ENERGY CONSERVATION FOR THE WEAK SOLUTIONS TO THE 3D COMPRESSIBLE NEMATIC LIQUID CRYSTAL FLOW

In this paper,we establish some regularity conditions on the density and velocity fields to guarantee the energy conservation of the weak solutions for the three-dimensional compressible nematic liquid crystal flow in the periodic domain.

Inducing Fe 3d Electron Delocalization and Spin‑State Transition of FeN_(4) Species Boosts Oxygen Reduction Reaction for Wearable Zinc–Air Battery

Transition metal-nitrogen-carbon materials(M-N-Cs),particularly Fe-N-Cs,have been found to be electroactive for accelerating oxygen reduction reaction(ORR)kinetics.Although substantial efforts have been devoted to design Fe-N-Cs with increased active species content,surface area,and electronic conductivity,their performance is still far from satisfactory.Hitherto,there is limited research about regulation on the electronic spin states of Fe centers for Fe-N-Cs electrocatalysts to improve their catalytic performance.Here,we introduce Ti_(3)C_(2) MXene with sulfur terminals to regulate the electronic configuration of FeN_(4) species and dramatically enhance catalytic activity toward ORR.The MXene with sulfur terminals induce the spin-state transition of FeN_(4) species and Fe 3d electron delocalization with d band center upshift,enabling the Fe(II)ions to bind oxygen in the end-on adsorption mode favorable to initiate the reduction of oxygen and boosting oxygen-containing groups adsorption on FeN_(4) species and ORR kinetics.The resulting FeN_(4)-Ti_(3)C_(2)Sx exhibits comparable catalytic performance to those of commercial Pt-C.The developed wearable ZABs using FeN_(4)-Ti_(3)C_(2)Sx also exhibit fast kinetics and excellent stability.This study confirms that regulation of the electronic structure of active species via coupling with their support can be a major contributor to enhance their catalytic activity.

Influence of architecture and temperature of alkali atom vapor cells on absorption spectra

Chip-sized alkali atom vapor cells with high hermeticity are successfully fabricated through deep silicon etching and two anodic bonding processes.A self-built absorption spectrum testing system is used to test the absorption spectra of the ru-bidium atoms in alkali atom vapor cells.The influence of silicon cavity size,filling amount of rubidium atoms and temperature on the absorption spectra of rubidium atom vapor in the atom vapor cells are studied in depth through a theoretical analysis.This study provides a reference for the design and preparation of high quality chip-sized atom vapor cells.

Maximizing Terahertz Energy Absorption with MXene Absorber

Achieving high absorption in broad terahertz bands has long been challenging for terahertz electromagnetic wave absorbers.Recently in Nature Photonics,Xiao et al.reported the high absorption approaching the theoretical upper limit across the whole terahertz band of MXene-based terahertz absorbers and,on this basis,constructed an applicable,updated alternating current impedance matching model.

An Air‑Rechargeable Zn Battery Enabled by Organic–Inorganic Hybrid Cathode

Self-charging power systems collecting energy harvesting technology and batteries are attracting extensive attention.To solve the disadvantages of the traditional integrated system,such as highly dependent on energy supply and complex structure,an airrechargeable Zn battery based on MoS_(2)/PANI cathode is reported.Benefited from the excellent conductivity desolvation shield of PANI,the MoS_(2)/PANI cathode exhibits ultra-high capacity(304.98 mAh g^(−1) in N_(2) and 351.25 mAh g^(−1) in air).In particular,this battery has the ability to collect,convert and store energy simultaneously by an airrechargeable process of the spontaneous redox reaction between the discharged cathode and O2 from air.The air-rechargeable Zn batteries display a high open-circuit voltage(1.15 V),an unforgettable discharge capacity(316.09 mAh g^(−1) and the air-rechargeable depth is 89.99%)and good air-recharging stability(291.22 mAh g^(−1) after 50 air recharging/galvanostatic current discharge cycle).Most importantly,both our quasi-solid zinc ion batteries and batteries modules have excellent performance and practicability.This work will provide a promising research direction for the material design and device assembly of the next-generation self-powered system.

Recent Progress of MXene-Based Nanomaterials in Flexible Energy Storage and Electronic Devices

The increasing demands for wearable electronics have stimulated the rapid development of flexible energy storage devices.MXenes are considered as promising flexible electrodes due to the ultrahigh volumetric specific capacitance,metallic conductivity,superior hydrophily,and rich surface chemistry.

Effect of Enzyme-assisted Refining on the Properties of Bleached Softwood Pulp

Enzymatic pretreatment of pulp is demonstrated to be potentially effective for decreasing the energy consumption in the refining process.Herein,a neutral cellulase was utilized for the pretreatment of bleached softwood pulp in order to improve the refining performance.Cellulase pretreatment effectively improved the drainability of the pulp and could thus reduce the energy consumption in the refining process.The beating degree of the pulp was significantly improved at 6000 PFI revolutions,at which a maximum increase of 70% could be obtained.The water retention value(WRV) of the pulp increased by 17% after treatment with cellulase at a dosage of 5 IU/g,and the fibers could be easily torn apart after enzymatic treatment.To achieve the same beating degree,the refining time could be shortened by 80% when the pulp was treated with cellulase.Using a low dosage of cellulase,the freeness of the pulp increased rapidly without deterioration of the mechanical properties.

A new dynamic subgrid-scale model using artificial neural network for compressible flow

The subgrid-scale(SGS)kinetic energy has been used to predict the SGS stress in compressible flow and it was resolved through the SGS kinetic energy transport equation in past studies.In this paper,a new SGS eddy-viscosity model is proposed using artificial neural network to obtain the SGS kinetic energy precisely,instead of using the SGS kinetic energy equation.Using the infinite series expansion and reserving the first term of the expanded term,we obtain an approximated SGS kinetic energy,which has a high correlation with the real SGS kinetic energy.Then,the coefficient of the modelled SGS kinetic energy is resolved by the artificial neural network and the modelled SGS kinetic energy is more accurate through this method compared to the SGS kinetic energy obtained from the SGS kinetic energy equation.The coefficients of the SGS stress and SGS heat flux terms are determined by the dynamic procedure.The new model is tested in the compressible turbulent channel flow.From the a posterior tests,we know that the new model can precisely predict the mean velocity,the Reynolds stress,the mean temperature and turbulence intensities,etc.

Entropy and its application in turbulence modeling

The entropy concept was introduced into the turbulence modeling strategy in the present work.First,the turbulent boundary-layer was described from the point of energy dissipation.Based on the theoretical analysis and direct numerical simulations,the relationship between the entropy increment and viscosity dissipation was systematically investigated.Then,an entropy function fswas proposed to distinguish the turbulent boundary-layer from the external flow.This function is universal,independent of the inflow conditions or any specific turbulence model.With this function,a new version of delayed-detached-eddy simulation method SDES was constructed and verified with the supersonic boundary-layer flow and the cavity-ramped flow.Initial results showed that this method could successfully avoid the modeled stress depletion problem inherited from the original DES method.

Ultra-light, high flexible and efficient CNTs/Ti3C2-sodium alginate foam for electromagnetic absorption application

Ultra-light carboxylic functionalized multi-walled carbon nanotubes(CNTs-COOH) and Ti3C2 MXene hybrids modified sodium alginate(CNTs/Ti3C2-SA) based composite foams were prepared through ice-templated freeze-drying method. The microstructure of the synthesized CNTs/Ti3C2 hybrids and CNTs/Ti3C2-SA foams is characterized by the presence of CNTs inserted between MXene layers which prevents their restacking. The resultant CNTs/Ti3C2 hybrids exhibit a unique sandwich-like hierarchical structure. Scanning electron microscopy(SEM) images show that the CNTs/Ti3C2-SA foam exhibits a heterogeneous anisotropic microstructure and CNTs/Ti3C2 hybrids are homogeneously dispersed in the skeleton of the porous foam. In case that the content of the hybrids amounts 40 mg/cm^3, the CNTs/Ti3C2-SA foam possesses excellent electromagnetic(EM) absorption performance with a minimum reflection coefficient(RCmin) as low as-40.0 dB. In case of a sample thickness of 3.95 mm, the RCminreaches-24.4 dB and the effective absorption bandwidth covers the whole X band from 8.2 to 12.4 GHz. A control test shows that, with the same absorbent content, the CNTs/Ti3C2-SA foam exhibits a far better EM performance than that of CNT-free Ti3C2-SA foam.

Direct numerical simulation of hypersonic boundary layer transition over a liftingbody model HyTRV

Direct numerical simulation(DNS)of transition over a hypersonic lifting body model HyTRV developed by China Aerodynamics Research and Development Center is performed.The free-stream parameters are:the free-stream Mach number is 6,the unit Reynolds number is 10000/mm,the free-stream temperature is 79 K,the angle of attack is 0,and the wall temperature is 300 K.Weak random blowing-and-suction perturbations in the leading range are used to trigger the transition.A high order finite-difference code OpenCFD developed by the authors is used for the simulation,and grid convergence test shows that the transition locations are grid-convergence.DNS results show that transition occurs in central area of the lower surface and the concaved region of the upper surface,and the transition regions are also the streamline convergence regions.The transition mechanisms in different regions are investigated by using the spectrum and POD analysis.

Decomposition of mean skin friction in incident shock wave/turbulent boundary layer interaction flows at Mach 2.25

The evolution characteristics of the mean skin friction beneath the supersonic turbulent boundary layer that interacts with incident shock waves at Mach 2.25 are analyzed using Direct Numerical Simulation(DNS). The separated and attached boundary layers in the interaction region that respectively correspond to 33.2° and 28° incident shock angles are considered. The mean skin friction recovery rate for the separated boundary layer is much gentler and distinctly less than that for the attached case where the skin friction completes its recovery within one boundary layer thickness. The novel mean skin friction decomposition method for compressible flows proposed by the recent research is applied in the interaction region to investigate the internal evolution characteristics quantitatively. The results reveal that the three decomposition components are distinctly unequal between the two cases. The contributions of the turbulent motions at different scales to the associated term are focused on using empirical mode decomposition technology. It indicates that the outer large-scale structures dominate separation and reattachment regions, while contributions from inner small-scale structures are limited. In contrast, contributions from the outer largescale structures are dramatically reduced in the attached case, which results in the outer large-scale and inner small-scale motions being of equal importance.

Direct numerical simulation of impinging shock wave and turbulent boundary layer interaction over a wavy-wall

The interaction of an impinging oblique shock wave with an angle of 30°and a supersonic turbulent boundary layer at Ma_(∞)=2.9 and Re_(θ)=2400 over a wavy-wall is investigated through direct numerical simulation and compared with the interaction on a flat-plate under the same flow conditions.A sinusoidal wave with amplitude to wavelength ratio of 0.26 moves in the streamwise direction and is uniformly distributed across the spanwise direction.The influences of the wavy-wall on the interaction,including the characterization of the flow field,the skin-friction,pressure and the budget of turbulence kinetic energy,are systematically studied.The region of separation grows slightly and decomposes into four bubbles.Local peaks of skin-friction are observed at the rear part of the interaction region.The low-frequency shock motion can be seen in the wall pressure spectra.Analyses of the turbulence kinetic energy budget indicate that both diffusion and transport significantly increase near the crests,balanced by an amplified dissipation in the near-wall region.Proper orthogonal decomposition analyses show that the most energetic structures are associated with the separated shock and the shear layer over the bubbles.Only the bubbles in the first two troughs are dominated by a low-frequency enlargement or shrinkage.

Subgrid-scale model based on the vorticity gradient tensor for rotating turbulent flows

A new subgrid-scale(SGS)stress model is proposed for rotating turbulent flows,and the new model is based on the traceless symmetric part of the square of the velocity gradient tensor and the symmetric part of the vorticity gradient tensor(or the so-called vorticity strain rate tensor).The new subgrid-scale stress model is taken into account the effect of the vortex motions in turbulence,which is reflected on the anti-symmetric part of the velocity gradient tensor.In addition,the eddy viscosity of the new model reproduces the proper scaling as O(y^3)near the wall.Then,the new SGS model is applied in large-eddy simulation of the spanwise rotating turbulent channel flow.Different simulating cases are selected to test the new model.The results demonstrate that the present model can well predict the mean velocity profiles,the turbulence intensities,and the rotating turbulence structures.In addition,it needs no a second filter,and is convenient to be used in the engineering rotational flows.

Characteristics of reattached boundary layer in shock wave and turbulent boundary layer interaction

The reattached boundary layer in the interaction of an oblique shock wave with a flatplate turbulent boundary layer at Mach number 2.25 is studied by means of Direct Numerical Simulation(DNS).The numerical results are carefully compared with available experimental and DNS data in terms of turbulence statistics,wall pressure and skin friction.The coherent vortex structures are significantly enhanced due to the shock interaction,and the reattached boundary layer is characterized by large-scale structures in the outer region.The space-time correlation of fluctuating wall shear stress and streamwise velocity fluctuation reveals that the structural inclination angle exhibits a gradual decrease during the recovery process.The scale interactions are analyzed by using a twopoint amplitude modulation correlation.A possible mechanism is proposed to account for the strong amplitude modulation in the downstream region.Moreover,the mean skin-friction is decomposed to understand the physically informed contributions.Unlike the upstream Turbulent Boundary Layer(TBL),the contribution associated with the Turbulence Kinetic Energy(TKE)production is greatly amplified,while the spatial growth contribution induced by the pressure gradient largely inhibits skin-friction generation.Based on bidimensional empirical mode decomposition,the turbulence kinetic energy production contribution is further split into different terms with specific spanwise length scales.

Low Infrared Emissivity and Strong Stealth of Ti-Based MXenes

Advanced scenario-adaptable infrared(IR)stealth materials are crucial for creating localized closed thermal environments.Low emissivity over the broadest possible band is expected,as is superior mechanical deformability.Herein,we report a series of Ti-based MXenes with naturally low emissivity as ideal IR shielding materials.Over a wavelength ranging from 2.5 to 25μm,Ti_(3)C_(2)T_(X) film delivers an average emissivity of 0.057 with the lowest point of 0.042.Such a low emissivity coupled with outstanding structural shaping capability is beyond the current grasp.The reflection-dominated mechanism is dissected.Also,some intriguing scenarios of IR stealth for wearable electronic devices and skin thermal control are demonstrated.This finding lights an encouraging path toward next-generation IR shielding by the expanding MXene family.

Effect of uniform blowing or suction on hypersonic spatially developing turbulent boundary layers

Dear Editors,Over the past decades,the reduction of drag has attracted considerable attention for its potential applications in engineering[1-3].Direct numerical simulations(DNSs) provide accurate data that can be used to study the underlying physics of drag reduction.Inarecent DNS[4] of an incompressible spatially developing turbulentboundary layer with uniform blowing or suction applied on the wall,the drag reduction

Complex Transition of Double-Diffusive Convection in a Rectangular Enclosure with Height-to-Length Ratio Equal to 4: Part I

This is the first part of direct numerical simulation(DNS)of double-diffusive convection in a slim rectangular enclosure with horizontal temperature and concentration gradients.We consider the case with the thermal Rayleigh number of 105,the Pradtle number of 1,the Lewis number of 2,the buoyancy ratio of composition to temperature being in the range of[0,1],and height-to-width aspect ration of 4.A new 7thorder upwind compact scheme was developed for approximation of convective terms,and a three-stage third-order Runge-Kutta method was employed for time advancement.Our DNS suggests that with the buoyancy ratio increasing form 0 to 1,the flow of transition is a complex series changing from the steady to periodic,chaotic,periodic,quasi-periodic,and finally back to periodic.There are two types of periodic flow,one is simple periodic flow with single fundamental frequency(FF),and another is complex periodic flow with multiple FFs.This process is illustrated by using time-velocity histories,Fourier frequency spectrum analysis and the phase-space trajectories.

Cathode Engineering for High Energy Density Aqueous Zn Batteries

CONSPECTUS:Frequent safety accidents of lithium-ion batteries(LIBs)originating from the utilization of flammable electrolytes urges the battery community to develop a safe substitute.This safety background is a boom for aqueous batteries(ABs)which employ aqueous electrolytes to address safety concerns.Recently,ABs have experienced a rapid advance because various battery chemistries have been successively developed,e.g.,aqueous Zn batteries(AZBs),aqueous LIBs,aqueous sodium-ion battery,etc.Impeded by the narrow voltage window of aqueous electrolytes,however,the majorities of cathode materials with high operation potential employed in traditional nonaqueous batteries are excluded from the range of ABs cathodes,leading to a low energy density.Directly using metal as an anode is likely to improve the energy density,whereas most of the reported metal anodes,e.g.,lithium,sodium,magnesium,etc.,cannot run in aqueous electrolytes.One exceptional case is the Zn metal anode that permits theoretically high energy density AZBs due to triple merits:(1)the Zn metal anode exhibits a low redox potential(−0.76 V vs standard hydrogen electrode,SHE),taking the best advantage of the limited voltage window of aqueous electrolytes;(2)Zn metal anode with mild protection can easily maintain its chemical stability in aqueous medium;(3)Zn metal anode releases a high specific capacity of 820 mAh g^(−1).AZBs thus exhibit a rapid development,especially in developing high specific capacity cathode materials such as MnO_(2)and V_(2)O_(5),and the corresponding structure modification.Despite these spurring achievements,the overall energy density of the whole AZB device is still unsatisfactory.

Electromagnetic interference shielding properties of polymer derived SiC-Si3N4 composite ceramics

SiC-Si3N4 composite ceramics are successfully fabricated by pyrolysis of ferrocene-modified polycarbosilane(PCS) mixed with inert filler Si3N4 powders, followed by thermal treatment from 1100℃ to 1400℃ in Ar atmosphere. The porosity of SiC-Si3N4 ceramics decreases to 6.4% due to the addition of inert filler Si3N4. And the content and crystallization degree of free carbon and SiC derived from PCS are improved simultaneously with the increase of thermal treatment temperature. Finally, the free carbon and SiC interconnect, forming the conductive network. As a result, the electromagnetic interference(EMI) shielding performance of the as-prepared ceramic annealed at 1400℃ reaches up to 36 d B, meaning more than99.9% of EM energy is shielded. The low porosity and high EMI shielding performance enable SiC-Si3N4 composite ceramics to be a promising electromagnetic shielding and structural material.