Parallel Walls Research Articles

Simultaneous Flow of n-Immiscible Fractional Maxwell Fluids with Generalized Thermal Flux and Robin Boundary Conditions

In a rectangular region, the multilayered laminar unsteady flow and temperature distribution of the immiscible Maxwell fractional fluids by two parallel moving walls are studied. The flow of the fluid occurs in the presence of Robin’s boundaries and linear fluid-fluid interface conditions due to the motion of the parallel walls on its planes and the time-dependent pressure gradient. The problem is defined as a mathematical model which focuses on the fluid memory, which is represented by a constituent equation with the Caputo time-fractional derivative. The integral transformations approach (the Laplace transform and the finite sine-Fourier transform) is used to determine analytical solutions for velocity, shear stress, and the temperature fields with fluid interface, initial, and boundary conditions. For semianalytical solutions, the algorithms of Talbot are used to calculate the Laplace inverse transformation. We used the Mathcad software for graphical illustration and numerical computation. It has been observed that the memory effect is significant on both fluid motion and temperature flow.

Coarse-grained molecular dynamics simulation of wettability and bleed out of capillary underfill

Coarse-grained molecular dynamics simulations were performed to elucidate the capillary flow process of liquid state capillary underfill (CUF), a sealing resin material. First, we ran a wettability simulation with the CUF consisting of a monomer with small and large fillers. We observed that a certain amount of the monomer spreads ahead on the substrate, while many fillers are left inside the droplet. This was confirmed by subsequent mean square deviation (MSD), which showed that the monomer had a higher MSD, 25–45 σ2, than the small and large fillers, which were 0.4–1.4 σ2 and 0.02–0.2 σ2, respectively. When one part of large fillers was replaced with small fillers, small fillers helped accelerate the wetting dynamics because they could move fast. However, when the small filler ratio was high (20%), the MSD of small fillers decreased. Next, we performed a capillary flow simulation in which the CUF flowed between parallel walls and observed that it formed a ridgeline at the upper wall edge. Small fillers contributed to a decreased flow time. However, when the small filler ratio was even higher, the flow time increased. Then, the small fillers slowed themselves down, as shown in the MSD. This is due to an increase in monomer interactions and less space to move. We also found that the bleed length decreased with an increase in the small filler ratio. This study clarified the effects of filler usage on the flow time and bleed length and contributed to new insight into the capillary actions and material design relevant to CUF.

Dynamics of a sphere in inertial shear flow between parallel walls

Abstract

Analysis of couple stress fluid flow with variable viscosity using two homotopy-based methods

Abstract In this article, the generalized plane Couette flow of Vogel’s model of incompressible, non-isothermal, couple stress fluid flowing steadily between two parallel walls is investigated. The governing equations are reduced to ordinary differential equations. To investigate the non-linear coupled system of differential equations, the optimal homotopy asymptotic method with DJ polynomial and asymptotic homotopy perturbation method have been used. Important flow properties are presented and discussed. We have obtained expressions for velocity, average velocity, shear stress, volume flux and temperature. The results gained employing these techniques are in the form of infinite series; thus, the results can be easily calculated. Comparison of various results, obtained through the suggested approaches, is carried out and an excellent agreement is achieved.

Micro-modelling of shear localization during quasi-static confined granular flow in silos using DEM

The paper deals with the quasi-static confined flow of cohesionless sand in a plane strain model silo with parallel walls and a slowly movable bottom. Numerical modelling was carried out by the discrete element method (DEM) using spheres with contact moments to approximately capture a non-uniform shape of sand particles. Different initial void ratios of sand and silo wall roughness grades were employed. Regular triangular grooves (asperities) with the same inclination and a different height were used to describe the varying wall surface topography. The emphasis was on the formation and evolution of both wall and internal shear zones during sand flow. DEM simulation outcomes were compared with corresponding model experiments and theoretical solutions. In addition, the particle displacements, particle rotations, normal contact forces, void ratios and wall stresses were evaluated. The numerical findings enhance the understanding of shear localization at the grain level during confined flow in silos and its strong impact on the magnitude, distribution and oscillation of wall stresses.

Anisotropy-independent packing of confined hard ellipses

We have examined the close-packed structure and ordering properties of hard ellipses between two parallel hard walls by using geometrical conjectures and Monte Carlo simulations. Starting from closely packed hard disk structures, we discover two competing densest packings of ellipses with the stretching of hard disks into different directions. We find that parallel and tilted close-packed structures alternate with widening pore width up to the two-dimensional bulk limit, where both structures produce the same close packing value (η=π/12). Our results highlight that the densest packing does not depend on the aspect ratio of the ellipse. This universal behavior is confirmed for three different aspect ratios (κ=2, 3, and 4) using replica exchange Monte Carlo simulation. Our simulation results show that even the global phase diagram is universal, where layering transitions between parallel structures and tilted-parallel structural changes are present.

The laser welded joints properties of cryogenic austenitic nitrogen-containing steel*

ABSTRACT Formation, structure and properties of welded joints in cryogenic austenitic nitrogen containing steel of chromium – nickel – manganese – nitrogen alloying system with reduced nickel content are investigated. Under optimal laser welding conditions, a weld of small width with almost parallel walls is formed. This helps to reduce residual stresses and strains. There is a significant reduction in the structure size parameters of the weld compared to arc welding. The mechanical properties of laser welds are within acceptable limits, so the tensile strength is 0.84–0.96 of the base metal. Fractographic analysis of the fracture surface of welded samples after testing showed the presence of developed zones of plastic deformation, which characterizes the laser-welded joint as highly plastic.

Flow-mediated interactions between two self-propelled flexible fins near sidewalls

Abstract

How Small Is Too Small for the Capillarity Theory?

We perform molecular dynamics (MD) simulations of nanoscale water capillary bridges (WCBs) expanding between two parallel walls and determine the smallest separation between the walls above which t...

Structural and dynamical fingerprints of the anomalous dielectric properties of water under confinement

There is a long-standing question about the molecular configuration of interfacial water molecules in the proximity of solid surfaces, particularly carbon atoms, which plays a crucial role in electrochemistry and biology. In this study, the dielectric, structural, and dynamical properties of confined water placed between two parallel graphene walls at different interdistances from the angstrom scale to a few tens of nanometer have been investigated using molecular dynamics. For the dielectric properties of water, we show that the dielectric constant of the perpendicular component of water drastically decreases under sub-2-nm spatial confinement. The dielectric constant data obtained through linear response and fluctuation-dissipation theory are consistent with recent reported experimental results [L. Fumagalli et al., Science 360, 1339 (2018)]. By determining the charge density as well as fluctuations in the number of atoms, we provide a molecular rationale for the behavior of the perpendicular dielectric response function. We also interpret the behavior of the dielectric response in terms of the presence of dangling O-H bonds of water. By examining the residence time and lateral diffusion constant of water under confinement, we reveal that the water molecules tend to keep their hydrogen bond networks at the interface of water-graphene. We also found consistency between lateral diffusion and the $z$-component of variance in the center of mass of the system as a function of confinement.

Investigation of Ring and Star Polymers in Confined Geometries: Theory and Simulations.

The calculations of the dimensionless layer monomer density profiles for a dilute solution of phantom ideal ring polymer chains and star polymers with arms in a -solvent confined in a slit geometry of two parallel walls with repulsive surfaces and for the mixed case of one repulsive and the other inert surface were performed. Furthermore, taking into account the Derjaguin approximation, the dimensionless layer monomer density profiles for phantom ideal ring polymer chains and star polymers immersed in a solution of big colloidal particles with different adsorbing or repelling properties with respect to polymers were calculated. The density-force relation for the above-mentioned cases was analyzed, and the universal amplitude ratio B was obtained. Taking into account the small sphere expansion allowed obtaining the monomer density profiles for a dilute solution of phantom ideal ring polymers immersed in a solution of small spherical particles, or nano-particles of finite size, which are much smaller than the polymer size and the other characteristic mesoscopic length of the system. We performed molecular dynamics simulations of a dilute solution of linear, ring, and star-shaped polymers with , 300 (360), and 1201 (4 × 300 + 1-star polymer with four arms) beads accordingly. The obtained analytical and numerical results for phantom ring and star polymers are compared with the results for linear polymer chains in confined geometries.

Designing Interrupted Microchannel Heat Sink with Ribbed Microchambers by Single and Bi-objective Optimizations of Numerical Results

The heat load from electronic chips increases with betterment and diversification of their applications. This demands more innovations in designing microchannel heat sink (MCHS). Interrupted MCHS (IMCHS) with ribbed microchambers is considered here as a possible innovation. The numerical solution of the continuum equations is exploited in subsequent optimization studies. Besides the rectangular rib (RR) and its adaptation to paired rib forms of DR, CR, CDR and DCR signifying diverging, converging, converging–diverging and diverging-converging internal channel between a rib pair, one more paired structure of ODCR is considered. The latter pair with parallel internal walls possesses outwardly diverging-converging form. First, an exploratory parametric study has been carried out by employing three-dimensional finite-volume method. The structure with CR has shown the promise of yielding the highest thermal performance (TP) over an identified parameter space involving Reynolds number, number and length of the microchambers, ratio of the maximum to the minimum areas between the paired ribs as well as position of the pair. Results indicate strong role of the relative amount of discharges through and past the pair. Next, the numerical tool is used in combination with response surface method and particle swarm optimization for executing both maximization of a single-objective TP and fulfilling the conflicting objectives of maximizing heat transfer and minimizing pressure drop. By examining the results, the TP maximization design has been found to provide the design with concurrent combination of the high heat transfer and low penalty of pressure loss achieved by the IMCHS-CR design.

A measurement method of motion parameters in aircraft ground tests using computer vision

The purpose of this work is aiming at the problem of high precision and large-scale measurement of motion parameters of aircraft ground test. Based on the traditional vision measurement method of motion parameters, a new measurement method of motion parameters based on intersected planes is discussed. Firstly, the measurement of 3D coordinates of laser spots was performed using the 3 D vision technique combined with plane constraints of curtain wall, the images of laser spot projected on the parallel curtain wall were processed using the centroid method, the calibration of multi-coordinates were unified by the unique coordinate system method, then the pose was solved using the proposed method that the normal vector of the intersect plane fixed on the vehicle is used. Finally, according to the measurement principle, the influence of spot center positioning, camera parameter calibration and target geometric error on the calculation results of aircraft motion parameters is analyzed, and the error propagation model is given. The experimental results show that in the measurement range of 8000 mm × 4000 mm × 4000 mm, the measurement error of attitude parameters is less than 0.14°, and the measurement error of position parameters is less than 2 mm. Compared with binocular stereo vision, the accuracy of attitude angle measurement is improved by 100%. Therefore, the measurement method proposed in this paper can achieve high precision and large-scale measurement of the motion parameters of the test aircraft.

Free Convection Heat Transfer in a Micropolar Fluid Confined Between a Long Vertical Wavy Wall and a Parallel Flat Wall

The free convection heat transfer in a micropolar fluid confined between a long vertical wavy wall and a parallel flat wall has been studied. Analysis of fluid flow over a wavy wall is of interest because of its physical applications such as transpiration cooling of re-entry vehicles, rocket booster and film vaporization, in combustion chambers etc. The equations governing the flow and the heat transfer have been solved subject to the relevant boundary conditions by assuming that the solution consists of two parts viz. a mean part and a perturbed one. To obtain the perturbed part of the solution, use has been made of the long Wavy approximation. The sets of differential equations have been solved by Finite Element Method. The zeroth order, the first order and total solution of the problem have been evaluated numerically for several sets of values of the various parameters entering the problem and are depicted graphically.

Dual row retaining walls in dry sand: influence of wall stiffness on seismic response

Dual row retaining walls can form efficient port and embankment structures, or even be used as coastal defence against tsunamis. The system of parallel sheet pile walls can have a large lateral capacity within serviceability limit states due to the combined strength and stiffness of the walls and confined soil. Optimising the design by reducing the wall section and ensuring greater utilisation of the soil capacity has economic and environmental benefits but requires a deeper understanding of the dynamic soil–structure interaction. Centrifuge modelling and numerical modelling are used to elucidate the mechanics of two systems with relatively flexible and stiff walls. Considering the fraction of the wall’s plastic moment capacity mobilised alongside the peak deflections illustrates the advantages of using relatively flexible retaining walls in these systems. More fundamentally, the importance of vertical variations of both the stress and strain during the horizontal dynamic loading is shown. The limiting horizontal stresses and phasing of the stress components around the walls are better understood by considering the mobilisation of earth pressure coefficients, reinforcing previous work that recommends a move away from conventionally defined limiting dynamic earth pressure coefficients.

Thrombosis of pulmonary vasculature despite anticoagulation and thrombolysis: The findings from seven autopsies

BackgroundVenous thromboembolism (VTE) with the prevalence of pulmonary microcirculatory thrombosis is considered a common complication of novel coronavirus disease (COVID-19) that develops despite anticoagulation. MethodsThe clinical course of the disease and the autopsy findings of seven deceased patients with verified COVID-19 were analyzed. The chest computed tomography (CT) scan was routinely performed while CT pulmonary angiography and a duplex ultrasound scan (DUS) of the lower limbs were used in cases of suspected VTE. The VTE prophylaxis was administered to all patients with intermediate or therapeutic doses of low-molecular-weight heparin. The histological examination of the lung tissue and other organs was performed with particular attention paid to the pulmonary vasculature. ResultsVenous thromboembolism, including deep vein thrombosis in one patient and pulmonary artery thrombosis in two patients, was confirmed by imaging tests despite anticoagulation. Systemic thrombolysis was performed in two patients with putative and confirmed pulmonary embolism. An autopsy revealed the signs of acute respiratory distress syndrome in all seven patients. Abnormalities of lung vessels were found in all cases and were represented by dystrophy and necrosis in the endothelium and muscle fibers, and by infiltration by plasmatic cells, neutrophils, and lymphocytes. Multiple clots of variable maturity were observed. All those changes developed despite anticoagulation and were preserved after systemic thrombolysis. ConclusionInflammatory and prothrombotic changes in the arterial wall in parallel with the lack of lung perfusion may cause diffuse arterial thrombosis in the lungs. This background may be responsible for the low response to systemic anticoagulation and thrombolysis in severe forms of COVID-19.

Unsteady Flow of a Viscous Electrically Conductive Fluid between Rotating Parallel Walls with Blowing (Suction) of the Medium and a Magnetic Field

The motions of a viscous electrically conductive incompressible fluid, which initially rotates as a solid with a constant angular velocity together with the parallel walls bounding it, under the action of suddenly beginning longitudinal vibrations of one of the walls have been studied. The angle between the walls and the axis of rotation is arbitrary. An unsteady flow is induced by longitudinal vibrations of one of the walls, injection (suction) of the medium produced perpendicular to the surface of the porous plate, and a magnetic field suddenly switched on and directed normal to the wall. Analytical solutions of the equations of magnetohydrodynamics have been constructed. The velocity field and vectors of tangential stresses of a viscous conductive fluid that acts on the walls of the gap have been determined. The induced magnetic field in the flow of an electrically conductive fluid has been determined. Some particular cases of wall motion are considered. Based on the results, the individual structures of the boundary layers adjacent to the walls of the gap have been investigated.

Graphite-Based Blade-Type Field Emission Cathodes

In this work, the blade-type field emission cathodes (BFECs) of original construction have been developed and produced. As the basis of the cathodes the (110)-cut silicon wafers were used. On the upper surface of Si substrate the parallel grooves of the 20 μm width and 50 μm depth of 1.7 mm in length were etched in a concentrated aqueous 44% KOH solution. As a result the parallel Si walls of the rectangular shape were created, which after imparting them a wedge shape form were used as a pedestals for supporting of emitting graphite blades, obtained by the electron beam evaporation of graphite. The blades were sharpened to the thickness not exceeding 10 nm. It was shown that our BFECs can operate at field emission current up to 6 mA, depending on the sharpness of a blade. It was found that the cathodes, which had previously worked on low currents, then showed high limiting currents, in comparison with the cathodes, which were initially forced to work on extreme emission currents.

Spatially and Temporally Resolved Measurements of NO Adsorption/Desorption over NOx-Storage Catalyst.

The two‐dimensional (2D) temporal evolution of the NO‐concentration over a NOx‐storage catalyst is investigated in situ with planar laser‐induced fluorescence (PLIF) in an optically accessible parallel wall channel reactor. Signal accumulated phase‐correlated 2D‐recordings of repetitive adsorption/desorption cycles are obtained by synchronizing the switching of the NO gas flow (on/off) with the laser and detection system, thereby significantly increasing the signal‐to‐noise ratio. The gas compositions at the reactor outlet are additionally monitored by ex‐situ analytics. The impacts of varying feed concentration, temperature and flow velocities are investigated in an unsteady state. Transient kinetics and the mass transfer limitations can be interpreted in terms of the NO concentration gradient changes. The technique presented here is a very useful tool to investigate the interaction between surface kinetics and the surrounding gas flow, especially for transient catalytic processes.

Coupling of turbulence wall models and immersed boundaries on Cartesian grids

An improved coupling of immersed boundary method and turbulence wall models on Cartesian grids is proposed, for producing smooth wall surface pressure and skin friction at high Reynolds numbers. Spurious oscillations are frequently observed on these quantities with most immersed boundary wall modeling methods, especially for the skin friction which is found to be very sensitive to the solid surface's position and orientation against the Cartesian grids. The problem originates from the irregularity of the wall distance on the stair-step grid boundaries where the immersed boundary conditions are applied. To reduce this directional error, several modifications are presented to enhance the near wall solution. First, the commonly used interpolation for the flow velocity is replaced by one for the friction velocity, which has much less variation near wall. The concept of using a fictitious point to retrieve flow fields in the wall normal direction is abandoned and the interpolation is performed in the wall parallel plane with existing fluid points. Secondly, the velocity gradients at the approximated boundary are computed with advanced schemes and the normal gradient of the tangential velocity is reconstructed from the wall laws. To further protect the near wall solution, the normal velocity gradient and the working viscosity from the Spalart-Allmaras turbulence model are enforced by their theoretical solutions in the interior fluid close to the wall. Additionally, various post-processing algorithms for reconstructing wall surface quantities and force integrations are investigated. Other related factors are also discussed for their effects on the results. The validity of present method has been demonstrated through numerical benchmark tests on a flat plate at zero pressure gradient, both aligned and inclined with respect to the grid, as well as aerodynamic cases of NACA 23012 airfoil and NASA trap wing.