Flat Wall Research Articles

Numerical study of bubble dynamics near a solid wall with a gas-entrapping hole

Bubble dynamics with highly nonlinear interactions between an oscillating bubble and gas-liquid interface in the vicinity of a rigid wall with a gas-entrapping hole were numerically investigated in this study. The evolution of a cavitation bubble and its interactions with the gas-liquid interface were simulated using a fully compressible mixture model for three-phase flow. The solver was developed based on a dual-time preconditioning technique coupled with an interface capturing method on a general curvilinear grid. To assess the capability of the solver, validation was done with experimental data on bubble growth and collapse near a flat rigid wall (γ=2.0), and near a rigid wall with a gas-entrapping hole (γ=1.2). Good agreements in comparisons of the bubble shape and equivalent bubble radius were achieved. Numerical simulations of bubble growth and collapse in proximity to a rigid wall with a hole at different standoff distances were then conducted. The examined results showed that the gas entrapped inside the hole has considerable effects on the formation and redirection of the liquid jet, as well as on the dynamics of the cavitation bubble. Unlike the bubble collapse near a flat solid wall, the liquid jet was directed far away from the solid wall with a gas-entrapping hole, and the jet velocity was significantly smaller than that of the flat solid wall. The simulated results also revealed a potential for improving the design and production of body surfaces, which are often damaged by the impact of the liquid jet caused by bubble collapse.

Steady Gas-Droplet Flow Pattern and Heat Transfer Behind the Point of Incidence of a Shock Wave on a Flat Wall

The structure of a steady 2D gas-droplet flow in the near-wall region behind the point of incidence of a normal or an oblique shock wave on a plane wall is investigated. In this case, the normal wave corresponds to the Mach stem in the Mach reflection mode, and the oblique wave corresponds to the regular reflection mode of the incident oblique wave. The main aim of the study is to evaluate the effect of small liquid droplets present in the free stream on the equilibrium temperature of the adiabatic wall behind the point of wave reflection. The question is investigated: to what extent the presence of an oblique shock wave incident on the wall can enhance the effect of reducing the equilibrium wall temperature by small droplets present in the flow. The flow region is split into the outer region of “effectively inviscid flow” and the region of an asymptotic laminar boundary layer. Flow calculations in each region are based on a two-fluid model of a gas-droplet mixture, taking into account the phase transition (evaporation) on the droplet surface. The most interesting wave configurations from the point of view of heat transfer, corresponding to “fully and partially dispersed waves” with incomplete evaporation of droplets behind the reflected wave, are studied. A simple limiting scheme of the formation of a liquid film by droplets deposited on the wall is adopted, with the effects of film instability and spattering being ignored. Based on numerical calculations, the estimates are obtained for the possible decrease in the equilibrium temperature of the adiabatic wall behind the point of incidence of a shock wave in a steady supersonic gas flow containing a low concentration of liquid droplets.

Molecular dynamics simulation of the evaporation of thin liquid sodium film on the conical nanostructure surface

Deeply understanding the evaporation of the nanoscale thin liquid film on the nanostructure substrates is significant for further constructing the novel wick structure with nanostructure surfaces, which would be used in the optimal design of the sodium heat pipe. For this purpose, this paper investigates the evaporation of thin liquid film on conical nanostructured surfaces by the molecular dynamics method. A cuboid evaporation system is constructed. It consists of the bottom solid substrate and thin liquid sodium film. A flat solid wall is set as the reference. By changing the nanoscale cone's number and height, five conical nanostructure surfaces are built. The number of gas atoms and the total energy of fluid atoms are observed respectively. The evaporation rate and the average heat flux (q) of six cases are compared. The conical nanostructure suppresses evaporation. With the roughness of the conical nanostructures, the evaporation rate decrement maximum can reach 83% and the q decrement maximum can reach 58%. Based on the thermal resistances in the solid-liquid contact region, the D (Self-diffusion coefficient), and the peak values of RDF (Radial distribution function), the mechanisms of the conical nanostructure surfaces are analyzed. The suppressing effects of the conical nanostructure surfaces are achieved by weakening the heat conduction in the solid-liquid contact region and mitigating the collision heat transfer of liquid atoms.

Theoretical investigation on integrating a heaving WEC in the semicircle opening of a vertical breakwater

Despite so many investigations on the WEC-breakwater integration system, the practical wave energy extraction efficiency still needs to be improved. Recently, using specially designed arc breakwaters to focus the wave energy to a certain area was found to be a feasible method. In this paper the integration strategy of setting a heaving WEC at the central of a semicircle opening of a vertical solid breakwater is investigated based on the linear potential flow theory, with a semicircle thin curtain wall set outside the opening to give protection on the heaving WEC. The boundary value problem is solved using the method of separation of variables and eigenfunction expansion matching, and the multi-term Galerkin method is adopted to handle the singularity at the thin curtain wall. After successful validation of the proposed theoretical model, a comparison study between the proposed integration systems with different permeability of the curtain wall is conducted, and the cases with a heaving WEC in front of a flat vertical wall and an isolated heaving WEC are considered as a comparison. Then a parametric study is carried out on the effects of the incident wave angle, the porosity, radius, and draft of the semicircle curtain wall, and the dimension of the heaving WEC.

3D Reconstruction of Sawtooth 180° Tail‐to‐Tail Domain Walls in Single Crystal BiFeO3

AbstractPrevious studies of single crystal BiFeO3 have found a dense domain structure with alternating sawtooth and flat domain walls (DWs). The nature of these domains and their 3D structure has remained elusive to date. Herein, several sections taken at different orientations are used to examine the structure in detail, concentrating here on the sawtooth DWs using diffraction contrast transmission electron microscopy, electron diffraction, and aberration‐corrected scanning transmission electron microscopy (STEM). All DWs are found to be 180° type; the flat walls have head‐to‐head polarity while the sawtooth DWs are tail‐to‐tail with peaks elongated along the polar [111] axis, formed by neutral () DW facets and slightly charged facets with orientations close to () and (). The neutral DW facets are Ising type and very abrupt, while the charged DW facets have mixed Néel/Bloch/Ising character with a chiral nature and a width of about 2 nm.

Homogenization of thin-structured surfaces for acoustics in the presence of a two-dimensional low Mach potential flow

A surface homogenization method for acoustic waves over thin microstructured surfaces in the presence of a fluid in a potential flow is presented. Sound hard surfaces are considered, the flow is considered two-dimensional and slow and a low Mach approximation is introduced. We consider acoustic waves with a typical wavelength 1 / k much larger than the array spacing h and thickness e . Owing to the small parameter ε = k h , with e / h = O ( 1 ) , a matched asymptotic expansion technique is applied to the low Mach potential wave equation in the frequency domain. A boundary condition is obtained on an equivalent flat wall, which links the acoustic velocity to its normal and tangential derivatives (of the Myers type). The accuracy of the effective model is tested numerically for various periodic shapes and the accuracy of the model in O ( ε 2 ) is validated.

Pacific lamprey inspired climbing

Snakes and their bio-inspired robot counterparts have demonstrated locomotion on a wide range of terrains. However, dynamic vertical climbing is one locomotion strategy that has received little attention in the existing snake robotics literature. We demonstrate a new scansorial gait and robot inspired by the locomotion of the Pacific lamprey. This new gait allows a robot to steer while climbing on flat, near-vertical surfaces. A reduced-order model is developed and used to explore the relationship between body actuation and the vertical and lateral motions of the robot. Trident, the new wall climbing lamprey-inspired robot, demonstrates dynamic climbing on a flat near vertical carpeted wall with a peak net vertical stride displacement of 4.1 cm per step. Actuating at 1.3 Hz, Trident attains a vertical climbing speed of 4.8 cm s−1 (0.09 Bl s−1) at specific resistance of 8.3. Trident can also traverse laterally at 9 cm s−1 (0.17 Bl s−1). Moreover, Trident is able to make 14% longer strides than the Pacific lamprey when climbing vertically. The computational and experimental results demonstrate that a lamprey-inspired climbing gait coupled with appropriate attachment is a useful climbing strategy for snake robots climbing near vertical surfaces with limited push points.

Mixed convection flow in a channel with a dimpled section and adiabatic cylindrical obstacle under the influence of magnetic and Joule effects

This study investigates the mixed convection flow in a channel with a dimpled section and an adiabatic cylindrical obstacle under the influence of a magnetic field. The channel has a heated dimpled portion, while the flat horizontal walls are adiabatic. The energy equation considers the Joule heating effect, and the resulting momentum and thermal transport equations are numerically solved using the Galerkin weighted residual finite element method. The study discusses the flow and heat transfer characteristics for different values of the Joule heating parameter, Reynolds, Hartman, and Rayleigh numbers. The results show that increasing Hartmann and Joule heating parameters reduce average heat transfer rates while increasing Prandtl, Reynolds, and Rayleigh numbers enhances it. This analysis's outcomes will help design Ohmic heating systems like magnetic fluid power actuators and thermal processing gadgets for particulate food products. The novel quantitative findings in this study include enhancement in heat transfer with increasing Prandtl, Reynolds, and Rayleigh numbers. Furthermore, the adiabatic cylindrical obstacle near the dimpled section significantly affects the heat transfer rates, resulting in complex flow patterns. The results of this study can be used in magnetic fluid power actuators, thermal processing gadgets for particulate food products, and heat exchangers.

Effect of free boundary on the performance of single expansion nozzle

Abstract Effect of free boundary on the flow field characteristics of a single expansion nozzle has been studied experimentally and computationally. The single expansion nozzles studied in this investigation are a convergent-divergent nozzle of rectangular cross-section with convergent-divergent wall on one side and a flat wall on the opposite side, and an identical convergent-divergent ramp with its top open to atmosphere. The studies have been carried out at nozzle pressure ratios 2, 3, 4 and 5. The results show that the single expansion nozzle with wall boundary is able to deliver the flow with Mach number around 1.5, at nozzle pressure ratios of 4 and 5 even with the single expansion. The nozzle with free boundary, the wall static pressure is appreciably lower than that of nozzle with closed boundary and the exit Mach number is 1.5 for NPR 4 and 1.75 for NPR 5. That is, at the exit, the single expansion nozzle with free boundary delivers higher Mach number compared to single expansion nozzle with wall boundary for NPR 5.

Study of the thickness of the liquid film formed by a round water jet impinging on a curved cylindrical wall

An experimental setup has been established to investigate the thickness profiles of the liquid film formed by an oblique round water jet impinging on curved walls. The influence of the jet velocity, jet angle, and radius of curvature of the curved wall on the thickness profile of the liquid film has been investigated. Both on the flat and curved walls, as the jet velocity increases, the mode of the liquid film transitions from laminar to turbulent, and the thickness of the liquid film decreases first and then increases along the flow direction. The transition velocity range for the curved walls with the radius of curvature R = 30 mm is 19.1–25.08 m/s (Re = 10 946–14 373). Both on the flat and curved walls, the thicknesses of the liquid film increase in the downstream part of the liquid film while decreasing in the upstream part, as the jet angle increases. The laminar and turbulent thickness prediction models of liquid film on flat walls were extended to curved walls by replacing the distance away from a stagnation point with the radius of curvature. Predictions obtained by the present models agree well with measurements. Errors of the film thickness between the predictions and measurements along the centerline are mainly less than 20%, and the correlation coefficients (σc) are mainly located in 0.85–0.99.

Theoretical analysis of a novel PCHE with enhanced rib structures for high-power supercritical CO2 Brayton cycle system based on solar energy

A printed circuit heat exchanger (PCHE) is one of the most significant components in the supercritical CO2 (SCO2) Brayton cycle system based on concentrated solar power, whose performance significantly affects the efficiency and compactness of the system. To improve the performance of PCHE with semicircular-straight channels, rib structures placed on the top flat wall of the semicircular channel are proposed, which are achieved by the double sides etched heat transfer plates, and the effects of the distributions of rib structures on the performance of channels are also investigated. The thermal-hydraulic performance of the conventional semicircular channel and the semicircular channel with different rib structures are compared. The results indicate that rib structures lead to an apparent increase of turbulence kinetic energy in channels, and improve the synergy between velocity and temperature gradient fields, leading to the heat transfer enhancement. Among the channels with different rib structures, the comprehensive performance of the semicircular channel with the spacing distribution of short rib structures (channel DR3) is the best, which is relatively 19.3–19.8% higher than that of the conventional semicircular channel. When the channel DR3 is adopted in PCHE, the effectiveness of PCHE could reach 98.4–98.7%, the efficiency of the SCO2 Brayton cycle system based on solar power could be increased by 15.3%, and the compactness of the system could be improved by 3.8%. This present work is of great significance for deepening the understanding of PCHE heat transfer mechanism and performance optimisation, as well as improving the overall performance and compactness of large-scale SCO2-based power systems.

Analysis of the remote microphone probe technique for the determination of turbulence quantities

Measuring the statistics of the wall-pressure fluctuations (WPFs) is essential for understanding and evaluating laminar/turbulent boundary layers and flow-induced noise. The remote microphone probe (RMP) technique is one of the most successful experimental approaches to measuring WPFs. Despite its extensive use in academic and industrial research, the scientific literature currently lacks an evaluation of the RMP influence on the determination of turbulence quantities, such as the WPF spectrum, the convection velocity, and the turbulence coherence length. To fulfill this necessity, this paper analyses the effects of the RMP on the determination of turbulence quantities, including comparisons between the results obtained using RMPs and flush-mounted microphones (FMMs). The RMPs and the FMMs measure the WPFs under a turbulent boundary layer developed on a zero-pressure-gradient flat wall. This study shows that the turbulence quantities determined from RMP measurements are influenced by the RMP setup, i.e., tube length and inner diameter. The tube length affects the phase between RMPs, which impacts the turbulent structure convection velocity estimation, which can be determined correctly by considering the calibrated phase. Furthermore, the tube inner diameter influences the coherence between a pair of RMPs, affecting the calculation of the turbulence correlation length. This research also shows that the turbulence quantities determined from the RMP and the FMM measurements differ, mainly in the coherence between pairs of microphones.

Heat transfer on a flat wall due to a rectangular turbulent jet

We present Large Eddy Simulation (LES) results of heat transfer and flow evolution of a turbulent two-dimensional rectangular jet of air, of height H, impinging upon a flat wall. Rectangular jet is quasi-two dimensional, i.e. planar jet with infinite slot-width. Exit of jet is set at four distances, D, from wall, D = 4H, 3H, 2H, H. Distance of slot exit to wall modifies flow evolution, reported as mean and instantaneous, compared to experimental data. In mean flow Undisturbed Region of Flow, URF, and, in instantaneous flow, Negligible Disturbances Flow, NDF, and Small Disturbances Flow, SDF, are reported for the various distances D/H. In order to evaluate influence of flow parameters on heat transfer and flow evolution, two levels of turbulence are imposed on slot jet exit, i.e. 1% and 6%. The level of turbulence modifies heat transfer results, as far as convective heat transfer and Nusselt number are concerned. The shape of Nusselt number distribution along impinged wall is different according to level of turbulence. The paper presents a very good comparison with experimental Nusselt numbers, for the first time in literature.

Investigation of heat transfer in a porous material based on triply periodic surfaces of minimal energy

The subject of research: is a mathematical model of thermal conductivity in a porous flat wall, the structure of which is based on the three times periodic minimal Sean surface I-WP(TPMS).
 The purpose of research: to study the course of the heat conduction process in a porous plate at a given porosity. At the same time, the material from which the porous plate is made is PETG plastic.
 Methods of research: in this work, the solution was carried out by the finite difference method in the Mathcad software.
 Results of the research: temperature distributions were obtained along the spatial coordinate and in time, as well as the distribution of heat flow in time depending on the change in the plate porosity coefficient.

Sedimentation and migration dynamics of a spherical particle in an elastoviscoplastic fluid near a wall

The sedimentation of a spherical particle in an elastoviscoplastic fluid in proximity of a flat wall is investigated by direct numerical simulations. The governing equations under inertialess conditions are solved by the finite element method with an Arbitrary Lagrangian-Eulerian formulation to manage the particle motion. The fluid is modeled with the Giesekus constitutive equation modified as proposed by Saramito (2007).

Lamb dilatation and its hydrodynamic viscous flux in near-wall incompressible flows

In this paper, we introduce a new physical concept referred to as the wall-normal Lamb dilatation flux (WNLDF), which is defined as the wall-normal derivative of the Lamb dilatation (namely, the divergence of the Lamb vector) multiplied by the dynamic viscosity for incompressible viscous flows. It is proved that the boundary Lamb dilatation flux (BLDF, namely the wall-normal Lamb dilatation flux at the wall) is determined by the boundary enstrophy flux (BEF) and the surface curvature-induced contribution. As the first step to explore this new concept, the present study only considers flow past a stationary no-slip flat wall without curvature-fluid dynamic coupling effects. It is found that the temporal–spatial evolution rate of the WNLDF is contributed by four source terms, which can be explicitly expressed by using the fundamental surface quantities including skin friction (or surface vorticity) and surface pressure. Therefore, the instantaneous near-wall flow structures are directly related to the WNLDF in both laminar and turbulent flows. As an example, for the turbulent channel flow at Reτ=180, the intensification of the temporal–spatial evolution rate of the WNLDF is caused by the strong wall-normal velocity event (SWNVE) associated with quasi-streamwise vortices and high intermittency of the viscous sublayer. In addition, near the SWNVE, this evolution rate is mainly contributed by the dot product of skin friction and surface enstrophy gradient, as well as the coupling between the skin friction divergence and the boundary enstrophy. The exact results presented in this paper could provide new physical insights into complex near-wall flows.

Thermal analysis of ferromagnetic nanofluid flow in a channel over a dimpled cavity

The water-based nanofluid analysis of a ferromagnetic fluid flow in a channel has been conducted over a dimpled cavity. The horizontal flat walls are assumed to be adiabatic, whereas the dimpled part and the inner rectangular-finned-shaped obstacle are heated. The system of Partial Differential Equations (PDEs) along with boundary conditions have been solved via Galerkin Weighted Residual Finite Element Method. To predict the streamlines isotherms, local and average Nusselt numbers, all the simulations are performed for various Reynold and Hartmann numbers. High Reynolds numbers, results in significant improvement in heat transfer at the dimpled surface. It is believed that such investigation might be helpful to make a base for finding the experimental results for mixed convection of ferromagnetic nanofluid problem over a dimpled cavity.

Evidence of Formation of 1–10 nm Diameter Ice Nanotubes in Double-Walled Carbon Nanotube Capillaries

Water exhibits rich phase behaviors in nanoscale confinement. Since the simulation evidence of the formation of single-walled ice nanotubes (INTs) in single-walled carbon nanotubes was confirmed experimentally, INTs have been recognized as a form of low-dimensional hydrogen-bonding network. However, the single-walled INTs reported in the literature all possess subnanometer diameters (<1 nm). Herein, based on systematic and large-scale molecular dynamics simulations, we demonstrate the spontaneous freezing transition of liquid water to single-walled INTs with diameters reaching ∼10 nm when confined to capillaries of double-walled carbon nanotubes (DW-CNTs). Three distinct classes of INTs are observed, namely, INTs with flat square walls (INTs-FSW), INTs with puckered rhombic walls (INTs-PRW), and INTs with bilayer hexagonal walls (INTs-BHW). Surprisingly, when water is confined in DW-CNT (3, 3)@(13, 13), an INT-FSW freezing temperature of 380 K can be reached, which is even higher than the boiling temperature of bulk water at atmospheric pressure. The freezing temperatures of INTs-FSW decrease as their caliber increases, approaching to the freezing temperature of two-dimensional flat square ice at the large-diameter limit. In contrast, the freezing temperature of INTs-PRW is insensitive to their diameter. Ab initio molecular dynamics simulations are performed to examine the stability of the INT-FSW and INT-PRW. The highly stable INTs with diameters beyond subnanometer scale can be exploited for potential applications in nanofluidic technologies and for mass transport as bioinspired nanochannels.

Droplet-mediated long-range interfacial correlations. Exact field theory for entropic repulsion effects

We consider near-critical two-dimensional statistical systems at phase coexistence on the half plane with boundary conditions leading to the formation of a droplet separating coexisting phases. General low-energy properties of two-dimensional field theories are used in order to find exact analytic results for one- and two-point correlation functions of both the energy density and order parameter fields. The subleading finite-size corrections are also computed and interpreted within an exact probabilistic picture in which interfacial fluctuations are characterized by the probability density of a Brownian excursion. The explicit analysis of the closed-form expression for order parameter correlations reveals the long-ranged character of interfacial correlations and their confinement within the interfacial region. The analysis of correlations is then carried out in momentum space through the notion of interface structure factor, which we extend to the case of systems bounded by a flat wall. The presence of the wall and its associated entropic repulsion leads to a specific term in the interface structure factor which we identify.

Investigation of the overall stability of welded I-beams with a cross-corrugated wall of the Zeman range

Introduction. Welded I-beams with a transversely corrugated wall when loaded in the wall plane are calculated for overall stability in accordance with paragraph 20.6.3.11 of СП 294.1325800.2017, change 2 from 06/15/2021. Here, a separate centrally compressed girder belt is calculated for overall longitudinal stability from the plane of the beam wall as an element pivotally supported at the ends. This does not take into account the joint work of the compressed belt with the wall, supporting ribs, stretched belt.The authors set a goal to show by calculation using the examples of six beams that if, in comparison with the calculation according to СП 294.1325800.2017, the joint work of the compressed belt with the wall, support ribs, stretched belt is taken into account, then the calculated critical load of the total loss of stability of the corrugator will be greater. And this increase is the more significant, the lower the height of the corrugator. At the same time, the authors limited themselves to considering beams with a ratio of sizes and critical loads that ensure the operation of steel in the elastic stage with loss of overall stability.Materials and methods. The general stability of split beams with a transversely corrugated wall was studied by calculation in three ways: according to СП 294.1325800.2017, in the LIRA-CAD PC by modelling beams with shell elements, including belts, walls and support ribs, and according to CП 16.13330.2017, considering a welded I-beam with a flat wall equivalent to the criterion of general stability.The results of the study. The data of calculation of critical loads of the first form of loss of general stability of six split beams of the Zeman range with height are given 333, 500, 750, 1000, 1250, 1500 with a span of 6.0 m in three proposed ways with loading of the upper belt with a uniformly distributed load in the wall plane without loosening the compressed belt in the span and loosening the support sections from the wall plane and from rotation relative to the axis of the beams.Discussion and conclusions. For beams with a wall height of 333 mm, taking into account the joint work of the compressed belt with the wall, stretched belt, support ribs showed an increase in critical load in comparison with the calculation according to СП 294.1325800.2017 by 24%. With an increase in the height of the beams to 900-1000 mm, the difference in the values of critical loads calculated taking into account the joint work of the beam elements and according to СП 294.1325800.2017 non-linearly decreases to 3%. And for corrugated rollers with a height of 1000-1500 mm, this difference is less than 3%, which shows that it is possible to perform calculations of beams with a transversely corrugated wall of the Zeman range with heights of 1000-1500 mm for overall stability with sufficient accuracy for engineering calculations according to СП 294.1325800.2017.