cylinders leads to significant lift fluctuation in the downstream cylinder owing to periodic vortex shedding. To address such research issues, we present herein a numerical study that uses deep reinforcement learning to perform active flow control (AFC) on two tandem cylinders with a low Reynolds number of 100, where the actuator causes the rotation of the downstream cylinder. First, the cylinder center spacing ratio L * varies from 1.5 to 9.0, and the variation of L * leads to the quasi-steady reattachment regime ( L * ≤ 3.5) and the co-shedding regime ( L * ≥ 4.0). The fluctuating lift of the downstream cylinder is maximum when L * = 4.5. Next, we train an optimal AFC strategy that suppresses 75% of the lift fluctuation in the downstream cylinder. This approach differs from using direct-opposition control to change the vortex-shedding frequency or strength, as reported in previous studies. This strategy modifies the phase difference between the lift fluctuations of the two cylinders by delaying the merging with the upstream cylinder wake and accelerating the formation of recirculating bubbles after the vortex merging. With the new phase difference, the effect of the additional lift from the upstream cylinder is significantly mitigated. The results of the dynamic mode decomposition show that the vortices surrounding the downstream cylinder in mode 1 that contribute to the lift fluctuation are weakened. To the best of our knowledge, this investigation can provide new ideas and physical insights into the problem of AFC under disturbed incoming flow. [ABSTRACT FROM AUTHOR]

Detailed and reliable flow information is the basis for understanding and further mediating turbulent flows. Due to experimental limitations, such as the absence of seeding particles owing to an inhomogeneous tracer distribution or obstructed optical paths, gappy flow-field data frequently appear with diverse shapes. To resolve this problem, we propose herein the use of a convolutional neural network (CNN) model to reconstruct the velocity field with the missing information of wall-confined turbulent flows. We consider the example of a turbulent channel flow with a frictional Reynolds number R e τ = 180 and use machine learning to attain the given objective. High-fidelity numerical data obtained by direct numerical simulation based on the lattice Boltzmann equation are used to generate the datasets required for network training, where data in randomly located square or rectangular regions are masked to provide a maximally realistic instantaneous gappy flow field. The results show that the missing information in gappy regions can be effectively reconstructed for both instantaneous and temporally continuous flow fields. Furthermore, the results are insensitive to the missing locations, even if the locations vary with time. The L2 relative error of the reconstructed instantaneous flow field is generally around 2%. Furthermore, an analysis based on the kinetic-energy spectrum and proper orthogonal decomposition verifies that the reconstructed data are physically consistent with the ground truth. The extracted dominating modes have a maximum relative error level of 10 − 3 . The results obtained herein verify that the proposed CNN model provides complete and reliable data for gappy flows and are physically consistent with physical data. [ABSTRACT FROM AUTHOR]

Scientific prediction of net carbon emissions in the Beijing–Tianjin–Hebei (BTH) region is of significance to examine carbon emission reduction in the context of the "double carbon" target. In this study, the carbon peak and carbon neutrality states in the BTH region are determined through a regional double carbon target analysis framework, and a logistic chaotic sparrow search algorithm backpropagation neural network hybrid model (LCSSA-BP) optimized by a logistic chaotic sparrow search algorithm (LCSSA) is used to forecast the net carbon emissions. The findings reveal that the net carbon emissions in the BTH region generally increased during the study period; Beijing, Tianjin, and Hebei are in different stages toward the realization of the double carbon target; population size, affluence, and urbanization rate are positively correlated, whereas the proportion of foreign direct investment and energy intensity is negatively associated with net carbon emissions; the prediction accuracy of the LCSSA-BP is superior to that of the SSA-BP and BP neural network, and it can be used to forecast the net carbon emissions in the BTH region. [ABSTRACT FROM AUTHOR]

A new Monte Carlo method has been implemented to describe the angular and polarization distributions of anisotropic liquids, such as water and linear alkylbenzene (LAB), by considering orientational fluctuations of polarizability tensors. The scattered light of anisotropic liquids is depolarized with an angular distribution of 1 + (1 − ρv)/(1 + 3ρv) cos2 θ, which is modified by the depolarization ratio ρv. A standalone experiment has validated the simulation results of LAB. The new method can provide more accurate knowledge on light propagation in large liquid detectors, which is beneficial to the development of reconstruction for detectors. [ABSTRACT FROM AUTHOR]

This paper describes a new experimental setup designed for the direct measurement of the Rayleigh ratio and Rayleigh scattering length for linear alkylbenzene, a solvent commonly used in liquid scintillator detectors for neutrino experiments. Using the new approach, the perpendicularly polarized Rayleigh ratio was determined to be (4.52 ± 0.28) × 10−6 m−1 sr−1 at 405 nm and (3.82 ± 0.24) × 10−6 m−1 sr−1 at 432 nm, and the corresponding Rayleigh scattering length was LRay = 22.9 ± 0.3(stat.) ± 1.7(sys.) m at 405 nm and LRay = 27.0 ± 0.9(stat.) ± 1.8(sys.) m at 432 nm. These results are consistent with both previous results determined using other experimental strategies and theoretical predictions. [ABSTRACT FROM AUTHOR]

Transition metal dichalcogenides (TMDs) have attracted broad interest in photonics owing to their unique electric band structures, which triggers various applications for functional devices. However, the optical absorbance of TMDs is relatively low because of the atomic-scale thickness, limiting further development of TMDs-based nonlinear optical devices. Here, we propose an effective method to enhance the nonlinear optical properties of TMDs using plasmons, which are from embedded silver (Ag) nanoparticles (NPs) inside the fused silica substrate. In such a configuration, the third-order nonlinear absorption coefficient of MoS2 with non-contact Ag NPs is one order of magnitude higher than that of pure monolayer MoS2 under excitation of 515 nm light, and at 1030 nm, the reverse saturable absorption switches to the saturable absorption due to the plasmonic implication. In addition, the mechanism of plasmon-enhanced nonlinear optical properties is confirmed by results of both transient absorption spectroscopy and near-field electromagnetic field simulation. This study on plasmon-enhanced third-order nonlinearity of MoS2 expands the boundaries of TMDs-based optical nonlinearity engineering. [ABSTRACT FROM AUTHOR]

Efficient nonlinear frequency conversion, such as second-harmonic generation in ultracompact structures, is essential for the development of modern nanophotonic devices. Here, we demonstrate intense second-harmonic emission in scalable embedded Ag nanoparticle arrays fabricated by ion implantation into BK7 glass. The interparticle coupling effect significantly enhances the local field at the nanogap (gap size ∼ 1 nm) of two neighboring Ag nanoparticles and finally amplifies second-harmonic emission generated at the surface of plasmonic nanoparticles. Notably, the intensity of second-harmonic emission in embedded Ag nanoparticle arrays is comparable to that of two-dimensional transition metal dichalcogenides under the excitation of a fundamental wave at 1064 nm and independent of the incident polarization angles. Our work offers a promising strategy on the rapid fabrication of low-cost nonlinear optical nanostructures with great environmental stability. [ABSTRACT FROM AUTHOR]

The electric and viscous correction of viscous potential flow (EVCVPF) is developed for analyzing the electrohydrodynamic instability of an electrified leaky-dielectric viscous jet. The EVCVPF model is based on the viscous potential flow (VPF) and the viscous correction of VPF (VCVPF), proposed by Joseph and Wang ["The dissipation approximation and viscous potential flow," J. Fluid Mech. 505, 365–377 (2004)]. The purpose is to resolve the discrepancy between the non-zero irrotational viscous and the electric tangent stresses. The power of the pressure correction is introduced to compensate the neglected viscous dissipation in the flow bulk in VPF, which is equal to the average power of the irrotational viscous and the electric tangent stresses. The model has been validated by comparing it to the exact normal-mode solution of the linearized Navier–Stokes equations (fully viscous flow, FVF). The energy budget is also performed to assist in understanding underlying mechanisms. Results show that EVCVPF is accurate for charged jets with low and moderate viscosities, i.e., the Ohnesorge number approximately Oh ≤ 0.1. The inaccuracy for highly viscous jets are the limitations of VPF itself. The electric field has less influence compared to the fluid viscosity. To achieve more accurate approximations, VCVPF and VPF are chosen for axisymmetric and non-axisymmetric modes under weak electric fields. EVCVPF is in remarkably good agreement with FVF under moderate and strong electric fields. In general, as VCVPF extends the applicability in fluid viscosity of VPF, EVCVPF further improves the adequacy when studying the electrohydrodynamic instability. [ABSTRACT FROM AUTHOR]

We propose a novel active-flow-control strategy for bluff bodies to hide their hydrodynamic traces, i.e., strong shears and periodically shed vortices, from predators. A group of windward-suction-leeward-blowing (WSLB) actuators are adopted to control the wake of a circular cylinder submerged in a uniform flow. An array of velocity sensors is deployed in the near wake to provide feedback signals. Through the data-driven deep reinforcement learning, effective control strategies are trained for the WSLB actuation to mitigate the cylinder's hydrodynamic signatures. Only a 0.29% deficit in streamwise velocity is detected, which is a 99.5% reduction from the uncontrolled value. The same control strategy is found also to be effective when the cylinder undergoes transverse vortex-induced vibration. The findings from this study can shed some light on the design and operation of underwater structures and robotics to achieve hydrodynamic stealth. [ABSTRACT FROM AUTHOR]

Developing efficient methods for boosting light–matter interactions is critical to improve the functionalities of two-dimensional (2D) transition metal dichalcogenides toward next-generation optoelectronic devices. Here, we demonstrate that the light–matter interactions in tungsten disulfide (WS2) monolayer can be significantly enhanced by introducing an air-stable functional substrate (fused silica with embedded plasmonic Ag nanoparticles). Distinctive from conventional strategies, the Ag nanoparticles are embedded under the surface of fused silica via ion implantation, forming a functional substrate for WS2 monolayer with remarkably environmental stability. A tenfold photoluminescence enhancement in WS2 monolayer has been achieved due to the plasmonic effect of Ag nanoparticles. This work offers a strategy to fabricate the plasmon-2D hybrid system at low cost and large scale and paves the way for their applications in optoelectronics and photonics. [ABSTRACT FROM AUTHOR]

Machine learning has recently become a promising technique in fluid mechanics, especially for active flow control (AFC) applications. A recent work [Rabault et al., J. Fluid Mech. 865, 281–302 (2019)] has demonstrated the feasibility and effectiveness of deep reinforcement learning (DRL) in performing AFC over a circular cylinder at Re = 100, i.e., in the laminar flow regime. As a follow-up study, we investigate the same AFC problem at an intermediate Reynolds number, i.e., Re = 1000, where the weak turbulence in the flow poses great challenges to the control. The results show that the DRL agent can still find effective control strategies, but requires much more episodes in the learning. A remarkable drag reduction of around 30% is achieved, which is accompanied by elongation of the recirculation bubble and reduction of turbulent fluctuations in the cylinder wake. Furthermore, we also perform a sensitivity analysis on the learnt control strategies to explore the optimal layout of sensor network. To our best knowledge, this study is the first successful application of DRL to AFC in weakly turbulent conditions. It therefore sets a new milestone in progressing toward AFC in strong turbulent flows. [ABSTRACT FROM AUTHOR]

A second-order perturbation analysis has been performed on the nonlinear temporal instability of para-sinuous disturbances on annular viscous sheets moving in an inviscid stationary gas medium. The mathematical expressions of second-order interface disturbances, velocity, and pressure have been derived. The nonlinear instability of annular viscous sheets has several characteristics which differ from that of planar viscous sheets: (1) both the first-order interface disturbances and the second-order interface disturbances contribute to breakup; (2) the zero-wavenumber component of interface disturbances in the second-order solution is nonzero; (3) the second-order interface disturbance is para-varicose in most cases, but para-sinuous for some cases. As with planar viscous sheets, it was found that viscosity plays a dual role in the nonlinear instability of annular viscous sheets. However, with the decrease in the ratio of inner radius to sheet thickness, the interval between the upper and the lower critical Reynolds numbers shrinks, and when the ratio of inner radius to sheet thickness is less than a certain value, the dual effect of viscosity vanishes. [ABSTRACT FROM AUTHOR]

The development of environmentally friendly, efficient, and universal access renewable energy technology is the key to achieve the goal of sustainable development. (Photo)electrochemical energy storage and conversion technology is an important part. Therefore, to realize the practical application of (photo)electrochemical energy technology, nanostructured catalytic materials need to be reasonably designed, synthesized, and modified. Ion beam technology is a powerful and versatile physical modification method. Modification of various catalytic materials from the surface to interface and thin films can be realized by controlling the species, energy, and fluence of implanted ions. Ion beam technology has its unique advantages, including its compulsivity of element doping and its high controllability, accuracy, and repeatability. It can realize arbitrary element doping and defect control of almost any material and finely control its concentration. This makes it possible for the ion beam technology to adapt to the modification requirements of catalytic materials to tailor the electronic structure, interface structure, and morphology of the materials more finely. Besides, a variety of strategies for material design can be realized using ion beams, including element doping, defect control, heterostructure construction, and micro/nanostructure formation, which may bring novel changes in catalytic materials. In this Review, we briefly introduce the principle of ion beam technology and introduce various ion beam technologies that can be applied to different catalytic material modification applications. We systematically review the research progress on the application of ion beam technology in photocatalytic, photoelectrocatalytic, and electrocatalytic materials for water splitting including bandgap engineering, defect engineering, heterostructure formation through ion doping, ion irradiation, ion sputtering, and their combined effects. The applications of ion beam technology on modification of fuel oxidation reaction and oxygen reduction reaction electrocatalysts for fuel cells are also introduced. The advantages of ion beam technology in the modification of catalytic materials are summarized. Several promising topics are proposed to look forward to the future development of ion beam technology in the field of catalytic materials. [ABSTRACT FROM AUTHOR]

We demonstrate the use of high-fidelity computational fluid dynamics simulations in machine-learning based active flow control. More specifically, for the first time, we adopt the genetic programming (GP) to select explicit control laws, in a data-driven and unsupervised manner, for the suppression of vortex-induced vibration (VIV) of a circular cylinder in a low-Reynolds-number flow (Re = 100), using blowing/suction at fixed locations. A cost function that balances both VIV suppression and energy consumption for the control is carefully chosen according to the knowledge obtained from pure blowing/suction open-loop controls. By implementing reasonable constraints to VIV amplitude and actuation strength during the GP evolution, the GP-selected best ten control laws all point to suction-type actuation. The best control law suggests that the suction strength should be nonzero when the cylinder is at its equilibrium position and should increase nonlinearly with the cylinder's transverse displacement. Applying this control law suppresses 94.2% of the VIV amplitude and achieves 21.4% better overall performance than the best open-loop controls. Furthermore, it is found that the GP-selected control law is robust, being effective in flows ranging from Re = 100 to 400. On the contrary, although the P-control can achieve similar performance as the GP-selected control at Re = 100, it deteriorates in higher Reynolds number flows. Although for demonstration purpose the chosen control problem is relatively simple, the training experience and insights obtained from this study can shed some light on future GP-based control of more complicated problems. [ABSTRACT FROM AUTHOR]

Transition metal dichalcogenides (TMDs) have attracted broad interest in photonics owing to their unique electric band structures, which triggers various applications for functional devices. However, the optical absorbance of TMDs is relatively low because of the atomic-scale thickness, limiting further development of TMDs-based nonlinear optical devices. Here, we propose an effective method to enhance the nonlinear optical properties of TMDs using plasmons, which are from embedded silver (Ag) nanoparticles (NPs) inside the fused silica substrate. In such a configuration, the third-order nonlinear absorption coefficient of MoS2 with non-contact Ag NPs is one order of magnitude higher than that of pure monolayer MoS2 under excitation of 515 nm light, and at 1030 nm, the reverse saturable absorption switches to the saturable absorption due to the plasmonic implication. In addition, the mechanism of plasmon-enhanced nonlinear optical properties is confirmed by results of both transient absorption spectroscopy and near-field electromagnetic field simulation. This study on plasmon-enhanced third-order nonlinearity of MoS2 expands the boundaries of TMDs-based optical nonlinearity engineering. [ABSTRACT FROM AUTHOR]

We report the fabrication of large-area, well-ordered, uniform-sized noble metal bowtie nanoantenna arrays used as an ultra-sensitive surface-enhanced Raman scattering (SERS) substrate by a refined combination of colloid lithography and subsequent oxygen plasma processing. The tip-to-tip distance between neighboring nanotriangles is made as small as 10 nm by accurately controlling the etching time. Owing to the nanoscale property, the as-fabricated Ag bowtie nanoantenna arrays exhibit a local SERS enhancement larger than 107. The experimental results were confirmed by theoretical calculations. [ABSTRACT FROM AUTHOR]

This paper demonstrates a substantial enhancement in radiation tolerance for small period-thickness of CrN/AlTiN multilayered nanofilms. CrN/AlTiN multilayered nanofilms with period-thicknesses of 3, 5, 7, and 9 nm were irradiated by 190 keV Ar+ ions to fluences ranging from 1 to 5 × 1016 ions/cm2. Nanofilm with 3 nm period-thickness begins to be amorphized under 5 × 1016 ions/cm2, while those with larger period-thicknesses are amorphized under 3 × 1016 ions/cm2. Our results show that multilayered ceramic nanofilms are potential radiation tolerant materials with good properties. The interfaces in the multilayered nanofilms act as good sinks to absorb the radiation-induced defects. [ABSTRACT FROM AUTHOR]