Transient Solver Research Articles

Dual control technique for mitigating water-hammer phenomenon in pressurized steel-piping systems

This paper examined a dual technique -based branching strategy to sustain the conventional technique skills in terms of limitation of wave oscillation period spreading. This technique consists in splitting the single plastic short-penstock used in the conventional one into dual plastic sub-short penstocks placed upstream each of the connections of the original steel piping system to other hydraulic parts. Numerical computations used the method of characteristics for the discretization of unconventional water-hammer model based on the Vitkovsky and the Kelvin-Voigt formulations. The robustness of the transient solver was evaluated by comparing the obtained numerical results with pertinent experimental results. Moreover, the efficiency of the dual technique was explored for a series of operating condition tests including up- and- down surge- initiated water hammer events. Additionally, two plastic material types were utilized for sub- short-penstock pipe-walls, including (HDPE) or (LDPE) plastic materials. Results demonstrated that dual technique is a useful tool to soften both first hydraulic-head peak and crest. Moreover, examination of wave oscillation periods confirmed that the dual technique could markedly improve the efficiency of the conventional one, providing acceptable trade-off between hydraulic-head peaks or crests attenuation, and wave oscillation period spreading limitation. Furthermore, results demonstrated that the amortization of pressure head -rise and -drop was sensitive to the plastic sub- short-penstocks dimensions.

Effects of air injection throat width on a non-premixed rotating detonation engine

Rotating detonation engines are widely studied because of their compact configurations and high thermal cycle efficiency. For simplification, most of the numerical simulations of rotating detonation engines used premixed reactant mixtures. The rotating detonation waves under non-premixed conditions are not studied enough. The width of air injection throat is an important design parameter for a non-premixed rotating detonation engine. Here, a series of three-dimensional numerical simulations of a non-premixed rotating detonation engine with different air injection throat widths are performed. Cases with three different throat widths of 0.4 mm, 1.0 mm and 2.0 mm are calculated. The transient explicit density-based solver in ANSYS Fluent is used to perform the simulations. Detonation combustion happens in different radial domains for different air injection throat widths. When the air injection throat width is 1.0 mm, there is only one rotating detonation wave in the combustion chamber when the total mass flow rate is 272.3 g/s or 500 g/s. When the total mass flow rate is further increased to 1000 g/s, there are two co-rotating detonation waves in the combustion chamber. While for air injection throat width 0.4 mm, the transition to two-wave mode happens at a smaller total mass flow rate of 500 g/s. When the air injection throat width is too large, the interaction between the injection process and the propagating of rotating detonation wave becomes very strong, and the detonation wave quenches due to the insufficient injected reactants. The engine requires a larger total mass flow rate to sustain the continuous propagation of the rotating detonation wave.

Numerical investigation and parametric analysis of a photovoltaic thermal system integrated with phase change material

In this paper, a comprehensive three-dimensional model of photovoltaic thermal system integrated with phase change material (PVT/PCM) is developed and simulated. The effect of some key parameters using parametric analysis on performance of PVT/PCM system with water as working fluid is investigated. Parameters considered in this study include the properties of PCM (i.e. melting temperature, enthalpy of fusion and thermal conductivity), solar radiation and mass flow rate. The parametric analysis ranges are selected according to the properties of the most of available PCMs on the market, which shows the practical application of the numerical research. Furthermore, a three-dimensional model of PVT system is simulated as well to compare its performance with PVT/PCM system. An enthalpy-porosity method is used to simulate the solidification and melting of PCM. To solve the governing equations, the pressure-based finite volume method using transient solver in ANSYS Fluent 16.2 is employed. Moreover, the SIMPLE algorithm is selected to provide the coupling between the pressure and velocity components. The results present that the PVT/PCM system has lower surface temperature and coolant outlet temperature compared to the PVT only system. The results also shows that enhancing the melting temperature of PCM from 40 °C to 65 °C increases the surface temperature from 51.53 °C to 58.78 °C, while it reduces the percentage of melted PCM from 82.7% to 9.6%. It is also concluded that as the thermal conductivity of PCM enhances, both electrical and thermal energy efficiency of PVT/PCM system increase.

Numerical parameter characterisation of a buried mine blast event with further emphasis on IED shapes and soil bed conditions

The modelling of a buried charge and its impact on vehicle structures is a complex simulation task since numerous structural variables, physical properties and numerical parameters have to be determined to provide accurate estimation of the structure's response. A number of variables in question are directly related to the numerical approach chosen to perform the analysis while others relate to the overall modelling process and the detail used to describe the physical processes. This paper documents the results of a comprehensive sensitivity study of the structural response of a vehicle subjected to the impulse from a buried charge using the discrete particle method (DPM) to model the soil and high explosive (HE) coupled to a finite element solver for the structure. Fourteen design variables and numerical parameters were studied requiring in excess of 1000 computational hours. The response parameter was chosen to be the total blast impulse (TBI) on the structure. The non-linear transient dynamic explicit finite element solver used for the analysis was the IMPETUS Afea Solver® which has implemented the DPM for blast simulations and is called iDPM. The study includes soil characteristics and charge related parameters. The depth of burial (DOB) and number of discrete particles were also considered in the study. As a natural extension of the sensitivity study the effect of an improvised explosive device (IED) made from an oil can is investigated as well as the effect of having rocks in the soil bed making it a non-homogeneous soil bed.

Numerical parameter characterisation of a buried mine blast event with further emphasis on IED shapes and soil bed conditions

The modelling of a buried charge and its impact on vehicle structures is a complex simulation task since numerous structural variables, physical properties and numerical parameters have to be determined to provide accurate estimation of the structure's response. A number of variables in question are directly related to the numerical approach chosen to perform the analysis while others relate to the overall modelling process and the detail used to describe the physical processes. This paper documents the results of a comprehensive sensitivity study of the structural response of a vehicle subjected to the impulse from a buried charge using the discrete particle method (DPM) to model the soil and high explosive (HE) coupled to a finite element solver for the structure. Fourteen design variables and numerical parameters were studied requiring in excess of 1000 computational hours. The response parameter was chosen to be the total blast impulse (TBI) on the structure. The non-linear transient dynamic explicit finite element solver used for the analysis was the IMPETUS Afea Solver® which has implemented the DPM for blast simulations and is called iDPM. The study includes soil characteristics and charge related parameters. The depth of burial (DOB) and number of discrete particles were also considered in the study. As a natural extension of the sensitivity study the effect of an improvised explosive device (IED) made from an oil can is investigated as well as the effect of having rocks in the soil bed making it a non-homogeneous soil bed.

Transient Core-Loss Simulation for Ferrites With Nonuniform Field in SPICE

A methodology to develop a subcircuit model for core loss simulation in LTspice is presented. The subcircuit implements the dynamic core loss model used in the transient solver of finite-element analysis (FEA) in order to provide equivalent results in the time domain for ferrite materials. The wipe-out rule is applied in the simulation so that the core loss during the transient behavior can be predicted. A field factor is derived from an effective flux density for a nonuniform flux distribution to simplify the relationship between the current and the field for arbitrary magnetic core shapes. The interface of the subcircuit is capable of being integrated to any power stage simulations. Simulation results of core loss equivalent to the FEA on an exemplary plate-core inductor are obtained from the subcircuit that significantly reduces computational cost.

An improved method to null-fill H-plane radiation pattern of graphene patch THz antenna utilizing branch feeding microstrip line

This paper proposes a new design for graphene patch antenna in THz band frequency range. Graphene supports surface plasmon polariton (SPP) behavior and also provides various conductivity derived from some tunable parameters. The numerical simulations with CST, Computer Simulation Technology transient solver, is applied to the proposed antenna. Here, branch feeding microstrip line is used to exciting antenna. This approach fills some nulls of H-plane radiation pattern.

Compound technique -based inline design strategy for water-hammer control in steel pressurized-piping systems

The inline design strategy was recognized as being an effective tool for water-hammer control in pressurized-pipe flow. Principally, this strategy is based on replacing a short-section of the existing steel-piping system by another made of polymeric material. However, this strategy leads to an excessive radial-strain amplification and a large spread-out of wave oscillation period. Alternatively, an innovative compound technique -based inline design strategy was reported in this paper to enhance the foregoing limitations. The proposed technique is based on splitting the single short-section used in the conventional technique into a couple of two sub short-sections made up of two distinct material types. The materials demonstrated in this study include high- and low-density polyethylene (HDPE) and (LDPE). The transient solver was based on the 1-D unconventional water-hammer model embedding the Vitkovsky et al. and Kelvin-Voigt formulation, while the numerical discretization was performed using the Fixed Gird Method of Characteristics (FG-MOC). The proposed method is validated through a comparison with experimental results. Further, detailed numerical results obtained from several scenarios are presented and discussed. Results illustrated the reliability of the proposed technique in mitigating excessive high- or low-pressures, and evidenced that the (HDPE–LDPE) sub short-sections combination (where the former is attached to hydraulic parts and the latter to the steel pipe) is the most prominent configuration providing an acceptable trade-off between piezometric-head and circumferential-stress attenuation (from one side), and limitation of the excessive spreading of oscillation period and amplification of radial-strain (from the other side). The findings of a parametric study of the sensitivity of pressure wave damping and spreading to the employed short-section length and diameter resulted in estimation of the near-optimal design values of the short-section size.

Interfacing electromagnetic model of tower-footing impedance with the EMTP software package

This paper presents a new approach for modeling of time domain multi-port grounding systems, considering the frequency-dependent behavior of the soil. The presented method provides accurate results for the calculation of the lightning-related overvoltages. To this aim, first, the method of moment is used for solving the Maxwell equations, which leads to the extraction of the impedance matrix of the grounding system over the frequency range of interest. In the next step, a logical approximation of the impedance matrix is made using the Vector Fitting (VF) method. The used VF method has led to the fitting of a set of poles-residues for all the arrays of the impedance matrix of the grounding system. Finally, considering that all the electromagnetic transient solvers perform the simulation based on the admittance matrix, the proper model of the time domain single-port and multi-port grounding system is executed by state-space equations. From the simulation results, it is found that the modeling type of the grounding system is significantly effective on the lightning overvoltages, the outage rates of transmission lines (TLs), unavailability of the TLs and the lifetime of surge arresters (SAs). The advantage of the proposed model consists of higher accuracy and lower complexity, as it covers all the frequency range of interest and does not need any matrix inversion. Also, it can be implemented directly in a transient analysis solver, e.g. EMTP.

Latency Insertion Method Based Real-Time Simulation of Power Electronic Systems

In this paper, we demonstrate how latency insertion method (LIM) can be used for real-time simulation of high switching frequency power electronics systems and how this approach can be implemented for scalable Field Programmable Gate Array (FPGA) execution. We first present a summary of the LIM and how this method can be developed into a solver for high-performance FPGA execution. We then present how common power electronics topologies—buck and boost—can be modeled using the LIM approach, followed by how the LIM model of a three-phase dc/ac converter is created. Afterward, we demonstrate the accuracy of the developed FPGA solver through a set of power electronic system examples, with each example compared with near ideal results of same example provided by a traditional electromagnetic transient type solver. We complete the paper analyzing the scalability of the proposed approach on FPGA devices, both in terms of achievable time step as well as of resource usage.

A Minimally Intrusive Low-Memory Approach to Resilience for Existing Transient Solvers

We propose a novel, minimally intrusive approach to adding fault tolerance to existing complex scientific simulation codes, used for addressing a broad range of time-dependent problems on the next generation of supercomputers. Exascale systems have the potential to allow much larger, more accurate and scale-resolving simulations of transient processes than can be performed on current petascale systems. However, with a much larger number of components, exascale computers are expected to suffer a node failure every few minutes. Many existing parallel simulation codes are not tolerant of these failures and existing resilience methodologies would necessitate major modifications or redesign of the application. Our approach combines the proposed user-level failure mitigation extensions to the Message-Passing Interface (MPI), with the concepts of message-logging and remote in-memory checkpointing, to demonstrate how to add scalable resilience to transient solvers. Logging MPI communication reduces the storage requirement of static data, such as finite element operators, and allows a spare MPI process to rebuild these data structures independently of other ranks. Remote in-memory checkpointing avoids disk I/O contention on large parallel filesystems. A prototype implementation is applied to Nektar++, a scalable, production-ready transient simulation framework. Forward-path and recovery-path performance of the resilience algorithm is analysed through experiments using the solver for the incompressible Navier–Stokes equations, and strong scaling of the approach is observed.

Further investigation on the water-hammer control branching strategy in pressurized steel-piping systems

The branching design strategy was recognized as being a practical technique for water-hammer surge control in pressurized steel-piping systems. This strategy is based on adding a branched polymeric short-section at the transient sensitive region of an existing steel piping system. On the other hand, design practices require numerical solvers that are both accurate and computationally efficient. Accordingly, this paper revisited the implementation of the branching design strategy using a 1-D unconventional water-hammer model based upon the Ramos formulation to benefit from its simplified representations of unsteady friction effects and pipe wall behavior. The transient solver was performed using the Fixed Gird Method of Characteristics (FG-MOC). The transient solver effectiveness was demonstrated by comparing the obtained numerical results with pertinent experimental ones quoted in the literature; further, computational savings were significantly carried out via the selected formulation. The proposed design strategy was implemented within two kinds of boundary conditions initiating water-hammer up- and down-surge waves. In addition, two types of polymeric materials, including high- or low-density polyethylene (HDPE or LDPE), were utilized for the branched short-section. Results evidenced the potential of the branching design strategy to attenuate excessive pressure -rise and -drop, while safeguarding existing pressurized piping system configurations. Furthermore, the study of the dependency of the short-section material and size on the attenuation rate of pressure -rise and -drop evidenced that a higher wave speed of the branched short-section provided a higher attenuation rate of pressure -rise and -drop, and helped estimate near-optimal values for the short-section length and diameter.

Ember: An open-source, transient solver for 1D reacting flow using large kinetic models, applied to strained extinction

Simulation of quasi one-dimensional reacting flow is a standard in many combustion studies. Here Ember, a new open-source code for efficiently performing these calculations using large, detailed chemical kinetic models is presented. Ember outperforms other standard software, such as Chemkin, in computation time by leveraging rebalanced Strang operator splitting which does not suffer the steady-state inaccuracies of most splitting methods. The splitting approach and implementation used in Ember are described. Ember is validated for computation of flame extinction through imposed strain, extinction strain rate (ESR), and shown to be capable of modeling three typical experimental strained flame configurations: premixed twin flames, premixed single flames opposing inert, and diffusion flames. As further demonstration, Ember is used to investigate Lewis number effects on ESR using a detailed chemical kinetic model with 500 species for simulation of strained extinction of lean (Le > 1) and rich (Le < 1) propane/air flames. Primary trends predicted by Law (2006) using asymptotic theories of strained flames are accurately reproduced with the large, detailed chemical kinetic model. However, the complicated chemistry introduces some subtle phenomena not seen with single-step models. The Ember software is open-source and freely available to any user online.

High directivity plasmonic graphene-based patch array antennas with tunable THz band communications

This paper proposes patch array antennas based on graphene with plasmonic characteristics in THz application. Here, the proposed structure is based on two graphene rectangular patch elements in horizontal plane which presents behavior of Surface Plasmon Polariton (SPP) waves. Also, analyzing the graphene characteristics are studied in details. The numerical simulations with CST, Computer Simulation Technology transient solver, and COMSOL are presented in array antenna structure. The efficiency is improved by using array antenna and also by increasing the graphene layers. Parametric study method is used to achieve desired tunable low terahertz band communication.

Vibroacoustic Characterization of a Permanent Magnet Synchronous Motor Powertrain for Electric Vehicles

The vibroacoustic characterization of an electric powertrain which consists of a permanent magnet synchronous motor and a gearbox for electric vehicles is analyzed in this paper. First, the vibroacoustic origins of the electric powertrain are investigated. The influence of current harmonics on electromagnetic force harmonics is presented. The spatial distribution of electromagnetic force exerting on stator inner surfaces is investigated by a two-dimensional magnetic transient solver. The spectrum of gear meshing force is then presented. After that, the vibration response of the electric powertrain system is predicted by a mode superposition method, based on which the acoustic noise of the electric powertrain is obtained using a direct boundary element method. Furthermore, the influence of electromagnetic force harmonics and gearbox on the vibroacoustic behavior of the electric powertrain is analyzed. To verify the calculating results, a vibroacoustic test is conducted in a semi-anechoic room and the results show that both electromagnetic forces and gear meshing forces contribute to a considerable proportion of the vibroacoustic behavior due to the integrated structure.

Design of a high gain and high efficiency W-band folded waveguide TWT using phase-velocity taper

The folded-waveguide (FW) slow-wave structure (SWS) is a very promising beam-wave interaction circuit for high-frequency traveling wave tubes (TWTs). This paper presents the design methodology for enhancing the gain and efficiency of a practical W-band FWTWT using phase-velocity taper. Initially, phase-velocity, interaction impedance, and gain-growth of a single-section FWTWT are predicted theoretically. Further, these parameters are optimized using the eigenmode solver and transient solver of 3-D CST Studio. The simulated results agreed with theoretically predicted results within ~1% discrepancy. Then the single-section SWS along with the input and output couplers consisting of quarter-wave impedance transformers and pillbox windows are simulated using transient solver and a return-loss of around −20 dB is obtained over the desired band. Finally, the output power, gain and electronic efficiency for a two-section TWT with the phase-velocity taper are obtained using particle-in-cell simulation of CST Studio. At an applied beam voltage of 20 kV and a current of 0.1 A, an output power of 130 W, corresponding to a saturated gain of 39.4 dB, an instantaneous 3-dB bandwidth of around 5.3%, and electronic efficiency of around 6.5% at 94 GHz has been obtained. It was found that at 94 GHz the ‘positively and negatively phase-velocity tapered’ sections enhanced the gain by almost 3 dB and doubles the electronic efficiency of the two-section FWTWT design as compared to the two-section FWSWS with the same interaction circuit length with no phase-velocity taper.

A HIERARCHICAL APPROACH TO DETERMINING ACOUSTIC ABSORPTION PROPERTIES OF POROUS MEDIA COMBINING PORE-RESOLVED AND MACROSCOPIC MODELS

Acoustic properties of porous media are very important for numerous industrial applications, the typical goal being to maximize broadband absorption to decrease the sound pressure level of the engineering system under consideration. Up to now acoustic absorption for porous media with complex inner geometry is determined experimentally, as acoustic simulations on the pore scale are computationally challenging due to the tedious geometric reconstruction of computer tomography (CT) data and the corresponding mesh generation as well as substantial computational requirements for the corresponding transient 3D solvers. The lattice Boltzmann method (LBM), which is an established computational approach to simulate pore-resolved porous media transport problems, has been used successfully for aeroacoustic setups and is utilized in this work to fill this gap. This paper presents a comparison of different experimental and numerical approaches to determine the acoustic absorption of different porous media. Experimental work with an impedance tube was carried out for comparison and CT scans were conducted to supply the detailed numerical simulation with geometry data of the porous samples. Results of LB simulations for the acoustic impedance of a microperforated plate and a felt are shown. Finally we demonstrate how microscopic parameters determined by a pore scale approach can be used to feed homogenized models to bridge the gap towards simulations of components where acoustic absorbers are applied to, e.g., wing flaps of airplanes.

Modelling fragmentation of a 155 mm artillery shell IED in a buried mine blast event

The shapes of improvised explosive devices (IEDs) used in conflicts are complex and take many forms. As device fragmentation and soil interaction must be included to provide accurate system physics, modelling unique IED device, artillery shell, and high explosive (HE) shapes is critical for the successful evaluation of protective structures for soldiers and civilians. Finite element and discrete particle method (DPM) techniques using the explicit non-linear transient finite element software IMPETUS Afea Solver® are documented for a series of studies ranging from a simple cylinder explosion to a complex full vehicle blast. Explored in detail are detonation results of a soil buried M795 artillery shell and its ejecta interaction with the TARDEC Generic Vehicle Hull containing a seated IMPETUS Afea hybrid III 50th percentile dummy. These studies also evaluate sensitivity to nine design variables and structure fragmentation response resulting from element type and the use of a node splitting algorithm.

Plasmonic patch antenna based on graphene with tunable terahertz band communications

This paper proposes a plasmonic graphene-based patch antenna for THz application in detailed analysis. Here, a rectangular patch antenna based on graphene presenting the Surface Plasmon Polariton (SPP) waves behavioral in graphene sheet is proposed and analyzed. In addition, the characteristics of graphene are presented and analyzed. In the Ffollowing, the SPP wave propagation in graphene is numerically and analytically studied. The numerical simulations with COMSOL and CST, Computer Simulation Technology transient solver, are presented in proposed structure. These solvers demonstrate that the efficiency of a rectangular graphene patch drastically improved by increasing the graphene layers. In order to achieve the desired low tunable terahertz band communication, parametric study method is applied on the proposed antenna.

Detailed dynamic refrigerant migration characteristics in room air-conditioner with R290

Propane (R290) as flammable refrigerant posed additional fire and explosion risk in room air conditioner (RAC), refrigerant migration and redistribution have been recognized as essential factors for leakage characteristics and charge optimization researches. Hence, a detailed understanding of the dynamic behavior of RAC systems is beneficial to future charge analysis and model-based control design, and then a system-level finite-volume dynamic model was developed in this paper. For the purpose of realistic modelling, dynamic lubricant solubility and absorption rate, finite-volume model integrated with geometry configuration and fin equivalent simplification were adopted. On the other hand, transient solver and public domain software YSMP were derived from SINDA/FLUINT (S/F) platform to make models converge and reduce the CPU time. From the initial state assumptions given in this paper, simulated results like dynamic mass, temperature and pressure distribution were validated by experiments; and they had good conformity. It was found that refrigerant migration process before equilibrium could be divided into dramatic change phase and smooth change phase. Dramatic change phase was dominated by mass transfer, while smooth change phase was dominated by heat transfer. Finally, the sensitivity of conditions and geometries to each component charged fraction was studied. Under reasonable charged circumstances, greater displacement, less mass charged and larger volume ratio of condenser to evaporator will be beneficial for improving system stability during startup.