Fluctuating Boundary Conditions Research Articles

Direct observation on the phase separation of polymer membrane embryos confined by microfluidics chips

In this paper, the growth of finger-like droplets inside various polymer membrane embryos induced by non-solvent phase separation (NIPS) is studied in details by using a standardized optical microscopic observation method. The boundary conditions of the phase separation are well-defined by microfluidics technology. The growth of finger-like droplets and Marangoni convection inside them are tracked and recorded by high-speed camera of sharp resolutions. Effects of the geometry of the membrane embryos (thickness/shape/size), humidity of the atmosphere and the viscosity of the embryos on the growth of finger-like droplets are systematically studied. Except the qualitative observations that are consistent to previous relative reports, more quantitative data are provided in this paper. The diffusing speed of the interface of the developing large droplets is measured by template matching algorithm. Strikingly, the time-dependent diffusing speed of the developing large droplets shows a wave-like declining pattern, which indicates that fluctuating boundary conditions are surrounding the membrane embryos. The Reynolds number of the flow inside the developing large droplets is estimated at the order of 10−5, the magnitude of which is 2 order smaller than that of a swimming sperm.

Three-dimensional Heisenberg model with Dzyaloshinskii-Moriya interaction: A Monte Carlo study.

The three-dimensional classical Heisenberg model on a simple cubic lattice with Dzyaloshinskii-Moriya (DM) interactions between nearest-neighbors in all directions has been studied using Monte Carlo simulations. The Metropolis algorithm, combined with single histogram reweighting techniques and finite-size scaling analyses, has been used to obtain the thermodynamic behavior of the system in the thermodynamic limit. Simulations were performed with the same set of interaction parameters for both shifted boundary conditions (SBC) and fluctuating boundary conditions (FBC). Because of an incommensurability caused by the DM interaction, the SBC incorporated a fixed shift angle at the boundary which varies as a function of the DM interaction and lattice size. This SBC method decreases the simulation time significantly, but the distribution of states is somewhat different than that obtained with FBC. The ground state for nonzero DM interaction is a spiral configuration where the spins are restricted to lie in planes perpendicular to the DM vector. We found that this spiral configuration undergoes a conventional second-order phase transition into a disordered, paramagnetic state with the transition temperature being a function of the magnitude of the DM interaction. The limiting case with only DM interaction in the model has also been considered. The critical exponent ν, the critical exponent ratios α/ν, β/ν, γ/ν, as well as the critical temperature T_{c} and fourth-order cumulant of the order parameter U_{4}^{*} at T_{c} have been estimated for different magnitudes of DM interaction. The critical exponents and cumulants at the transition are different from those for the three-dimensional Heisenberg model, but the ratios α/ν, β/ν, γ/ν, U_{4}^{*}/ν are the same, implying that weak universality is valid for all values of DM interaction. Structure factor calculations for particular cases have been performed considering SBC and FBC in the simulations with different lattice sizes at the critical temperatures.

Numerical model of predicting surge boundaries in high-speed centrifugal compressors

A numerical model of the whole compression system is established in this paper. The introduction of the Laval nozzle and opening boundary methods adaptively simulate the physical fluctuating boundary conditions during the surge without any artificial hypothesis. Based on the model, the steady simulation method has the capacity to well predict the low-order systematical oscillations during the surge by monitoring flow parameters on several representative surfaces. The surge boundaries of a high-speed centrifugal compressor, including the mild surge and the deep surge, are predicted based on the current numerical model in different flow regimes. The discrepancies of the mild surge boundary between experimental data and numerical predictions are within 5% in the transonic regime, but within 0.5% in the subsonic and supersonic regimes. The sensitivities of the mild surge to the throttle opening degree differ much in different flow regimes: the highest is in the transonic regime and the lowest is in the supersonic regime. In the supersonic regime, there exist various mild surges in a wide range of throttle opening degrees and an unstable equilibrium point (UEP) which is the most typical characteristic of the two-regime surge. To the best knowledge of the authors, it is the first time that all the oscillation characteristics of various mild surges and the existence of UEP in the supersonic regime are well predicted by one numerical method.

On the operation of switchable oxygen depolarized cathodes

The electrochemical chlor-alkali electrolysis remains an energy-intensive production process even though many improvements have been developed over the last decades. Oxygen depolarized cathodes (ODC) reduce energy consumption by approximately 25% while oxygen is consumed instead of hydrogen being evolved. The switchable ODC (sODC) facilitates the oxygen-consuming and the conventional hydrogen-evolving modes with the same electrode. In this study, we investigate sODCs for demand-side electrolysis by switching between modes. Experimental investigations covered current densities ranging from 50mAcm−2 to 525mAcm−2, and we evaluated the long-term effect of switching between the two modes on the system's stability. In addition, cell potential, Faraday efficiency and contact angle measurements were compared for pristine and used sODCs after up to 1600 switching cycles. The lab cell was implemented in 3D-CFD simulations to investigate a nitrogen flushing process between the two modes to prevent the formation of explosive mixtures in the electrolyzer. sODCs showed a stable and continuously high Faraday efficiency. However, an increase in cell potential over 1000 switching cycles of up to 7.8% was observed, which was attributed to electrolyte flooding. Still, the sODC is well comparable to the conventional ODC with a marginal difference in cell potential (0.07V), which demonstrates its high potential for industrial application. 3D-CFD simulations were compared to experimental flushing time measurements. We implemented the resulting flushing times for a safe operation in the subsequent switching cycles. The verified CFD simulations can help to further optimize the flushing procedure for an economically feasible switching process in industry-sized cells. Switchable electrodes enable flexibility to react to fluctuating boundary conditions such as the electricity price and can be implemented in many processes beyond the chlor-alkali electrolysis.

Diffusion Thermo and Chemical Reaction Effects on Magnetohydrodynamic Jeffrey Nanofluid Over an Inclined Vertical Plate in the Presence of Radiation Absorption and Constant Heat Source

This article investigates the Diffusion thermo and chemical reaction effects on the free convection heat and mass transfer flow of Jeffrey nanofluids (Cu and TiO2) over an inclined porous vertical plate embedded in a porous medium in the presence of radiation absorption and constant heat source under fluctuating boundary conditions. The plate is moved with a constant velocity U0, temperature and the concentration are assumed to be fluctuating with time harmonically from a constant mean at the plate. Perturbation technique is applied to solve the governing equations of the flow and pointed out the variations in velocity, temperature and concentration with the use of graphical presentations. The impact of several parameters on local skin friction, Nusselt number and Sherwood number is also noticed and discussed. It is concluded that the resultant velocity reduces with increasing Jeffrey parameter and Suction parameter, velocity and Temperature enhances with increasing Radiation absorption parameter. Also it is noticed that the solutal boundary layer thickness decreases with an increase in chemical reaction parameter. It is because chemical molecular diffusivity reduces for higher values of Kr.

Incommensurate phases in the two-dimensional XY model with Dzyaloshinskii-Moriya interactions.

The two-dimensional XY model with Dzyaloshinskii-Moriya interaction has been studied through extensive Monte Carlo simulations. A hybrid algorithm consisting of single-spin Metropolis and Swendsen-Wang cluster-spin updates has been employed. Single histogram techniques have been used to obtain the thermodynamic variables of interest and finite-size-scaling analysis has led to the phase transition behavior in the thermodynamic limit. Fluctuating boundary conditions have been utilized in order to match the incommensurability between the spin structures and the finite lattice sizes due to the Dzyaloshinskii-Moriya interaction. The effects of the fluctuating boundary conditions have been analyzed in detail in both commensurate and incommensurate cases. The Berezinskii-Kosterlitz-Thouless transition temperature has been obtained as a function of the Dzyaloshinskii-Moriya interaction and the results are in excellent agreement with the exact equationfor the transition line. The spin-spin correlation function critical exponent has been computed as a function of the Dzyaloshinskii-Moriya interaction and temperature. In the incommensurate cases, optimal sizes for the finite lattices and the distribution of the boundary shift angle have been extracted. Analysis of the low temperature configurations and the corresponding vortex-antivortex pairs have also been addressed in some regions of the phase diagram.

Casson nanofluid performance on MHD convective flow past a semi‐infinite inclined permeable moving plate in presence of heat and mass transfer

AbstractThis article investigates the radiation and chemical reaction effects on the free convection heat and mass transfer flow of Casson nanofluids (Cu and TiO2) over an inclined porous vertical plate embedded in a porous medium in the presence of radiation absorption and constant heat source under fluctuating boundary conditions. The plate is moved with a constant velocity of U0, and temperature and the concentration are assumed to be fluctuating with time harmonically from a constant mean at the plate. The perturbation technique is applied to solve the governing equations of the flow and pointed out the variations in velocity, temperature, and concentration with the use of graphical presentations. The impact of several parameters on local skin friction, Nusselt number, and Sherwood number is also noticed and discussed. It is concluded that the resultant velocity reduces with increasing Casson parameter and suction parameter, velocity, and temperature enhanced with increasing radiation absorption and diffusion thermo parameters. Also, it is noticed that the solutal boundary layer thickness decreases with an increase in chemical reaction parameters. It is because chemical molecular diffusivity reduces for higher values of Kr.

Digital Twins in deep drawing for virtual tool commissioning and inline parameter optimization

Fluctuating boundary conditions and the highly nonlinear process behavior of deep drawing operations make experience-based selection of suitable control parameters difficult. Nowadays, commissioning deep drawing tools, i.e., die spotting and identification of suitable control parameters for defect-free parts, is conducted on real world try-out presses. Transferring the tools to production machines entails adaption of these initial parameters. Once production is ramped up, any changes to material properties, lubrication and press behavior require continuous manual machine parameter tuning. Virtual tool commissioning and utilizing these simulation models in the production phase to adapt control parameters would reduce time and cost over the entire life cycle of the machine and the tool sets. The virtual representative, which provides services for a plant over several life cycle phases, is also referred to as Digital Twin. In this paper, the authors present a Digital Twin concept for deep drawing presses to predict the state of the system and optimize the control parameters during the production. The integration of all involved subsystems into one system simulation and its efficient calculation is the biggest challenge. The authors combine a virtual commissioning simulation tool with a finite element model to implement all relevant properties of the deep drawing press and the interaction of its subsystems. It is shown, how the idea of a system simulation makes predictions of system parameters specific to a production situation possible, and therefore, can help to select suitable control parameters that leads to a reduction of the error rate in deep drawing.

The effect of height variations on cyclone performance under fluctuating boundary conditions

This study has been investigated the effect of the height variation of the cyclone components under the non-uniform flow. Oscillating flow is an approach to consider the real flow condition in the use of cyclones as an inlet air filter for the internal combustion engine. Accordingly, an oscillating flow with an inlet velocity of 15 (m/s) along with a range of pressure based on the motor inlet is applied to the cyclone’s dirty and clean outlets. Seven cyclone models including a base model, and models with height change in vortex finder, cylinder, and cone have been created and gridded. The numerical solution has been run using the Large Eddy Simulation and the Eulerian–Lagrangian model. Having validated, the flow field and cyclone performance have been considered. The results indicate that under the non-uniform flow, a decrease in the vortex finder height and an increase in the height of the cyclone’s cylinder and cone minimize the pressure drop. Applying oscillations along with the 30% change in the vortex finder height increases vortices power in the cone, cylinder, and vortex finder zones by 4.23, 8.99, and 21.42 percent, respectively. Additionally, the efficiency dependence on the change in length of cyclone components under the uniform flow is much higher than that of under the non-uniform flow.

Numerical simulation of an acoustically excited plane jet in an incompressible framework and comparison with experimental data.

This paper is concerned with the interaction of a two-dimensional plane jet with transverse plane acoustic waves, which occur, for example, in flue instruments in the vicinity of the nozzle exit. The acoustic excitation is modeled with fluctuating boundary conditions within the framework of an incompressible simulation. This method can be easily implemented in commercial CFD software. The simulations are compared with well-documented measurement data from other authors. It is shown that the model can predict not only the growth rate but also the amplitude of the perturbation, thus it captures the key features of the jet behavior in the receptive and in the linear growth region.

Numerical study on a crossing probability for the four-state Potts model: Logarithmic correction to the finite-size scaling

Abstract A crossing probability for the critical four-state Potts model on an $L\times M$ rectangle on a square lattice is numerically studied. The crossing probability here denotes the probability that spin clusters cross from one side of the boundary to the other. First, by employing a Monte Carlo method, we calculate the fractal dimension of a spin cluster interface with a fluctuating boundary condition. By comparison of the fractal dimension with that of the Schramm–Loewner evolution (SLE), we numerically confirm that the interface can be described by the SLE with $\kappa=4$, as predicted in the scaling limit. Then, we compute the crossing probability of this spin cluster interface for various system sizes and aspect ratios. Furthermore, comparing with the analytical results for the scaling limit, which have been previously obtained by a combination of the SLE and conformal field theory, we numerically find that the crossing probability exhibits a logarithmic correction ${\sim} 1/\log(L M)$ to the finite-size scaling.

Incommensurability and phase transitions in two-dimensional XY models with Dzyaloshinskii-Moriya interactions.

The Dzyaloshinskii-Moriya (DM) interaction in magnetic models is the result of a combination of superexchange and spin-orbital coupling, and it can give rise to rich phase-transition behavior. In this paper, we study ferromagnetic XY models with the DM interaction on two-dimensional L×L square lattices using a hybrid Monte Carlo algorithm. To match the incommensurability between the resultant spin structure and the lattice due to the DM interaction, a fluctuating boundary condition is adopted. We also define a different kind of order parameter and use finite-size scaling to study the critical properties of this system. We find that a Kosterlitz-Thouless-like phase transition appears in this system and that the phase-transition temperature shifts toward higher temperature with increasing DM interaction strength.

Fluctuating chemohydrodynamics and the stochastic motion of self-diffusiophoretic particles.

The propulsion of active particles by self-diffusiophoresis is driven by asymmetric catalytic reactions on the particle surface that generate a mechanochemical coupling between the fluid velocity and the concentration fields of fuel and product in the surrounding solution. Because of thermal and molecular fluctuations in the solution, the motion of micrometric or submicrometric active particles is stochastic. Coupled Langevin equations describing the translation, rotation, and reaction of such active particles are deduced from fluctuating chemohydrodynamics and fluctuating boundary conditions at the interface between the fluid and the particle. These equations are consistent with microreversibility and the Onsager-Casimir reciprocal relations between affinities and currents and provide a thermodynamically consistent basis for the investigation of the dynamics of active particles propelled by diffusiophoretic mechanisms.

The importance and challenge of hyporheic mixing

AbstractThe hyporheic zone is the interface beneath and adjacent to streams and rivers where surface water and groundwater interact. The hyporheic zone presents unique conditions for reaction of solutes from both surface water and groundwater, including reactions which depend upon mixing of source waters. Some models assume that hyporheic zones are well‐mixed and conceptualize the hyporheic zone as a surface water‐groundwater mixing zone. But what are the controls on and effects of hyporheic mixing? What specific mechanisms cause the relatively large (>∼1 m) mixing zones suggested by subsurface solute measurements? In this commentary, we explore the various processes that might enhance mixing in the hyporheic zone relative to deeper groundwater, and pose the question whether the substantial mixing suggested by field studies may be due to the combination of fluctuating boundary conditions and multiscale physical and chemical spatial heterogeneity. We encourage investigation of hyporheic mixing using numerical modeling and laboratory experiments to ultimately inform field investigations.

Effect of turbulent model closure and type of inlet boundary condition on a Large Eddy Simulation of a non-reacting jet with co-flow stream

In this paper, the behavior and turbulence structure of a non-reacting jet with a co-flow stream is described by means of Large Eddy Simulations (LES) carried out with the computational tool OpenFoam. In order to study the influence of the sub-grid scale (SGS) model on the main flow statistics, Smagorinsky (SMAG) and One Equation Eddy (OEE) approaches are used to model the smallest scales involved in the turbulence of the jet. The impact of cell size and turbulent inlet boundary condition in resulting velocity profiles is analyzed as well. Four different tasks have been performed to accomplish these objectives. Firstly, the simulation of a turbulent pipe, which is necessary to generate and map coherent turbulence structure into the inlet of the non-reacting jet domain. Secondly, a structured mesh based on hexahedrons has been built for the jet and its co-flow. The third task consists on performing four different simulations. In those, mapping statistics from the turbulent pipe is compared with the use of fluctuating inlet boundary condition available in OpenFoam; OEE and SMAG approaches are contrasted; and the effect of changing cell size is investigated. Finally, as forth task, the obtained results are compared with experimental data. As main conclusions of this comparison, it has been proved that the fluctuating boundary condition requires much less computational cost, but some inaccuracies were found close to the nozzle. Also, both SGS models are capable to simulate this kind of jets with a co-flow stream with exactitude.

A combination method to generate fluctuating boundary conditions for large eddy simulation

A Combination Random Flow Generation (CRFG) technique for obtaining the fluctuating inflow boundary conditions for Large Eddy Simulation (LES) is proposed. The CRFG technique was developed by combining the typical RFG technique with a novel calculation of k and e to estimate the length- and time-scales (l, t) of the target fluctuating turbulence field used as the inflow boundary conditions. Through comparatively analyzing the CRFG technique and other existing numerical/experimental results, the CRFG technique was verified for the generation of turbulent wind velocity fields with prescribed turbulent statistics. Using the turbulent velocity fluctuations generated by the CRFG technique, a series of LESs were conducted to investigate the wind flow around S-, R-, L- and U-shaped building models. As the pressures of the models were also measured in wind tunnel tests, the validity of the LES, and the effectiveness of the inflow boundary generated by the CRFG techniques were evaluated through comparing the simulation results to the wind tunnel measurements. The comparison showed that the LES accurately and reliably simulates the wind-induced pressure distributions on the building surfaces, which indirectly validates the CRFG technique in generating realistic fluctuating wind velocities for use in the LES. In addition to the pressure distribution, the LES results were investigated in terms of wind velocity profiles around the building models to reveal the wind flow dynamics around bluff bodies. The LES results quantitatively showed the decay of the bluff body influence when the flow moves away from the building model.

Transient Moisture Distribution and Hygrothermal Stress Analysis of Composite Laminates Using Equivalent Diffusion Methods

It often takes several decades to reach moisture equilibrium state in composite laminates. Therefore, hourly varying fluctuating boundary moisture and temperature conditions require several millions of time steps to reach a state of equilibrium. A new transient analysis method is proposed to efficiently solve moisture distribution of composite laminates with rapidly fluctuating moisture and temperature boundary conditions. First, effective constant diffusivity and maximum moisture content were obtained by averaging transient temperature-dependent moisture diffusivity. In this way, a simple analytic solution can be effectively used to determine the transient moisture distribution under hourly fluctuating boundary conditions. Its efficiencies are demonstrated in three different conditions by comparing this method with a conventional finite difference method. Hygrothermal stress and material degradation were consequently discussed to demonstrate that we must avoid a simple knock-down factor or 40% devastating penalty and use more rational simulation and supporting data.

Feinentschwefelung von Biogas mit dotierter Aktivkohle

Doped activated carbon is a special developed activated carbon for the desulfurization of biogas. Because of its special properties it is able to bond a big amount of hydrogen sulfi de. After many laboratory tests it was possible to demonstrate the performance of doped activated carbon for desulfurization in practical use The advantages and the specifi c functioning of doped activated carbon for desulfurization were here exactly as in previous laboratory studies. Despite fluctuating boundary conditions a continuous complete desulfurization was possible. By using the desulfurization system the concentration of hydrogen sulfide is lowered to less than 1 ppm. The damages or interferences that are often caused by hydrogen sulfi de could not be identified. A directly visible positive impact of the full desulfurization is the doubling of oil using time.

Anisotropies in thermal Casimir interactions: Ellipsoidal colloids trapped at a fluid interface

We study the effective interaction between two ellipsoidal particles at the interface of two fluid phases which are mediated by thermal fluctuations of the interface. In this system the restriction of the long-ranged interface fluctuations by particles gives rise to fluctuation-induced forces which are equivalent to interactions of Casimir type and which are anisotropic in the interface plane. Since the position and the orientation of the colloids with respect to the interface normal may also fluctuate, this system is an example of the Casimir effect with fluctuating boundary conditions. In the approach taken here, the Casimir interaction is rewritten as the interaction between fluctuating multipole moments of an auxiliary charge-density-like field defined on the area enclosed by the contact lines. These fluctuations are coupled to fluctuations of multipole moments of the contact line position (due to the possible position and orientational fluctuations of the colloids). We obtain explicit expressions for the behavior of the Casimir interaction at large distances for arbitrary ellipsoid aspect ratios. If colloid fluctuations are suppressed, the Casimir interaction at large distances is isotropic, attractive, and long ranged (double logarithmic in the distance). If, however, colloid fluctuations are included, the Casimir interaction at large distances changes to a power law in the inverse distance and becomes anisotropic. The leading power is 4 if only vertical fluctuations of the colloid center are allowed, and it becomes 8 if also orientational fluctuations are included.

Cluster simulation of relativistic fermions in two space–time dimensions

For Majorana–Wilson lattice fermions in two dimensions we derive a dimer representation. This is equivalent to Gattringer's loop representation, but is made exact here on the torus. A subsequent dual mapping leads to yet another representation in which a highly efficient Swendsen–Wang type cluster algorithm is constructed. It includes the possibility of fluctuating boundary conditions. It also allows for improved estimators and makes interesting new observables accessible to Monte Carlo. The algorithm is compatible with the Gross–Neveu as well as an additional Z ( 2 ) gauge interaction. In this article numerical demonstrations are reported for critical free fermions.