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The Beijing-Tianjin-Hebei urban agglomeration is one of the regions in China with the most severe air pollution. Using aircraft observations collected over Xingtai in May 2016 and multi-source data, such as aerosol chemical composition and lidar data, we analyzed aerosol composition and optical properties, vertical pollution characteristics of gases within the boundary layer, and their interactions with meteorological parameters. This study focuses on investigating the transport and evolution mechanisms of pollutants during transitions from polluted research flight No.7(RF7) to clean research flight No.8 (RF8) periods in summertime Xingtai. Results show that during RF7, the near-surface submicron aerosol (PM1) mass concentration was generally low (37.5 µg m−3), with the contribution of inorganic salts far exceeding that of organic matter. Aircraft observations indicated weak cold-air activities above 1000 m during RF8, while the southeasterly wind still prevailed below 1000 m, with a slight increase in wind speed. From RF7 to RF8, the overall vertical atmosphere gradually transitioned from polluted to clean conditions. During RF8, an inversion layer appeared in the temperature profile between 1100 and 1300 m, with ~ 21.3% of this inversion coming from black carbon heating. Changes in meteorological and pollution transport conditions led to significant differences in PM1 and gaseous components between RF7 and RF8. Compared to RF7, the contribution of nitrates decreased markedly during RF8, with the organic matter becoming the dominant component of PM1. During RF7, sulfur dioxide (SO2), nitrogen dioxide (NO2), and carbon monoxide (CO) were strongly correlated with the scattering coefficient, with correlation coefficients of 0.92 and 0.96 for SO2 and NO2, respectively. By contrast, during RF8, the correlations between SO2, NO2, and CO and the scattering coefficient decreased, with SO2 having the highest correlation with the scattering coefficient (coefficient of determination = 0.88). From RF7 to RF8, the concentrations of various pollutants within the boundary layer, except for ozone (O3), had decreasing trends. When the relative humidity was below 50%, O3 generally contributed positively to extinction. There was no pronounced correlation between O3 and relative humidity when the relative humidity exceeded 50%. [ABSTRACT FROM AUTHOR]

Shock Wave Boundary Layer Interactions represent a complex flow feature combining high-speed inertial force - dominated flows with low-speed viscous force dominated regions. In this research, planar oblique shock impingement on finned missile body (slender body) have been studied, involving a complex flow field of shocks and expansion waves. Computational studies are used to perform quantitative and qualitative analysis of multi body configurations and investigate the associated flow physics. There is change in induced forces and moments of finned body with change in location of shock impingement, due to the combined effect of enhanced compression, expansion and flow pitch angle on different regions of the missile. For fixed lateral separation between the bodies, changes in coefficient of forces and moments of missile body are very small for shock impingement locations close to fin leading edge. The fins contribute about 50-60% to coefficient of forces and moments of finned missile body. With change in angle of incidence, there is change in polarity of forces and moments illustrating extreme sensitivity of missile body to location of shock impingement, which is due to the combined effect of enhanced compression and expansion on different regions of the missile. [ABSTRACT FROM AUTHOR]

The early stages of hydrogen–air and methane–air flame dynamics and the development and evolution of tulip flames in closed tubes of various aspect ratios and in a semi-open tube are studied by solving the fully compressible reactive Navier–Stokes equations using a high-order numerical method coupled to detailed chemical models for stoichiometric hydrogen/air and methane/air mixtures. The use of adaptive mesh refinement (AMR) provides adequate resolution of the flame reaction zone, pressure waves, and flame–pressure wave interactions. The purpose of this study is to gain a deeper insight into the influence of chemical kinetics on the combustion regimes leading to the formation of a tulip flame and its subsequent evolution. The simulations highlight the effect of the flame thickness, flame velocity, and reaction order on the intensity of the rarefaction wave generated by the flame during the deceleration phase, which is the principal physical mechanism of tulip flame formation. The obtained results explain most of the experimentally observed features of tulip flame formation, e.g., faster tulip flame formation with a deeper tulip shape for faster flames compared to slower flames. [ABSTRACT FROM AUTHOR]

AbstractReactions catalyzed with immobilized enzymes are widely used in biotechnology. Such reactions occur in a heterogeneous system because the reaction takes place in the solid phase represented by the biocatalyst, while substrates and products are generally soluble in the liquid medium where the course of reaction is being monitored. This reaction system confronts heat and mass transfer problems, which consequently affect the observed rate of reaction. If the enzyme is immobilized on the surface of a non-porous support, external diffusional restrictions are likely to occur, due to the formation of a stagnant layer in the proximity of the biocatalyst solid surface. This review analyzes the influence of external mass and heat transfer rates on the reaction kinetics of immobilized enzymes under agitation, presenting and discussing the different mathematical models that have been proposed and their underlying assumptions. [ABSTRACT FROM AUTHOR]

This article aims at the construction and analysis of a computational method for a class of singularly perturbed nonlinear differential equations of convection diffusion type with Cauchy data. A computational method, which is a combination of finite difference operators and a Shishkin mesh, is constructed to solve the class of problems. An algorithm involving the continuation technique together with the suggested computational method is developed to solve the class of problems numerically. The newly developed computational method is proved to be essentially first-order parameter-independent convergent. Computational experiments included support the theoretical results. [ABSTRACT FROM AUTHOR]

This work studies the influence of a growing boundary layer on the process of supersonic flow around an aerodynamic body. The task is to select and implement in an experiment the parameters of a supersonic flow and to study the flow pattern near the surface of an aerodynamic body at different viscosity values for the incoming flow. Visualization of the shock wave configuration in front of the body and studying the change in the pressure field in the flow region under these conditions is the main goal of this work. The experiment was carried out on an experimental stand created on the basis of a shock tube. The aerodynamic body under study (a semi-cylinder pointed along a circle or an ellipse) was placed in a supersonic nozzle. The model was clamped by lateral transparent walls, which were simultaneously a source of boundary layer growth and the viewing windows for visualizing the flow. For selected modes with Reynolds numbers from 8200 to 45,000, schlieren flow patterns and pressure distribution fields near the surface of the streamlined models and the plate of the growing boundary layer were obtained. The data show a complex, unsteady flow pattern realized near the model which was caused by the viscous-inviscid interaction of the boundary layer with the bow shock wave near the wall. [ABSTRACT FROM AUTHOR]

A class of singularly perturbed semilinear time-dependent problems with a delay term in time is considered. A numerical approach is formulated and analysed to unravel the complexity for such problems due to the presence of a small perturbation parameter and a semilinear term simultaneously. We use Shishkin mesh to capture the boundary layer. We estimate errors in the energy and balanced norms occurred in orthogonal spline collocation discretization for the spatial direction. For the full discretization an extrapolated Crank–Nicolson formula is used in time direction. Some numerical results are presented to confirm the effectiveness of the method. The computed convergence corroborated the predicted theory in different norms. [ABSTRACT FROM AUTHOR]

Modeling the boundary layer flow of ternary hybrid nanofluids is important for understanding and optimizing their thermal performance, particularly in applications where enhanced heat transfer and fluid dynamics are essential. This study numerically investigates the boundary layer flow of alumina-copper-silver/water nanofluid over a permeable stretching/shrinking sheet, incorporating both first and second-order velocity slip. The mathematical model is solved in MATLAB facilitated by the bvp4c function that employs the finite difference scheme and Lobatto IIIa formula. The solver successfully generates dual solutions for the model, and further analysis is conducted to assess their stability. The findings reported that only one of the solutions is stable. For the shrinking sheet case, increasing the first-order velocity slip delays boundary layer separation and enhances heat transfer, while, when the sheet is stretched, the second-order velocity slip accelerates separation and improves heat transfer. Boundary layer separation is most likely to occur when the sheet is shrinking; however, this can be controlled by adjusting the velocity slip with the inclusion of boundary layer suction. [ABSTRACT FROM AUTHOR]

[Display omitted] The friction and interfacial nanostructure of a water-in-surface-active ionic liquid mixture, 1.6 M 1-butyl-3-methylimidazolium 1,4-bis-2-ethylhexylsulfosuccinate ([BMIm][AOT]), can be tuned by applying potential on Au(1 1 1) and stainless steel. Atomic force microscopy (AFM) was used to examine the friction and interfacial nanostructure of 1.6 M [BMIm][AOT] on Au(1 1 1) and stainless steel at different potentials. Superlubricity (vanishing friction) is observed for both surfaces at OCP+1.0 V up to a surface-dependent critical normal force due to [AOT]− bilayers adsorbing strongly to the positively charged surface thus allowing AFM tip to slide over solution-facing hydrated anion charged groups. High-resolution AFM imaging reveals ripple-like features within near-surface layers, with the smallest amplitudes at OCP+1 V, indicating the highest structural stability and resistance to thermal fluctuations due to highly ordered boundary [AOT]− bilayers templating robust near-surface layers. Exceeding the critical normal force at OCP+1.0 V causes the AFM tip to penetrate the hydrated [AOT]− layer and slide over alkyl chains, increasing friction. At OCP and OCP-1.0 V, higher friction correlates with more pronounced ripples, attributed to the rougher templating [BMIm]+ boundary layer. Kinetic experiments show that switching from OCP-1.0 V to OCP+1.0 V achieves superlubricity within 15 s, enabling real-time friction control. [ABSTRACT FROM AUTHOR]

Present research work explores the three‐dimensional nanofluid flow across a horizontal surface with bi‐directional deviation of velocity in power‐law index n. Porosity, the ohmic effect with other parameters, thermophoresis, Prandtl number, Schmidt number, and Brownian motion in the flow geometry are taken into consideration. Dimensional nonlinear formulations are changed into dimensionless expressions by employing the appropriate similarity transformation. Utilizing shooting procedure and the fourth‐fifth‐order Runge–Kutta integration approach, the numerical result has been achieved, and velocity, temperature, and concentration profiles are presented through graphs to illustrate the findings. The current outcome has numerous industrial applications, including the production of paper, metal extrusion, and optical fiber. The major findings of the present research are good agreement compared to the data that are accessible in the limiting instance compared with the earlier results. [ABSTRACT FROM AUTHOR]

The turbulent boundary layer in a viscous incompressible fluid developing longitudinally past the surface of a thin cone or cylinder at a finite distance from the laminar–turbulent transition zone is studied. The characteristic Reynolds number, determined from the external flow velocity and the length of the body, is assumed to be large, and the thickness of the boundary layer is small and comparable to the radius of the body. The asymptotic method of multiple scales is used to find solutions to the Navier–Stokes equations. Instead of the traditional decomposition of the solution into time-averaged values and their fluctuations, the velocities and pressure are expressed as an asymptotic series consisting of steady and perturbed terms. As a result, the viscous steady flow ('secondary') that arises in the boundary layer as a mandatory component of fast turbulent fluctuations was described. Analytical and numerical solutions for the radial steady velocity are presented, describing the self-induced suction of fluid from the external flow into the boundary layer. Further analytical solutions are obtained for the longitudinal and circumferential velocities, which differ markedly from the laminar regime. The solutions found are somewhat similar to the degenerate (one-dimensional) case of self-sustaining longitudinal thin structures in turbulent shear flows. A qualitative comparison with direct numerical simulations is presented. [ABSTRACT FROM AUTHOR]

The behavior of two eddy-diffusivity mass-flux (EDMF) planetary boundary layer schemes used in versions 15 and 16 of NOAA's operational Global Forecast System (GFS) is examined in terms of local and nonlocal mixing processes in a one-dimensional (1D) model configuration. Diagnosis using process perturbation sensitivity experiments indicates that the nonlocal mixing represented by the mass-flux term is much more dominant in one scheme than the other. Quantitative aspects of the local eddy diffusivity are different between the two schemes, pointing to uncertainty in the physical partition of local and nonlocal mixing in the EDMF formulation. One of the two schemes is also shown to produce unphysical thermal and tracer tendencies due to the scheme's specific numerical treatment of moist nonlocal mixing. To resolve this problem, a physically and numerically consistent approach is proposed to calculate these tendencies. [ABSTRACT FROM AUTHOR]

This paper presents a robust fitted mesh finite difference method for solving a dynamical system of two parameter convection–reaction–diffusion delay differential equations defined on the interval [ 0 , 2 ] . The method incorporates a piecewise uniform Shishkin mesh to accurately resolve the solution behavior caused by small perturbation parameters and delay terms. The proposed numerical scheme is proven to be parameter-robust and achieves almost first-order convergence. Numerical illustrations are provided to showcase the method's effectiveness, highlighting its capability to address boundary and interior layers with improved accuracy. The results, supported by symmetrical considerations in the figures, enhance the precision and serve as validation for the theoretical results. [ABSTRACT FROM AUTHOR]

Dispersion within idealized urban environments was studied in a simulated neutrally buoyant, 1:200 scale boundary layer with the Meteorological Wind Tunnel at the EPA’s Fluid Modeling Facility. The measurements are used to offer a baseline of performance for the mechanical turbulence formulation and concentration predictions of AERMOD, the EPA’s preferred Gaussian dispersion model. Scaled meteorological conditions and dispersion characteristics were studied for both uniform and nonuniform building arrays oriented at 0° and 30° with respect to the flow and were compared to baseline, “rural”, measurements without the presence of buildings. Particle image velocimetry (PIV) measured velocity and shear stress profiles within each model configuration, whereas hydrocarbon analyzers (HCAs) measured ethane concentrations at defined points throughout the model. Four source locations were examined for each building array, with two in the urban core and two in a street canyon, each with a source within and above the building canopy. Experimental profiles, regardless of their shape, were fitted to Gaussian profiles to determine lateral and vertical plume spread and shift from the wind tunnel centerline. These parameters were compared against a no-building reference case. Concentration predictions using the formulations in AERMOD are computed for 3 variations of modeled velocity profiles for each source, using factor of 2 (FAC2) and fractional bias (FB) as the governing model evaluation parameters. The two urban configurations were found to decrease the FAC2 performance by 34.1% and 30.1% from the no-building reference for the uniform and nonuniform cases, respectively, while producing modeled concentrations of only 48.1% and 62.4% of the 10 highest observed concentrations. These results encouraged simple first-order corrections to improve model performance with an emphasis on predicting maximum concentrations for regulatory purposes. These corrections proved successful for the uniform cases, mitigating FB, and improving the FAC2 percentage by 11.4% with more mixed results in nonuniform configurations, highlighting the difficulty in applying uniformly derived parameterizations in realistic, nonuniform environments. [ABSTRACT FROM AUTHOR]

Nanoparticles exhibit diverse effects when added as additives to oily medium, enhancing tribological properties and surface characteristics. Studies have shown that many oxide ceramic nanoparticles improve friction and wear, while mixtures also demonstrate favorable tribological properties. This study explores the tribological effect of an yttria–silica (Y2O3, SiO2) nanoparticle mixture in a Group III base oil medium. The results reveal that the yttria–silica mixture significantly reduces friction (−8–17%), mean wear scar diameter (−32%), and wear volume (−94%), while increasing load-bearing capacity (+114%) by creating a durable boundary layer. Observations from scanning electron microscopy revealed the original surface is protected. EDX analyses highlight the boundary layer's elemental composition, which is high in yttrium, silicon, and oxygen and found in higher areas. XRD analysis could not detect the yttria nanoparticle additive within the boundary layer, suggesting that it fragmented due to sliding stress, resulting in an amorphous structure for the new boundary layer. TEM imaging confirmed that the boundary layer thickness is 40–45 nm. These findings demonstrate significant potential for industrial applications in developing advanced, high-performance lubricants for demanding mechanical systems. [ABSTRACT FROM AUTHOR]