State Boundary Conditions Research Articles

PEGASUS Centrifugal Particle Receiver CentRec300S - Optimization, Manufacturing and Test

The centrifugal particle receiver (CentRec®) developed by DLR for high-temperature solar applications, previously tested on sun in a 500 kWth prototype at the solar tower in Jülich was further developed and tested at slightly smaller scale of 300 kWth. The test with artificial sunlight provided more stationary and controllable boundary conditions. Outlet temperature of 681 °C was reached at an irradiating input power of 214 kW. The performance was determined with particle mass flow and temperature measurement systems. The data confirms the thermodynamic model for this receiver, indicating an extrapolated performance of 81% at nominal conditions.

Effects of a Finite Volume in the Phase Structure of QCD

Working in the SU(2) flavor version of the NJL model, we study the effect of taking a finite system volume on a strongly interacting system of quarks, and, in particular, the location of the chiral phase transition and the CEP. We consider two shapes for the volume, spherical and cubic regions with different sizes and different boundary conditions. To analyze the QCD phase diagram, we use a novel criterion to study the crossover zone. A comparison between the results obtained from the two different shapes and several boundary conditions is carried out. We use the method of Multiple Reflection Expansion to determine the density of states and three kinds of boundary conditions over the cubic shape. These boundary conditions are: periodic, anti-periodic and stationary boundary conditions on the quark fields.

IMPROVING THE ENERGY EFFICIENCY OF BUILDINGS BY PROVIDING BETTER PROTECTIVE STRUCTURES BASED ON THE UNIVERSITY'S KNOWLEDGE HUB

BACKGROUND AND OBJECTIVES. Modern university buildings use a large number of resources, such as heat, cold and hot water, as well as electricity, which is the main consumed energy resource and is used for lighting, office equipment, ventilation and air conditioning systems. In order to improve the energy efficiency of university buildings, it is necessary to carry out heat and energy modernization of internal and external envelopes, which will allow for internal billing and qualitative analysis of consumption, which contribute to prompt decision-making on heat and energy modernization of the premises.METHODS. To assess the potential for improving the energy efficiency of buildings by improving the quality of protective structures, methods for assessing the temperature and humidity conditions of multilayered enclosing structures in a wide range of humidity under stationary boundary conditions were developed on the basis of the University Hub of Knowledge.FINDINGS. On the basis of the University Hub of Knowledge, Kyiv National University of Technologies and Design, the moisture content profile was calculated for the general estimation of the moisture condition for building No. 4, the planes of the highest moisture content were determined to find the most dangerous, from the moisture condition point of view, section of the structure, the calculation of the enclosing structure modernization according to the maximum allowable moisture condition for the analysis of moisture accumulation in the coldest month of the year was made.CONCLUSION. The advantage of the proposed method of increasing the energy efficiency of buildings by improving the quality of protective structures based on the University Hub of Knowledge is the possibility of calculation in a wide range of moisture content of materials, including supersorption moisture zone, as well as applicability to structures with multizone condensation of moisture. The clarity and simplicity of the proposed method makes it available for the practical implementation of energy efficiency improvements in all university buildings.

Strain localization criteria for viscoplastic geomaterials

AbstractThis work presents a viscoplastic localization criterion to detect quasi‐instantaneous (i.e., load‐induced) and delayed (creep‐induced) strain localization in rate‐dependent solids. The study is based on the theory of controllability and a viscoplastic description of the mechanical response. Analytical precursors of unstable states are defined through systems of ordinary differential equations (OEDs). The use of the proposed criteria is illustrated at the material point level through a set of strain localization analyses simulating active strain localization of a porous rock. In addition, full‐field finite element simulations of compression tests conducted under various pressures are reported to demonstrate the role of local unstable viscoplasticity in the spontaneous propagation of deformation bands under stationary boundary conditions. The study shows that the viscoplastic localization criterion maintains a negative sign as long as the behavior is unstable, that is, the rate of deformation is accelerating. The sign switch coincides with the transition to decelerating deformation. The analyses revealed that pulses of overstress always emerge in correspondence with the growth of unstable behavior, and the peak matches the transition to stable behavior. The local responses recovered from full‐field analyses were consistent with those observed in analyses at material point level and the predictions of the presented theory.

Assessing long-term evolution of the fine sediment budget in the Iffezheim reservoir: temporal upscaling of numerical simulations

ABSTRACT Since the construction of a weir at Iffezheim (Rhine-km 334) in 1977, a continuous deposition of sediments in the weir channel has been observed. The aim of this paper’s underlying investigations was to assess the long-term riverbed evolution of the Iffezheim reservoir in terms of deposition and erosion. The computational fluid dynamic model SSIIM-3D was applied to simulate the morphological changes in the weir channel. To reduce the computation time of long-term simulation (e.g. several decades), a reduced complexity approach called ‘classification of the boundary conditions for discharges (Q) and suspended sediment concentrations (SSC)’ was developed and applied to the Iffezheim reservoir. The basic idea of the approach was to replace instationary simulations by a precalculated series of simulations with stationary boundary conditions. For doing so, daily volume changes were precalculated by combining representative discharge and suspended sediment concentration regimes. To determine the results of the instationary simulation, the number of days in each bin of Q and SSC was counted, multiplied with the precalculated result and added up. The amount of the volume change in the reservoir for a certain period could then be obtained by adding up the calculated daily volume changes. The results of this method showed an acceptable agreement with measurements for long-term periods (years) and for short-term periods (months) with remarkably shortened computation times compared to the instationary simulations (of 97%).

ОСНОВНЫЕ ПОЛОЖЕНИЯ В РАЗРАБОТКАХ МОДЕЛЕЙ ПО ТЕЧЕНИЮ ОТКРЫТЫХ ВОДНЫХ ПОТОКОВ

For the design of hydraulic structures, it is necessary to use special methods for calculating the water flow to determine the kinetic energy acting on the structures. A mathematical model of a two-dimensional in terms of stationary open water flow and boundary conditions in the problem of free spreading are formulated. The main solved and to be solved problems of determining the flow parameters are determined, its reduction to a dimensionless form by various transformations of coordinates and flow parameters. The method proposed by I.A. Sherenkov. The solution of the problem is described, which depends on the dimensionless parameter - the Froude criterion at the outlet of the flow from the pipe. With Froude numbers exceeding one or close to it, it is required to build a series of graphs or develop a unified theory, an algorithm for solving the problem. The general conclusions on the work are as follows: - the need for further research has been proven theoretically and experimentally. - tasks are formulated that must be performed, solved in order to obtain a result adequate to the real process of solving the problem of free spreading of a turbulent flow to the entire spectrum of parameters. - substantiated the need to continue research to determine the entire spectrum of parameters of a stationary open two-dimensional potential flow in terms of its outflow from a free-flow pipe into a wide horizontal smooth channel. - the requirements for the model taking into account the conjugation of a uniform flow with a radial flow in the form of a simple wave are determined The work was written with a critical assessment of existing methods for solving the problem and to substantiate the relevance of further scientific research.

THE USE OF THE METHOD OF SEPARATION OF VARIABLES (FOURIER) FOR SOLVING BOUNDARY PROBLEMS OF UNSTATIONARY HEAT CONDUCTIVITY IN A MULTILAYER MEDIUM

Anticorrosive coating of pipes, precipitation of hardness salts on the walls of pipes in hot water supply systems, heating, refrigeration systems, in coils for charging and discharging heat accumulators, coils of heat exchangers and a multilayer structure of metal-plastic pipes for the heating system and for the floor heating systems is an additional thermal resistance for the transfer of heat from the coolant to the environment. This resistance must be taken into account when calculating heat transfer from the pipe surface, both to increase the efficiency of heat exchangers and to calculate unproductive heat losses to the environment from main pipelines and risers of engineering systems. During the design process, the specialists are faced with the question of how to reduce or increase the heat transfer of 1 m of a steel pipe. To increase, you need to change the infrared radiation up. This is done with paint. Red color enhances heat dissipation. It is better if the paint is matte. Another approach is to install ribs. They are mounted externally. This will increase the heat transfer area. And for a "warm floor" system or a coil immersed in a solid medium (for example, a ground collector of a heat pump or a coil for charging / discharging a heat accumulator with a solid heat storage material), it is necessary to correctly calculate the heat transfer of 1 m of the pipe in order to ensure heating or cooling of this pipe to the required parameters. When do you need to decrease the parameter? The need arises when optimizing a section of the pipeline located outside the residential area, in an unheated volume. Then experts recommend insulating the site - isolating it from the external environment. According to M. Jacob's data, the formulas for heat transfer by thermal conductivity through flat walls can often be applied with sufficient accuracy for cylindrical walls, if the heat transfer surface is taken along the average thickness. Moreover, a large number of practical problems of calculating temperature fields in multilayer objects can be calculated as one-dimensional. Previously, an analytical solution to the homogeneous problem of non-stationary heat conduction in multilayer objects with stationary boundary conditions of the third kind was proposed. This article discusses the issues of unsteady heat conduction in multilayer objects. A solution to the homogeneous boundary problem with nonstationary boundary conditions of the third kind is proposed. The solution is based on the Fourier variable separation method by the eigenfunctions of the problem and the Duhamel integral. The proposed solution formula has an explicit form and, due to the recurrent form of writing the basic relations, can be useful in numerous calculations.

Analysis of transient thermal sensation for “overheating” and “subcooling” conditions in a room

Thermal comfort is a very important issue for the design of HVAC systems. The aspects of thermal comfort are also significant in the operation of the heating/cooling systems in rooms. Many investigations studied the stationary boundary conditions, while transient conditions were not in focus. Therefore, this paper describes measurements carried out in a climate room under transient conditions. After an introduction, the second part of this paper gives an overview about the experimental setup and the methodology of the dynamic investigations. The third part describes the main results of the analyzes. The fourth part includes a discussion of these results and the contribution to potential regulation systems and further investigations. As a result of these analysis, it can be stated, that the individual comfort temperature as set by the user is mostly (operative room temperature). With regard to the dynamic behavior, it must be concluded that most subjects accept a room temperature increase of without directly expressing a change request. When the operative temperature falls below the individual comfort temperature, the desire for an increase in the operative temperature is immediately expressed, even in the case of small deviations. The paper ends with a summary of the results.

A $q$-Robinson-Schensted-Knuth Algorithm and a $q$-Polymer

In Matveev-Petrov (2017) a $q$-deformed Robinson-Schensted-Knuth algorithm ($q$RSK) was introduced.In this article we give reformulations of this algorithm in terms of the Noumi-Yamada description, growth diagrams and local moves. We show that the algorithm is symmetric, namely the output tableaux pairs are swapped in a sense of distribution when the input matrix is transposed. We also formulate a $q$-polymer model based on the $q$RSK, prove the corresponding Burke property, which we use to show a strong law of large numbers for the partition function given stationary boundary conditions and $q$-geometric weights.We use the $q$-local moves to define a generalisation of the $q$RSK taking a Young diagram-shape of array as the input. We write down the joint distribution of partition functions in the space-like direction of the $q$-polymer in $q$-geometric environment, formulate a $q$-version of the multilayer polynuclear growth model ($q$PNG) and write down the joint distribution of the $q$-polymer partition functions at a fixed time.

MicroHH 1.0: a computational fluid dynamics code for direct numerical simulation and large-eddy simulation of atmospheric boundary layer flows

Abstract. This paper describes MicroHH 1.0, a new and open-source (www.microhh.org) computational fluid dynamics code for the simulation of turbulent flows in the atmosphere. It is primarily made for direct numerical simulation but also supports large-eddy simulation (LES). The paper covers the description of the governing equations, their numerical implementation, and the parameterizations included in the code. Furthermore, the paper presents the validation of the dynamical core in the form of convergence and conservation tests, and comparison of simulations of channel flows and slope flows against well-established test cases. The full numerical model, including the associated parameterizations for LES, has been tested for a set of cases under stable and unstable conditions, under the Boussinesq and anelastic approximations, and with dry and moist convection under stationary and time-varying boundary conditions. The paper presents performance tests showing good scaling from 256 to 32 768 processes. The graphical processing unit (GPU)-enabled version of the code can reach a speedup of more than an order of magnitude for simulations that fit in the memory of a single GPU.

Contribution of coherent structures to momentum and concentration fluxes over a flat vegetation canopy modelled in a wind tunnel

Coherent structures dominate the shear flow in and above the vegetation canopy, affecting the transport of passive scalars. Their detailed understanding is therefore of great interest for a number of environmental studies such as organic gas exchange, pollution dispersion, or forest fire propagation. In the present study, a forest embedded in an atmospheric boundary layer was reproduced in a wind tunnel. An area source was installed to mimic the volatile organic compounds emission coming from the vegetation. A fast gas analyser combined to a triple hot-wire anemometer were used to measure simultaneously and at the same point the momentum and the concentration fluxes above the canopy. This particular set-up enabled the complex scalar exchange mechanism to be studied in the well defined and stationary boundary conditions of a laboratory experiment simulating neutral atmospheric conditions. Measurements showed that the contribution of coherent structures to the momentum and the concentration flux was 80% and 60% respectively. Contributions were found to be nearly constant with height. The combination of velocity and concentration measurements enabled the determination of the mean concentration of the coherent structures. Results highlights the preponderant role of ejections in releasing highly concentrated gas pockets above the forest canopy. These releases were measured to be, in average, 40% more concentrated than the average gas concentration at the same height. It is shown that 70% of the extreme events observed are linked to an ejection process.

Influence of temperature and moisture content on the thermal conductivity of wood-based fibreboards

Temperature and moisture content are important parameters affecting thermal properties of wood and wood-based materials. Quantitative data are required to better characterize the thermal performance of fibreboards. The aim of this work was to assess thermal conductivity of insulation fibreboards at different temperatures (ranging from −10 to +60 °C at 10 °C increments) and moisture contents (gravimetrically determined after conditioning at 15, 50, 85 % RH). Thermal measurements were done conducted with at stationary boundary conditions. The relationship between thermal conductivity, temperature and MC of samples was found by fitting data to a second order polynomial function. Moisture behavior was characterized by applying the Hailwood–Horrobin (H–H) sorption model. The single-hydrate model was used with the equilibrium moisture data at each conditioned climate step, starting at 10 % RH, up to 90 % RH, at 10 % RH increments. Total sorption as a composite of hydrated and dissolved water sorption was quantified for adsorption as well as desorption. Modelled data were in close agreement with the experimental data. Accurate datasets on the thermal behaviour of insulation materials are crucial in numerical modelling approaches, which will improve the proper design of building envelopes.

Flow Induced Energy Losses in the Exhaust Port of an Internal Combustion Engine

A numerical study of the flow in the exhaust port geometry of a Scania heavy-duty diesel engine is performed using the large eddy simulation (LES) and an unsteady Reynolds-Averaged Navier–Stokes (URANS) simulation approach. The calculations are performed at fixed valve positions and stationary boundary conditions to mimic the setup of an air flow bench experiment, which is commonly used to acquire input data for one-dimensional engine simulations. The numerical results are validated against available experimental data. The complex three-dimensional (3D) flow structures generated in the flow field are qualitatively assessed through visualization and analyzed by statistical means. For low valve lifts, the major source of kinetic energy losses occurs in the proximity of the valve. Flow separation occurs immediately downstream of the valve seat. Strong helical flow structures are observed in the exhaust manifold, which are caused due an interaction of the exhaust port streams in the port geometry.

LES of a Spatially Developing Atmospheric Boundary Layer: Application of a Fringe Method for the Stratocumulus to Shallow Cumulus Cloud Transition

AbstractAn arrangement of a large-eddy simulation (LES) is described that facilitates a spatially developing thermally stratified atmospheric boundary layer (ABL). When the inflow and outflow boundary conditions are specified, the LES of stably stratified ABL turns out to be challenging because spurious reflections of waves at the boundary accumulate inside the domain. To tackle this problem, a fringe method with an auxiliary LES running concurrently is applied to enforce upstream/downstream boundary conditions. An artificial forcing term is applied within a fringe region located at the beginning of the main LES domain in order to ensure statistically stationary inflow boundary conditions. The auxiliary LES, which is horizontally homogeneous in a doubly periodic domain, is used to determine the inflow condition of the main LES domain. The present scheme is used to provide an Eulerian perspective of the stratocumulus to shallow cumulus cloud (Sc–Cu) transition, one of the key cloud regimes over the subtropical ocean. In this study, the transition is triggered by increasing the sea surface temperature (SST) and the LES runs until a statistically steady evolution of the Sc–Cu transition is achieved. The flow statistics are compared with those from a recycling-type method and it is found that the fringe method is more suitable for the current applications.

Analytical flow study of a conducting Maxwell fluid through a porous saturated channel at various wall boundary conditions

Hydrodynamics of a conducting visco-elastic fluid in a porous medium sandwiched between two parallel plates, under the effect of the Lorentz force, for both moving and stationary wall boundary conditions, is considered in this paper. The non-linear momentum equation is solved analytically using the optimal homotopy analysis method (OHAM) and the effect of existing parameters in the physics of the problem is demonstrated on the dimensionless velocity profile and skin friction coefficient. The robustness of the analytical solution is checked by comparison with numerical results and plotting the residual errors. Results show that there is excellent agreement between numerical and analytical solutions. Furthermore, the error diagrams show that OHAM yields accurate results in all values of the different effective parameters, such as porous medium shape factor, Forchheimer number, visco-elastic parameter, Reynolds number and the parameters related to the Lorentz force.

A Reduced-Order Meshless Energy Model for the Vibrations of Mistuned Bladed Disks—Part I: Theoretical Basis

In this paper, a reduced order model for the vibrations of bladed disk assemblies was achieved. The system studied was a 3D annulus of shroudless, “custom-tailored,” mistuned blades attached to a flexible disk. Specifically, the annulus was modeled as a spectral-based “meshless” continuum structure utilizing only nodal data to describe the arbitrary volume in which the system's dynamical energy was minimized. An extended Ritz variational procedure was used to minimize this energy, subjected to constraints imposed by an assumed 3D displacement field of mathematically complete, orthonormal “blade-disk” polynomials multiplied by generalized coefficients. The coefficients were determined by constraining the polynomial series to satisfy the extended Ritz stationary equations and essential boundary conditions of the bladed disk. From this, the governing equations of motion were generated into their usual dynamical forms to calculate upper-bounds on the actual free and forced responses of bladed disks. No conventional finite elements and element connectivity or component substructuring data were needed. This paper, Part I, outlines the theoretical foundation of the present model, and through extensive Monte Carlo simulations, establishes the analytical basis, predictive accuracy, and re-analysis efficiency of the present technology in the prediction of 3D maximum response amplitude of mistuned bladed disks having increasing numbers of nodal diameter excitations. Further applications validating the 3D approach against conventional finite element procedures of free and forced response prediction of a mistuned Integrally-Bladed Rotor used in practice is presented in a companion paper, Part II (Fang, McGee, and El-Aini, 2013, “A Reduced-Order Meshless Energy Model for the Vibrations of Mistuned Bladed Disks—Part II: Finite Element Benchmark Comparisons, ASME J. Turbomach., to be published.

Simulation of the Conjugate Heat Trasfer on the Process Incipient State in a Bridgman Method

Numerically the conjugate convective heat transfer in a flat-bottomed graphite crucible at a stage previous an onset of solidification of a silicon melt in a vertical variant of a Bridgman method is researched. The finite element method on rectangular mesh solves set of equations of thermogravitational convection in Boussinesq approximation in variables temperature, a stream function and a vorticity in cylindrical co-ordinates. At the fixed geometry of calculation region evolution of the space configuration of melt flow of silicon of the thermogravitational nature in a range of Grashof numbers Gr from 1 to 1,3 × 107 is studied. Self-consistent fields of temperature in a melt, walls of a crucible and in a gas interlayer are calculated at stationary boundary conditions

Validating LES-based flow and dispersion models

Validating LES-based flow and dispersion models for the purpose of predicting transient flow and dispersion phenomena is more demanding than validating RANS-based codes. Since the model output is no longer related to stationary or quasi-stationary boundary conditions, and since the model results are not meant to be used for predicting mean flow and dispersion patterns, an evaluation of the model based on mean results is not meaningful for most of the model-specific applications. A more sophisticated but also more complex validation approach based on statistically representative ensembles is required. By comparing frequency/probability distributions of flow and dispersion results with qualified reference data the reliability of complex models can be evaluated. Based on a careful comparison of FAST3D-CT simulation results with corresponding systematic wind tunnel data from the JU2003 experiments, an example for an application specific evaluation procedure for instantaneous puff dispersion modeling is given.

A Mixed Problem for Quasilinear Impulsive Hyperbolic Equations with Non Stationary Boundary and Transmission Conditions

A Mixed Problem for Quasilinear Impulsive Hyperbolic Equations with Non Stationary Boundary and Transmission Conditions

A Mixed Problem for Quasilinear Impulsive Hyperbolic Equations with Non Stationary Boundary and Transmission Conditions

The initial-boundary value problem for a class of linear and nonlinear equations in Hilbert space is considers. We prove the existence and uniqueness of solution of this problem. The results of this investigation are applied to solvability of initial-boundary value problems for quasilinear impulsive hyperbolic equations with non-stationary transmission and boundary conditions.