Two-level Operation Research Articles

One-level and two-level operator splitting methods for the unsteady incompressible micropolar fluid equations with double diffusion convection

In this paper, a one-level operator splitting method (OOSM) and a two-level operator splitting method (TOSM) for the two-dimensional or three-dimensional (2D/3D) unsteady incompressible micropolar fluid equations with double diffusion convection (IMFDDC) are proposed and analyzed. Firstly, a OOSM is constructed, which consists of five steps. The projection strategy is adopted to get the values of linear velocity and pressure in the first two steps, then the three elliptic subproblems about the angular velocity, the temperature, and the concentration variables are solved in the last three steps. The original variables with the predefined boundary conditions need to be solved separately in each step. Next, in order to solve the problem efficiently, a TOSM is presented, which only solves the nonlinear equations in the coarse level subspaces and the Oseen type linearized equations in the fine level. The numerical analysis of stability and error estimates of the fully discrete methods show that TOSM can reach the same accuracy as the OOSM with H∼O(h2/3). Numerical examples are provided to confirm the effectiveness and reliability of the proposed methods.

Vaccinating humanitarian workers against COVID-19.

To describe the United Nations' (UN's) coronavirus disease 2019 (COVID-19) vaccination programme and its efforts to vaccinate frontline humanitarian personnel stationed in locations where access to COVID-19 vaccine was limited or absent. The vaccination programme was structured as a two-level operation: a global vaccine deployment support team and local vaccine deployment teams in each participating country, territory or administrative area. The central group, led by a global vaccine coordinator, oversaw medical, legal, financial, logistical, data, technological and communication aspects. Local vaccine deployment teams were led by coordinators who managed registration, logistics, communication and vaccine administration. The programme used World Health Organization-approved COVID-19 vaccines and developed prioritization criteria for distributing vaccine supplies. The programme ensured that vaccines for the UN personnel were not diverted from the populations they were intended to serve. The programme successfully formed 120 deployment teams across 152 eligible countries, territories and administrative areas, targeting approximately 673 000 individuals. By April 2023, 72 countries, territories and administrative areas had received over 470 000 doses, of which 337 072 doses were administered. Almost half of the doses administered (167 616) were to individuals in five UN hardship countries. Ninety-five severe adverse events were reported, but none led to any reported medical evacuation, permanent disability or death. The programme demonstrated effective global coordination and local implementation, adapting to diverse contexts and operational challenges. The model can serve as a guide for global actors for future health emergencies, or for deploying health aid at a regional or global scale.

Local Search For Satisfiability Modulo Integer Arithmetic Theories

Satisfiability Modulo Theories (SMT) refers to the problem of deciding the satisfiability of a formula with respect to certain background first-order theories. In this article, we focus on Satisfiablity Modulo Integer Arithmetic, which is referred to as SMT(IA), including both linear and non-linear integer arithmetic theories. Dominant approaches to SMT rely on calling a CDCL-based SAT solver, either in a lazy or eager flavour. Local search, a competitive approach to solving combinatorial problems including SAT, however, has not been well studied for SMT. We develop the first local-search algorithm for SMT(IA) by directly operating on variables, breaking through the traditional framework. We propose a local-search framework by considering the distinctions between Boolean and integer variables. Moreover, we design a novel operator and scoring functions tailored for integer arithmetic, as well as a two-level operation selection heuristic. Putting these together, we develop a local search SMT(IA) solver called LocalSMT. Experiments are carried out to evaluate LocalSMT on benchmark sets from SMT-LIB. The results show that LocalSMT is competitive and complementary with state-of-the-art SMT solvers, and performs particularly well on those formulae with only integer variables. A simple sequential portfolio with Z3 improves the state-of-the-art on satisfiable benchmark sets from SMT-LIB.

Development and experimental realization of an adaptive neural-based discrete model predictive direct torque and flux controller for induction motor drive

This paper develops a neural network-based discrete predictive direct torque and flux control (NNPDTFC) for induction motor drive with the space vector modulation (SVM) technique. Moreover, this SVM technique with NNPDTFC is implemented to activate the inverter in the two-level operation and the performance is compared with the conventional PI direct torque and flux control (PIDTFC) technique. The PSO based model predictive control incorporated with the neural network is developed here in the NNPDTFC and is analyzed using MATLAB software. Disturbance reduction, simple control, and real-time implementation are the major features of NNPDTFC and it also enhances the transient performance of the motor drive by reducing settling time and peak overshoot. In addition, the flux and the torque ripples are significantly improved using the proposed NNPDTFC technique which is extensively used for the fast dynamic response of the induction motor drives as compared to PIDTFC. In order to show the potentiality of the proposed controller, a prototype controller is developed and validated with the laboratory setup and the control signals are generated for both NNPDTFC and PIDTFC using a low-cost Digital signal processor (DSP) controller which is fed to the induction motor of 3.7 kW capacity in the real-time platform. It is observed that the results with NNPDTFC are not only found to be extremely satisfactory even with the system and parameter uncertainties and external load perturbations but also, it produces enhanced dynamic as well as steady-state performance along with the reduced ripples in the signal flux, torque, and current compared to that of PIDTFC.

Scheduling optimization for smart microgrids considering twolevels transactions of electric vehicles and energy markets

Smart microgrid planning poses some challenges, including bi-directional energy flow both inside and outside the smart microgrid. This research studies at the internal flow level energy transactions with the internal resources of the microgrid, and at the external flow level the energy transactions with external stakeholders to the microgrid. Electric vehicles can operate bidirectionally and participate at the internal and external level of smart microgrids. In this context, this study analyzes electric vehicles both levels. (1) At the level of internal participation, electric vehicles operate in a residential area of a microgrid that is also connected to energy storage systems and elements with uncertainty, such as photovoltaic generation systems and residential loads. Prices are set by considering operating costs within the smart microgrid. (2) The level of external participation refers to an electric vehicle station that offers three services: charging, discharging and swapping battery. Prices are stochastic and come from the spot market prices of electric vehicles. Consequently, it is proposed that the aggregator must plan the optimal scheduling for the visit scenarios of two-level stochastic electric vehicles. The system can also trade energy that come from a local and wholesale energy market. Results demonstrate that two-level operation can help to evaluate the economic impact of electric vehicles.

A conservative generalized multiscale finite volume/element method for modeling two-phase flow with capillary pressure

In this paper we investigate a numerical method used for solving a two-phase flow model within the context of a physics-based two-level operator splitting scheme. The governing equations involve an elliptic pressure equation coupled with a convection-dominated parabolic equation which poses a significant restriction on constructing fully implicit numerical solutions. In addition to the complications associated with solving the model equations, the underlying permeability of the porous medium is assumed to exhibit high variation over many length scales which leads to poor approximation properties in standard multiscale finite element spaces. In order to effectively solve the system we construct an approach that combines a mass-conservative generalized multiscale finite element method with an operator splitting scheme. In particular, we split the coupled system into three subsystems: the elliptic, the hyperbolic, and the parabolic pieces. This type of two-level splitting allows for the use of respective numerical discretizations for each type of equation, and also accounts for the scale-dependent exchange of information between them. In order to solve the elliptic equation we propose the use of a mass-conservative generalized multiscale finite element method (GMsFEM-FV), for the parabolic equation we use a mass-conservative multiscale finite element method (MsFEM-FV), and for the hyperbolic equation we use a standard non-oscillatory finite-volume scheme. The performance of the methodology is validated through a variety of numerical experiments.

Coupled Inductor Based Regenerative Cascaded Multicell Converter for Drives With Multilevel Voltage Operation at Both Input and Output Sides

Cascaded multicell converter (CMC) is proved to be an effective solution for medium voltage drives. Three-phase diode bridge rectifiers are normally used in this converter, which do not have regeneration capability. Pulsewidth-modulated (PWM) rectifier-based power cells enable the drives with regeneration capability. However, due to their two-level voltage operation at the input side, an inductor is required for each cell to filter out the switching frequency components from the input current. In this paper, a new configuration with single-phase PWM rectifier-based power cell is proposed to achieve multilevel voltage operation at both the input and output sides of the converter. Therefore, the devices at the rectifier side are also operated at reduced switching frequency without using any filter inductor for each cell. In the proposed converter, simple modular transformers are used instead of a single large and complex transformer that is used in the conventional CMC. Further, input phase currents of the converter are controlled in the synchronously rotating d–q frame, which requires only two current sensors at the input side instead of individual current sensors for each cell. Experimental validation of the proposed converter configuration is carried out on a 4.5-kVA prototype, and the results are presented.

Comprehensive Analysis and Design of a Quasi Two-Level Converter Leg

Quasi two-level operation of voltage source converters, intended to operate either within high/medium voltage DC networks or low output frequency applications, has caught the attention of both academia and industry. Sequential insertion of the so-called submodules (cells) into the circuit alleviates potential problems related to high voltage slopes (dv/dt), whereas the ease of stacking them together provides the means for satisfactory performance in terms of voltage scalability. Although resembling the modular multilevel converter, quasi two-level converter operates in a different manner, leading to the completely different sizing, operating and balancing principles. So far, no publication has analyzed the transition process occurring within the quasi two-level converter leg, which can be considered crucial from the cell capacitance sizing perspective. Therefore, this paper provides thorough and generalized mathematical description of converter leg dynamics during the voltage transition process. Comprehensive method giving the possibility for cell capacitance determination is presented. With the aim of validating the conducted analysis, simulation results covering the operation of a few different topologies, with varying number of cells, are presented.

Trapezoidal Current Modulation for Bidirectional High-Step-Ratio Modular DC–DC Converters

A modular dc–dc converter (MDCC) has been proposed for high-step-ratio interconnection in dc grid applications. To further optimize the performance of the MDCC, this article presents a trapezoidal current modulation with bidirectional power flow ability. By giving all the submodule (SM) capacitors an equal duty to withstand the stack dc voltage, their voltages are balanced without additional feedback control. Moreover, based on soft-switching performance and circulating current analysis, three-level and two-level operation modes featured with high efficiency conversion and large power transmission, respectively, are introduced. The control schemes of both modes are designed to minimize the conduction losses. Besides, the SM capacitor voltage ripples with different switching patterns are compared, and the option for ripple minimization is presented. A full-scale case study is provided to introduce the design process and device selection of the MDCC. The experimental tests based on a downscaled prototype are finally presented to validate the theoretical analysis.

Experimental demonstration of a digital quantum simulation of a general PT -symmetric system

$\mathcal{PT}$-symmetric systems are one of the most interesting research fields in modern quantum physics, where various theoretical and experimental progress has been made. In our work, we experimentally demonstrate how to realize a general $\mathcal{PT}$-symmetric two-level operation in a quantum computation frame with a universal circuit model. It is based on enlarging the system with ancillary qubits and encoding the subsystem with the non-Hermitian Hamiltonian with postselection. Furthermore, we use the general formula to demonstrate one particular interesting issue, entanglement restoration induced by local $\mathcal{PT}$-symmetric operation in our four-qubit liquid nuclear magnetic resonance platform, and a theoretical explanation for the physical characteristics has been proposed. We also extend the protocol to the realization of an arbitrary two-level Hamiltonian evolution without a Hermitian restriction by an appropriate modification of the original quantum circuit. Our work lays the foundation for quantum simulation of the general $\mathcal{PT}$-symmetric problem in realistic quantum simulators, and the scheme can be extended to other quantum computation platforms.

Stepped Two-Level Operation of Nonisolated Modular DC/DC Converter Applied in High-Voltage DC Grid

Modular dc/dc converter (MDCC) based on cascaded submodules (SMs) has been an attractive converter topology for the interconnection of high-voltage dc (HVDC) grids for its low cost and high efficiency. The MDCC with sine-wave modulation has been developed and poses satisfactory performance; however, the high-frequency sorting of SM capacitor voltages, which aims to achieve their balance, results in quite large computation burden in HVDC application. To resolve the problem, the stepped two-level operation of the MDCC is proposed in this paper. Compared with the traditional sine-wave modulation, the stepped two-level modulation offers the following merits: 1) smaller SM capacitance requirement; 2) smaller ac circulating current; 3) the switching frequency is equal to the fundamental frequency, which avoids the extra switching actions existing in the sine-wave modulation; and 4) the sorting algorithm of the SM capacitor voltages needs only to be executed twice in a period, which poses a significant reduction of the computation burden, especially for ultrahigh voltage power conversion requiring hundreds of SMs. The simulation and experimental results verify the theoretical analysis.

The Experimental Study on the Principal Aerodynamic Characteristics and Their Influence on the Efficiency in a Cylindrical Two-Level Six-Channel Cyclone to Remove Dissimilar Types of Particles

The analysis of the two-level multi-channel cyclone operation requires the investigation of the treated air flow movement. The two-level multi-channel cyclone was designed for separating solid particles from the air flow and constructed at the Institute of the Environmental Protection of Vilnius Gediminas Technical University. The study was aimed at determining airflow distribution in a two-level six-channel cyclone and measuring air treatment efficiency. To determine the velocity of airflow in the channels, a multi-functional measuring instrument Testo-400 and the dynamic Pitot tube were used. The concentration of particles in the air inlet and outlet ducts was determined by the weight method. The obtained results showed that the same air volumes on both cyclone levels are equal and the velocity of the airflow was uniformly distributed in the channels. The maximum airflow velocity was recorded in the first channel. The mean airflow velocity in the channels did not differ significantly.

Three-Phase Three-Level Flying Capacitors Split-Source Inverters: Analysis and Modulation

Single-stage dc–ac power converters with boost capabilities offer an interesting alternative compared to two-stage architecture. One of the recently proposed single-stage dc–ac power converters is the split-source inverter (SSI). This paper extends the SSI topology from the two-level operation to the three-level one using the flying capacitors (FCs) configuration. Among the diode-clamped SSI (DC-SSI) and FC-SSI, the second one is proposed in order to avoid the need of two isolated dc sources. This paper shows that the FC-SSI provides additional merits compared to the two-level SSI in terms of input inductance requirements using the same switching frequency, voltage stresses across the active switches, and total harmonic distortion of the output voltage. Such results are obtained by discussing the analysis and the modulation of this topology, considering some simulation results, and comparing it to some other existing topologies to show its properties and limitations. Finally, a reduced-scale 1.5-kVA three-level FC-SSI is implemented experimentally to validate its functionality and verify the proposed analysis and modulation properties.

A robust approach to the two-level humanitarian relief operation

A robust approach to the two-level humanitarian relief operation

A robust approach to the two-level humanitarian relief operation

During the initial disaster response phase, an important issue in humanitarian relief logistics is to tackle the complexity of decision-making that covers the appropriate relief allocation and the associated transportation plan. We propose a two-level modelling methodology to alleviate that challenge, where the upper-level model focuses on the strategic relief distribution policy among the recipients whereas the lower-level model deals with the transportation routes, vehicle deployment and loading problems at the operational level based on the upper-level solution. Furthermore, in view of the demand uncertainty in impacted areas, a robust model is developed to guarantee the feasibility of relief distribution solutions. Also, a case study is conducted to illustrate the benefit of the presented robust framework, which is not only favourable in terms of the reliability and equity level relative to the deterministic one but also capable of offering advice on the vehicle arrangement.

Intraoperative blood loss, postoperative drainage, and recovery in patients undergoing lumbar spinal surgery

BackgroundSpine surgery is widely accepted as an effective management for patients with lumbar disc herniation; however, the factors influencing intraoperative procedure and prognosis are not fully understood. The present study was aimed to identify the factors influencing intraoperative blood loss, postoperative drainage volume, and recovery in patients undergoing spinal surgery.MethodsWe retrospectively analyzed the clinical data of 183 consecutive patients with lumbar disc herniation who underwent spine surgery. The clinical characteristics, operation procedure, and outcome were documented and the correlations were analyzed.ResultsThere were significant differences between one-level and two-level operations in the bleeding volumes of male (P = 0.005) and female (P = 0.002) patients, and in final drainage of male (P = 0.043) and female (P = 0.003) patients. The blood loss was correlated with the operation duration. There were differences in intraoperative bleeding and final drainage between groups with one-level and two-level operations. Additionally, there were differences in intraoperative autologous blood transfusion among various groups. There were significant differences in intraoperative bleeding between autologous blood transfusion and non-transfusion groups.ConclusionsThe key factors affecting the intraoperative blood loss and postoperative drainage volume include operation methods, operation duration, blood-transfusion modes, and usage of anticoagulants. These results should be taken into consideration in the attempt to optimize operation procedure and improve post-operative recovery.

Quasi Two-Level Operation of Modular Multilevel Converter for Use in a High-Power DC Transformer With DC Fault Isolation Capability

DC fault protection is one challenge impeding the development of multiterminal dc grids. The absence of manufacturing and operational standards has led to many point-to-point HVDC links built at different voltage levels, which creates another challenge. Therefore, the issues of voltage matching and dc fault isolation are undergoing extensive research and are addressed in this paper. A quasi two-level operating mode of the modular multilevel converter is proposed, where the converter generates a square wave with controllable dv/dt by employing the cell voltages to create transient intermediate voltage levels. Cell capacitance requirements diminish and the footprint of the converter is reduced. The common-mode dc component in the arm currents is not present in the proposed operating mode. The converter is proposed as the core of a dc to dc transformer, where two converters operating in the proposed mode are coupled by an ac transformer for voltage matching and galvanic isolation. The proposed dc transformer is shown to be suitable for high-voltage high-power applications due to the low-switching frequency, high efficiency, modularity, and reliability. The dc transformer facilitates dc voltage regulation and near instant isolation of dc faults within its protection zone. Analysis and simulations confirm these capabilities in a system-oriented approach.

WeFiLab: A Web-Based WiFi Laboratory Platform for Wireless Networking Education

Remote access to physical laboratories for education has received significant attention from both researchers and educators as it provides access at reduced cost in sharing manner of real devices and gives students practical training. With the rapid growing of wireless technologies, it has become an essential of learning to have the hand-on experience on wireless networking for the proliferated number of students in engineering. Some of current implementations for wireless networking are either using simulations, which lose the reality, or too complicated for undergraduate students to control experiments. In this paper, we present a practical online laboratory platform, Web-based WiFi Laboratory (WeFiLab). WeFiLab mainly focuses on providing students hand-on experience of doing experiments on real devices through webpage anytime anywhere. It uses the structure of two-level operations, which facilities increasing the scale of wireless devices and allows WeFiLab to be extended to more complicated operations. The schedule schemes of WeFiLab let more students share and make efficient use of wireless devices. A prototype of WeFiLab has been implemented and used successfully as complements for an undergraduate course for two years in a university. In total, 315 computer science students attended the evaluation.

Operator Splitting Multiscale Finite Volume Element Method for Two-Phase Flow with Capillary Pressure

A numerical method used for solving a two-phase flow problem as found in typical oil recovery is investigated in the setting of physics-based two-level operator splitting. The governing equations involve an elliptic differential equation coupled with a parabolic convection-dominated equation which poses a severe restriction for obtaining fully implicit numerical solutions. Furthermore, strong heterogeneity of the porous medium over many length scales adds to the complications for effectively solving the system. One viable approach is to split the system into three sub-systems: the elliptic, the hyperbolic, and the parabolic equation, respectively. In doing so, we allow for the use of appropriate numerical discretization for each type of equation and the careful exchange of information between them. We propose to use the multiscale finite volume element method (MsFVEM) for the elliptic and parabolic equations, and a nonoscillatory difference scheme for the hyperbolic equation. Performance of this procedure is confirmed through several numerical experiments.

Non-Fusion Rates in Anterior Cervical Discectomy and Implantation of Empty Polyetheretherketone Cages

A prospective analysis. Our aim was to assess the radiographically detectable bony fusion in patients with anterior cervical discectomy (ACD) and polyetheretherketone (PEEK)-cage implantation without additional filling. Furthermore, clinical data of patients with and without fusion were compared. PEEK-cage implantation is performed in cervical spinal surgery because of its benefits. However, fusion rates without filling of the cage have not been reported. Patients selected for ACD with PEEK-cage implantation prospectively underwent plain radiography in anterior-posterior and lateral projections during the postoperative hospital stay and at follow-up. Furthermore, clinical status was evaluated using the Odom scale, the Short Form-36, the Visual Analog Scale (VAS) for arm and neck pain, and the cervical Oswestry score. Fusion status, migration, and subsidence of the PEEK cage were evaluated on the basis of the lateral radiographs. Fusion was confirmed by presence of continuous trabecular bone bridges in the disc space. To exclude an influence of the cage on the evaluation of fusion rates, fusion was evaluated in analogous fashion retrospectively in a control group. A total of 52 patients underwent ACD and interbody fusion. One-level surgery was performed in 44 patients and 2-level surgery in 8 patients. A total of 60 ACD and interbody fusions with a PEEK cage were analyzed. A majority of operations were at the C5/6 level (40 patients, 77%). Cage height was 4 mm in 32 cases, 5 mm in 23 cases, and 6 mm in 5 cases. Bony fusion was present at 43 treated levels (71.7%), whereas at 17 levels (28.3%) no fusion was found. Statistical analysis revealed no significant difference between the fusion and non-fusion groups regarding time to follow-up, implanted cage height. Short Form-36, cervical Oswestry score, VAS arm and neck, or Odom criteria. In the control group, ACD was performed in 29 patients (42 levels; 18 one-level and 12 two-level operations). Bony fusion was present at 30 levels (71.4%), whereas non-fusion was present at 12 treated levels (28.6%). Statistically analysis revealed no significant difference between the study group and the control group regarding time to follow-up or fusion rates. Implantation of empty PEEK cages after ACD shows an unexpectedly low rate effusion according to radiologic criteria, although no statistically significant difference could be observed clinically.