Point Source Model Research Articles

Prediction of the failure probability of the overhead power line exposed to large-scale jet fires induced by high-pressure gas leakage

A thermal failure model (TFM) is proposed to predict the failure probability of Aluminum Conductor Steel-Reinforced (ACSR) typed power line close to a large-scale jet fire of leaked high-pressure gases. It introduces a newly developed method for heat transfer from jet fires and a distribution model for conductor failure probability via IEEE Standard 738–2012. Comparisons covering van der Waals equation, jet flame length correlations (Chamberlain, Schefer, Molkov and Bradley) and thermal radiation models (point source, multi-point source and line source) were made to illustrate priority with respect to experimental measurement of large hydrogen and natural gas jet fires. Results show that a theoretical framework incorporating van der Waals equation, Molkov's correlation for jet flame length, radiative fraction model and point source model is adequately precise to predict high-pressure leakage process, total flame length and received radiant heat flux (far-field). Predicted total flame lengths of a large jet fire for nearby power lines within 50–200 m to the accident site correspond well to reported results and the conservative hazard ranges are predicted based on harm criteria of wood and Probit equations. In simulations, an acceptable safety distance for power line carrying 907 A and below is determined to be 150 m.

Source Process Analysis of the Northern Osaka Earthquake of 18 June, 2018, and Strong Ground Motion Simulation with the Pseudo Point-Source Model

Source Process Analysis of the Northern Osaka Earthquake of 18 June, 2018, and Strong Ground Motion Simulation with the Pseudo Point-Source Model

Buildup with Bremsstrahlung in the Martian Atmosphere

Photon buildup is a function of energy, medium, and geometry and therefore must be specifically calculated for the case of interest. The Martian atmosphere, mostly comprising carbon dioxide, is becoming more relevant to radiation researchers and therefore warrants the study of this gas mixture’s buildup properties for ionizing photon flux resulting from the secondary effects of galactic cosmic rays and solar flares. Specifically, this work uses the MCNP6 code to develop energy absorption buildup factors in finite slab models for energies ranging from 40 keV to 15 MeV with Martian regolith as the backscattering medium. The Martian carbon dioxide cycle is accounted for by determining maximum and minimum mean densities as a function of orbital position. An isotropic point source model for the atmosphere is also developed using the geometric progression fitting function. Buildup is bounded to a factor of approximately 23 at 100 keV for normally incident photons at the top of the atmosphere. For conservatism, the design problem neglects coherent scattering but assumes bremsstrahlung effects and uses Klein-Nishina free-electron cross sections for Compton scattering.

Research on thermal response of pool fire of oil tank based on PyroSim

This paper presents a numerical investigation aimed at studying the thermal response of adjacent tank caused by different fire conditions. The analysis involves two-step procedure:(1) PyroSim model to obtain the temperature and the inclination variation of the large pool-fire flame, to determine the ultimate temperature distributions of the target tank, and to compare the ultimate temperature and the fuel ignition temperature, thus to determine the target tank is safety or not,(2) There are three kinds of semi-empirical models for describing the radiation e.g.(i) point source,(ii) Shokri-Beyler and (iii) Mudan models, to determine thermal radiation flux of the target tank. The analysis considered the influence of two parameters: (i) seven kinds of wind, (ii) five kinds of distances between the tanks. The paper compares three kinds of semi-empirical models and PyroSim model, so PyroSim model is more fit for relationships between thermal radiation flux. The simulation results show that the maximum temperature of the target tank is the connection between the wall and the top of the tank; the target tanks have failure risk within 0.3D and 0.6D tank distances and the tank spacing greater than or equal to 0.9D have no failure risk.

Coseismic electric and magnetic signals observed during 2017 Jiuzhaigou Mw 6.5 earthquake and explained by electrokinetics and magnetometer rotation

SUMMARY Very clear coseismic electric and magnetic signals accompanying seismic waves were observed during the 2017 Mw 6.5 Jiuzhaigou earthquake, which took place in western China. In order to understand the generation mechanism of these observed signals, we simulate electric and magnetic responses to this specific earthquake based on three mechanisms, namely, the electrokinetic effect, the motional induction effect and the rotation effect of the coil-type magnetometer. We conduct the simulations using a point source model and a realistic layered earth model and compare to the observed data in the frequency band 0.05–0.3 Hz. Our results show that the electrokinetic effect can explain the observed electric fields in both waveform and amplitude, but it cannot explain the magnetic signals accompanying the Rayleigh wave. The motional induction effect cannot explain either the coseismic electric or magnetic data because it predicts much weaker coseismic electric and magnetic fields than the observed data. The magnetic fields resulting from the rotation of the magnetometer agree with the observed data in the waveforms though their amplitudes are two to four times smaller than the observed data. Our simulations suggest that the electrokinetic effect is responsible for the generation of coseismic electric fields and that rotation of the coil magnetometer is likely the main cause of coseismic magnetic fields. The results improve our interpretation of the coseismic electromagnetic (EM) phenomenon and are useful for understanding other kinds of earthquake-associated EM phenomena.

Analytical modelling of drop heating and evaporation in drop clouds: Effect of temperature dependent gas properties and cloud shape

The conservation equations describing the heat and mass transfer from an arbitrary number of interacting drops floating in a gaseous environment are analytically solved under steady-state conditions using a point source model, accounting analytically for the temperature dependence of the thermo-physical properties in the gaseous mixture. The model accuracy is assessed by comparison with exact analytical solutions available for pairs of interacting drops, accurate numerical predictions for drop arrays and experimental measurements for drop streams. A statistical description of drop clouds is used to propose a correlation for evaluating the screening effect in clouds of arbitrary shape and drop numerosity. The model is used to study the influence of cloud compactness and shape on global and local drop evaporation characteristics and the effect of operating conditions and temperature dependence of thermo-physical properties on total mass and heat rates for different chemical species is discussed.

Sensitivity of InSAR and teleseismic observations to earthquake rupture segmentation

SUMMARYEarthquakes often rupture across more than one fault segment. If such rupture segmentation occurs on a significant scale, a simple point-source or one-fault model may not represent the rupture process well. As a consequence earthquake characteristics inferred, based on one-source assumptions, may become systematically wrong. This might have effects on follow-up analyses, for example regional stress field inversions and seismic hazard assessments. While rupture segmentation is evident for most Mw > 7 earthquakes, also smaller ones with 5.5 < Mw < 7 can be segmented. We investigate the sensitivity of globally available data sets to rupture segmentation and their resolution to reliably estimate the mechanisms in presence of segmentation. We focus on the sensitivity of InSAR (Interferometric Synthetic Aperture Radar) data in the static near-field and seismic waveforms in the far-field of the rupture and carry out non-linear and Bayesian optimizations of single-source and two-sources kinematic models (double-couple point sources and finite, rectangular sources) using InSAR and teleseismic waveforms separately. Our case studies comprises of four Mw 6–7 earthquakes: the 2009 L’Aquila and 2016 Amatrice (Italy) and the 2005 and 2008 Zhongba (Tibet) earthquakes. We contrast the data misfits of different source complexity by using the Akaike informational criterion (AIC). We find that the AIC method is well suited for data-driven inferences on significant rupture segmentation for the given data sets. This is based on our observation that an AIC-stated significant improvement of data fit for two-segment models over one-segment models correlates with significantly different mechanisms of the two source segments and their average compared to the single-segment mechanism. We attribute these modelled differences to a sufficient sensitivity of the data to resolve rupture segmentation. Our results show that near-field data are generally more sensitive to rupture segmentation of shallow earthquakes than far-field data but that also teleseismic data can resolve rupture segmentation in the studied magnitude range. We further conclude that a significant difference in the modelled source mechanisms for different segmentations shows that an appropriate choice of model segmentation matters for a robust estimation of source mechanisms. It reduces systematic biases and trade-off and thereby improves the knowledge on the rupture. Our study presents a strategy and method to detect significant rupture segmentation such that an appropriate model complexity can be used in the source mechanism inference. A similar, systematic investigation of earthquakes in the range of Mw 5.5–7 could provide important hazard-relevant statistics on rupture segmentation. In these cases single-source models introduce a systematic bias. Consideration of rupture segmentation therefore matters for a robust estimation of source mechanisms of the studied earthquakes.

Central DOA Estimation Method for Exponential-Type Coherent Distributed Source Based on Fourth-Order Cumulant

The performance of direction-of-arrival (DOA) estimation for sparse arrays applied to the distributed source is worse than that applied to the point source model. In this paper, we introduce the coprime array with a large array aperture into the DOA estimation algorithm of the exponential-type coherent distributed source. In particular, we focus on the fourth-order cumulant (FOC) of the received signal which can provide more useful information when the signal is non-Gaussian than when it is Gaussian. The proposed algorithm extends the array aperture by combining the sparsity of array space domain with the fourth-order cumulant characteristics of signals, which improves the estimation accuracy and degree of freedom (DOF). Firstly, the signal-received model of the sparse array is established, and the fourth-order cumulant matrix of the received signal of the sparse array is calculated based on the characteristics of distributed sources, which extend the array aperture. Then, the virtual array is constructed by the sum aggregate of physical array elements, and the position set of its maximum continuous part array element is obtained. Finally, the center DOA estimation of the distributed source is realized by the subspace method. The accuracy and DOF of the proposed algorithm are higher than those of the distributed signal parameter estimator (DSPE) algorithm and least-squares estimation signal parameters via rotational invariance techniques (LS-ESPRIT) algorithm when the array elements are the same. Complexity analysis and numerical simulations are provided to demonstrate the superiority of the proposed method.

Modeling pressure variations generated by short train passing in open air

The pressure variation generated when a high-speed train passes in open air affects structures on the ground and causes environmental problems. It was assumed in past studies that the effect of the train nose passage is independent of the tail passage for long trains (with 16 cars). In these studies, only the effect of the nose passage was analyzed with a point source model. However, as the superposition of the effect of the nose and tail is not negligible for short trains (consisting of less than 10 cars), the entire pressure variation should be analyzed. For this purpose, the positions of sources representing the nose and tail need to be considered. In this study, a model experiment and theoretical analysis on the pressure variation when a short train passes in an open air environment were performed. The experimental results show that the negative peak amplitude is greater than the positive peak amplitude for observation points located over 1/10 of the train length away from the train, because of superposition. The distance between the two zero-pressure points is different from the train length. The effects of nose and tail configurations are considered in the proposed line source model and point source model based on the typical potential theory; the prediction results of the models showed good agreement in the waveforms and peak values of the pressure variation compared with those in model experiments. Therefore, we herein demonstrate that the line source model is valid, and the point source model is applicable to short trains when considering the positions of the point sources.

Random field model of sequential ground motions

This paper proposes a random field model of sequential ground motions, which considers the spatial correlation between the mainshock and aftershock from a stochastic view. First, the correlation between the mainshock and aftershock under the physical “source–path–local site” mechanism is explained. Based on the mechanism, the point-source model, and uniform-isotropic-medium model, a random single-point model for sequential ground motion with eight basic parameters is presented. Second, the random field model is developed from the random single-point model. More than 1000 pairs of sequential ground motions are used to identify and analyze statistically the basic parameters in the model. Moreover, the probability distribution of each parameter is presented based on copula functions. The results show that the spatial correlations of sequential ground motions can be effectively simulated based on the proposed random field model. Furthermore, it is possible to realistically reproduce the attenuation and time lag of ground motions at a local site.

Modelling the thermal radiation from kitchen hob fires

Kitchen hob fires present a potential threat to occupants escaping from dwellings and calculations may be needed to assess the hazard. Determination of the thermal heat flux from flames to a target can be achieved through the use of hand calculation methods or computational tools. This article compares point source, parallel plane and cylindrical view factor hand calculations and computational simulations using B-RISK and Fire Dynamics Simulator of thermal heat flux with kitchen hob fire experiments presented in the literature. Knowing the level of accuracy of each method provides useful information to designers. Although the point source model is influenced by whether the radial distance is measured perpendicular to the heat flux target or is offset relative to the centre of the flame, the article concludes that it provides an adequate approach for the calculation of thermal heat flux in the case of kitchen hob fires.

Incorporation of envelope delays and amplifications into simulation of far-field long-period ground motions

Far-field long-period ground motions (long-period ground motions) are likely to excite significant responses from high-rise buildings. A method for performing stochastic long-period simulations that incorporate envelope delays and basin-induced amplifications is developed, improving upon the conventional point-source model, which generates ground motions without considering nonstationary properties or basin effects. A model for envelope delays is proposed to realize nonstationary properties and empirical amplifications obtained from attenuation relationships account for basin effects. Then, numerical simulations are performed to validate the feasibility of the proposed method by comparing the simulations with records. Furthermore, the parameters related to envelope delays are obtained as the result of fitting by proposed model to the records, and these parameters are treated as random variables represented by probabilistic models. The formulation of basin-induced amplification factors is established on the basis of an impedance ratio relating the site properties at shallow depths to those extending to the top of bedrock. Therefore, a parametric method for simulating long-period ground motions is developed to promote its applications in engineering practice.

A probabilistic approach to phase calibration – I. Effects of source structure on fringe-fitting

ABSTRACT We propose a probabilistic framework for performing simultaneous estimation of source structure and fringe-fitting parameters in very long baseline interferometry (VLBI) observations. As a first step, we demonstrate this technique through the analysis of synthetic short-duration Event Horizon Telescope observations of various geometric source models at 230 GHz, in the presence of baseline-dependent thermal noise. We perform Bayesian parameter estimation and model selection between the different source models to obtain reliable uncertainty estimates and correlations between various source and fringe-fitting related model parameters. We also compare the Bayesian posteriors with those obtained using widely used VLBI data reduction packages such as casa and aips, by fringe-fitting 200 Monte Carlo simulations of each source model with different noise realizations, to obtain distributions of the maximum a posteriori estimates. We find that, in the presence of resolved asymmetric source structure and a given array geometry, the traditional practice of fringe-fitting with a point source model yields appreciable offsets in the estimated phase residuals, potentially biasing or limiting the dynamic range of the starting model used for self-calibration. Simultaneously estimating the source structure earlier in the calibration process with formal uncertainties improves the precision and accuracy of fringe-fitting and establishes the potential of the available data, especially when there is little prior information. We also note the potential applications of this method to astrometry and geodesy for specific science cases and the planned improvements to the computational performance and analyses of more complex source distributions.

Simulation of strong ground motions modified by attenuation effects under specific soil conditions: modeling the 2017 Puebla, Mexico (Mw 7.1) earthquake

This research presents an updated scheme of simulation for strong ground motions based on stochastic summation methods and the source-to-site attenuation effects under particular soil conditions different from firm sites. Two approaches are proposed to modify and improve the empirical Green’s functions at different source distances from seismic station records: (1) a spectral shape based on far-field point-source spectra models and (2) through specific ground motion prediction equations (GMPEs) and random vibration theory (RVT). The ground motions simulated were compared with the September 19, 2017 Mexico records (normal-fault-intraslab event, Mw 7.1), finding that for soft soils, it will be possible to reproduce some characteristic of spectral peaks appropriately in high frequency compared with available similar approaches or using one-dimensional wave propagation models. On average, the spectral overestimations from available simulation approaches and for high-frequency bandwidth (i.e., 0.2 s and 0.3 s) can be reduced by a factor between 2 and 5 using proper attenuation corrections; this effect could be especially beneficial for small structures and low-rise buildings where the inertial design forces will be potentially reduced.

A Procedure for 3D Seismic Simulation from Rupture to Structures by Coupling SEM and FEM

ABSTRACTThis article aims at numerically simulating the 3D seismic wave propagation from rupture to structures. A two-step method coupling the spectral element method (SEM) and the finite-element method (FEM) is proposed based on the domain reduction method to simultaneously simulate the seismic wave propagation in large-scale regions and analyze the dynamic behavior of structures in local sites. First, the concept of the proposed two-step method is illustrated. In the first step, the seismic wave propagation of the entire area, involving the source, propagation media, and local region of interest, is simulated using the SEM. In the second step, the dynamic analysis of structure-foundation system with local geological and topographical conditions is implemented using the FEM in a fine mesh based on the results in the first step. Subsequently, the FEM grid size is evaluated to match the SEM results, and the proposed SEM–FEM procedure is verified using both point-source and finite-fault model in a layered flat model. Finally, two analysis examples are presented using the proposed procedure. The analysis results show that the proposed SEM–FEM procedure can well consider the effects of local geological and topographical conditions on synthesized ground motions and can be applied to the rupture-to-structure simulations in earthquake engineering.

Code-to-code verification for thermal models of melting and solidification in a metal alloy: comparisons between a Finite Volume Method and a Finite Element Method

Abstract. Verification, the process of checking a modelling output against a known reference model, is an important step in model development for the simulation of manufacturing processes. This manuscript provides details of a code-to-code verification between two thermal models used for simulating the melting and solidification processes in a 316 L stainless steel alloy: one model was developed using a non-commercial code and the Finite Volume Method (FVM) and the other used a commercial Finite Element Method (FEM) code available within COMSOL Multiphysics®. The application involved the transient case of heat-transfer from a point heat source into one end of a cylindrical sample geometry, thus melting and then re-solidifying the sample in a way similar to an autogenous welding process in metal fabrication. Temperature dependent material properties and progressive latent heat evolution through the freezing range of the alloy were included in the model. Both models were tested for mesh independency, permitting meaningful comparisons between thermal histories, temperature profiles and maximum temperature along the length of the cylindrical rod and melt pool depth. Acceptable agreement between the results obtained by the non-commercial and commercial models was achieved. This confidence building step will allow for further development of point-source heat models, which has a wide variety of applications in manufacturing processes.

Field sources near the southern-sky calibrator PKS B1934-638: effect on spectral line observations with SKA-MID and its precursors

ABSTRACT Accurate instrumental bandpass corrections are essential for the reliable interpretation of spectral lines from targeted and survey-mode observations with radio interferometers. Bandpass correction is typically performed by comparing measurements of a strong calibrator source to an assumed model, typically an isolated point source. The wide field-of-view and high sensitivity of modern interferometers means that additional sources are often detected in observations of calibrators. This can introduce errors into bandpass corrections and subsequently the target data if not properly accounted for. Focusing on the standard calibrator PKS B1934-638, we perform simulations to asses this effect by constructing a wide-field sky model. The cases of ASKAP (0.7–1.9 GHz), MeerKAT (UHF: 0.58–1.05 GHz; L-band: 0.87–1.67 GHz) and Band 2 (0.95–1.76 GHz) of SKA-MID are examined. The use of a central point source model during bandpass calibration is found to impart amplitude errors into spectra measured by the precursor instruments at the ∼0.2–0.5 per cent level dropping to ∼0.01 per cent in the case of SKA-MID. This manifests itself as ripples in the source spectrum, the behaviour of which is coupled to the distribution of the array baselines, the solution interval, the primary beam size, the hour-angle of the calibration scan, as well as the weights used when imaging the target. Calibration pipelines should routinely employ complete field models for standard calibrators to remove this potentially destructive contaminant from the data, a recommendation we validate by comparing our simulation results to a MeerKAT scan of PKS B1934-638, calibrated with and without our expanded sky model.

The Impact of Realistic Foreground and Instrument Models on 21 cm Epoch of Reionization Experiments

Abstract Predictions for the ability of 21 cm interferometric experiments to discriminate Epoch of Reionization (EoR) signal models are typically limited by the simplicity of data models, whereby foreground signals and characteristics of the instrument are often simplified or neglected. To move toward more realistic scenarios, we explore the effects of applying more realistic foreground and instrument models to the 21 cm signal, as well as the ability to estimate astrophysical parameters with these additional complexities. We use a highly optimized version of 21cmFAST, integrated into 21cmMC, to generate lightcones of the brightness temperature fluctuation for Bayesian parameter estimation. We include a statistical point-source foreground model and an instrument model based on the Murchison Widefield Array scaled in observation time to have an effective sensitivity similar to the future Square Kilometre Array. We also extend the current likelihood prescription to account for the presence of beam convolution and foregrounds, the two-dimensional power spectrum (PS), and the correlation of PS modes. We use frequency bands between 150 and 180 MHz to constrain the ionizing efficiency (ζ), the minimum virial temperature of halos ( ), the soft X-ray emissivity per unit star formation rate (SFR) (L X /SFR), and the X-ray energy threshold (E 0). We find that the inclusion of realistic foregrounds and instrumental components biases the parameter constraints, due to unaccounted for cross-power between the EoR signal, foregrounds, and thermal noise. This causes estimates of ζ to be biased by up to 5σ, but the estimates of T vir, L X /SFR and E 0 remain unaffected and are all within 1σ.

Configuration predictions of large liquefied petroleum gas (LPG) pool fires using CFD method

Liquefied petroleum gas (LPG) has potential pool fire risks due to its flammability. The configuration of pool fires plays a significant role when applying the solid flame model or point source model to assess the risks from heat radiation. However, no existing correlations can precisely predict the configuration of large LPG (100% propane) pool fires. To enhance the fundamental understanding on how pool diameter and wind velocity can influence the configuration of large LPG pool fires, an experimentally validated Computational Fluid Dynamics (CFD) model is employed to simulate fires using different burning rate models. Fire temperature profiles, flame heights, and flame tilts predicted by the CFD model were compared with empirical models and experimental data. Accordingly, new correlations for flame height and flame tilt as functions of pool diameter D and wind velocity uw have been developed. The comparisons demonstrate that the new correlations have the best overall accuracy in the prediction of flame height and tilt for large LPG pool fires under different conditions (10 m ≤ D ≤ 20 m, 0 ≤ uw ≤ 3 m·s−1).

Detection of time-varying heat sources using an analytic forward model

We present a simple, analytic point source model for both static and time-varying point-like heat sources and the resulting temperature profile that solves the heat equation in dimension three. Simple algorithms to detect the location and spectral content of these sources are developed and numerically tested using Finite Element Mesh simulations. The resulting framework for heat source reconstruction problems, which are ill-posed inverse problems, seems promising and warrants for future research. Possible fields of application for our work are material testing, to detect manufacturing defects, and medical imaging to detect abnormal health conditions.