Variety Of Boundary Conditions Research Articles

Temporal interfaces by instantaneously varying boundary conditions

Temporal metamaterials have been recently exploited as a novel platform for conceiving several electromagnetic and optical devices based on the anomalous scattering response arising at a single or multiple sudden changes of the material properties. However, they are difficult to implement in realistic scenarios by switching the permittivity of a material in time, and new strategies to achieve time interfaces in a feasible manner must be identified. In this paper, we investigate the possibility to realize a temporal metamaterial without acting on the material properties, but rather on the effective refractive index and wave impedance perceived by the wave during the propagation in an empty guiding structure by varying the boundaries in time. We demonstrate analytically and through numerical experiments that suddenly changing the physical distance between the metallic plates of a parallel-plate waveguide will induce an effective temporal interface. In addition to the standard backward and forward scattered fields at different frequencies due to the temporal interface, we also identify the presence of a static field necessary to satisfy the continuity of the electromagnetic field across the interface. The proposed concept can be extended to temporally controlled metasurfaces, opening an easier path to the design and realization of novel devices based on time-varying metamaterials at microwave and optical frequencies.

Performance comparison using different multilayer perceptron input–output formats to predict unsteady indoor temperature distribution

AbstractComputational fluid dynamics (CFD) is widely used to predict the indoor thermal environment; however, large time cost represents a significant disadvantage. Several deep learning approaches have been introduced to reduce prediction time in steady‐state predictions, though their feasibility under unsteady ones has yet to be investigated. Considering the flexibility of the multilayer perceptron (MLP) input–output format, this study compared the performance of two MLP input–output formats, MLP‐A (simultaneously outputting the values for all cells in a space in a single calculation run) and MLP‐B (outputting the values for a cell in each calculation run), when used to predict unsteady indoor temperature distribution in three scenarios: time interpolation, time extrapolation, and varying boundary conditions. The two considered input–output formats resulted in different prediction patterns in the time interpolation scenario: MLP‐B accurately predicted the spatiotemporal development of airflow compared to the CFD results, whereas MLP‐A did not. Both MLP‐A and MLP‐B performed poorly in the time extrapolation scenario but exhibited different error patterns. Finally, MLP‐A also generally provided a correct prediction of airflow development as well. This study contributes to an understanding of the prediction patterns provided by different MLP input–output formats for unsteady indoor airflow prediction.

Design optimization of a district heating and cooling system with a borehole seasonal thermal energy storage

The optimal design of borehole thermal energy storage systems can ensure their techno-economical goals are met. Current design optimization methods either employ detailed modelling unsuitable for numerical optimization or use simplified models that do not consider operational conditions. This paper proposes an optimization-oriented model and a non-convex optimization formulation that, differently from other studies in the literature, can consider the influence of the seasonal storage size and temperature on its capacity, losses, heat transfer rate, and efficiency of connected heat pumps or chillers. This methodology was applied to a case study, considering two scenarios: storing only the rejected heat from cooling and integrating solar thermal generation. Results show that, with varying boundary conditions such as the electricity CO2 intensity profile, cooling demand, and price of carbon emissions, not only the optimal seasonal storage size changes but also its optimal operating conditions. The potential reduction of CO2 emissions was found, under standard boundary conditions, to be limited (up to 6.7%), but an increase in cooling demand and an enhancement of the CO2 intensity seasonal variation led to a reduction of 27.1%. Integration of solar generation further improved it to 43.7%, with a comparably small increase in annual cost, up to 6.1%.

Linking atomic structural defects to mesoscale properties in crystalline solids using graph neural networks

Structural defects are abundant in solids, and vital to the macroscopic materials properties. However, a defect-property linkage typically requires significant efforts from experiments or simulations, and often contains limited information due to the breadth of nanoscopic design space. Here we report a graph neural network (GNN)-based approach to achieve direct translation between mesoscale crystalline structures and atom-level properties, emphasizing the effects of structural defects. Our end-to-end method offers great performance and generality in predicting both atomic stress and potential energy of multiple systems with different defects. Furthermore, the approach also precisely captures derivative properties which strictly observe physical laws and reproduces evolution of properties with varying boundary conditions. By incorporating a genetic algorithm, we then design de novo atomic structures with optimum global properties and target local patterns. The method would significantly enhance the efficiency of evaluating atomic behaviors given structural imperfections and accelerates the design process at the meso-level.

Compact Ammonia/Water Absorption Chiller of Different Cycle Configurations: Parametric Analysis Based on Heat Transfer Performance

Ammonia/water absorption chillers are driven by low-grade heat and cover wide refrigeration temperatures. This paper analyses single-stage ammonia/water absorption chillers. A numerical model was developed based on the heat exchanger performance. The model captures variational heat exchanger performances and describes the actual cycle with varying boundary conditions. The detrimental effects of refrigerant impurity were analysed quantitatively under different operating conditions. The model was validated with experimental data. A basic cycle and three advanced cycles were analysed for sub-zero refrigeration by comparing the thermodynamic performances. A compression-assisted cycle extended the activation temperature from 80 to 60 °C. At the heat source of 120 °C, when a counter-current desorber or bypassed rich solution was used, the COP increased from 0.51 to 0.58 or 0.57, respectively. The operating parameters included the heat source temperatures, heat sink temperatures, the mass flow rates and mass concentrations of rich solutions. Higher heat source temperatures increase cooling capacity. The increase was around 20 kW for the basic cycle of sub-zero refrigeration. There is an optimum heat source temperature maximising the COP. Higher heat source temperatures increased the refrigerant mass flow rate and reduced the mass concentration. The mass concentration can decrease from 0.999 to 0.960.

Acoustic Emission analysis of a comprehensive database of wire breaks in prestressed concrete girders

Acoustic Emission (AE) analysis has been established as a valuable monitoring technique for detecting wirebreaks, for example, in assessing the risk of Stress Corrosion Cracking (SCC) in older prestressed concrete bridges. A broad knowledge of the characteristics of the damage signal is crucial to reliable detection and subsequent classification. For this reason, extensive experiments were carried out on six prestressed concrete girders from three different bridges susceptible to SCC. Of these girders over 382 wirebreaks were provoked under a variety of boundary conditions, and a database of 2700 signals was collected. Typical AE features were calculated from the signals and used for analysis. For classification purposes reference data from real traffic noise and rebound hammer impacts were added to the database. The distributions of wirebreak features were found to be generally similar for all girders. Further, the local bonding conditions between the wire and the grout at the break location showed to affect the signals respective energy, which was mainly released below 50 kHz. The best separation of damage and noise signals was found for amplitude, energy, and RMS. Statistical parameters were extracted for use in deriving valid thresholds for monitoring applications.

Indicators of mantle control on the geodynamo from observations and simulations

There has been longstanding controversy about whether the influence of lateral variations in core-mantle boundary heat flow can be detected in paleomagnetic records of geomagnetic field behavior. Their signature is commonly sought in globally distributed records of virtual geomagnetic pole (VGP) paths that have been claimed to exhibit specific longitudinal preferences during polarity transitions and excursions. These preferences have often been linked to thermal effects from large low seismic velocity areas (LLVPs) in the lowermost mantle, but the results have been contested because of potential sensitivity to sparse temporal and spatial sampling. Recently developed time varying global paleofield models spanning various time intervals in 1–100 ka, three of which include excursions, allow us to complement assessments of spatial distributions of transitional VGP paths with distributions of minimum field intensity. Robustness of the results is evaluated using similar products from four distinct numerical dynamo simulations with and without variable thermal boundary conditions and including stable geomagnetic polarity, excursions and reversals. We determine that VGP distributions are less useful than minimum field intensity in linking the influences of thermal CMB structure to geographical variations in actual paleofield observables, because VGP correlations depend strongly on good spatial sampling of a sufficient number of relatively rare events. These results provide a basis for evaluating comparable observations from four paleofield models. The distribution of VGP locations provide unreliable results given the restricted time span and available data locations. Rough correlations of global distributions of minimum intensity with areas outside the LLVPs give some indications of mantle control during excursions, although the results for the eastern hemisphere are complex, perhaps highlighting uncertainties about the hemispheric balance between thermal and compositional variations in the lowermost mantle. However, access to other geomagnetic properties (such as intensity and radial field at the CMB) provides a strong argument for using extended and improved global paleofield models to resolve the question of mantle influence on the geodynamo from the observational side.

Off-Design Modeling and Operational Optimization of Trans-Critical Carbon Dioxide Heat Pumps

Abstract Industrial heat pumps, and specifically those using carbon dioxide (CO2) as a refrigerant, can play a key role in the decarbonization of the heating and cooling sector, due to their low global warming potential, toxicity and flammability. However, challenges arise when dealing with the modeling and optimization of CO2 heat pumps under different operating conditions. We address this challenge by presenting a modeling and optimization tool to predict and optimize the operation of heat pumps in off-design conditions. The tool improves on the current state-of-the-art in several ways. First, it describes a novel thermodynamic cycle, which features higher performance than conventional heat pumps. Also, it is based on a mathematical model that describes accurately the behavior of CO2 across a wide range of thermodynamic conditions, especially near its critical region, and takes into account effects of motor-cooling, leakages and performance limits. Furthermore, it maximizes the coefficient of performance (COP) of the heat pump via an accurate and computationally efficient optimization problem. The capabilities of the model are illustrated by looking at different typical heat pump applications based on real-world projects within the heating and cooling sector. Different case studies are considered, showing how the heat pump is optimally operated during the year to maximize its COP while meeting the varying boundary conditions.

Deep learning paradigm for prediction of stress distribution in damaged structural components with stress concentrations

Scientists and engineers agree that solving complex problems requires integrating traditional physics-based modeling techniques with state-of-the-art deep learning (DL) methods. This paper aims to integrate physics knowledge into a convolutional neural network (CNN) to boost learning within a feasible solution space in a specific domain. Our proposed method uses deep neural networks in the form of (CNNs) augmented with custom loss functions which uses physics rules to bypass the need for Finite Element Analysis and predict high-resolution stress distributions on damaged steel plates with variable loading and boundary conditions. We embedded physics constraints into the loss function to enforce the model training, precisely capturing stress concentrations around the tips of various structural damage configurations. The CNN was designed and trained to use the geometry, boundary conditions, and load as input and predict the stress contours. The proposed framework’s performance is compared to Finite-Element simulations using partial differential equation (PDE) solver. The trained DL model can predict the stress distributions of damaged steel plates with a mean absolute error of 0.22% percent and an absolute peak error of 1.5% for the Von Mises stress distribution

Coupled Model of Heat and Power Flow in Unventilated PV/PCM Wall-Validation in a Component Scale

The paper concerns the problem of the modeling of the thermal interaction between the phase change material (PCM) and photovoltaic (PV) panel, in the case of unventilated construction. The study aims to develop the numerical model of such a building element to support its proper future design under variable boundary conditions. The need for such a study comes from the realization of the research project which aims at developing a novel energy-activated thermal insulation composite system. Two different methods of PCM simulation using ESP-r software were compared. The model that was finally proposed was validated against experimental data, which proved its robustness. The MBE for the simulated and measured temperatures on the back of the PV panel did not exceed 2.0 °C and the maximum observed energy production difference was 4 Wh/m2.

Constraining Clouds and Convective Parameterizations in a Climate Model Using Paleoclimate Data

AbstractCloud and convective parameterizations strongly influence uncertainties in equilibrium climate sensitivity. We provide a proof‐of‐concept study to constrain these parameterizations in a perturbed parameter ensemble of the atmosphere‐only version of the Goddard Institute for Space Studies Model E2.1 simulations by evaluating model biases in the present‐day runs using multiple satellite climatologies and by comparing simulated δ18O of precipitation (δ18Op), known to be sensitive to parameterization schemes, with a global database of speleothem δ18O records covering the Last Glacial Maximum (LGM), mid‐Holocene (MH) and pre‐industrial (PI) periods. Relative to modern interannual variability, paleoclimate simulations show greater sensitivity to parameter changes, allowing for an evaluation of model uncertainties over a broader range of climate forcing and the identification of parts of the world that are parameter sensitive. Certain simulations reproduced absolute δ18Op values across all time periods, along with LGM and MH δ18Op anomalies relative to the PI, better than the default parameterization. No single set of parameterizations worked well in all climate states, likely due to the non‐stationarity of cloud feedbacks under varying boundary conditions. Future work that involves varying multiple parameter sets simultaneously with coupled ocean feedbacks will likely provide improved constraints on cloud and convective parameterizations.

Structural Characterization of a Toppling Rock Slab From Array‐Based Ambient Vibration Measurements and Numerical Modal Analysis

AbstractAccurate assessments of the internal structure and boundary conditions of unstable rock slopes are imperative for evaluating landslide hazard scenarios. However, instability characterization at depth remains challenging and is often limited by costly or invasive subsurface investigations. Here, we develop a new approach coupling array‐based ambient vibration modal analysis and numerical modeling to improve structural characterization of rock slope instabilities at depth. We used ambient noise cross‐correlation on 4 hr of seismic data recorded by an array of 30 nodal geophones at a 500‐m‐long toppling rock slab in Utah, USA to identify modal frequencies between 0.8 and 3.5 Hz and derive modal displacements. We show that transverse and longitudinal bending modes span the length of the instability, indicating an interconnected slab. Statistical comparison of field results with outputs from >1,000 finite element models with varying boundary conditions showed that the instability depth varies between 40–70 and 10–20 m in the middle and lateral regions, respectively. Our approach yields new information on the structural conditions of rock cliff and column instabilities at depth, which is not easily obtained by other means but is imperative for change detection monitoring and improved hazard assessments.

Recent variation in channel erosion efficiency of the Lower Yellow river with different channel patterns

• Both temporal and spatial declines existed in the channel erosion efficiency of the Lower Yellow River. • An excessive decrease in sediment transport capacity and a deficit of riverbed supply led to the temporal decline in channel erosion efficiency. • Cross-sectional geometry and bed material composition were the key factors to cause the differences in channel erosion efficiency of different reaches in the LYR. The operation of the Xiaolangdi Reservoir has exerted prominent effects on the processes of sediment transport and channel evolution in the Lower Yellow River (LYR) since 2000. The investigation into the spatio-temporal adjustment characteristics of channel evolution is of significance to the management of the LYR covering different channel-pattern reaches of braided, transitional and meandering. An indicator of channel erosion efficiency was firstly defined as the ratio of annual channel evolution volume to annual incoming water volume in a specified study reach. Furthermore, the values of channel erosion efficiency in different reaches of the LYR were calculated using the measured hydrological data and cross-sectional profiles during the post-dam stage. Calculated results show that the channel erosion efficiency in the LYR was exhibiting a decline tendency at both temporal and spatial scales, with the mean value of the whole reach decreasing from 6.0 (2000–2007) to 2.7 (2008–2020) and that of the braided reach four times higher than the meandering reach. Main factors influencing the channel erosion efficiency were investigated, covering the incoming flow-sediment regime, riverbed composition and cross-sectional geometry. The temporal decreases in these factors, including incoming sediment coefficient, bed material supply and geomorphic coefficient, resulted in the temporal decline in channel erosion efficiency. The discrepancies in channel erosion efficiency of different reaches were mainly ascribable to the variations in cross-sectional geometry and riverbed composition. The geomorphic coefficient in the braided reach decreased from 19.7 to 9.0 m −1/2 , and meanwhile the percentages of fine and medium grain sizes in the riverbed reduced by around 50% from 2000 to 2020, which led to the most dramatic variation in channel erosion efficiency and the different trajectory of channel evolution. In addition, the seldom varied channel slope played a minor role in the temporal variation of channel erosion efficiency, but partly accounted for the spatial reduction along the LYR. This study provides an illustrative example for the response of a large alluvial river with different channel patterns to the varying boundary conditions caused by upstream damming.

Разностные методы решения интегро-дифференциального уравнения конвекции-диффузии с граничными условиями третьего рода

An initial-boundary value problem for a multidimensional integro-differential convection-diffusion equation with variable coefficients and boundary conditions of the third kind is investigated. For the numerical solution of the multidimensional problem, a locally one-dimensional difference scheme is constructed. It is shown that the approximation error of the scheme is 𝑂(|ℎ|2+𝜏), where |ℎ|2=ℎ12+ℎ22+...+ℎ𝑝2. Using the method of energy inequalities, we obtain an a priori estimate in the 𝐿2 - norm, which implies uniqueness, stability with respect to the right-hand side and initial data, and convergence of the solution of the locally one-dimensional difference scheme to the solution of the original differential problem at a rate equal to the approximation error. Numerical calculations are carried out on a test example, illustrating the theoretical calculations obtained in this work.

The Sense and Non-Sense of PEDs—Feeding Back Practical Experiences of Positive Energy District Demonstrators into the European PED Framework Definition Development Process

This article discusses early developments of the Positive Energy District (PED) concept, both in terms of its definition and of its implementation in real world demonstrators. Based on the specific challenges for creating an operational definition for the European +CityxChange project, the feasibility of creating a PED was practically explored by identifying 4 possible subtypes that respond to varying constraints regarding the energy balance of the PED. This article provides the context and describes these 4 ambitions levels: PEDautonomous, PEDdynamic, PEDvirtual, and PrePED; and the 3 boundary modes: geographical, functional, and virtual. The work thus expands on the first general PED definitions as they were put forward in the SET-plan and by the European Commission, while allowing a better response to the specific boundary conditions of PEDs’ physical context. As such, it provides an operational, city-focused, bottom-up PED definition. The present study analyses how these efforts connect to current work being performed on the development of a European PED Framework Definition. In the latter, new elements such as context factors are introduced in order to account for the varying boundary conditions that PEDs must address, and in particular the difficulties of realising PEDs in existing and densely built-up urban areas. Hereby it can be argued that the approach with 4 subtypes is a bottom-up method of addressing the same challenges as a context factor based approach operating in a top-down manner, this time starting from the regional or national renewable energy potentials. Both approaches indeed strive towards an optimum setup of PEDs both within their geographical boundaries and in their interactions with the surrounding energy infrastructures and cities. These efforts are instrumental in helping to prevent that a PED is being regarded as a goal in se, functionally disconnected from its surroundings. There are strong arguments in favour of handling PEDs as building blocks for the broader realisation of carbon neutral cities and regions, thus contributing to the systemic change that is needed to futureproof the built environment as a whole. Without applying this integrating perspective, PEDs risk creating a sub-optimal lock-in within their sites and thus remain one-off experiments, lacking connection to the wider urban sustainability strategies that are needed to properly address today’s energy and climate emergencies. This holds even more when considering the quality-related requirements that come with sustainable urban design and governance. Therefore, this study further explores how PEDs can fully support such a deep urban sustainability transition, and what could consequently be the next steps towards successful and upscaled PED deployment.

Thermal buckling solutions of generic metallic and laminated structures: Total and updated Lagrangian formulations via refined beam elements

The thermal buckling behavior of metallic and laminated beams/plates is investigated using a linearized stability analysis. By selecting different reference frames, two distinct types of 3 D stability equations can be generated using total and updated Lagrangian formulations (TLF and ULF). Various beam theory kinematics can be obtained within the framework of 1 D Carrera Unified Formulation (CUF) by employing an arbitrary expansion of the generalized variables. More precisely, an improved hierarchical Legendre expansion (IHLE) is used to formulate the Layer-Wise (LW) model in a robust manner. Additionally, using a finite element approximation in conjunction with CUF-IHLE, the obtained stability equations are discretized into a set of algebraic equations. The critical temperatures predicted by TLF- and ULF-based CUF-IHLE models are compared using numerical examples of beams and plates with varying boundary conditions, lamination schemes, and thickness-to-width ratios. Both models are validated for correctness using the commercial software ABAQUS. Besides, the effect of strain distribution during the pre-buckling stage is evaluated in the plate-like structure using one- and two-step analyses.

Damage precise localization under varying operating conditions via the vibration-data-based Functional Model method: Formulation and experimental validation

A robust to varying Operating Conditions (OCs) vibration-data-based damage precise localization method is postulated. The method is, for the first time, capable of providing precise damage coordinate estimates despite the presence of varying OCs. Moreover, it is based on only partial models of the dynamics and a few vibration sensors, without resorting on large-scale Finite Element Models or requiring knowledge of the OCs in its real time operational phase. It is founded upon a proper formulation of the broader Functional Model Based Method and uses Functionally Pooled AutoRegressive with eXogenous excitation (FP-ARX) models in which the scheduling vector incorporates both the damage coordinates and the OCs. Both excitation-response and response-only versions are provided. The method is experimentally validated via damage precise localization on a laboratory plate structure under varying boundary conditions using only two vibration sensors. A comparative assessment of the two versions is also provided based on hundreds of experiments. The results reveal high achievable accuracy, which is slightly superior for the excitation-response version, despite considerable variations in the boundary conditions.

Effects of initial and boundary conditions on gravel-bed river morphology

Assuming that the equilibrium geometry of river channels does not depend on their initial state but solely on boundary conditions, several formulas have been derived that relate the channel depth and width to the river bankfull discharge and bed material. However, due to the existence of a threshold for sediment motion and the strong non-linearity between sediment transport and flow rate, this assumption might not be generally valid for gravel-bed rivers. This research clarifies the role of the initial conditions, more specifically the initial channel width, on the geometry of gravel-bed rivers considering a variety of boundary conditions. The approach includes laboratory experiments and two-dimensional modeling, reproducing the evolution of alluvial channels with different starting widths, discharge regimes and sediment input rates. The experiments represent the Arc River (France). Thus, the characteristics of this river were used in the numerical model to obtain a realistic virtual case complementing the experiments. Different boundary and starting conditions resulted in either braided or single-thread channels. We found that the initial width strongly influences the evolution process and leaves a footprint on the river braid-belt extension. The active width of braided systems and the width of single-thread channels do not depend on the starting condition. They depend on sediment input rather than on discharge variability. Different initial widths result in different final bed levels. This indicates that the initial channel width may affect the degree of channel incision or aggradation. The results of this study justify the use of equilibrium formulas for single-thread rivers.

Thermal treatment of ion‐exchanged glass

AbstractThermal treatments of ion‐exchanged glasses are often needed for postprocessing steps, including deposition or curing of inorganic or organic coatings deposited on the glass surface. In this study, we investigate the effects of post–ion exchange thermal treatment on the concentration profile, residual stress, and final strength of the ion‐exchanged glass. A new general solution of the diffusion problem with variable boundary conditions is presented together with its integration in a recent residual stress model, including fast and slow relaxation terms. Strength is evaluated as a function of the surface flaw depth in both solely ion‐exchanged glass and ion‐exchanged glass subjected to subsequent thermal treatments. Results indicate that the thermal treatment may have significant detrimental consequences in terms of strength, which should be carefully considered in the design of glass components to avoid unexpected immediate or time‐delayed breakages.

Constraints on Cosmic Rays in the Milky Way Circumgalactic Medium from O viii Observations

We constrain the cosmic-ray (CR) population in the circumgalactic medium (CGM) of the Milky Way by comparing the observations of absorption lines of O viii ions with predictions from analytical models of the CGM: the precipitation (PP) and isothermal (IT) models. For a CGM in hydrostatic equilibrium, the introduction of CR suppresses thermal pressure and affects the O viii ion abundance. We explore the allowances given to the ratio of CR pressure to thermal pressure (P CR/P th = η), with varying boundary conditions, CGM mass content, photoionization by extragalactic ultraviolet background, and temperature fluctuations. We find that the allowed maximum values of η are η ≲ 10 in the PP model and η ≲ 6 in the IT model. We also explore the spatial variation of η: rising (η = Ax) or declining (η = A/x) with radius, where A is the normalization of the profiles. In particular, the models with a declining ratio of CR to thermal pressure fare better than those with a rising ratio with suitable temperature fluctuation (higher σ lnT for PP and lower for IT). The declining profiles allow A ≲ 8 and A ≲ 10 in the case of the IT and PP models, respectively, thereby accommodating a large value of η (≃200) in the central region but not in the outer regions. These limits, combined with the limits derived from the γ-ray and radio background, can be useful for building models of the Milky Way CGM including the CR population. However, the larger amount of CRs can be packed in the cold phase, which may be one way to circumvent these constraints.