Dynamic Response In Time Domain Research Articles

Time-domain analysis for dynamic responses of non-classically damped composite structures

A complex mode superposition method based on equivalent viscous damping model (CEV) is proposed for time-domain dynamic responses of non-classically damped composite structures. The damping matrix of hysteretic non-classically damped composite structures can be easily obtained and complex mode superposition method based on hysteretic damping model is directly realized in the physical space. However, hysteretic damping model is not causal, and is only applied in frequency-domain analysis. By the aid of constitutive model of Kelvin-Voigt model, viscous damping model which is equivalent to the hysteretic damping model is obtained based on single degree of freedom system. Combined with complex mode superposition method of hysteretic damping model and mathematical simplicity of equivalent viscous damping model, CEV can be realized. Structural vibration responses of shaking table test under random excitations are measured, which are compared with the corresponding calculation results using CEV and traditional complex mode superposition method based on viscous damping model (CR), respectively. The calculation results obtained by the two complex mode superposition methods are consistent with the test results. Compared with CR, the calculation results of CEV are unique, computationalefficiency of CEV is higher and the problem of choosing reasonable modes has been avoided.

A continuous-discrete approach for pre-design of flexible-base tall buildings with fluid viscous dampers

An equivalent coupled-two-beam (CTB) discrete model is presented to compute the dynamic response in time domain of flexible-base buildings with linear fluid viscous dampers (FVDs). The equivalent model consists of a flexural cantilever beam and a shear cantilever beam connected in parallel by a finite number of axially rigid members that allows the consideration of intermediate modes of lateral deformation. The effects of different lateral deformations on the pre-design stage of two high-rise buildings are showed considering four soil types (hard rock, dense soil, stiff soil and soft soil) and three different distribution methods for linear FVDs: uniform-distribution (UD), story-shear-proportional-distribution (SSPD) and story-shear-strain-energy-distribution (SSSED). For UD method, the damping coefficients of FVDs increase as the flexural rigidity of the building decreases; whereas for SSPD and SSSED methods, the damping coefficients of FVDs vary along the height depending on the type of lateral deformation. For the three distribution methods and the same lateral deformation, the damping coefficients of FVDs decrease as the soil flexibility increases, which leads to a significant decrease in controlling earthquake-induced vibrations and wind-induced vibrations in the two high-rise buildings on a soft soil.

Concurrent topology optimization with uniform microstructure for minimizing dynamic response in the time domain

This paper aims to develop an efficient concurrent topology optimization approach for minimizing the maximum dynamic response of two-scale hierarchical structures in the time domain. Compared with statics problems, the dynamic response problems usually involve many time steps, which may lead to intensive computational burdens in both dynamic response and sensitivity analyses. This study thus proposes an enhanced decoupled sensitivity analysis method for concurrent topology optimization of the time-domain dynamic response problems. The mode acceleration method is incorporated for efficient dynamic response analysis. The three-field density-based approach is employed for topology optimization of macrostructure and microstructure. A previously proposed aggregation functional is employed to approximate the maximum dynamic response of the structure. The method of moving asymptotes (MMA) is employed to update the design variables. Three numerical examples are presented to demonstrate the effectiveness of the proposed approach. Some discoveries regarding the concurrent topology optimization for dynamics problems are presented and discussed. Furthermore, the potential of the concurrent topology optimization formulation for designing lightweight structures under dynamic loads is also demonstrated.

An approach based on level set method for void identification of continuum structure with time-domain dynamic response

• An identification approach based on level set and time-domain response is developed. • The shape derivative of the objective function for time-domain response is derived. • The effect of the model parameters on the proposed approach is studied. • Numerical examples show that the approach can well identify voids in structures. An approach based on the level set method has been developed to identify the position and geometry of voids in continuum structure using time-domain dynamic response. The level set method is employed in the proposed approach to represent the boundary of the voids implicitly. The voids are identified by solving an optimization problem which minimizes an objective function about the displacement error. The boundary of the voids is evolved by updating the level set function . The shape derivative of the objective function for the time-domain dynamic response is derived and used to construct the velocity field . Then, the level set function is updated through the velocity field. The proposed approach has been applied to several numerical examples of void identification in continuum structure. The results indicate that the proposed approach based on the level set method can identify voids effectively and accurately with time-domain dynamic response. Moreover, the effects of measure points, excitation force , noise, void distribution, numerical error, element size and boundary conditions on the approach are studied. Meanwhile, the computational costs of some examples are provided.

A Two-Step Transformation Approach for ESS Model of Viscoelastic Material to Time Domain

ESS (Equivalent Standard solid) model is proofed to be accurate for describing the physical properties of viscoelastic material under different temperatures and excitation frequencies. However the ESS model faces difficulties for calculating time history responses. Therefore a two-step transformation approach for ESS model to time domain is proposed, which can obtain precise time history response of structural finite element models of with dampers made of viscoelastic materials. In the first step of the approach, ESS model is extended from frequency domain to complex frequency domain. In the second step, a high-precision numerical inverse Laplace transform method is chosen to transform the model from complex frequency domain to time domain. Experimental and numerical tests are conducted to verify the effectiveness of the two-step transformation approach under simple harmonic external excitation. The comparisons indicate that the approach is accurate under different temperatures and excitation frequencies. Finally, the time domain dynamic responses of the viscoelastic dampers are obtained under earthquake excitations. Comparing to the equivalent linearization method for ESS model, the proposed two-step transformation approach has absolute advantage to obtain accurate results under random excitation, such as earthquake excitations.

Transient Response and Charge Transport in Organic and Polymer Thin-Film Transistors

Organic thin-film transistors (TFTs) are very promising for several large-area applications particularly when mechanical flexibility is important. A detailed model that directly correlates the TFT time-domain transient response with charge transport and material properties is very desirable for circuit applications. Temperature-dependent measurements of the field effect transistor (FET) mobility have provided numerous insights on the characteristics of the charge carriers and nature of transport phenomena. However, in disordered semiconductors such as organics and polymers, the FET mobility μFET is very different from the trap-free mobility μ as most of the carriers remain trapped in the band-tail states. In such systems, determining the drift mobility becomes very important. To study this drift mobility, we analyze the time-domain response of the transistor. The time-domain response of transistors can be obtained through channel formation time measurements. In such measurements, carriers are injected at the source of the transistor using a voltage pulse and extracted at the drain. A large-signal time domain analysis of these electronic time-of-flight measurements was first done by J.R. Burns [1] nearly 50 years ago for metal-oxide-semiconductor (MOS) transistors and was later extended to organic thin-film transistors by D. Basu et al. [2]. However, the mathematical models used in both works assumed that mobility is insensitive to the variance in carrier density and temperature. Therefore, these models cannot accurately predict complex time-domain response in organic and polymer FETs, hence there exists the need for more comprehensive models that account for mobility dependence on carrier concentration and temperature. Indeed, the main mechanisms of charge transport in organic and polymer semiconductors are commonly accepted to be variable-range hopping (VRH) and multiple-trap and release (MTR). In MTR, trapped carriers are thermally excited into the band, where charge transport takes place. Both charge transport models predict a mobility that depends on the trap density of states of the material, carrier density, and lattice temperature and, thus a complete model must account for these parameters. This paper presents an extension to the work of Burns and Basu et al. that considers the effect of the multiple trap and release charge transport model on mobility. With the underlying assumption that the trap density of states in most high-mobility organic and polymer transistors can be modeled by a single exponential function, we can write a closed form non-linear partial differential equation that describes the time and spatial evolution of current and voltage in an organic or polymer FET. In this way, we account for relevant material and device operation parameters such as trap density of states, carrier density, and temperature and therefore present a much more complete model of the time-domain response of organic and polymer thin-film transistors while retaining ease of use. Given sufficient knowledge of material parameters, we are able to accurately model electronic time-of-flight measurements in organic and polymer thin film transistors across different device operation regimes and temperatures. This model can also be used to extract material parameters from channel formation measurements. We apply this more complete time-domain dynamic response model to experimental data from high-mobility donor-acceptor polymer based thin-film transistors measured in our group. [1] J. R. Burns, RCA Review, 30, 15, 1969. [2] Basu, Debarshi & Dodabalapur, Ananth. (1970). Drift Velocity and Drift Mobility Measurement in Organic Semiconductors Using Pulse Voltage. 10.1007/12_2009_4. Figure 1

Multi-scale Dynamic System Reliability Analysis of Actively-controlled Structures under Random Stationary Ground Motions

This paper proposes a multi-scale dynamic system reliability analysis approach to assess the failure probability of an activelycontrolled structure subject to random stationary ground motions. The proposed approach employs a multi-scale hierarchical framework where a lower-scale system reliability analyses compute the failure probabilities of the structural members during the earthquakes and a higher-scale system reliability analysis computes the failure probability of the structural system based on results of the lower-scale analyses. The multi-scale framework facilitates the system reliability analysis of the large-sized complex system by decomposing the complex system into the manageable-sized subsystems. It also enables us to deal with statistical dependence both in spatial and temporal senses through the decomposition of the reliability analysis in space and time aspects. In these regards, the proposed approach performs a reliability analysis with the dynamic responses in time domain. This dynamic reliability analysis approach can consider uncertainties in system parameters and earthquake excitations simultaneously. In addition, the peak response over a time duration is used to describe the limit state of the structural system, which provides a more realistic measure of the failure probability of a structural system than instantaneous probability. In order to demonstrate the proposed approach, a 3-story shear-type building equipped with an optimal active control device is considered. The control performance under uncertainties is investigated through the reliability assessment by the proposed approach and the Monte Carlo Simulation (MCS) approach, respectively. The numerical study also investigates the influence of the uncertainties in the system parameters and the earthquake excitations on the system failure probability. The results of the numerical examples demonstrate that the proposed approach can efficiently estimate the system reliability and the failure probability of an actively-controlled structure.

Analysis of PSVR Transient Voltage Stability Based on RTDS

Power system voltage regulator (PSVR) is a kind of high-voltage side bus voltage control method, which maintains the high-voltage side bus voltage of the power plant at a high level, increases the reactive power limit of the generator, and improves the voltage stability of the power system. In this paper, a PSVR model with gain reduction is introduced. With PCS- 9410 excitation regulator, the semi-physical simulation system is constituted on RTDS. By setting a three-phase ground short-circuit on the high-voltage side transmission line, the time domain dynamic response of automatic voltage regulator (AVR) and PSVR is compared and analyzed in synchronous generator and synchronous condenser mode respectively, simulation results show that PSVR with gain reduction can increase the reactive power response speed of the excitation system in two modes.

Damage detection of sandwich panels with truss core based on time domain dynamic responses

A damage identification method, based on structural time domain dynamic responses and Teager energy operator, is presented for sandwich panels with truss core in the paper. The dimensionless structural dynamic responses, i.e., dimensionless velocity and displacement, are combined to construct damage index. Application of the method on sandwich panels in the cases of single damage and multiple damages with different extents are conducted. Effects of some factors on the method are discussed, including excitation location, excitation frequency, boundary condition, number of points N in Poincare maps. Numerical and experimental results show that the proposed method is effective in detecting both single damage and multiple damages with different extents. Excitation location plays a very important role in affecting the effectiveness of the method. Excitation frequency has little effect on the method, and there is a great selection space of excitation frequencies. Increasing the boundary condition constraint is beneficial for damage identification.

Power system stabiliser PSS4B‐W parameters optimisation and RTDS test verification

This study introduces the mathematical model of a new type of power system stabiliser PSS4B-W. The concept of crossbreed in genetic algorithm is introduced to particle swarm optimisation algorithm and applied to PSS4B-W parameter optimisation. With a PCS-9410 excitation regulator, the semi-physical simulation system is constituted on a real time digital simulator (RTDS) platform. By measuring the excitation system phase frequency characteristic without compensation, the PSS4B-W parameter optimisation is converted to a set of parameters optimisation process with inequality constraint. In the RTDS platform, the time-domain dynamic response of without PSS, adding PSS2B and PSS4B-W, is compared and analysed. Simulation results show that the optimised PSS4B-W can provide effective damping in different frequency bands, and have good inverse-regulation suppression effect.

Tuning Approach for Power System Stabilizer PSS4B using Hybrid PSO

As a multi-band PSS, PSS4B draws wide attention for its great potential in constraining the ultra-low frequency oscillations. In this paper, the crossbreed operation in genetic algorithm is introduced to particle swarm optimization (PSO) algorithm to form a hybrid PSO, so as to optimize PSS4B parameters. According to the single machine infinite system model, the phase frequency characteristic of excitation system without compensation is calculated by Heffron-Philips model. Based on phase frequency characteristic, the PSS4B parameters are tuned and excellent phase compensation effect is obtained. In Matlab/Simulink platform, the time domain dynamic response without PSS, adding PSS2B and PSS4B is compared and analyzed in detail. Simulation results show that the optimized PSS4B can provide effective damping in different frequency bands and have strong applicability.

Investigation about journal bearing wear effect on rotating system dynamic response in time domain

Analysis of vibration signal plays a fundamental role in fault diagnosis and monitoring of rotating machines. Diagnosis, identification and evaluation of bearing faults are critical aspects for a healthy operating system. Among bearing failures, wear is one of the most common, directly affecting the bearing clearance, resulting in changes of the rotating system characteristics. Therefore, time response of a rotor-bearing system, under different worn bearing condition, is analyzed in order to define a general vibration behavior that characterizes the presence of bearing wear. Spectrum response is used to compare healthy and worn bearings. Experimental tests were performed to validate the numerical findings. Results show a standard tendency in dynamic behavior related to the backward component of the shaft whirling movement.

Coupled Dynamic Characteristics of Wind Turbine Gearbox Driven by Ring Gear Considering Gravity

Gravity is usually neglected in the dynamic modeling and analysis of the transmission system, especially in some relatively lightweight equipment. The weight of wind turbine gearbox is up to tens of tons or even hundreds of tons, and the effects of gravity have not been explored and quantified. In order to obtain accurate vibration response predictions to understand the coupled dynamic characteristics of the wind turbine gear transmission system, a comprehensive, fully coupled, dynamic model is established by the node finite element method with gravity considered. Both time-domain and frequency-domain dynamic responses are calculated using the precise integration method with various excitations being taken into account. The results indicate that gravity has a significant impact on the vibration equilibrium position of central floating components, but the changing trends are different. Gravity does not change the composition of the excitation frequency, but will have a certain impact on the distribution ratio of the frequency components. And the high frequency vibrations are hardly affected by gravity. In addition, the load sharing coefficient is greater when gravity is taken into account, both of internal gearing and of external gearing system. When the planet gears have a certain position error in accordance with certain rules, the load sharing performance of the system will be better.

Dynamic analysis of a thick hollow cylinder made of two-dimensional functionally graded material using time-domain spectral element method

ABSTRACTIn this paper, the time-domain dynamic response of a thick hollow cylinder made of two-dimensional functionally graded material is studied. An axisymmetric solid spectral finite element model is proposed for the analysis of elastic wave propagation in the cylinder. The effectiveness of the proposed model is verified by comparing with the conventional finite element method. The effect of material grading pattern is analyzed. The results demonstrate the effectiveness of the present method for the analysis of elastic wave propagation in functionally graded solids with axial symmetry and the material composition variation has an important effect on structural wave propagation behavior.

Dynamic Response of the Lumbar Spine to Whole-body Vibration Under a Compressive Follower Preload.

A finite element study of dynamic response of the lumbar spine to whole-body vibration. The aim of this study was to develop and validate a finite element model for exploring the impact of whole-body vibration on the entire lumbar spine with a compressive follower preload applied. Several finite element studies have investigated the biodynamic characteristics of the human lumbar spine when exposed to whole-body vibration. However, very limited studies have been performed to quantitatively describe dynamic response in time domain of the entire lumbar spine to vibration loading under a compressive follower preload. A three-dimensional nonlinear finite element model of the human lumbar spine (L1-sacrum) subjected to the compressive follower preload was created. Transient dynamic analysis was conducted on the model to compute the spinal response to a sinusoidal vertical vibration load of ±40 N under a 400 N preload. The obtained dynamic response results at all spinal levels were collected and plotted as a function of time. As a comparison, the corresponding results for vertical static loads (-40 and 40 N) under the preload (400 N) were also computed. Plots of the dynamic response at all levels showed a cyclic response with time, and their vibration amplitudes (peak-to-bottom variations) were markedly higher than the corresponding changing amplitudes of static load cases. The increasing effect of the vibration load reached 314.5%, 263.2%, 242.4%, and 232.7%, respectively, in axial displacement of vertebral center, disc bulge, intradiscal pressure, and annulus stress (von-Mises stress). In addition, increasing the compressive follower preload led to an increase in the dynamic response and a decrease in their vibration amplitudes. This study may be useful to help quantify the effect of cyclic loading on the entire lumbar spine under physiologic compressive loading, and better understand vibration characteristics of the spine. 5.

Analysis on multi-frequency vortex-induced vibration and mode competition of flexible deep-ocean riser in sheared fluid fields

Multi-frequency vortex-induced vibration of flexible riser in lineally sheared fluid fields with different shearing parameters is explored by using the numerical approach. By combining the finite element method with a hydrodynamic model, the approach can consider mode competition based on modal energy and can carry out nonlinearly simultaneously dynamic response in time domain. Our analysis shows that multi-frequency VIV may occur both in non-uniform and uniform fluid fields. And, the behaviors of multi-frequency VIV are different from single-frequency VIV. Because several modes are involved and compete with each other, and consequently the determination of modal excitation region become more complicated. As the towing speed (or the shearing parameter) increases in sheared flow, the average RMS displacement does not regularly rise (or drop), but slightly fluctuates owing to changes of the participating mode and its excitation region. On the other hand, the average RMS stress gradually rises owing to higher-order modes being included. Moreover, it is found that the dominant frequency distributing along structural span significantly changes with the towing speed, and the length of the first dominant frequency gets smaller due to larger shearing parameter along with more intense competition between the participating modes.

Influence of different frequencies of axial cyclic loading on time-domain vibration response of the lumbar spine: A finite element study

Very few studies have quantitatively analyzed influence of the loading frequency on time-domain vibration response of the whole lumbar spine in the presence of a physiologic compressive preload. In this study, a three-dimensional non-linear finite element model of ligamentous L1–S1 segment was developed to predict time-domain dynamic response of the whole lumbar spine to axial cyclic loading with different frequencies. A compressive follower preload of 400 N was applied to the model to simulate the physiologic compressive load. Modal analysis was initially performed to extract axial resonant frequency of the model under a 40 kg upper body mass and the 400 N preload. The result showed that the axial resonant frequency was 7.77 Hz. Subsequently, transient dynamic analyses were performed on the model under a sinusoidal axial load of ±40 N at frequencies of 3, 5, 7, 9, 11, 13 and 15 Hz with the 400 N preload and 40 kg mass. The computational results (strains and stresses in the spinal components) were collected and plotted as a function of time. These predicted results were found to be frequency-dependent and consistent with the notion in engineering dynamics texts that the closer the loading frequency approaches the resonant frequency, the larger the response is. For example, the results for 5 Hz load compared to 3 Hz load showed a 68.6–111.5% increase in peak-to-bottom variations of the predicted response parameters, and the results for 13 Hz load compared to 11 Hz load showed a 26.4–37.8% decrease in these variations.

Efficient frequency controllers for autonomous two‐area hybrid microgrid system using social‐spider optimiser

In this study, an efficient load frequency controller (LFC) for standalone two-area hybrid microgrid system (HμGS) is addressed. Social-spider optimiser (SSO) is applied to fine tune the proposed proportional–integral–derivative (PID) controllers by generating their optimal settings. The integral time multiplied summation of absolute deviations and the gains of PID controllers define the fitness function and control variables, respectively. The performance of the proposed SSO-based method is demonstrated on an isolated HμGS complete with variety of energy storage systems under number of scenarios. The scenarios include load fluctuations, variations in wind speed, and sun irradiance employing real site measurements. In this study, photovoltaic generating arrays and wind turbine generators are not participating in system frequency regulations due the proposal of their maximum power tracking operation strategy. The signatures of time-domain dynamic responses and cropped numerical results ascertain that the proposed SSO-based LFC scheme is promising in diminuting the signal deviations and in short time. Further validations for cropped results produced by SSO are made compared with powerful optimisation tool such as genetic algorithm which signify the tuned PID gains.

Improved complex mode theory and truncation and acceleration of complex mode superposition

The traditional complex mode theory is substantially improved. By means of the conjugacy of the complex characteristic value and vector, the formulae to compute the accuracy solution to the time-domain dynamic response for a symmetrical system with generalized linear viscous damping are established. Based on the formulation constructed, the truncation of the complex mode superposition is investigated. By employing the complex mode expansion of the system flexibility matrix discovered in this work, an accelerating method for the complex mode superposition after truncation is proposed. According to such improved complex mode theory, the system dynamical parameters, for example, the natural frequency and the complex vibration shape, are all the functions with respect to the mass, stiffness, and damping. However, the influence of the damping on the natural frequency may always be relatively slight. The numerical simulating investigation shows that the precise dynamic response can be obtained simply and conveniently in line with the formulae established. Meanwhile, the truncating and accelerating methods are effective for both the narrow or wide band external excitation and can provide the approximate solution to the dynamic response with better precision even when the frequency of the excitation is higher. Under certain conditions, the accelerating approach can be used to boost the accuracy of the truncated response.

Influence of the backlash generated by tooth accumulated wear on dynamic behavior of compound planetary gear set

Backlash is a key internal excitation on the dynamic response of planetary gear transmission. After the gear transmission running for a long time under load torque, due to tooth wear accumulation, the backlash between the tooth surface of two mating gears increases, which results in a larger and irregular backlash. However, the increasing backlash generated by tooth accumulated wear is generally neglected in lots of dynamics analysis for epicyclic gear trains. In order to investigate the impact of backlash generated by tooth accumulated wear on dynamic behavior of compound planetary gear set, in this work, first a static tooth surface wear prediction model is incorporated with a dynamic iteration methodology to get the increasing backlash generated by tooth accumulated wear for one pair of mating teeth under the condition that contact ratio equals to one. Then in order to introduce the tooth accumulated wear into dynamic model of compound planetary gear set, the backlash excitation generated by tooth accumulated wear for each meshing pair in compound planetary gear set is given under the condition that contact ratio equals to one and does not equal to one. Last, in order to investigate the impact of the increasing backlash generated by tooth accumulated wear on dynamic response of compound planetary gear set, a nonlinear lumped-parameter dynamic model of compound planetary gear set is employed to describe the dynamic relationships of gear transmission under the internal excitations generated by worn profile, meshing stiffness, transmission error, and backlash. The results indicate that the introduction of the increasing backlash generated by tooth accumulated wear makes a significant influence on the bifurcation and chaotic characteristics, dynamic response in time domain, and load sharing behavior of compound planetary gear set.