Wide-band Process Research Articles

Dynamic responses for inhomogeneous hydrogen deflagrations in a 7 m3 box-type chamber with a vent: Effects of ignition position and equivalent volume concentration

The aim of this paper is to investigate the effects of ignition position and equivalent volume concentration C‾H2 on dynamic responses for vented inhomogeneous hydrogen deflagrations in a 7 m3 chamber. The results indicate that the narrow-band and wide-band processes of overpressure signals are formed during Helmholtz-type and thermo-acoustic oscillations, respectively. The displacement response depending on C‾H2 is characterized by the weak vibration or a sharp increase. The acceleration impulsive response is dominated by the forced vibration component with the high-frequency and high-amplitude, while the acceleration dynamic response is dominated by the free vibration component with the low frequency and low amplitude. The high-frequency component and frequency overlapping in structural responses become more significant with the increase of C‾H2. Structural responses generated by bottom ignition are the largest due to the most violent thermo-acoustic oscillation induced by a coupled flame structure of deflagration flame and jet fire.

Reliability of quasi integrable and non-resonant Hamiltonian systems under fractional Gaussian noise excitation

The reliability of quasi integrable and non-resonant Hamiltonian system under fractional Gaussian noise (fGn) excitation is studied. Noting rather flat fGn power spectral density (PSD) in most part of frequency band, the fGn is innovatively regarded as a wide-band process. Then, the stochastic averaging method for quasi integrable Hamiltonian systems under wide-band noise excitation is applied to reduce 2n-dimensional original system into n-dimensional averaged Ito stochastic differential equations (SDEs). Reliability function and mean first passage time are obtained by solving the associated backward Kolmogorov equation and Pontryagin equation. The validity of the proposed procedure is tested by applying it to an example and comparing the numerical results with those from Monte Carlo simulation.

A new tri-modal spectral model for evaluating fatigue damage under three or multi-random Gaussian loads

A tri-modal or multi-modal process is one of the main loads in marine and offshore engineering. These resulting fatigue problems should be focused. Some available tri-modal spectral methods for estimating fatigue damage are first reviewed, and then a new tri-modal spectral model is developed based on a generalization of the principle proposed by Han and Ma. In order to validate the accuracy of these methods, a parametric triangular tri-modal spectrum is constructed and time domain solution based on rainflow counting is considered as a reference. The results show that the new developed method provides quite good fatigue damage evaluation compared with RFC solution. Then, the developed tri-modal model is generalized to multi-modal processes and general wideband processes. The generalized principle is to divide a wideband spectrum into several narrowband segments. Different dividing principles are investigated, it is demonstrated that adopting equal-frequency-band principle can give good fatigue damage estimation.

Development of a novel fatigue damage model for Gaussian wide band stress responses using numerical approximation methods

A significant development has been made on a new fatigue damage model applicable to Gaussian wide band stress response spectra using numerical approximation methods such as data processing, time simulation, and regression analysis. So far, most of the alternative approximate models provide slightly underestimated or overestimated damage results compared with the rain-flow counting distribution. A more reliable approximate model that can minimize the damage differences between exact and approximate solutions is required for the practical design of ships and offshore structures. The present paper provides a detailed description of the development process of a new fatigue damage model. Based on the principle of the Gaussian wide band model, this study aims to develop the best approximate fatigue damage model. To obtain highly accurate damage distributions, this study deals with some prominent research findings, i.e., the moment of rain-flow range distribution MRR(n), the special bandwidth parameter μk, the empirical closed form model consisting of four probability density functions, and the correction factor QC. Sequential prerequisite data processes, such as creation of various stress spectra, extraction of stress time history, and the rain-flow counting stress process, are conducted so that these research findings provide much better results. Through comparison studies, the proposed model shows more reliable and accurate damage distributions, very close to those of the rain-flow counting solution. Several significant achievements and findings obtained from this study are suggested. Further work is needed to apply the new developed model to crack growth prediction under a random stress process in view of the engineering critical assessment of offshore structures. The present developed formulation and procedure also need to be extended to non-Gaussian wide band processes.

Variance of fatigue damage in stationary random loadings: comparison between time- and frequency-domain results

In this paper, the variance of fatigue damage under stationary random loadings is investigated by means of different case studies (linear oscillator system, random process with ideal unimodal or bimodal power spectral density). Monte Carlo method is used to evaluate the variance of fatigue damage in time-domain and frequency-domain approach. The variance from simulations is compared with analytical methods (e.g. Mark and Crandall’s method, Bendat’s method and Low’s method). Furthermore, for each case study, the paper investigates the relationship between the variance of fatigue damage and the bandwidth parameters. The purpose is to provide a simple closed-form solution for the case of ideal unimodal power spectral density. The closed-form solution has been compared with Monte Carlo simulation in time-domain. The results show good agreement not only for narrow-band but also for wide-band process.

Оptimal algorithm of radio brightness estimation in the spatial distributed radiometric sys-tems

Optimization of processing of radio thermal radiation noise processes in multichannel spatially-distributed radiometric systems is performed. The received algorithms applicable in the systems of aperture synthesis are received within the maximum likelihood method in the assumption that spectral brightness of radiation within the frequency band of the receiver is constant, but as the image of a radiating object it depends on spatial (angular) coordinates. Feature of the solved tasks consists in using spectral VF -transforms which do not require execution of the spatiotemporal band-limitedness (quasi monochromatic approximation) condition and make it possible to solve the problem of synthesis of algorithms for processing wideband and ultra wideband processes.

THE REPEATING FAST RADIO BURST FRB 121102: MULTI-WAVELENGTH OBSERVATIONS AND ADDITIONAL BURSTS

ABSTRACT We report on radio and X-ray observations of the only known repeating Fast Radio Burst (FRB) source, FRB 121102. We have detected six additional radio bursts from this source: five with the Green Bank Telescope at 2 GHz, and one at 1.4 GHz with the Arecibo Observatory, for a total of 17 bursts from this source. All have dispersion measures consistent with a single value (∼559 pc cm−3) that is three times the predicted maximum Galactic contribution. The 2 GHz bursts have highly variable spectra like those at 1.4 GHz, indicating that the frequency structure seen across the individual 1.4 and 2 GHz bandpasses is part of a wideband process. X-ray observations of the FRB 121102 field with the Swift and Chandra observatories show at least one possible counterpart; however, the probability of chance superposition is high. A radio imaging observation of the field with the Jansky Very Large Array at 1.6 GHz yields a 5σ upper limit of 0.3 mJy on any point-source continuum emission. This upper limit, combined with archival Wide-field Infrared Survey Explorer 22 μm and IPHAS Hα surveys, rules out the presence of an intervening Galactic H ii region. We update our estimate of the FRB detection rate in the PALFA survey to be FRBs sky−1 day−1 (95% confidence) for peak flux density at 1.4 GHz above 300 mJy. We find that the intrinsic widths of the 12 FRB 121102 bursts from Arecibo are, on average, significantly longer than the intrinsic widths of the 13 single-component FRBs detected with the Parkes telescope.

Stochastic stability of multi-nanobeam systems

The paper analyzes a stochastic stability problem of a multi-nanobeam system subjected to compressive axial loading. It is assumed that each pair of nanobeams is simply supported and continuously joined by a viscoelastic layer. Differential equations of nanobeams are given according to Eringen's nonlocal elasticity theory of Helmholtz and bi-Helmholtz type of kernel and Euler–Bernoulli beam theory. Each pair of axial forces consists of a constant part and a time-dependent stochastic function. By using the moment Lyapunov exponent method, regions of almost sure stability of a multi-nanobeam system are obtained in a function of different parameters of the viscoelastic medium, axial loadings and number of nanobeams. Using the regular perturbation method, an approximated analytical solution of the moment Lyapunov exponent is obtained for a single nanobeam subjected to the white noise process, where the results are successfully confirmed with numerical results using the Monte Carlo simulation method. Numerical determination of the moment Lyapunov exponents is further performed for a higher number of nanobeams and different models of wideband processes.

Comparison of Linear and Nonlinear Vibration Isolation System under Random Excitation

This article studies and compares the efficiency of linear and nonlinear vibration isolation systems. The estimation relies on the value of isolated mass standard deviation due to external oscillation with different spectral densities. The solution of nonlinear differential equation of isolated by nonlinear vibration isolator mass is based on harmonic linearization method. External excitation process is treated as white noise wideband process with different spectral densities. The main interest is attached to the definition of vibration criteria exceedance number for the isolated mass suspended on both types of vibration isolators. The article suggests the application of several practical formulas of exceedance number calculation.

A study on the fatigue damage model for Gaussian wideband process of two peaks by an artificial neural network

Calculations of the fatigue damage on marine structures with a wideband nature are difficult to be done in spectral approach point of view because the link between the spectrum of stress and the probability distribution is difficult to define. This paper addresses the methodology through which the functional relationship between the probability density function and the response spectrum of a bimodal wide-band process by using the artificial neural network technique. An artificial neural network scheme was used to identify the multivariate functional relationship between the two continuously varying functions. For this, the spectra were idealized as the superposition of two triangles with an arbitrary location, height and width and the probability density functions were represented by the linear combination of equally spaced Gaussian basis functions. To train the network under supervision, a variety of different wide-band spectra were assumed and the converged probability density function of the stress range was derived using the rainflow counting method and all these data sets were fed into the three layer perceptron model. It turned out that the network trained using the given data set could reproduce the probability density function of an arbitrary wide-band spectrum of two triangles with great success.

(Invited) Innovative Wide Band Gap Power Devices Achievements in ITRI

Wide band gap semiconductors, such as silicon carbide (SiC) and Gallium nitride (GaN), provide higher breakdown voltage, higher operation temperature, higher thermal conductivity and higher switching frequency than conventional silicon device, thus there are many research institutes and companies focus on wide band gap device development and explore the benefits of using those devices in power electronics application. In ITRI, there is a program to develop wide band gap device and we have established a wide band gap Power Electronics Consortium (WPEC) to link companies who is interested on this device and application and eventually form a supply chain in Taiwan. ITRI has achieved very good results on the wide band gap device design and process development, such as high uniformity and productivity of SiC SBD device with breakdown voltage > 1200V and current rating > 20A, two mask-level process of TMBS (trench MOS barrier schottky) device with breakdown voltage > 600V, SiC MOSFET with low interfacial defect density and breakdown voltage >1200V, and innovative normally-off GaN MIS-HEMT device with threshold voltage >2V.

광대역 정규 프로세스에 대한 주파수 영역 기반 피로해석법의 적용성에 관한 연구 II : 광대역 피로예측 모델

This is the final one of the two companion papers dealing with accuracy of accumulated fatigue damage estimation under wide band process. It is stated that four kinds of wide band models exist: typed of equivalent stress, combined PDF, correction factor, and damage combination. For the idealized ESDs from full scale measurement data on an 8100TEU container vessel, fatigue damages are compared for a narrow band prediction model based on Rayleigh PDF and five wide band fatigue prediction models of Dirlik, Wirsching-Light, Jiao-Moan, Benasciutti and DNV. DNV model consistently overestimates fatigue damages regardless of variation of ESDs. Predictions by Jiao-Moan model, which is understood as standard method for design of offshore platforms, are also in conservative side. Best accuracy is found from the results by Dirlik and Benasciutti models, but Benasciutti model is preferred since it can easily combined with narrow band fatigue damage based on Rayleigh PDF.

광대역 정규 프로세스에 대한 주파수 영역 기반 피로해석법의 적용성에 관한 연구 I : 레일리 PDF

Abstract This paper deals with accuracy of accumulated fatigue damage es timation using stochastic fatigue analysis method based on Rayl eighPDF. From full scale measurement data on an 8100TEU container v essel, zero-order spectral moments for wave- and vibration-indu cedenergy spectral densities are determined on the probability lev el of 99%. 80 simulation cases in total are prepared according to the variation of ratio of zero-order spectral moments and center fr equency of vibration ESD. By using inverse Fourier transformati on and rainflow cycle counting for the combined ESD of wave and vibrat ion, exact fatigue damages are derived. Fatigue damages in freq uencydomain based on Rayleigh PDF show large conservativeness compar ed to exact fatigue damages in times domain. The main cause of theexcessive conservativeness is analyzed by two aspects: ratio of zero crossing and peak frequencies and ratio of initial zero o rder spectralmoments and zero order spectral moments from rainflow stress ra nge distributions. Finally, a guideline of applicability of Ray leigh PDF is proposed for wide band processes.Keywords : Rayleigh probability density function(PDF, 레일리 PDF), Energy spectral density(ESD, 스펙트럼), Rainflow cycle counting (RFC, 레인플로우 집계), Invserse fourier transformation(역푸리에 변환), Wave process(파랑 프로세스), Vibration process(진동 프로세스), Combined process(조합 프로세스), Wide band process(광대역 프로세스), Narrow band process(협대역 프로세스)

On fatigue damage assessment in bimodal random processes

Rainflow fatigue damage in bimodal Gaussian processes is discussed. Some methods specifically developed for bimodal processes (e.g., single-moment, Jiao–Moan, Sakai–Okamura and Fu–Cebon method) are compared with the Tovo–Benasciutti method, a new approach valid for the broader class of wide-band processes. Further, an improvement of the Fu–Cebon method is also proposed. Numerically simulated bimodal random processes are used to compare the accuracies of all methods and to find the optimal range for their applicability. The single-moment, the modified Fu–Cebon and the Tovo–Benasciutti methods are seen as the most accurate over a wide combination of bimodal spectra.

STABILITY OF STOCHASTIC SYSTEMS WITH MEMORY

Many processes in physics, biology, ecology, mechanics etc. can be modeled by Volterra integro-differential equations (VIDEs) with "fading memory". Often the behaviour of corresponding systems is perturbed by random noises. One of the main problems for the theory of stochastic Volterra integro-differential equations (SVIDEs) is connected with their stability. The present paper is devoted to the numerical solution of the stability problem for linear SVIDEs. The method is based on the statistical simulation of input random wide-band stationary processes, which are assumed in the form of "colored" noises. For each realization the numerical solution of VIDEs is found. The conclusion about the stability of the considered system SVIDE with respect to statistical moments is made on the basis of Liapunov exponents, which are calculated for statistical moments of the solution.

Analysis and simulation of a cascaded delta delta–sigma modulator

This paper analysis and simulates a delta delta–sigma modulator, the cascade combination of a delta modulator and a delta–sigma modulator. Using prediction, the delta modulator reduces the dynamic range of a signal prior to quantisation. Using noise shaping, the delta–sigma modulator cancels the error of the delta modulator. Performance is evaluated with input signals from a random noise process, composed of a high dynamic range narrow-band process and a low dynamic range wide-band process. Integrator leakage in the delta modulator and DAC mismatch between the two modulators may complicate implementation. The cascaded modulator outperforms conventional modulators of similar order when there is a degree of correlation between consecutive samples of the input process.

Impulse response symmetry error fordesigning phase equalisers

A new approach to designing analogue phase equalisers based on the symmetry of the impulse response is proposed. Phase equalisation techniques based on group delay or phase delay are usually performed on a limited band, the time response of which may not be adequate for the transmission of pulsed signals. The proposed technique is inherently a wideband process, and therefore more adequate for systems that deal with pulsed signals.

Multiple Internal Resonance in Suspended Cables under Random In-Plane Loading

The random excitation of a suspended cable with simultaneous internal resonances is considered. The internal resonances can take place among the first in-plane and the first two out-of-plane modes. The external loading is represented by a wide-band random process. The response statistics are estimated using the Fokker-Planck-Kolmogorov (FPK) equation, together with Gaussian and non-Gaussian closures. Monte Carlo simulation is also used for numerical verification. The unimodal in-plane motion exists in regions away from the internal resonance condition. The mixed mode interaction is manifested within a limited range of internal detuning parameters, depending on the excitation power spectrum density and damping ratios. The Gaussian closure scheme failed to predict bounded solutions of mixed mode interaction. The non-Gaussian closure results are in good agreement with the Monte Carlo simulation. The on-off intermittency of the autoparametrically excited modes is observed in the Monte Carlo simulation over a small range of excitation levels. The influence of the cable parameters, such as damping ratios, sag-to-span ratio, internal detuning parameters, and excitation level on the autoparametric interaction, is studied. It is found that the internal detuning and excitation level are the two main parameters which affect the autoparametric interaction among the three modes. Due to the system's nonlinearity, the response of the three modes is strongly non-Gaussian and the coupled modes experience irregular modulation.

Stability of elastic and viscoelastic systems under a random perturbations of their parameters

The present paper is concerned with the investigation of the almost sure stability of elastic and viscoelastic systems, when their parameters assume a random wide-band stationary process. The parameters are parametric loads, characteristics of external damping and material viscosity. With the help of Liapunov's direct method, the sufficient condition of the almost sure asymptotic stability for distributed parameter systems with respect to perturbations of initial conditions of an arbitrary form is obtained. It is shown that, in some cases, this condition coincides with a similar condition derived from the assumption that the form of sure and required perturbations coincides with the first eigenfunction of system oscillations. However, an example is given for the stability of a viscoelastic rod, when the perturbations of initial conditions are more dangerous, if their form differs from the first eigenfunction.

Nonlinear deterministic and random response of shallow shells

Deterministic and random vibrations are considered for the case of shear deformable shallow shells composed of multiple perfectly bonded layers. The nontrivial generalization of the flat plate vibrations is expressed by the fact of “small amplitude” vibrations existing about the curved equilibrium position together with the snap-through and snap-buckling type large amplitude vibrations about the flat position. The geometrically nonlinear vibrations are treated by applying Berger’s approximation to the generalized von Karman-type plate equations considering hard hinged supports of the straight boundary segments of skew or even more generally shaped polygonal shells. Shear deformation is considered by means of Mindlin’s kinematic hypothesis and a distributed lateral force loading is applied. Application of a multi-mode expansion in the Galerkin procedure to the governing differential equation, where the eigenfunctions of the corresponding linear plate problem are used as space variables, renders a coupled set of ordinary time differential equations for the generalized coordinates with cubic and quadratic non-linearities. For reasons of convergence, a light viscous modal damping is added. The nonlinear steady-state response of shallow shells subjected to a time-harmonic lateral excitation is investigated and the phenomenon of primary resonance is studied by means of the “perturbation method of multiple scales”. The use of a nondimensional formulation and introduction of the eigen-time of the basic mode of the associated linearized problem provides a unifying result with respect to the planform of the shell. Within the scope of random vibrations, it is assumed that the effective forces can be modelled by uncorrelated, zero-mean wide-band noise processes. Considering the set of modal equations to be finite, the Fokker-Planck-Kolmogorov (FPK) equation for the transition probability density of the generalized coordinates and velocities is derived. Its stationary solution gives the probability of eventual snapping after a long time has elapsed. However, the probability of first occurrence follows from the (approximate) integration of the nonstationary FPK equation. The probability of first dynamic snap-through is derived for a single mode approximation with the influence of higher modes taken into account. Using the two-mode expansion, the probability distribution of the asymmetric snap-buckling is also evaluated.