Flow Calculations Research Articles

A kinetic energy and entropy preserving (KEEP) finite volume scheme on unstructured meshes for compressible flows

A kinetic energy and entropy preserving (KEEP) finite-volume scheme on unstructured meshes is proposed for non-dissipative and stable compressible flow computations. The KEEP finite volume scheme proposed in the present study is based on the existing KEEP schemes developed for finite difference methods. The KEEP schemes have shown superior numerical robustness without numerical dissipation in previous studies. The KEEP schemes are discretized such that the numerical fluxes discretely replicate the key analytical relations that the governing equations hold, leading to preservation of kinetic energy and a superior entropy preservation property in the incompressible and inviscid limits. This study proposes using cell-vertex discretization for the proposed KEEP finite volume scheme in order to maintain this characteristic property of the KEEP scheme and achieve second-order accuracy in space on simplex meshes and arbitrary meshes that are not highly irregular. In the numerical tests performed in this study, the proposed KEEP finite volume scheme successfully performs long-time stable computations on various unstructured meshes. Also, the proposed KEEP scheme shows much better numerical stability and spatial accuracy than the KEEP scheme with cell-centered discretization.

A Computationally Efficient Method for the Thermo-Structural Analysis of a Turbocharger Turbine Wheel Under Engine Transient Operation

Abstract The accurate evaluation of thermally-induced stresses is of particular importance for life-cycle analysis of modern turbochargers. Conjugated heat transfer (CHT), steady-state calculations represent the state-of-the-art, whose temperature distribution is usually imported into a structural solver as a thermal load for carrying out thermo-structural analyses. In the case of transient analyses, however, issues arise. Indeed, due to the different timescales of convective and conductive heat-transfer mechanisms, unsteady CHT simulations are burdened by high computational costs. Moreover, in the specific case of turbocharged engines, the timescales of engine load transients and turbocharger response may differ by a factor of up to 105. The present study proposes an approach for carrying out transient thermo-structural analyses of the solid domain only, characterized by a physical time in the order of 101 s, without the need for unsteady fluid flow calculations, which would imply the use of a solver time-step in the order of 10−5 s. In the proposed methodology, the effects of the fluid flow in the heat-transfer process are modeled by imposing adequate boundary conditions to wet surfaces of the solid domain with minimum computational effort. Indeed, by performing two steady-state CHT calculations and one steady-state computational fluid dynamics (CFD) calculation, time-dependent boundary conditions can be derived in terms of convective heat transfer coefficient and near-wall fluid temperature. The case study used for model development is a high-performance turbocharger turbine for automotive applications, responding to engine load transients of tenths of seconds. Initially, steady-state CFD and CHT models of the turbine stage are validated against experimental data. Then, transient analyses on the turbine wheel are carried out, and their results are imported as thermal loads into an unsteady structural solver, allowing for accurate transient thermo-structural analyses at sustainable computational cost.

An inverse Jiles-Atherton model of nanocrystalline magnetic core for nanoseconds square pulsed magnetization.

The magnetic core is a key component of a linear transformer driver (LTD), and the accuracy of the core model affects the calculation of the LTD power flow and the prediction of the output waveform. In this paper, a magnetization model based on the inverse Jiles-Atherton (inverse J-A) model is developed and a particle swarm algorithm is used to identify the parameters and to obtain the variation of the parameters with the excitation characteristic. A nanoseconds square wave LTD magnetic core test platform was built to obtain the magnetization characteristics of nanocrystalline magnetic cores under different excitation characteristic parameters. Under square wave pulses, due to the presence of harmonic components, core loss is more complex. In view of the fitting deviation caused by the traditional J-A model not considering harmonic factors and anisotropy, a dynamic loss correction factor is proposed. Through a comparison of experimental and simulation results, this model can well reflect the magnetization process and has high accuracy in fitting dynamic hysteresis loops and predicting losses, which is important for guiding the design of a square pulse LTD.

Unified Power Flow Solver for Hybrid AC/DC Distribution Networks

Hybrid ac/dc distribution grids are gaining popularity due to their potential to conflate the benefits of both ac and dc grids. However, the complex structure of these grids imposes new challenges in power flow (PF) calculations due to multiple ac/dc interconnections, variety of converter operations, and constant dc power loads. This paper proposes a unified PF algorithm incorporating different types of ac/dc and dc/dc converters. The proposed approach is based on the Newton Raphson (NR) algorithm and uses the Modified-Augmented-Nodal-Analysis (MANA) formulation. The proposed formulation can handle arbitrary hybrid ac/dc configurations. The modular structure allows incorporating arbitrary devices and converter controllers. The formulation is built on the summation of line flows and their associated derivatives instead of bus power injections to handle connections with different types of buses equipped with various converters in a straightforward manner. The proposed algorithm is implemented in MATLAB and tested on the modified IEEE 33 bus test system for various configurations. Electromagnetic Transient (EMT) simulations confirmed the accuracy of the proposed method.Another original contribution of the paper is the demonstration of the improved convergence characteristics of MANA based formulation over classical NR formulation through comparisons of conditioning number.

Analysis of the characteristics of valveless piezoelectric pump with crescent dune bluff body.

To improve the output flow characteristics of the piezoelectric pump in one direction, a new valveless piezoelectric pump with a crescent dune bluff body has been proposed. The pump can achieve low damage to the active substance on the premise that the active cell can guarantee the transport volume. By comparing with the hemispherically deficient and imitated meniscus resistance fluid, the barchan dune resistance fluid which can effectively improve the unidirectional output of the piezoelectric pump is obtained. Combined with the pump theoretical flow calculation formula, these influencing parameters, the degree of inclination, the sand ridge radius and the order of the crescent dune were analyzed. Finally, an experimental prototype of a valveless piezoelectric pump has been fabricated by 3D printing technology, and the pump flow test is being conducted. The relationship between frequency, voltage and output flow has been obtained. The test results show that with a dune inclination of 37.5, a sand ridge radius of 6.75 mm and a dune order of 4, the flow rate of the piezoelectric pump is best at 194.7 mL/min. The experimental results agree with the simulation results, showing the effectiveness of the valveless piezoelectric pump structure.

Adaptive calculation of river ecological flow considering the variable lifting volume under changing conditions

Abstract Ecological flow is a restrictive condition set for human water consumption in water resources management to protect river ecosystems. However, uncertainties in hydrological conditions, changes in regulatory capacity, and temporal-spatial differences bring difficulties to the implementation and supervision of ecological flows. By analyzing the relevant concept and connotation of ecological flow, it is concluded that the ecological flow regimes suitable for practical application should be a dynamic interval rather than a fixed value. Therefore, an improved ecological flow calculation method was proposed, which is to increase the feasible lifting volume for different ecological service objects based on the determined ecological baseflow. Taking two cross-sections of the Weihe River as an example, the ecological flow was calculated by comprehensively considering the factors of inflow, fish survival, landscape, and agricultural irrigation. The result shows the lifting volume occurred in the high flow season (April–June), and the maximum ecological flow of LJC and WJB cross-sections was 20.2 and 16.0 m3/s, respectively. Moreover, it is a generalized approach that can be extended to a more complicated river system, and the calculation results have reliability and adaptability in integrated water regulation.

Thermohydraulic Analysis of Ocean Thermal Energy Conversion

Thermohydraulic analysis is a mandatory analysis for power plants, including in this case marine thermal power plants (OTEC = ocens thermal energy conversion). The application to OTEC shows that temperature and pressure are things that must be considered, the higher the surface temperature, the greater the output power. Calculation of heat flow is a variable that is determined before a plant is built. This study aims to make a mathematical analysis of heat flow or thermohydraulic. The study uses the study of the heritage of Mendalan so that modeling and determination of design variables are formed, including heat flow in the pipe, diameter, hot and cold water discharge and boiler variables

Ocena ryzyka ograniczen generacji w OZE dla stanow awaryjnych sieci elektroenergetycznych

The risk of emergencies in power grids may result in the need to repeatedly curtail the power generated from renewable energy sources (RES). The frequency of network emergency states occurring during the year can be determined based on available failure rate statistics. The power redispatch signal (this term means reducing generation in a renewable energy source and correspondingly increasing it in a centrally controlled source) may be issued by the appropriate network operator. The article presents the results of analyses, the aim of which was to assess the probable effects of annual generation reduction in a selected wind and photovoltaic power plant connected at the same grid node. An original method was proposed using Monte Carlo simulation, taking into account the generation technology and its annual distribution, and an external computational "engine" implementing sequences of flow calculations.

The domestication and breeding history of Castanea crenata Siebold et Zucc. estimated by direction of gene flow and approximate Bayesian computation

The domestication and breeding history of Castanea crenata Siebold et Zucc. estimated by direction of gene flow and approximate Bayesian computation

Grid-Characteristic Method for Calculation of Discontinuous Non-Steady Flows of a Multicomponent Reacting Gas in Channels

This paper is devoted to the description of computational algorithms for modeling quasi-one-dimensional non-steady flows of a multicomponent reacting gas. The particularity of the developed modeling technique is that the paths of strong and weak discontinuities are mobile computational nodes, and the parameters for them are calculated using special algorithms. A set of programs has been developed, which can be used to solve the problems of the reacting gas dynamics that are of applied importance, as well as serve as an illustrator for physical gas dynamics training courses. The paper provides the results of the numerical modeling of the supersonic flow in a flat channel simulating the operation of experimental facilities of the Institute for Problems in Mechanics and the Institute of Physics and Technology. A satisfactory correlation between the calculated and experimental data has been obtained.

Research on intelligent control theory and strategy of gas drainage pipe network based on graph theory

To address the issues related to the working conditions of the mine gas drainage system and achieve adaptive adjustment of negative pressure during gas drainage, this study proposes a gas drainage pipe network structure model based on graph theory principles. The model is built upon the air-gas mixed flow model of the pipe network. The boundary conditions of the pipe network are determined, and both the pipe network flow model and resistance model are constructed. An assignment iterative adjustment method is used to establish the pipe network solution model, enabling the solution of the gas drainage pipe network. The mathematical model for regulating and controlling the gas drainage pipe network is formulated, considering the objective function, decision variables, and constraint conditions based on the drainage parameters. The control model, along with the flow calculation model of the gas drainage pipe network system, is employed to calculate the optimal gas control scheme. This scheme enables remote intelligent control of the control valve opening on the ground, significantly improving the safety and efficiency of the gas drainage system. The engineering application of Liuzhuang Coal Mine demonstrates the successful realization of the rational distribution of negative pressure and intelligent dynamic control of the drainage pipe network. As a result, the total gas concentration in the drainage pipe network has increased by 12%, yielding favorable outcomes. The findings of this study have significant theoretical and practical implications for the design, development, and improvement of intelligent control systems for mine drainage pipe networks.

Research on an Improved Carbon Emission Flow Model Considering Electric Vehicle Charging Fluctuation and Hybrid Power Transaction

With the update of the power transaction mode and the access of increasing electric vehicles with high randomness to the power grid, the existing carbon emission flow calculation method cannot consider the influence of carbon emission due to the fluctuation in electric vehicle charging and various transaction modes. Given the above shortcomings, this paper proposes an improved carbon emission flow model considering electric vehicle charging fluctuations and hybrid power transactions. The model first considers different transaction modes, allocates the network loss to both sides of power generation, and forms a lossless network, realizing the decoupling of the bilateral transaction mode, pool transaction mode, and network loss and then calculating the day-ahead network’s carbon emission. Then, the changes in different transaction modes caused by the fluctuation in electric vehicle charging under different transaction modes are analyzed and the nodes of ‘spontaneous change’ are found to form a day-ahead intra-day deviation network and calculate its carbon emission flow. Finally, the calculation results are combined to obtain an improved power system carbon emission flow considering electric vehicle charging fluctuations and transaction modes. In this paper, the 33-node system is used for verification.

Comparative Analysis of the Particle Swarm Optimization and Primal-Dual Interior-Point Algorithms for Transmission System Volt/VAR Optimization in Rectangular Voltage Coordinates

Optimal power flow (OPF) is one of the most widely studied problems in the field of operations research, as it applies to the optimal and efficient operation of the electric power system. Both the problem formulation and solution techniques have attracted significant research interest over the decades. A wide range of OPF problems have been formulated to cater for the various operational objectives of the power system and are mainly expressed either in polar or rectangular voltage coordinates. Many different solution techniques falling into the two main categories of classical/deterministic optimization and heuristic/non-deterministic optimization techniques have been explored in the literature. This study considers the Volt/VAR optimization (VVO) variant of the OPF problem formulated in rectangular voltage coordinates, which is something of a departure from the majority of the studies, which tend to use the polar coordinate formulation. The heuristic particle swarm optimization (PSO) and the classical primal-dual interior-point method (PDIPM) are applied to the solution of the VVO problem and a comparative analysis of the relative performance of the two algorithms for this problem is presented. Four case studies based on the 6-bus, IEEE 14-bus, 30-bus, and 118-bus test systems are presented. The comparative performance analysis reveals that the two algorithms have complementary strengths, when evaluated on the basis of the solution quality and computational efficiency. Particularly, the PSO algorithm achieves greater power loss minimization, whereas the PDIPM exhibits greater speed of convergence (and, thus, better computational efficiency) relative to the PSO algorithm, particularly for higher-dimensional problems. An additional distinguishing characteristic of the proposed solution is that it incorporates the Newton–Raphson load flow computation, also formulated in rectangular voltage coordinates, which adds to the efficiency and effectiveness of the presented solution method.

Calibration of a Near-Wall Differential Reynolds Stress Model Using the Updated Direct Numerical Simulation Data and Its Assessment

In the article, a differential Reynolds stress model is recalibrated using turbulent channel flow direct numerical simulation data in the range of friction Reynolds numbers 550–5200. The calibration aims to produce a RANS sublayer model for use within the hybrid RANS/LES framework. The model is designed to capture the average field of a thin near-wall part of a boundary layer as accurately as possible. An a posteriori procedure is employed in which one-dimensional channel flow calculations are performed for all variations of the model coefficients at each stage of the optimization procedure. The coefficients are initialized with their original values and then optimized by minimizing the appropriately chosen norm. An improved representation of the mean velocity profile and peak Reynolds stress values is demonstrated. Both models—baseline and recalibrated—are implemented in an in-house CFD code, and several simulations, including a channel flow, a flat plate boundary layer and a boundary layer separation from a rounded step, are performed. The latter benchmark flow is also simulated in hybrid RANS/LES mode. The updated model is compared to the original one, demonstrating improvements over the baseline model in the cases it was designed for.

Experimental and Numerical Investigations on the Thermal Performance of Three Different Cold Plates Designed for the Electrical Vehicle Battery Module

The thermal performance of battery modules has a crucial role in the performance, safety, and lifetime of battery cells. Commonly, battery models are validated through experimental data to ensure the correctness of model behavior; however, the influences of experimental setups are often not considered in the laboratory environment, especially for prismatic cells such as lithium titanate oxide (LTO) battery cells used in electric vehicles. For this purpose, both experimental and numerical studies of the thermal performance of the battery module consisting of LTO cells was investigated using different cold plates used in electrical and hybrid vehicles. Three different discharging rates were applied to the battery module to obtain comparative results of the cooling performance. In the numerical simulations, heat generation models are typically used to observe the thermal behavior of the battery module; however, in the numerical study, dual potential multi-scale multi-domain (MSMD) battery models were used, with transient flow and heat transfer calculations performed. The numerical results were in good agreement with the experimental data. A new high-performance cold plate was developed for the thermal management of LTO battery cells. In comparison with the other two cold plate configurations, the proposed cold plate configuration dropped the maximum temperature up to 45% for the same operating conditions.

Optimal allocation of distributed generation in active distribution power network considering HELM-based stability index

Traditionally, assessment of static voltage stability for distribution power networks with distributed generation (DG) utilizes the continuous power flow (CPF) method by calculating load margin index (LMI) at various load levels, which suffers from slow calculation speed and low accuracy when power flow calculation becomes ill-conditioned. To overcome this issue, this paper presents a novel holomorphic embedding load flow method (HELM)-based approach that calculates voltage stability indices for various types of nodes in a distribution network. Different orders of sensitivity information are calculated using the HELM method, and the criteria for assessing voltage stability are derived, which are used as constraints to optimize the location and capacity of DG. Such formulation of the optimization problem does not need to calculate the maximum load margin using CPF. Moreover, it can be effectively solved using the Genetic Algorithm and easily adaptable to various operating conditions of DG, which improve the calculation speed of the stability index. The effectiveness of the proposed approach is verified on IEEE 33-node and IEEE 69-node distribution power network models. Results show that considering the HELM VSI, the location, and capacity of DG in the distribution network can be better optimized. For the IEEE 33-node model, the total cost is reduced by approximately 2.33%, the network loss is reduced by 1.2%, and the HELM voltage stability criteria index is increased by about 4.23%; while for the IEEE 69-node system, the total cost is reduced by about 2.98%.

Vulnerable Area Identification of Islanded Combined Electrical and Heat Networks Based on Static Sensitivity Analysis

In combined electrical and heat networks (CEHNs) under the Islanded mode, the district heating network (DHN) is more vulnerable to fluctuations in the electrical load, resulting in the transgression of the CHEN power flow. Identifying vulnerable areas in islanded CEHNs is necessary. In this paper, we introduce a static sensitivity analysis method into islanded CHENs, which can identify vulnerable areas susceptible to these impacts, and explore the energy interaction mechanisms between the electrical network (EN) and DHN. We established a power flow model of the islanded CEHN, and developed the sensitivity matrix. Then, the decomposition model is solved, based on which the static sensitivity matrices can be calculated. The case study shows that the sensitivity can effectively represent the impact of EN load changes on the mass flow rate of the DHN, thus we can locate the weak areas of the CEHN. It can provide auxiliary information for the safe and stable operation of an islanded CEHN, with fewer calculations compared to the power flow calculation method. In addition, the results present the enhancement of islanded CHEN stability by using a kind of combination of CHP units.

Bi-level modeling and multi-energy flow calculation of main grid and microgrid cluster based on electro-thermal coupling analysis

In this study, considering the characteristics of multi-energy coupling and multi-level interconnection, a bi-level steady-state physical model is developed. Both decomposed and unified solution methods are employed for the energy flow calculation under the bi-level and multi-energy coupling structure. A numerical study is executed to examine the validity of the algorithms. In addition, the load change within the microgrid and the system operation state under both on-grid and off-grid modes are examined. According to the simulation results, compared with the unified solution method, the decomposed method has faster solution speed and higher data privacy protection, and is more adaptive to the further expansion of the scale of the interconnected microgrids. The microgrids can realize energy interaction and energy mutual aid through grid-connecting operation or inter-network interconnection.

Case Report: Arterial Spin Labeling Perfusion MRI Contribution for Clinical Implementation of Brain Disease

Currently, ASL is widely used as an additional breakthrough sequence in MRI due to acquiring reliable results. The case report aims to prove the efficacy and effectiveness of a quantitative method of ASL sequence through the calculation of the cerebral blood flow (CBF) on CBF maps in different cases. ASL sequence has been done on four patients with different cases. Then, the authors put 2 regions of interest (ROI) for measurement in normal and different regions on CBF maps and then calculated the average value result from CBF maps. ASL has been proven as a reliable and breakthrough sequence in MRI for detecting brain disease with a non-invasive method through the calculation of CBF value. ASL should be used as an additional protocol in brain examinations because it allows radiologists to assess the significance of CBF values using a quantitative method that is more reliable and non-invasive.

Brain-like position measurement method based on improved optical flow algorithm

In this paper, a brain-like navigation scheme based on fuzzy kernel C-means (FKCM) clustering assisted pyramid Lucas Kanade (LK) optical flow algorithm is developed to measure the position of vehicle. The Speed Cell and Place Cell in animals’ brain are introduced to construct the brain-like navigation mechanism which involves the optical flow method and image template matching to imitate the cells above-mentioned separately. To eliminate the singular values during optical flow calculation, the output of pyramid LK algorithm is clustered by FKCM algorithm firstly. Then, the velocity is calculated and integrated to get the position of the vehicle, and the brain-like navigation scheme is introduced to correct the position measurement errors by eliminating the accumulated errors resulting from velocity integration. The prominent advantages of the presented method are: (i) a pure visual brain-like position measurement method based on the concept of speed cells and place cells is proposed, making visual navigation more accurate and intelligent; (ii) the FKCM algorithm is used to eliminate the singular value of the pyramid LK algorithm, which improves the calculated velocity accuracy. Also, experimental comparison with classical pyramid LK algorithm is given to illustrate the superiority of the proposed method in position measurement.