Numerical Simulation Code Research Articles

Discrete evolution model based on mean spatial density for space debris environment

Concerning the increasing space debris and its collision threat on operational satellites, a simple and less computation consuming model is built to implement the long-term evolution and prediction of the Low-Earth orbit space debris environment. Space debris is divided into groups according to the orbital height and the area-mass ratio. For each group, the distribution state is described by the mean spatial density, and the dynamics of the evolution process under the effect of mutual collision and the perturbation of atmosphere drag is modeled by a partial differential equation (PDE). A two-dimensional analytical solution is obtained when the collision-associated terms of those PDEs are excluded. Moreover, the analytical results are shown compatible with the results of a numerical simulation code, which means that the mechanical model in the evolution model is reliable. In a hypothetical scenario, i.e. without future launch activities, the collisional “chain reaction” of the space debris environment will be reached, as shown by the numerically obtained solution of the general evolution model.

CPS construction for laser coating with a general-purpose three-dimensional thermohydraulics numerical simulation code SPLICE

CPS construction for laser coating with a general-purpose three-dimensional thermohydraulics numerical simulation code SPLICE

AN IMPROVED PRESSURE CALCULATION METHOD FOR SIMULATIONS OF GAS–LIQUID TWO-PHASE FLOWS ON UNSTRUCTURED MESHES

The authors have developed a numerical simulation code for gas–liquid two-phase flows with a high-precision volume-of-fluid–type interface-tracking method on unstructured meshes. In this paper, we propose an improved pressure calculation method in the vicinity of a gas–liquid interface based on the balanced-force algorithm, which was originally developed on structured meshes. To achieve accurate calculations for interfacial dynamics, we introduce the concept of external force potentials to take into account the physically appropriate mechanical balance between the pressure and the external forces, i.e., the surface tension and the gravitational force, at the gas–liquid interfaces. The validity of the improved pressure calculation method is checked by simulating a spherical bubble in stationary liquid and a rising bubble in liquid. As a result, the improved pressure calculation method succeeds in highly suppressing unphysical behavior, i.e., the spurious velocity, compared to the conventional simulation method. Therefore, the improved pressure calculation method on unstructured meshes is considered to work well in numerical simulations of gas–liquid two-phase flows.

Theoretical and Experimental Investigations on Transient Run-Up Procedures of Journal Bearings Including Mixed Friction Conditions

This paper focuses on the operating behavior of journal bearings for industrial machinery application during run-ups. For this purpose, a numerical simulation code that is based on a two-dimensional extended and generalized Reynolds equation and a full three-dimensional energy equation, was advanced by a theoretical model considering the effects of mixed friction and warming of journal components during start-up. The mixed friction routine contained the elastic half-spaces model proposed by Boussinesq, which considers the influence of rough surfaces by implementing flow factors and calculates additional stiffness and dissipation in areas with solid interactions. Furthermore, a transient term was added in the energy equation to consider the thermal inertia of journal, and bearing to ensure a realistic heating during run-ups. Results of the prediction were compared to experimental data taken from a special test rig built up for validation procedures. Besides the conventional sensors for temperature, oil flow, and relative motion between shaft and stator, a contact voltage measurement was installed to determine the intensity of mixed friction. The evaluation of experimental data by Stribeck curves, based on a shaft torsion measurement, indicated a significant influence of run-up time on frictional moment. The friction coefficient of the rotor bearing system was strongly influenced by the run-up time. A short run-up time reduced the frictional coefficient in the mixed lubrication regime while the opposite behavior was observed in the hydrodynamic lubrication regime. The numerical code predicted these tendencies in good agreement with experimental data, however, only if the transient energy model was applied.

Time-Domain Hydro-Elastic Analysis of a SFT (Submerged Floating Tunnel) with Mooring Lines under Extreme Wave and Seismic Excitations

Global dynamic analysis of a 700-m-long SFT section considered in the South Sea of Korea is carried out for survival random wave and seismic excitations. To solve the tunnel-mooring coupled hydro-elastic responses, in-house time-domain-simulation computer program is developed. The hydro-elastic equation of motion for the tunnel and mooring is based on rod-theory-based finite element formulation with Galerkin method with fully coupled full matrix. The dummy-connection-mass method is devised to conveniently connect objects and mooring lines with linear and rotational springs. Hydrodynamic forces on a submerged floating tunnel (SFT) are evaluated by the modified Morison equation for a moving object so that the hydrodynamic forces by wave or seismic excitations can be computed at its instantaneous positions at every time step. In the case of seabed earthquake, both the dynamic effect transferred through mooring lines and the seawater-fluctuation-induced seaquake effect are considered. For validation purposes, the hydro-elastic analysis results by the developed numerical simulation code is compared with those by a commercial program, OrcaFlex, which shows excellent agreement between them. For the given design condition, extreme storm waves cause higher hydro-elastic responses and mooring tensions than those of the severe seismic case.

Manipulation of Laser Distribution to Mitigate the Space-Charge Effect for Improving the Performance of a THz Coherent Undulator Radiation Source

A THz coherent undulator radiation (THz-CUR) source has been developed at the Institute of Advanced Energy, Kyoto University. A photocathode Radio-Frequency (RF) gun and a bunch compressor chicane are used for generating short-bunch electron beams. When the electron beam energy is low, the space-charge effect strongly degrades the beam quality, such as the bunch length and the energy spread at the high bunch charge condition at around 160 pC, and results in the reduction of the highest frequency and the maximum radiated power of the THz-CUR. To mitigate the space charge effect, we have investigated the dependence of the electron beam quality on the laser distribution in transverse and longitudinal directions by using a numerical simulation code, General Particle Tracer GPT. The manipulation of the laser distribution has potential for improving the performance of the THz-CUR source. The electron bunch was effectively compressed with the chicane magnet when the laser transverse distribution was the truncated Gaussian profile, illuminating a cathode. Moreover, the compressed electron bunch was shortened by enlarging the laser pulse width. Consequently, an enhancement of the radiated power of the THz-CUR has been indicated.

Development and validation of the hybrid code for numerical simulation of detonations

The present paper describes a numerical code for simulation of detonation flows on hybrid computational clusters (CPU/GPU). The code can solve 1D, 2D and 3D unsteady Euler equations for multispecies chemically reacting flows using high-order shock-capturing TVD schemes and a finite-rate chemistry solver. The implementation is based on the OpenMP, MPI and CUDA technologies allowing for both flexibility and computational efficiency of the code. The program is verified on the analytical Zeldovich-von Neumann-Doering solution for the 1D detonation wave in H2/O2 mixture. Some examples of 2D computations are also given.

Thermal decomposition of solid fuels. Objectives, challenges and modelling

The progress of computing means, the complexity of the situation concerning fires, the need to describe with more accuracy all the different phases of the fire behaviour (such as the ignition, its growing, its extinction…) has led to a great effort in the past few years of the international research community to improve the performance of the numerical simulation codes. Those codes consist of different sub-models in order to simulate the different physical, thermal and chemical aspects of the fire, such as the thermal decomposition.This one plays a significant role on the fire evolution since it represents the fuel quantity that supplies the combustion process. Then, the requirement of a more detailed characterization and description is one actual and important challenge. This is the challenge of this keynote lecture which is dealing with thermal decomposition of solid fuels in the conditions of fire, addressing both the experimental investigations and the modelling development aspects.After a presentation of the objectives and challenges of thermal decomposition, this lecture details the different steps required for its description, based on an experimental and numerical scaling-up approach. Examples of application are done. Special focus is highlighted on different scientific bolts and issues for thermal decomposition modelling.

General relativistic numerical simulation of sub-Keplerian transonic accretion flows on to rotating black holes: Kerr space–time

We study time evolution of sub-Keplerian transonic accretion flows onto black holes using a general relativistic numerical simulation code. We perform simulations around the black holes having non-zero rotation. We first compare one-dimensional simulation results with theoretical results and validate the performance of our code. Next, we present results of axisymmetric, two-dimensional simulation of advective flows. In the literature, there is no solution which describes steady shock solutions in two dimensions. However, our simulations produce these centrifugal force supported steady shock waves even in presence of strong dragging of inertial frames. Since the post-shock region could be hot and upscatter photons through Comptonization, these shock would put imprints on the spectra. Thus, our solutions, which represent truly new results, could be useful to measure spins through radiation spectrum of accreting Kerr black holes.

Using ensemble Kalman filter to determine parameters for computational crowd dynamics simulations

PurposeIt is of paramount importance to ensure safe and fast evacuation routes in cities in case of natural disasters, environmental accidents or acts of terrorism. The same applies to large-scale events such as concerts, sport events and religious pilgrimages as airports and to traffic hubs such as airports and train stations. The prediction of pedestrian is notoriously difficult because it varies depending on circumstances (age group, cultural characteristics, etc.). In this study, the Ensemble Kalman Filter (EnKF) data assimilation technique, which uses the updated observation data to improve the accuracy of the simulation, was applied to improve the accuracy of numerical simulations of pedestrian flow.Design/methodology/approachThe EnKF, one of the data assimilation techniques, was applied to the in-house numerical simulation code for pedestrian flow. Two cases were studied in this study. One was the simplified one-directional experimental pedestrian flow. The other was the real pedestrian flow at the Kaaba in Mecca. First, numerical simulations were conducted using the empirical input parameter sets. Then, using the observation data, the EnKF estimated the appropriate input parameter sets. Finally, the numerical simulations using the estimated parameter sets were conducted.FindingsThe EnKF worked on the numerical simulations of pedestrian flow very effectively. In both cases: simplified experiment and real pedestrian flow, the EnKF estimated the proper input parameter sets which greatly improved the accuracy of the numerical simulation. The authors believe that the technique such as EnKF could also be used effectively in other fields of computational engineering where simulations and data have to be merged.Practical implicationsThis technique can be used to improve both design and operational implementations of pedestrian and crowd dynamics predictions. It should be of high interest to command and control centers for large crowd events such as concerts, airports, train stations and pilgrimage centers.Originality/valueTo the authors’ knowledge, the data assimilation technique has not been applied to a numerical simulation of pedestrian flow, especially to the real pedestrian flow handling millions pedestrian such as the Mataf at the Kaaba. This study validated the capability and the usefulness of the data assimilation technique to numerical simulations for pedestrian flow.

A new Intel Xeon Phi accelerated hydrodynamic code for numerical simulations of interacting galaxies

In this paper, a new hydrodynamics code to simulate of interacting galaxies on Intel Xeon Phi processors with KNL architecture is presented. A new vector numerical method implemented in the form of a program code for massively parallel architectures is proposed in details. A detailed description is given and a parallel implementation of the code is made. A 92 per cent scalability is reached with 64 processors. The scenarios of interacting galaxies S+E is presented.

Numerical simulation of complex hexagonal structures to predict drop behavior under submerged and fluid flow conditions

Abstract This study simulated a control rod assembly (CRA), which is a part of reactor shutdown systems, in immersed and fluid flow conditions. The CRA was inserted into the reactor core within a predetermined time limit under normal and abnormal operating conditions, and the CRA (which consists of complex geometric shapes) drop behavior is numerically modeled for simulation. A full-scale prototype CRA drop test is established under room temperature and water-fluid conditions for verification and validation. This paper describes the details of the numerical modeling and analysis results of the several conditions. Results from the developed numerical simulation code are compared with the test results to verify the numerical model and developed computer code. The developed code is in very good agreement with the test results and this numerical analysis model and method may replace the experimental and CFD method to predict the drop behavior of CRA.

Extended MHD modeling of the steady solar corona and the solar wind

The history and present state of large-scale magnetohydrodynamic modeling of the solar corona and the solar wind with steady or quasi-steady coronal physics is reviewed. We put the evolution of ideas leading to the recognition of the existence of an expanding solar atmosphere into historical context. The development and main features of the first generation of global corona and solar wind models are described in detail. This historical perspective is also applied to the present suite of global corona and solar wind models. We discuss the evolution of new ideas and their implementation into numerical simulation codes. We point out the scientific and computational challenges facing these models and discuss the ways various groups tried to overcome these challenges. Next, we discuss the latest, state-of-the art models and point to the expected next steps in modeling the corona and the interplanetary medium.

Nonlinear Flow Characteristics of a System of Two Intersecting Fractures with Different Apertures

The nonlinear flow regimes of a crossed fracture model consisting of two fractures have been investigated, in which the influences of hydraulic gradient, surface roughness, intersecting angle, and scale effect have been taken into account. However, in these attempts, the aperture of the two crossed fractures is the same and effects of aperture ratio have not been considered. This study aims to extend their works, characterizing nonlinear flow through a system of two intersecting fractures with different apertures. First, three experiment models with two fractures having different apertures were established and flow tests were carried out. Then, numerical simulations by solving the Navier-Stokes equations were performed and the results compared with the experiment results. Finally, the effects of fracture aperture on the critical pressure difference and the ratio of hydraulic aperture to mechanical aperture were systematically analyzed. The results show that the numerical simulation results agree well with those of the fluid flow tests, which indicates that the visualization techniques and the numerical simulation code are reliable. With the increment of flow rate, the pressure difference increases first linearly and then nonlinearly, which can be best fitted using Forchheimer’s law. The two coefficients in Forchheimer’s law decrease with the increasing number of outlets. When increasing fracture aperture from 3 mm to 5 mm, the critical pressure difference increases significantly. However, when continuously increasing fracture aperture from 5 mm to 7 mm, the critical pressure difference changes are negligibly small. The ratio of hydraulic aperture to mechanical aperture decreases more significantly for a fracture that has a larger aperture. Increasing fracture aperture from 5 mm to 7 mm, that has a negligibly small effect on the critical pressure difference will however significantly influence the ratio of hydraulic aperture to mechanical aperture.

A Better Understanding of the Very Low-Pressure Plasma Polymerization of Aniline by Optical Emission Spectroscopy Analysis

In the wavelength range from 200 up to 1000 nm, optical emission from electronically excited fragments (CN, CH, NH, C2, H2, N2, N2+, and H-Balmer) is detected when aniline plasmas are generated in a multi-dipolar microwave reactor. The optical emission spectrum monitored in very low-pressure conditions (~ 1 mTorr) shows important characteristics. The Hα, Hβ and CN species are the most radiating systems and according to the NH/N2 intensity ratio, two different operating regimes are observed suggesting a change in the reaction pathways. These two regimes are correlated to changes of the plasma characteristics (electron temperature and density) deduced from Langmuir probe measurements. The plasma thermodynamic state is quantified by implementing numerical simulation codes for synthetic spectra calculations. The rovibrational temperatures (Tr, Tv) are determined for some neutral species (CN, CH). The obtained values of Tr and Tv show the non-local thermodynamic equilibrium of the vibrational and rotational states (Tr < Tv). Moreover, the very low pressure aniline-based plasmas deviate substantially from the Boltzmann distribution. Correlation between the optical emission data and the solid phase analysis allows proving that the in situ characterization of the plasma phase is an important predictive tool of physico-chemical properties of the film. From these correlated data, we deduce preponderant chemical reaction pathways which help to better understand the plasma generation. Relative contributions of the dehydrogenation of C–H and N–H groups are established in order to deduce the leading initiation reaction for H-Balmer line formation.

A simplified method to predict fatigue damage of offshore riser subjected to vortex-induced vibration by adopting current index concept

In the present study, an innovative method for estimating fatigue performance of risers under vortex-induced vibration (VIV) is proposed. Generally, fatigue performance is affected by the surrounding environment such as wind, wave, and current. It is well known that current is the most influential load among all for offshore risers. In structural safety aspect, strength and fatigue are the most important factors for riser design. In addition, fatigue design is affinitive to the VIV phenomenon. For the analysis of fatigue performance of riser, SHEAR7 numerical simulation code which is commonly used in offshore industry is applied. In order to identify the relation between current load and fatigue performance of riser, Fatigue damage versus Current index (F-C) diagram has been proposed. F-C diagram may cover change of current profiles and help predict the fatigue damage under VIV. In case of current profile, a total of sixty cases of current scenarios are considered based on six representative sea-states. The obtained results from this study will be a useful guideline to predict the effect of current on the fatigue performance of riser.

Simflowny 2: An upgraded platform for scientific modelling and simulation

Simflowny is an open platform for scientific dynamical models which automatically generates parallel code for different simulation frameworks. Here we present major upgrades on this software to support an extended set of model families, in particular: (i) a new family for the evolution of partial differential equations, which can include spatial derivatives of any order, (ii) a new family for agent based models to study complex phenomena — either on a spatial domain or on a graph. Additionally we introduce an upgraded flexible graphical user interface to accommodate these and future families of equations and models. This paper describes the new architecture and summarizes the formal representation and implementation of these new families, providing several validation results. Program summaryProgram Title: SimflownyProgram Files doi:http://dx.doi.org/10.17632/g9mcw8s64f.1Licensing provisions: Apache License, 2.0Programming language: Java, C++ and JavaScriptJournal Reference of previous version: Comput. Phys. Comm. 184 (2013) 2321–2331Does the new version supersede the previous version?: YesReasons for the new version: Additional featuresSummary of revisions: Expanded support for Partial Differential Equations, Support for Agent Based Models, New Graphical User Interface.Nature of problem: Simflowny generates numerical simulation code for a wide range of models.Solution method: Any discretization scheme based on either Finite Volume Methods, Finite Difference Methods, or meshless methods for Partial Differential Equations. Agent Based Model simulations execute their own algorithm as set in their models.Additional comments: The software architecture is easily extensible for future additional model families and simulation frameworks. Full documentation is available in the wiki home of the Simflowny project [2].

An FEM-BEM method for halo current problem and its application to HL-2M Tokamak

Halo current in Tokamak device during plasma vertical displacement events (VDEs) may damage the in-vessel components due to huge electromagnetic (EM) force. It is important to simulate the distribution of halo current in order to evaluate the EM force and the corresponding structural dynamic response. In this paper, an FEM-BEM method was proposed at first to simulate the current distribution in conductors excited by a sinusoidal conducting current injection. The response under a transient excitation was then realized by using the strategy of the frequency domain summation (FDS). A numerical simulation code was developed based on the proposed numerical scheme. The validity of the numerical method and corresponding code were proved by comparing numerical results of the developed code with those of the ANSYS software using step by step time domain integration. Finally, the proposed method was applied to the simulation of the halo current in a simplified vacuum vessel model of the HL-2M Tokamak. The simulation results provide good reference for the subsequent analysis of mechanical behaviors of in-vessel structures.

Direct numerical simulations of reacting flows with detailed chemistry using many-core/GPU acceleration

A new direct numerical simulation (DNS) code for multi-component gaseous reacting flows has been developed at KAUST, with the state-of-the-art programming model for next generation high performance computing platforms. The code, named KAUST Adaptive Reacting Flows Solver (KARFS), employs the MPI+X programming, and relies on Kokkos for “X” for performance portability to multi-core, many-core and GPUs, providing innovative software development while maintaining backward compatibility with established parallel models and legacy code. The capability and potential of KARFS to perform DNS of reacting flows with large, detailed reaction mechanisms is demonstrated with various model problems involving ignition and turbulent flame propagations with varying degrees of chemical complexities.

Large-Scale Simulation of Magnetization Process of HTS Undulator for X-Ray FEL Based on ${T}$ -Method

In a development of the next-generation light source such as X-ray free-electron laser (FEL), it is necessary to achieve very intense undulator magnetic field with compact magnets. For this purpose, the pure-type high-Tc superconductor (HTS) undulator was proposed. In general, it is very difficult to adjust the magnetic field distribution after the HTSs are magnetized all together in a cryostat; therefore, a very careful design of the undulator magnets using a numerical analysis is required. The authors have been working on a development of the numerical simulation code of the magnetization process of the HTS undulator by using the current vector potential method ( $T$ -method). In this paper, the previously developed numerical code is modified to be used for much larger scale simulations by using a method of a multipole expansion for calculations of magnetic field aiming a practical FEL undulator design.