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The oblique shock impinging on the supersonic turbulent boundary layer leads to a mixing layer and the emergence of large-scale coherent structures within the interaction zone which leave significant velocity defect and turbulence amplification downstream. In the present study, we investigate the turbulence recovery in the post-shock region by exploiting direct numerical simulation data of the oblique-shock/turbulent boundary layer interaction flow at the incoming Mach number of $2.28$ and the shock angle of $33.2^\circ$ , with special attention paid to the contribution of the mixing layer and large-scale structures to flow dynamics. For that purpose, we propose to split the mean velocity, Reynolds stresses and spanwise spectra into a canonical portion that is constructed according to the statistics of canonical turbulent boundary layers, and a mixing-layer-induced portion. We found that the hidden mixing layer grows with the boundary layer thickness and that the induced mean shear and Reynolds stresses decay at different rates. The mean velocity recovers to the canonical profiles at a distance of 13 boundary layer thicknesses downstream where the mixing-layer-induced mean shear ceases to have strong impacts. The recovery of Reynolds stresses requires 10 boundary layer thicknesses in the near-wall region but a much longer streamwise extent in the outer region due to the slow decay of large-scale motions. These large-scale motions superpose on the near-wall turbulence, intensifying the turbulent fluctuations, yet having a trivial impact on the skin friction, for the contribution of the mixing-layer-induced mean shear and Reynolds shear stress are balanced by the advection term. We further establish a simple physical model capable of approximately predicting the streamwise evolution of mixing-layer-induced mean shear and turbulent kinetic energy. This model suggests that the complete recovery of turbulence in the outer region requires a streamwise extent of approximately 50 boundary layer thicknesses.

Single-atom catalysis is recognized as a frontier of heterogeneous catalysis for its efficient utilization of metals and the possibility to engender unusual reactivity. Yet, despite the observation of single atoms, understanding their coordination structures and developing structure–property relationships remains challenging due to the structural complexity of support surfaces. Here, using single-crystalline MgO(111) two-dimensional nanosheets and a surface organometallic chemistry method, we describe the formation of highly dispersed Ir(III) sites (isolated at 0.1 wt%, and Ir pairs and trimers at 1 wt%) with well-defined coordination structures. These species display unique catalytic properties in the coupling reaction of benzene and ethylene to form styrene, a reactivity that contrasts with conventional homogeneous and heterogeneous iridium catalysts that yield ethylbenzene. The similar activities for high- and low-loading catalysts suggest that iridium sites, whether isolated or in the form of clusters (for example Ir3), have similar activity, consistent with the involvement of surface dynamics. The fine tuning of the interface between single atoms and their supports may open advantageous reactivity scenarios, although it remains challenging. Now, Liu, Coperet and colleagues address this problem by dispersing Ir(III) species on well-defined single-crystalline MgO(111) 2D nanosheets, achieving a unique reactivity for the coupling of benzene and ethylene.

In the present study, we perform direct numerical simulations to investigate the spatial development and basic flow statistics in the supersonic turbulent boundary layers at the free-stream Mach number of 2.0 over smooth and disturbed walls, the latter of which enforces extra Reynolds shear stress in the streamwise direction to emulate the drag increment and mean streamline curvature effects of rough walls. Such disturbances escalate the growth rate of turbulent boundary layer thickness and the shape factor. It is found that under the rescaled global coordinate, the mean velocity, Reynolds stress, and pressure fluctuation variance manifest outer-layer similarity, whereas the average and fluctuation variances of temperature and density do not share such a property. Compressibility effects are enhanced by the wall disturbances, yet not sufficiently strong to directly impact the turbulent kinetic energy transport under the presently considered flow parameters. The generalized Reynolds analogy that relates the mean velocity and temperature can be satisfied by incorporating the refinement in modifying the generalized recovery coefficient, and that associates the fluctuating velocity and temperature work reasonably well, indicating the passive transport of temperature fluctuations. The dispersive motions are dominant and decay exponentially below the equivalent sand grain roughness height ks, above which the wall disturbances are distorted to form unsteady motions responsible for the intensified density and pressure fluctuations in the free-stream traveling isentropically as the acoustic radiations.

Synthesizing high-quality two-dimensional nanomaterials of nonlayered metal oxide is a challenge, especially when long-range single-crystallinity and clean high-energy surfaces are required. Reported here is the synthesis of single-crystalline MgO(111) nanosheets by a two-step process involving the formation of ultrathin Mg(OH)2 nanosheets as a precursor, and their selective topotactic conversion upon heating under dynamic vacuum. The defect-rich surface displays terminal -OH groups, three-coordinated O2- sites and low-coordinated Mg2+ sites, as well as single electrons trapped at oxygen vacancies, which render the MgO nanosheets highly reactive, as evidenced by the activation of CO molecules at low temperatures and pressures with formation of strongly adsorbed red-shifted CO and coupling of CO molecules into C2 species.

The wall effect on the flow structures of the three-dimensional rotating detonation wave in an annular chamber is investigated by utilizing premixed hydrogen/air at a stoichiometric ratio as the reactant and a 7-species-8-reaction finite-rate chemical reaction model for evaluating the chemical source term. Three cases are constructed with different flow solvers, i.e., Euler, Navier-Stokes and filtered Navier-Stokes with Improved Delayed Detached-Eddy Simulation to identify the wall effects. Both slip and non-slip wall boundary conditions are considered to quantify the viscous effect near the wall. It is found that the flow structures of the rotating detonation wave change notably when the effect of the viscous wall is taken into consideration, resulting in the extension of the deflagration zone to the upstream along the wall, the severe deformation of the detonation front at the unbounded side, and the formation of the reactant deficit. The mechanism of such alteration in the flow structures is analyzed, and the formation procedure of the reactant deficit zone is proposed. The comparison of specific impulse shows that our simulation is about 10% lower than the ideal two-dimensional rotating detonation engine model, and the practical experiment model is about 5.8% lower than our simulation. The wall effect, the three-dimensional effect and the turbulent mixing would lead to the performance losses.

Catalytic hydrogenation is one of the backbones of the chemical industry. Controlling the reaction behaviour of the activated hydrogen species over oxide-supported metal catalysts is essential. Aside from the expected addition to substrates, the activated hydrogen species would also destroy the active structures. Here we show that, with the assistance of alkali cations, the atomically dispersed Ru(iii) on Al2O3 exhibits enhanced performance in the hydrogenation of a broad range of substrates. The alkali cations facilitate the hydrogenation mediated by heterolytic hydrogen species, which not only restrain the hydride species from migrating to interfacial oxygen, thus suppressing the reduction and aggregation of ruthenium, but also stabilize the negatively charged transition states and intermediates through enhanced Columbic attraction. Distinctively, an inverse H/D isotope effect related to H2 splitting as the rate-determining step over the atomically dispersed ruthenium-catalysed hydrogenation is predicted and confirmed. Alkali metals have been traditionally used to promote heterogeneous catalysts, albeit their mode of action remains controversial. Now, the authors demonstrate the multifaceted role of sodium ions in promoting atomically dispersed Ru(iii) on Al2O3, resulting in a superior hydrogenation catalyst.

Understanding the generation mechanism of the heat flux is essential for the design of hypersonic vehicles. We proposed a novel formula to decompose the heat flux coefficient into the contributions of different terms by integrating the conservative equation of the total energy. The reliability of the formula is well demonstrated by the direct numerical simulation results of a hypersonic transitional boundary layer. Through this formula, the exact process of the energy transport in the boundary layer can be explained and the dominant contributors to the heat flux can be explored, which are beneficial for the prediction of the heat and design of the thermal protection devices.

In the present study, we investigate influences of shock intensity on wall pressure fluctuations by performing direct numerical simulations of supersonic turbulence boundary layers over compression ramps with different turning angles. We found that as the turning angle increases, low-frequency motions of the separation shock are enhanced, accompanied by enlarged energetic pressure structures with lower convection velocities. By inspecting wavenumber-frequency spectra under the assumption of streamwise homogeneity, we further identified two energetic modes convected at different velocities. The one with the lower convection velocity, namely, the “slow mode,” inherited from the upstream pressure fluctuations of the turbulent boundary layer, is decelerated when passing through the oblique shock, during which the “rapid mode” with pressure fluctuations convected at higher speeds are generated. The increasing turning angle decelerates the slow mode and intensifies the fast mode. The reconstruction of the flow field suggests that the rapid mode is associated with the shear layer generated adjacent to the interaction zone, while the slow mode is associated with the Görtler vortices on the ramp.

In this paper, we investigate the differences in wall heat transfer between the low- and high-enthalpy turbulent boundary layers by exploiting direct numerical simulation databases of hypersonic turbulent boundary layers at the free-stream Mach number of 4.5 and the friction Reynolds number of 800. For that purpose, we refine the integral formula of decomposing the wall heat flux proposed by Sun et al. [“A decomposition formula for the wall heat flux of a compressible boundary layer,” Adv. Aerodyn. 4, 1–13 (2022)], enabling us to scrutinize the contribution of different physical processes. Statistical results show that the mean wall heat transfer is primarily contributed by the heat conduction, the turbulent heat transfer, viscous dissipation of mean kinetic energy, and turbulent kinetic energy production. Among these processes, the contribution of the turbulent heat flux in the high-enthalpy case is 10% higher than that in the low-enthalpy case. Such discrepancy is caused by the turbulent–chemistry interaction consisting of velocity and species mass fraction fluctuations. Coherent structures in the conditionally averaged fields related to this process reveal that the sweep in the viscous sublayer and ejection in the logarithmic layer bringing the hot fluid downward and upward, respectively, significantly alter the distribution of the species mass fraction. The wall heat flux fluctuations are slightly enhanced in the high-enthalpy flows, which is ascribed to be the intensification of traveling wave packets.

Atomically dispersed catalysts have demonstrated superior catalytic performance in many chemical transformations. However, limited success has been achieved in applying oxide-supported atomically dispersed catalysts to semihydrogenation of alkynes under mild conditions. By utilizing various metal oxides (e.g., Cu2O, Al2O3, ZnO, and TiO2) as supports for atomically dispersed Pd catalysts, we demonstrate herein the critical role of the oxidation state and coordinate environment of Pd centers in their catalytic performance, thus leading to the discovery of an “oxide-support effect” on atomically dispersed metal catalysts. Pd atomically dispersed on Cu2O exhibits far better catalytic activity in the hydrogenation of alkynes, with an extremely high selectivity toward alkenes, compared to catalysts on other oxides. Pd species galvanically displace surface Cu(I) sites on Cu2O to create two-coordinated Pd(I), which is a critical step for the activation and heterolytic splitting of H2 into Pd-H− and O-H+ species for the selective hydrogenation of alkynes. Moreover, the adsorption of alkenes on H2-preadsorbed Pd(I) is relatively weak, preventing deeper hydrogenation and increased selectivity during semihydrogenation. We demonstrate that the local coordinate environment of active metal centers plays a crucial role in determining the catalytic performance of an oxide-supported atomically dispersed catalyst.

This two-part study investigates the effects of Mach number and wall temperature on the statistics of wall shear stress, pressure, and heat flux fluctuations in compressible wall-bounded turbulence. In the first part, we focus on their one-point statistics, including the root mean square (r.m.s.), skewness factor (third-order moment), flatness factor (fourth-order moment), and their correlations. By exploiting the direct numerical simulation databases, we found that the r.m.s. of the streamwise wall shear stress and pressure, the skewness factor of all the flow quantities considered, and the flatness factor of streamwise wall shear stress monotonically vary with the friction Mach number ([Formula: see text]), while for the rest, the wall heat flux and global temperature parameters should be taken into account as well for a monotonic trend of variation. The correlation coefficients between wall shear stress, pressure, and heat flux fluctuations increase with the Mach number [Formula: see text], suggesting the underlying interactions between dynamic and thermodynamic processes. The distributions of spectra and probability density functions indicate that the increased correlation is induced by the highly intermittent traveling wave packets among the streaky structures, as reflected by the “double-peak” feature of the spectra that gradually emerges with the increasing compressibility effects. The probability density distribution also manifests the alteration of the occurrence of extreme events caused by these structures. By accordingly decomposing the fluctuations with cutoff filtering, it is found that the root mean squares of streamwise wall shear stress and heat flux fluctuations related to the streaky structures are Mach number-independent, while those related to the traveling wave packets monotonically increase with the friction Mach number.

To enhance the performance of the third-order weighted essentially non-oscillatory (WENO) scheme on the four-point stencil, a new hybrid central-type scheme is proposed in this paper. Developments are conducted in the following two aspects. First, a new fourth-order central WENO scheme, named as WENO4-LC, is devised for shock capturing, and a low dissipative third-order grid-centered upwind scheme is adopted for the smooth region. Second, a discontinuity indicator is extended to the four-point stencil version, which is simpler than its counterpart in our former article (Guo et al. in J Sci Comput 83, 28, 2020), for switching between the linear and nonlinear branch of the hybrid scheme. Numerical tests of the Euler and Navier–Stokes benchmarks show that the new indicator is good on detecting the discontinuities. Moreover, the performances of the central-type WENO4-LC and its corresponding hybrid scheme have obvious improvements compared with those of WENO3-JS and WENO3-L, and even behave slightly better than classical WENO5-JS on resolving the fluid structures. Meanwhile, the hybrid scheme is efficient which only costs 76% CPU time of WENO3-JS and about 58% of WENO5-JS.

Single atom catalysis has been recently recognized as an efficient utilization of metals in heterogeneous catalysis with the possibility to engender unusual reactivity. Yet, despite the observation of single atoms, controlling the uniformity in the coordination structures of supported species and understanding the structure-property relationships remains a grand challenge due to the surface structural complexity of the supports. Here, we combined the use of single-crystalline MgO(111) 2D nanosheets with surface organometallic chemistry to generate highly dispersed Ir(III) sites. The MgO(111) surfaces enable the formation of isolated Ir(III) single-sites stabilized by three 3-coordinated surface -O(H) anions at low loading (0.1 %wt) as well as Ir pairs and trimers at higher loading (1 %wt). These materials show unique catalytic properties and enable the coupling of benzene and ethylene into styrene, in contrast to the expected ethylbenzene, formed with the corresponding Ir-based homogeneous catalysts or with atomically dispersed Ir on MgO nanoparticles.

Few-shot learning is the key step towards human-level intelligence. Prototypical Network is a promising approach to address the key issue of over-fitting for few-shot learning. Nevertheless, the original Prototypical Network only uses one or few labeled instances to represent the corresponding class, which easily deviates from the real class distribution leading to the imprecise classification results. In this paper, we propose Transductive Prototypical Network (Td-PN), a universal transductive approach that refines the class representations by merging scarce labeled samples and high-confidence ones of target set. Our proposed Td-PN first maps the samples to a classifying-friendly (discriminative) embedding space by redesigning a weighted contrastive loss, then utilizes the transductive inference to obtain the powerful prototype representation for each class. Experiments demonstrate that our approach outperforms the state-of-the-art algorithms.

Synthesizing high-quality two-dimensional nanomaterials of non-layered metal oxide is a grand challenge because it requires long range single-crystallinity and clean high-energy surfaces. Here, we report the synthesis of single-crystalline MgO(111) nanosheets via a two-step process involving the formation of ultrathin Mg(OH)2 nanosheets as precursor and their selective topotactic conversion upon heating under dynamic vacuum. The defect-rich surface displays terminal -OH groups, 3-coordinated O2- sites, low-coordinated Mg2+ sites as well as single electrons trapped at oxygen vacancies, that render MgO nanosheets highly reactive as evidenced by the activation of CO molecules at low temperatures and pressures, with formation of strongly adsorbed red-shifted CO and coupling of CO molecules into C2 species.

In the present work, we propose a new discontinuity indicator for constructing shock-capturing finite difference schemes based on the self-adaptation algorithm in the WENO-OS4 scheme (Li et al. in J Sci Comput 71:109–143, 2017). The new discontinuity indicator can identify the discontinuities as accurate as the original self-adaptation algorithm through using a more straightforward numerical condition. The high-resolution and the problem-independency of the original algorithm for identifying discontinuities are maintained, but the excessive expenses are greatly reduced by the decoupled implementation and the employment of the local characteristic projection. Moreover, new hybrid WENO schemes are constructed based on this new discontinuity indicator. The hybrids are carried out between nonlinear WENO schemes with shock-capturing ability and linear upwind or central schemes with high resolution. Both the effectiveness of the indicator and the good resolution of the hybrid schemes are demonstrated by the numerical tests. The results of the tests indicate that the new hybrid WENO schemes have significant improvements on both resolution and computational efficiency when compared with the WENO-JS5 and WENO-OS4.

Herein we report a drug delivery system based on hollow iron silicate nanospheres. Fe3+ on the nanospheres’ surface can effectively bind with doxorubicin (DOX), an anticancer drug, through coordination bonds. The bonds are fairly stable in a neutral environment but could easily break up in an acid environment. The release of DOX from hollow iron silicate nanospheres into cancer cells can be therefore triggered by a pH drop caused by endocytosis. The iron silicate shell allows a DOX loading content of up to 50.2% in weight, which is significantly higher than most drug delivery systems reported. Cell experiments show that DOX-loaded hollow iron silicate nanospheres exhibit a higher efficiency in killing cancer cells than free DOX, and a higher cytotoxicity for human hepatoma cells than hepatocyte cells at the same DOX-loaded nanospheres’ concentration. Confocal laser scanning microscopy (CLSM) experiments show the releasing and transportation process of DOX, and confirm the enrichment of DOX in cell nuclei.

Hydrogen spillover is a well-known phenomenon in heterogeneous catalysis; it involves H2 cleavage on an active metal followed by the migration of dissociated H species over an ‘inert’ support1–5. Although catalytic hydrogenation using the spilled H species, namely, spillover hydrogenation, has long been proposed, very limited knowledge has been obtained about what kind of support structure is required to achieve spillover hydrogenation1,5. By dispersing Pd atoms onto Cu nanomaterials with different exposed facets, Cu(111) and Cu(100), we demonstrate in this work that while the hydrogen spillover from Pd to Cu is facet independent, the spillover hydrogenation only occurs on Pd1/Cu(100), where the hydrogen atoms spilled from Pd are readily utilized for the semi-hydrogenation of alkynes. This work thus helps to create an effective method for fabricating cost-effective nanocatalysts with an extremely low Pd loading, at the level of 50 ppm, toward the semi-hydrogenation of a broad range of alkynes with extremely high activity and selectivity. Spillover hydrogenation is facet specific and occurs on atomically dispersed Pd catalyst on Cu(100). Knowing this, cost-effective catalysts with extremely low Pd loading are fabricated that successfully catalyse the semi-hydrogenation of a broad range of alkynes with high activity and selectivity.

Size effect plays a crucial role in catalytic hydrogenation. The highly dispersed ultrasmall clusters with a limited number of metal atoms are one candidate of the next generation catalysts that bridge the single-atom metal catalysts and metal nanoparticles. However, for the unfavorable electronic property and their interaction with the substrates, they usually exhibit sluggish activity. Taking advantage of the small size, their catalytic property would be mediated by surface binding species. The combination of metal cluster coordination chemistry brings new opportunity. CO poisoning is notorious for Pt group metal catalysts as the strong adsorption of CO would block the active centers. In this work, we will demonstrate that CO could serve as a promoter for the catalytic hydrogenation when ultrasmall Pd clusters are employed. By means of DFT calculations, we show that P d n n = 2 ‐ 147 clusters display sluggish activity for hydrogenation due to the too strong binding of hydrogen atom and reaction intermediates thereon, whereas introducing CO would reduce the binding energies of vicinal sites, thus enhancing the hydrogenation reaction. Experimentally, supported Pd 2 CO catalysts are fabricated by depositing preestablished [Pd 2 ( μ -CO) 2 Cl 4 ] 2- clusters on oxides and demonstrated as an outstanding catalyst for the hydrogenation of styrene. The promoting effect of CO is further verified experimentally by removing and reintroducing a proper amount of CO on the Pd cluster catalysts.

We gratefully acknowledge the financial support from King Abdullah University of Science and Technology; Competitive Research Grant (URF/1/1378) and Baseline Funding, and from Chinese NSFC (51672309) and the Fundamental Research Funds for the Central Universities (18CX07009A).

Instabilities in rotating detonation are concerned because of their potential influence on the stability of operation. Previous studies on instability of 2-D rotating detonation mainly cared about the one of the contact discontinuity originated from the conjunction of the detonation and oblique shock. Hishida et al. first found the rippled structure existed in the interface between fresh injections and burnt product from the previous cycle (Shock Waves 19, 2009), and a mechanism of Kelvin-Helmholtz instability was suggested as well. Similar structures were observed as well in simulations by current authors, where a fifth-order WENO-type scheme with improved resolution and 7-species-and-8-reaction chemical model were used. In order to achieve a deep understanding on the flow mechanism, more careful simulations are carried out by using three grids with increasing resolution. The results show that besides the previously-mentioned Kelvin-Helmholtz instability, there are two other mechanisms which take effect in the interface instability, i.e., the effect of baroclinic torque and Rayleigh-Taylor instability. Occurrence conditions for two instabilities are checked and testified. Especially, the spike- and bubble-like structures are observed at the interface, which show appearances different from canonical structures by Kelvin-Helmholtz instability.

Atomically dispersing metal atoms on supports has been emerging as an effective strategy to maximize the atom utilization of metals for catalysis. However, due to the lack of effective tools to characterize the detailed structure of metal-support interface, the chemical functions of supports in atomically dispersed metal catalysts are hardly elucidated at the molecular level. In this work, an atomically dispersed Pd1/TiO2 catalyst with Ti(III) vicinal to Pd is prepared and used to demonstrate the direct involvement of metal atoms on support in the catalysis of dispersed metal atoms. Systematic studies reveal that the Ti(III)-O-Pd interface facilitates the activation of O2 into superoxide (O2−), thus promoting the catalytic oxidation. The catalyst exhibits the highest CO turn-over frequency among ever-reported Pd-based catalysts, and enhanced catalysis in the combustion of harmful volatile organic compound (i.e., toluene) and green-house gas (i.e., methane). The demonstrated direct involvement of metal atoms on oxide support suggests that the real active sites of atomically dispersed metal catalysts can be far beyond isolated metal atoms themselves. Metal atoms on oxide supports in the vicinity serve as another vector to promote the catalysis of atomically dispersed metal catalysts.

Atomically dispersed catalysts have shown promising prospects in catalysis studies. Among all of the developed methods for synthesizing atomically dispersed catalysts, the photochemical approach has recently aroused much attention owing to its simple procedure and mild preparation conditions involved. In the present study, we demonstrate the application of the photochemical method to synthesize atomically dispersed Pd catalysts on (001)-exposed anatase nanocrystals and commercial TiO2 (P25). The as-prepared catalysts exhibit both high activity and stability in the hydrogenation of styrene and catalytic oxidation of CO.

Attaining high activity with high selectivity at low temperature is challenging in the selective hydrogenation of phenol to cyclohexanone due to its high activation energy ( E a, 55–70 kJ/mol). Here we report a simple and efficient strategy for phenol hydrogenation catalyzed by Pd in aqueous phase at 30 ° C by introducing air to promote the catalysis. With the assistance of air, >99% conversion and >99% selectivity were achieved over Pd(111)/Al2O3 with an overall turnover frequency (TOF) of 621 h−1, ~80 times greater than that of the state-of-art Pd catalyst at 30 °C. Mechanism studies revealed that phenol was activated to generate phenoxyl radicals. The radicals were yielded from the reaction between phenol and hydroxyl radicals in the presence of hydrogen, oxygen and protic solvent on Pd. The phenoxyl pathway resulted in a low apparent E a (8.2 kJ/mol) and thus high activity. More importantly, this strategy of activating substrate by air can be adapted to other Pd based catalysts, offering a new thinking for the rational design of cyclohexanone production in industry.

The largest Ln-Fe metal cluster [Gd12 Fe14 (μ3 -OH)12 (μ4 -OH)6 (μ4 -O)12 (TEOA)6 (CH3 COO)16 (H2 O)8 ]⋅(CH3 COO)2 (CH3 CN)2 ⋅(H2 O)20 (1) and the core-shell monodisperse metal cluster of 1 a@SiO2 (1 a=[Gd12 Fe14 (μ3 -OH)12 (μ4 -OH)6 (μ4 -O)12 (TEOA)6 (CH3 COO)16 (H2 O)8 ]2+ ) were prepared. Experimental and theoretical studies on the magnetic properties of 1 and 1 a@SiO2 reveal that encapsulation of one cluster into one silica nanosphere not only effectively decreases intermolecular magnetic interactions but also significantly increases the zero-field splitting effect of the outer layer Fe3+ ions.

The importance of eliminating errors in grid-metric evaluation for high-order difference schemes has been widely recognized in recent years, and it is known from the proof by Vinokur and Yee (NASA TM 209598, 2000) that when conservative derivations of grid metric are used by Thomas, Lombard and Neier (AIAA J., 1978, 17(10) and J. Spacecraft and rocket, 1990, 27(2)), errors caused by metric evaluation could be eliminated by linear schemes when flux splitting is not considered. According to the above achievement, central schemes without the use of flux splitting could fulfill the requirement of error elimination. Difficulties will arise for upwind schemes to attain the objective when the splitting is considered. In this study, further investigations are made on three aspects: Firstly, an idea of central scheme decomposition is introduced, and the procedure to derive the central scheme is proposed to evaluate grid metrics only. Secondly, the analysis has been made on the requirement of flux splitting to acquire free-stream preservation, and a Lax-Friedrichs-type splitting scheme is proposed as an example. Discussions about current study with that by Nonomura et al. (Computers and Fluids, 2015, 107) have been made. Thirdly, for half-node- or mixed-type schemes, interpolations should be used to derive variables at half nodes. The requirement to achieve metric identity on this situation is analyzed and an idea of directionally consistent interpolation is proposed, which is manifested to be indispensable to avoid violations of metric identity and to eliminate metric-caused errors thereafter. Two numerical problems are tested, i.e., the free-stream and vortex preservation on wavy, largely randomized and triangular-like grids. Numerical results validate aforementioned theoretical outcomes.

Surface coordination chemistry of nanomaterials deals with the chemistry on how ligands are coordinated on their surface metal atoms and influence their properties at the molecular level. This Perspective demonstrates that there is a strong link between surface coordination chemistry and the shape-controlled synthesis, and many intriguing surface properties of metal nanomaterials. While small adsorbates introduced in the synthesis can control the shapes of metal nanocrystals by minimizing their surface energy via preferential coordination on specific facets, surface ligands properly coordinated on metal nanoparticles readily promote their catalysis via steric interactions and electronic modifications. The difficulty in the research of surface coordination chemistry of nanomaterials mainly lies in the lack of effective tools to characterize their molecular surface coordination structures. Also highlighted are several model material systems that facilitate the characterizations of surface coordination structures, including ultrathin nanostructures, atomically precise metal nanoclusters, and atomically dispersed metal catalysts. With the understanding of surface coordination chemistry, the molecular mechanisms behind various important effects (e.g., promotional effect of surface ligands on catalysis, support effect in supported metal nanocatalysts) of metal nanomaterials are disclosed.

Aimed at decreasing the numerical dissipation of weighted non-oscillatory and non-free-parameter dissipation (WNND) scheme, we present an improved counterpart for shock-capturing. The new algorithm is based on the framework of Z-type weighting procedure with new local and global smoothness indicators. The performances of the proposed scheme are evaluated on several numerical tests governed by one-dimensional Euler equations. Numerical results indicate that the improved WNND scheme has advantages over the original WNND and third-order WENO-JS and WENO-Z schemes.