Changes In Boundary Conditions Research Articles

Numerical simulation study of the hydrodynamic regimes in large to micro fluidized beds based on a Two-Fluid Model

Although the hydrodynamic properties of fluidized bed reactors have been investigated experimentally in the macro- to micro-scale range, the measurements and diagnostic tools needed for this analysis are complex. This in turn increases the difficulties experienced when trying to account for various specific phenomena, which occur within very small, i.e. Micro-Fluidized Beds (MFBs). In the present study, a numerical approach has been used to better understand the hydrodynamic behaviour of microreactors. The influence of a reduced value of bed diameter (Dt), the value of the static bed height (Hs), and the role of changing boundary conditions on various hydrodynamic parameters, bubble behaviours and particle circulation patterns were simulated using a Two-Fluid Model (TFM), and compared with our previous experimental results. It was found that a reasonable setting of the solid volume fraction in a fixed bed (εs,0) was key to successful simulation of the onset of minimum fluidization (and minimum bubbling), which is delayed for decreasing values of Dt. Setting the specularity coefficient (ϕ) to 1 resulted in bubble and particle movements being more consistent with those observed in the MFB experiment. In the simulation, a round-nosed slug rose along the center of the bed for small values of Hs, whereas it transformed into a wall slug in the upper region at greater values of Hs, which is in good agreement with our experimental observations. In addition, the experimental phenomenon in which slug is prevented from rising upwards within the MFB was successfully reproduced in the simulation.

Quantifying the impacts of the Three Gorges Dam on the spatial–temporal water level dynamics in the upper Yangtze River estuary

Abstract. Understanding the alterations in spatial–temporal water level dynamics caused by natural and anthropogenic changes is essential for water resources management in estuaries, as this can directly impact the estuarine morphology, sediment transport, salinity intrusion, navigation conditions, and other factors. Here, we propose a simple triple linear regression model linking the water level variation on a daily timescale to the hydrodynamics at both ends of an estuary. The model was applied to the upper Yangtze River estuary (YRE) to examine the influence of the world's largest dam, the Three Gorges Dam (TGD), on the spatial–temporal water level dynamics within the estuary. It is shown that the regression model can accurately reproduce the water level dynamics in the upper YRE, with a root mean squared error (RMSE) of 0.061–0.150 m seen at five gauging stations for both the pre- and post-TGD periods. This confirms the hypothesis that the response of water level dynamics to hydrodynamics at both ends is mostly linear in the upper YRE. The regression model calibrated during the pre-TGD period was used to reconstruct the water level dynamics that would have occurred in the absence of the TGD's freshwater regulation. Results show that the spatial–temporal alterations in water levels during the post-TGD period are mainly driven by the variation in freshwater discharge due to the regulation of the TGD, which results in increased discharge during the dry season (from December to March) and a dramatic reduction in discharge during the wet-to-dry transitional period. The presented method to quantify the separate contributions made by changes in boundary conditions and geometry to spatial–temporal water level dynamics is particularly useful for determining scientific strategies for sustainable water resources management in dam-controlled or climate-driven estuaries worldwide.

Quantifying the Effect of Index-Based Operation Logic for Building Environmental Control System—Taking Shading as Example

Dynamic control of building environment control systems (BECSs) is an important procedure to realize energy consumption reduction while ensuring the occupant’s comfort. Two types of BECSs operation logic exist: parameter-based and index-based. This research concluded that based on the literature review and argumentation, index-based operation logic, advanced from parameter-based operation logic, can better fit the dynamic and complex needs of occupants. However, existing index-based operation logic is generally based on a single performance index, while the BECS operation affects the indoor environment in multiple dimensions, thus, a single index cannot describe the operation comprehensively and accurately. Therefore, this study takes shading as an example, summarizes the performance indices of index-based operation logic for shading from two dimensions, and sorts out six typical control strategies according to different control objectives. The operation effect was analyzed and quantified through simulation. The results demonstrate that the index-based operation strategy has positive effects. It is not sensitive to changes in boundary conditions and the control effect is not affected by individual factors. Meanwhile, advice on the index selection for shading is proposed.

Generated datasets from dynamic reproduction of projectiles in ballistic environments for advanced research (DROPBEAR) testbed

High-rate dynamics occur when a system’s acceleration is larger than 100 gn over durations less than 100 ms. Structural health monitoring algorithms must be created for high-rate dynamic systems to maximize safety and minimize economic losses. There is a need to evaluate these algorithms for precision and accuracy prior to real-world implementation. An experimental testbed was created to simulate large-magnitude events while maintaining repeatability to accurately and robustly assess various structural health monitoring algorithms’ capability to monitor high-rate dynamic systems. All previous datasets created on the experimental testbed are discussed, examining various sensor setups, excitations, and boundary condition changes to properly simulate near-high-rate events and provide robust experimental data to evaluate structural health monitoring algorithms.

Modelling variability in vibration-based PBSHM via a generalised population form

Structural health monitoring (SHM) has been an active research area for the last three decades, and has accumulated a number of critical advances over that period, as can be seen in the literature. However, SHM is still facing challenges because of the paucity of damage-state data, operational and environmental fluctuations, repeatability issues, and changes in boundary conditions. These issues present as inconsistencies in the captured features and can have a huge impact on the practical implementation, but more critically, on the generalisation of the technology. Population-based SHM has been designed to address some of these concerns by modelling and transferring missing information using data collected from groups of similar structures. In this work, vibration data were collected from four healthy, nominally-identical, full-scale composite helicopter blades. Manufacturing differences (e.g., slight differences in geometry and/or material properties), among the blades presented as variability in their structural dynamics, which can be very problematic for SHM based on machine learning from vibration data. This work aims to address this variability by defining a general model for the frequency response functions of the blades, called a form, using mixtures of Gaussian processes.

Ising model in a boundary magnetic field with random discontinuities

We consider a two-dimensional Ising field theory on a space with boundary in the presence of a piecewise constant boundary magnetic field which is allowed to change value discontinuously along the boundary. We assume zero magnetic field in the bulk. The positions of discontinuities are averaged over as in the annealed disorder. This model is described by a boundary field theory in which a superposition of the free spin boundary condition is perturbed by a collection of boundary condition changing operators. The corresponding boundary couplings give the allowed constant values of the magnetic field as well as the fugacities for the transitions between them. We show that when the value of the magnetic field is allowed to take only two different values which are the same in magnitude but have different signs the model can be described by a quadratic Lagrangian. We calculate and analyse the exact reflection matrix for this model. We also calculate the boundary entropy and study in detail the space of RG flows in a three-parameter space and with four different infrared fixed points. We discuss the likely breakdown of integrability in the extended model which allows for two generic values of the boundary magnetic field, backing it by some calculations.

Effects of boundary and spatial heterogeneity on nonlocal dispersal equations

This paper is concerned with the positive solutions of some nonlocal dispersal equations. We study the effects of boundary condition and spatial heterogeneity on the asymptotic behavior of positive solutions. By analyzing the perturbation equations, we obtain the asymptotic profiles and limiting behaviors. The results of the paper exhibit how the boundary condition and spatial heterogeneity change the patterns of nonlocal dispersal equations.

Factors affecting the numerical response and fracture location of the GHBMC M50 rib in dynamic anterior-posterior loading

Rib fractures are common traumatic injuries, with links to increased morbidity and mortality. Finite element ribs from human body models have struggled to predict the force-displacement response, force and displacement at fracture, and the fracture location for isolated rib tests. In the current study, the sensitivity of a human body model rib with updated anisotropic and asymmetric material models to changes in boundary conditions, material properties, and geometry was investigated systematically to quantify contributions to response. The updated material models using uncalibrated average material properties from literature improved the force-displacement response of the model, whereas the cross-sectional geometry was the only parameter to effect fracture location. The resulting uncalibrated model with improved material models and cross-sectional geometry closely predicted experimental average force-displacement response and fracture location.

The stability of Prendergast magnetic fields

ABSTRACT Convection in massive main-sequence stars generates large-scale magnetic fields in their cores that persists as they evolve up the red giant branch. The remnants of these fields may take the form of the Prendergast magnetic field, a combination of poloidal and toroidal field components that are expected to stabilize each other. Previous analytic and numerical calculations did not find any evidence for instability of the Prendergast field over short time-scales. In this paper, we present numerical simulations which show a long time-scale, linear instability of this magnetic field. We find the instability to be robust to changes in boundary conditions and it is not stabilized by strong stable stratification. The instability is a resistive instability, and the growth rate has a power-law dependence on the resistivity, in which the growth rate decreases as the resistivity decreases. We estimate the growth rate of the instability in stars by extrapolating this power law to stellar values of the resistivity. The instability is sufficiently rapid to destabilize the magnetic field on time-scales shorter than the stellar evolution time-scale, indicating that the Prendergast field is not a good model to use in studies of magnetic fields in stars.

Hydrogen isotope behavior during rhyolite glass hydration under hydrothermal conditions

The diffusion of molecular water (H2Om) from the environment into volcanic glass can hydrate the glass up to several wt% at low temperature over long timescales. During this process, the water imprints its hydrogen isotope composition (δDH2O) to the glass (δDgl) offset by a glass-H2O fractionation factor (ΔDgl-H2O = δDgl – δDH2O) which is approximately −33‰ at Earth surface temperatures. Glasses hydrate much more rapidly at higher, sub-magmatic temperatures as they interact with H2O during eruption, transport, and emplacement. To aid in the interpretation of δDgl in natural samples, we present hydrogen isotope results from vapor hydration experiments conducted at 175–375 °C for durations of hours to months using natural volcanic glasses. The results can be divided into two thermal regimes: above 250 °C and below 250 °C. Lower temperature experiments yield raw ΔDgl-H2O values in the range of −33 ± 11‰. Experiments at 225 °C using both positive and negative initial ΔDgl-H2O values converge on this range of values, suggesting this range represents the approximate equilibrium fractionation for H isotopes between glass and H2O vapor (103lnαgl-H2O) below 250 °C. Variation in ΔDgl-H2O (−33 ± 11‰) between different experiments and glasses may arise from incomplete hydration, analytical uncertainty, differences in glass chemistry, and/or subordinate kinetic isotope effects. Experiments above 250 °C yield unexpectedly low δDgl values with ΔDgl-H2O values of ≤–85‰. While alteration alone is incapable of explaining the data, these run products have more extensive surface alteration and are not interpreted to reflect equilibrium fractionation between glass and H2O vapor. Fourier transform infrared spectroscopy (FTIR) shows that glass can hydrate with as much as 5.9 wt% H2Om and 1.0 wt% hydroxl (OH−) in the highest P-T experiment at 375 °C and 21.1 MPa. Therefore, we employ a 1D isotope diffusion–reaction model of glass hydration to evaluate the roles of equilibrium fractionation, isotope diffusion, water speciation reactions internal to the glass, and changing boundary conditions (e.g. alteration and dissolution). At lower temperatures, the best fitting model results to experimental data for low silica rhyolite (LSR) glasses require only an equilibrium fractionation factor and yield 103lnαgl-H2O values of −33‰ ± 5‰ and −25‰ ± 5‰ at 175 °C and 225 °C, respectively. At higher temperatures, ΔDgl-H2O is dominated by boundary layer effects during glass hydration and glass surface alteration. The modeled bulk δDgl value is highly responsive to changes in the δDgl boundary condition regardless of the magnitude of other kinetic effects. Observed glass dissolution and surficial secondary mineral formation are likely to impose a disequilibrium boundary layer that drives extreme δDgl fractionation with progressive glass hydration. These results indicate that the observed ΔDgl-H2O of ∼−33 ± 11‰ can be cautiously applied as an equilibrium 103lnαgl-H2O value to natural silicic glasses hydrated below 250 °C to identify hydration sources. This approximate ΔDgl-H2O may be applicable to even higher temperature glasses hydrated on short timescales (of seconds to minutes) in phreatomagmatic or submarine eruptions before H2O in the glass is primarily affected by boundary layer effects associated with alteration on the glass surface.

A dynamic operative framework for allocation in automated storage and retrieval systems

Two very important aspects in automated storage and retrieval systems (AS/RS) are productivity (or performance) and maintenance costs. In the literature, as in industry, it is very difficult to find a solution that guarantees satisfactory results in terms of both. Moreover, all the solutions that the scientific community has proposed are static, i.e., the system’s behavior does not change as boundary conditions change. In this paper, we propose an innovative solution known as a dynamic operative framework (DOF), which allows the system to react to changes in operating conditions and guarantees good results in terms of productivity, with a consistent reduction in the distance that handling machines travel and a consequent reduction in maintenance and energy consumption costs. To test the proposed solution, we focused on the shuttle-lift crane AS/RS, a common configuration used to store bundles of long metal bars, and we compared the DOF with four benchmark policies that exploit a class-based reorganization of the stock performed during non-working shifts. Many simulation runs’ results indicated that the DOF ensures a throughput time aligned to that of the benchmarks, but without needing to reorganize the stock during nonworking shifts. In this way, it leads to consistent savings in terms of energy consumption and maintenance costs.

Quaternary E-W Extension Uplifts Kythira Island and Segments the Hellenic Arc.

Several crustal and lithospheric mechanisms lead to deformation and vertical motion of the upper plate during subduction, but their relative contribution is often enigmatic. Multiple areas of the Hellenic Forearc have been uplifting since Plio-Quaternary times, yet spatiotemporal characteristics and sources of this uplift are poorly resolved. The remarkable geology and geomorphology of Kythira Island, in the southwestern Hellenic forearc, allow for a detailed tectonic reconstruction since the Late Miocene. We present a morphotectonic map of the island, together with new biostratigraphic dating and detailed analyses of active fault strikes and marine terraces. We find that the Tortonian-Pliocene stratigraphy in Kythira records ∼100m of subsidence, and a wide coastal rasa marks the ∼2.8-2.4Ma maximum transgression. Subsequent marine regression of ∼300-400m and minor E-W tilt are recorded in ∼12 marine terrace levels for which we estimate uplift rates of ∼0.2-0.4mm/yr. Guided by simple landscape evolution models, we interpret the coastal morphology as the result of initial stability or of slow, gradual sea-level drop since ∼2.8-2.4Ma, followed by faster uplift since ∼1.5-0.7Ma. Our findings on- and offshore suggest that E-W extension is the dominant mode of regional active upper crustal deformation, and N-S normal faults accommodate most, if not all of the uplift on Kythira. We interpret the initiation of E-W extension as the result of a change in plate boundary conditions, in response to either propagation of the North Anatolian Fault, incipient collision with the African plate, mantle dynamics or a combination thereof.

Discussion of “Stress-Displacement Response of Sand–Geosynthetic Interfaces under Different Volume Change Boundary Conditions” by Aliyeh Afzali-Nejad, Ali Lashkari, and Alejandro Martinez

Discussion of “Stress-Displacement Response of Sand–Geosynthetic Interfaces under Different Volume Change Boundary Conditions” by Aliyeh Afzali-Nejad, Ali Lashkari, and Alejandro Martinez

Closure to “Stress-Displacement Response of Sand–Geosynthetic Interfaces under Different Volume Change Boundary Conditions” by Aliyeh Afzali-Nejad, Ali Lashkari, and Alejandro Martinez

Closure to “Stress-Displacement Response of Sand–Geosynthetic Interfaces under Different Volume Change Boundary Conditions” by Aliyeh Afzali-Nejad, Ali Lashkari, and Alejandro Martinez

Molecular dynamics study of sucrose aqueous solutions: From solution structure to transport coefficients

In this work, we study the applicability of the equilibrium molecular dynamics methods for the prediction of solution structure, equation of state and transport properties of sucrose aqueous solution for various concentrations and temperatures. The OPLS-AA force field with 1.14*CM1A charge corrections is used to describe the interactions in the model. We analyze the prediction power of the model against the experimental data and simulation results, obtained previously in CHARMM, OPLS-AA, GLYCAM06 and their modifications. The glycosidic linkage conformations of sucrose in solution are found. Their independence from the sugar concentration and the appearance of the conformation region at temperatures from 283 K to 308 K are observed. Sucrose solution densities are calculated for different sugar concentrations and temperatures. The obtained viscosities and sucrose diffusion coefficients match the experimental data at low sucrose concentrations, and diverge within 41% from the experiment as the concentration increases. The change of boundary condition in Stokes–Einstein formula is observed with an increase of the sucrose concentration above 40%.

Sensitivity of non-Hermitian systems

Understanding the extreme sensitivity of the eigenvalues of non-Hermitian Hamiltonians to the boundary conditions is of great importance when analyzing non-Hermitian systems, as it appears generically and is intimately connected to the skin effect and the breakdown of the conventional bulk boundary correspondence. Here we describe a method to find the eigenvalues of one-dimensional one-band models with arbitrary boundary conditions. We use this method on several systems to find analytical expressions for the eigenvalues, which give us conditions on the parameter values in the system for when we can expect the spectrum to be insensitive to a change in boundary conditions. By stacking one-dimensional chains, we use the derived results to find corresponding conditions for insensitivity for some two-dimensional systems with periodic boundary conditions in one direction. This would be hard by using other methods to detect skin effect, such as the winding of the determinant of the Bloch Hamiltonian. Finally, we use these results to make predictions about the (dis)appearance of the skin effect in purely two-dimensional systems with open boundary conditions in both directions.

Development of a High-Fidelity Framework to Describe the Process-Dependent Viscoelasticity of a Fast-Curing Epoxy Matrix Resin including Testing, Modelling, Calibration and Validation.

Fast-curing epoxy resins enable substantial reduction of cycle times during production of thermoset polymer matrix composites. Due to the snap-cure behaviour, both characterisation and processing of these resins are associated with high complexity which motivates the development of a high-fidelity framework for the prediction of the process-dependent behaviour ranging from experiment to model validation. In order to determine influence of time, temperature, and degree of cure, a multitude of rheometer and dynamic mechanical analysis experiments are conducted and evaluated. Building on the experimental results, a material model based on a generalised Maxwell model is developed. It is calibrated on the results obtained in the tests and shown to describe the material’s behaviour with high accuracy under all investigated conditions. The model’s predictive capabilities are further tested by applying it to a dynamic mechanical analysis, exposing the model to previously unknown loading and temperature conditions. It is demonstrated that the model is capable of predicting such changing boundary conditions with high accuracy.

Detecting Changes in Boundary Conditions based on Sensitivity-based Statistical Tests

Structural health monitoring is a promising technology to automatically detect structural changes based on permanently installed sensors. Vibration-based methods that evaluate the global system response to ambient excitation are suited to diagnose changes in boundary conditions, i.e. changes in member prestress or imposed displacements. In this paper, these changes are evaluated based on sensitivity-based statistical tests, which are capable of detecting and localizing parametric structural changes. The main contribution is the analytical calculation of sensitivity vectors for changes in boundary conditions (i.e., changes in prestress or support conditions) based on stress stiffening, and the combination with a numerically efficient algorithm, i.e. Nelson’s method. One of the main advantages of the employed damage diagnosis algorithm is that, although it uses physical models for damage detection, it considers the uncertainty in the data-driven features, which enables a reliability-based approach to determine the probability of detection. Moreover, the algorithm can be trained and the probability of detecting future damages can be predicted based on data from the undamaged structure—in an unsupervised learning mode—making it particularly relevant for unique structures, where no data from the damaged state is available. For proof of concept, a numerical case study is presented. The study assesses the loss of prestress in a two-span reinforced concrete beam and showcases suitable validation approaches for the sensitivity calculation.

Assessing accuracy, reliability, and efficiency of combustion models for prediction of flame dynamics with large eddy simulation

A variety of combustion models for large eddy simulation of premixed turbulent flames have been developed and validated over the years. Validation studies concentrate on relevant mean quantities and turbulent fluctuations; however, the prediction of flame dynamics is typically not taken into account. Furthermore, it is difficult to meaningfully compare the computational efficiency of model formulations due to different compute resources, meshes, code bases, and numerics. The present study compares turbulent combustion models on the same code base, keeping boundary conditions, meshes, and numerical settings constant. The reliability and versatility of two turbulent combustion models, i.e., the artificially thickened flame and flame surface density formulations, are assessed by applying them to a variety of operating conditions and burner configurations. In particular, for a premixed methane swirl burner, we consider three power ratings by changing the inflow velocity, which increases the demand on the sub-grid scale model due to increased sub-grid scale wrinkling. A change in swirler position modifies the interference of swirl and acoustic perturbations, with a significant impact on flame dynamics. Changes in thermal boundary condition and combustion chamber size provide insight into the consequences of quenching effects resulting from heat losses on flame anchoring and flame topology.

Determining foaming regularities enabled by a five-nozzle foam generator for drilling technologies under conditions of abnormally low pressures

When studying the construction of oil and gas wells, it was found that the volume of drilling wells with abnormally low reservoir pressures increases over the years. This is due to significant difficulties such as a large absorption of drilling mud, possible clogging of a productive reservoir with drilling mud, and a failure to obtain the expected effect. These complications can be prevented by using gas-liquid mixtures as drilling mud, which have a number of advantages compared to washing liquids and make it possible to eliminate the above-mentioned negative phenomena. When opening productive horizons at low anomaly coefficients, foams must be used to flush wells. It has been established that at present in the practice of drilling oil and gas wells with foam there is no equipment that makes it possible to form foam with certain specified structure and dispersion. The use of a modernized foam generator for the preparation of foam has been proposed. To study the foaming process, computer modeling was carried out, with the help of which the processes that take place during the movement of flows of liquid, gas, and gas-liquid mixture along the foam generator were investigated, namely the distribution of pressure and speed in the longitudinal cross-section of the foam generator under changing boundary conditions, that is, at a pressure in the supply pipe of 10 and 7.5 MPa. Computer studies have confirmed the possibility of using an improved foam generator design to increase the efficiency of foaming. The results could be the basis for the development of foam generators and their experimental and industrial research and testing.