Unit Impulse Response Research Articles

The Method of Random Vibration Signal Transformation from Frequency to Time Domain Based on FIR Digit Filter

Based on finite unit impulse response, FIR digit filter is designed according as the given power spectral density (PSD) characteristics beforehand. By controlling the parameters of filter, amplitude-phase characteristics that can meet completely the preconditions and demand is obtained. And then white noise is inputted to filter, random vibration time signal is generated. Moreover, the method of parameters analysis of AR autoregressive model is used to estimate and validate power spectrum of the gotten random vibration time domain signal. The result shows that the accuracy and error of PSD between AR model and the given function are within the range of the permitted value. And random vibration time signal obtained by using the redesigned FIR filter can meet absolutely the precision demand beforehand in the simulation and test.

Parallel Computation of 3-D Soil-Structure Interaction in Time Domain with a Coupled FEM/SBFEM Approach

This paper introduces a parallel algorithm for the scaled boundary finite element method (SBFEM). The application code is designed to run on clusters of computers, and it enables the analysis of large-scale soil-structure-interaction problems, where an unbounded domain has to fulfill the radiation condition for wave propagation to infinity. The main focus of the paper is on the mathematical description and numerical implementation of the SBFEM. In particular, we describe in detail the algorithm to compute the acceleration unit impulse response matrices used in the SBFEM as well as the solvers for the Riccati and Lyapunov equations. Finally, two test cases validate the new code, illustrating the numerical accuracy of the results and the parallel performances.

Green’s functions for a loaded rolling tyre

A new formulation to determine the unit impulse response (Green’s) functions of a loaded rotating tyre in the vehicle-fixed (Eulerian) reference frame for tyre/road noise predictions is presented. The proposed formulation makes use of the set of eigenfrequencies and eigenmodes for the statically loaded tyre obtained from a finite element (FE) model of the tyre. A closed-form expression for the Green’s functions of a rotating tyre in the Eulerian reference system as a function of the eigenfrequencies and eigenmodes of the statically loaded tyre is found. Non-linear effects during loading are accounted for in the FE model, while the frequency shift due to the rotational velocity is included in the calculation of the Green’s functions. In the literature on tyre/road noise these functions are generally used to determine the tyre response during tyre/road contact calculations. The presented formulation opens the possibility to solve the contact problem directly in the Eulerian reference frame and to include local tyre softening due to non-linear effects while keeping the computational advantage of describing the tyre dynamics as a set of impulse response functions. The advantage of obtaining the Green’s functions in the Eulerian reference system is that only the Green’s functions corresponding to the potential contact zone need to be determined, which significantly reduces the computational cost of solving the tyre/road contact and since the mesh is fixed in space, a finer mesh can be used for the potential contact zone, improving the accuracy of the contact force calculations. Although these effects might be less pronounced if a more accurate tyre model is used, it is found that using the Green’s functions of the loaded tyre in a contact force calculation leads to smaller forces than in the unloaded case, lower frequencies are present in the response and they decrease faster as the rotational velocity increases.

Optimization of Release Kinetics from Sustained-Release Formulations using Model-Independent Pharmacokinetic Simulation

In the present work, a single model-independent approach was developed to optimize the release kinetics of drugs from sustained-release formulations, using stavudine (d4T) as a model drug. This approach is based on the pharmacokinetic simulation of drug plasma levels through a semiparametric approach of the input function and on convolution with an empirical polyexponential unit impulse response function. Input functions were evaluated using different zero-order and first-order release constants. Optimum drug release to obtain a specific pharmacokinetic profile was approached using target model-independent pharmacokinetic parameters such as C(max)(SS), C(min)(SS), t(max)(SS), and peak-trough fluctuations. A Monte Carlo simulation was performed to estimate the fractional attainment of d4T plasma concentrations over therapeutic d4T levels. Zero-order (K(0) = 4 mg/h) and first-order (K(1) = 0.05 h(-1)) release constants were optimal for the formulation of sustained-release d4T tablets, plasma concentrations within the therapeutic range being achieved.

Substructural Response Reconstruction in Wavelet Domain

An approach for structural response reconstruction in a substructure using the unit impulse response function in the wavelet domain is presented. The forces at the interface degrees-of-freedom are taken as input excitations on the substructure. The response reconstruction is conducted by transforming the measured responses into responses at other selected locations. The response reconstruction using the proposed wavelet domain method is then compared with that from an existing frequency domain method. Numerical studies on a seven-story plane frame structure are carried out to investigate the accuracy and effectiveness of the proposed method, and the effects of influencing parameters such as the number of sampled data, sampling rate, sensor numbers, and measurement noise are studied.

In Vivo Bioequivalence and In Vitro Similarity Factor (f2) for Dissolution Profile Comparisons of Extended Release Formulations: How and When Do They Match?

To investigate how likely two extended release formulations are to be bioequivalent when they demonstrate f2 similarity. Dissolution profiles were simulated using the Weibull model and varying model parameters around those of a reference profile. The f2 values were calculated for the comparisons of each simulation with the reference profile. The in vivo inputs obtained from an in vitro-in vivo correlation model were convolved with a unit impulse response function. The AUC, Cmax, and Tmax from each simulated in vivo concentration profile were compared to the reference profile. The AUCR (AUC ratio) and CmaxR (Cmax ratio) were determined. The consistency between f2 and bioequivalence was investigated. The relationships between AUCR, CmaxR, f2 and the Weibull model parameters demonstrate that the bioequivalence regions enclosed by the contour lines of 80% and 125% of AUCR and CmaxR were generally close to the regions enclosed by the f2 = 50 contour line, but did not exactly match, especially when Dmax and B deviated from the reference values. When f2 is used for in vitro dissolution profile comparison, the completeness of the dissolution profiles should not differ more than 10%, and the shapes of the dissolution profiles should not be significantly different.

A semi-analytical artificial boundary for time-dependent elastic wave propagation in two-dimensional homogeneous half space

This paper presents a time-dependent semi-analytical artificial boundary for numerically simulating elastic wave propagation problems in a two-dimensional homogeneous half space. A polygonal boundary is considered in the half space to truncate the semi-infinite domain, with an appropriate boundary condition imposed. Using the concept of the scaled boundary finite element method, the wave equation of the truncated semi-infinite domain is represented by the partial differential equation of non-constant coefficients. The resulting partial differential equation has only one spatial coordinate variable and time variable. Through introducing a few auxiliary functions at the truncated boundary, the resulting partial differential equations are further transformed into linear time-dependent equations. This allows an artificial boundary to be derived from the time-dependent equations. The proposed artificial boundary is local in time, global at the truncated boundary and semi-analytical in the finite element sense. Compared with the scaled boundary finite element method, the main advantage in using the proposed artificial boundary is that the requirement for solving a matrix form of Lyapunov equation to obtain the unit-impulse response matrix is avoided, so that computer efforts are significantly reduced. The related numerical results from some typical examples have demonstrated that the proposed artificial boundary is of high accuracy in dealing with time-dependent elastic wave propagation in two-dimensional homogeneous semi-infinite domains.

A high performance scaled boundary finite element method

Dynamic soil-structure interaction in the time domain may be described by the scaled boundary finite element method (SBFEM). Unfortunately the non locality in time and space implies significant numerical effort. Two essential improvements are demonstrated, boosting the performance of this method. The original discretisation scheme assuming a constant change of the acceleration unit-impulse response matrix within each time step is only conditionally stable, requiring a rather small minimum step size. Here a new scheme assuming the acceleration unit-impulse response matrix to vary linearly within each time step and an extrapolation parameter provide more stability to the solution. For large problems the linearization of the acceleration unit-impulse response matrix for late times is employed. Then, based on integration by parts, a new and very efficient recursive integration scheme for the evaluation of the soil-structure interaction vector described by convolution integral is developed, regaining a strong locality in time. The combination of the two enhancements leads to a very significant reduction of computational effort and linear dependency with respect to the number of time steps, and allows also the use of larger time steps. Examples analyzed by authors did not show instability of the solution for a wide range of parameters.

An efficient numerical method for estimating reliabilities of linear structures under fully nonstationary earthquake

Abstract The Conte–Peng’s equations for a fully nonstationary model of an earthquake are reformulated as modulated filtered white-noise processes so that the reliabilities of a linear structure under the fully nonstationary earthquake model can be estimated through calculating the first passage time probabilities of several time-varying linear systems subjected to independent white-noise processes. Each time-varying linear system consists of a second-order single-degree-of-freedom (SDOF) linear oscillator, a deterministic time-modulating function applied at the stationary oscillator’s responses and the linear structure considered. The unit impulse response functions of these time-varying linear systems are derived, from which the useful spectral characteristics of system responses in estimating the crossing rates and limiting decay rates can be numerically calculated. To save computation time, efficient matrix procedures in the time domain are proposed. The reliability analysis of a moment-resisting steel frame subjected to El-Centro-like earthquake demonstrates the effectiveness, accuracy and application of the numerical method.

Changes in [18F]Fluorothymidine Pharmacokinetics Following Capecitabine Treatment in Human Breast Cancer Detected by Positron Emission Tomography.

Abstract Background: Pre-clinical models used by our group have demonstrated that thymidylate synthase (TS) inhibition leads to redistribution of the nucleoside transporter, ENT1, to the cell membrane and hence increases the tissue uptake of [18F]fluorothymidine (FLT).Methods: In this study we assessed, for the first time, the altered pharmacokinetics of FLT in patients following TS inhibition. We analyzed 10 lesions from 6 breast cancer patients by positron emission tomography (PET) before and after treatment with capecitabine.Results: Whereas drug treatment did not alter tumor delivery pharmacokinetic variables or blood flow, tumor FLT retention variables increased with drug treatment in all but one patient. The baseline average standardized uptake value (SUV) at 60 min, rate constant for the net irreversible transfer of radiotracer from plasma to tumor (Ki) and unit impulse response function (IRF) at 60 min were 11.11 x 10-5 m2/ml, 4.38 x 10-2 ml plasma/min/ml tissue and 4.93 x 10-2 /min, respectively. At 1 h after capecitabine, the SUV was 13.55 x 10-5 m2/ml (p=0.004), Ki 7.40 x 10-2 ml plasma/min/ml tissue (p=0.004) and IRF 7.40 x 10-2 /min (p= 0.002).Conclusion: FLT pharmacokinetics did not change in normal tissues suggesting that the effect was largely restricted to tumor (p=0.55). In summary, we have identified FLT-PET retention parameters that could be used in future early clinical studies to measure the pharmacodynamics of TS inhibitors, as well as for identifying patients who are unlikely to benefit from TS inhibition. Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr 5003.

Modeling Impacts Between a Continuous System and a Rigid Obstacle Using Coefficient of Restitution

In this work, we discuss the limitations of the existing collocation-based coefficient of restitution method for simulating impacts in continuous systems. We propose a new method for modeling the impact dynamics of continuous systems based on the unit impulse response. The developed method allows one to relate modal velocity initial conditions before and after impact without requiring the integration of the system equations of motion during impact. The proposed method has been used to model the impact of a pinned-pinned beam with a rigid obstacle. Numerical simulations are presented to illustrate the inability of the collocation-based coefficient of restitution method to predict an accurate and energy-consistent response. We also compare the results obtained by unit impulse-based coefficient of restitution method with a penalty approach.

Aeroservoelastic Analysis for Transonic Missile Based on Computational Fluid Dynamics

Aeroservoelastic Analysis for Transonic Missile Based on Computational Fluid Dynamics

Altered Tissue 3′-Deoxy-3′-[18F]Fluorothymidine Pharmacokinetics in Human Breast Cancer following Capecitabine Treatment Detected by Positron Emission Tomography

We showed in preclinical models that thymidylate synthase (TS) inhibition leads to redistribution of the nucleoside transporter, ENT1, to the cell membrane and hence increases tissue uptake of [(18)F]fluorothymidine (FLT). In this study, we assessed for the first time the altered pharmacokinetics of FLT in patients following administration of capecitabine, a drug whose mode of action has been reported to include TS inhibition. We analyzed 10 lesions from six patients with breast cancer by positron emission tomography before and after treatment with capecitabine. Although drug treatment did not alter tumor delivery pharmacokinetic variables (K1 and permeability product surface area) or blood flow, tumor FLT retention variables increased with drug treatment in all but one patient. The baseline average standardized uptake value at 60 minutes, rate constant for the net irreversible transfer of radiotracer from plasma to tumor (Ki), and unit impulse response function at 60 minutes were 11.11 x 10(-5) m(2)/mL, 4.38 x 10(-2) mL plasma/min/mL tissue, and 4.93 x 10(-2)/min, respectively. One hour after capecitabine administration, the standardized uptake value was 13.55 x 10(-5) m(2)/mL (P = 0.004), Ki 7.40 x 10(-2) mL plasma/min/mL tissue (P = 0.004), and impulse response function was 7.40 x 10(-2)/min (P = 0.002). FLT pharmacokinetics did not change in normal tissues, suggesting that the effect was largely restricted to tumors (P = 0.55). We have identified FLT positron emission tomography retention parameters that could be used in future early clinical studies to measure the pharmacodynamics of TS inhibitors, as well as for identifying patients who are unlikely to benefit from TS inhibition.

In vitro–in vivo correlation for nevirapine extended release tablets

An in vitro-in vivo correlation (IVIVC) for four nevirapine extended release tablets with varying polymer contents was developed. The pharmacokinetics of extended release formulations were assessed in a parallel group study with healthy volunteers and compared with corresponding in vitro dissolution data obtained using a USP apparatus type 1. In vitro samples were analysed using HPLC with UV detection and in vivo samples were analysed using a HPLC-MS/MS assay; the IVIVC analyses comparing the two results were performed using WinNonlin. A Double Weibull model optimally fits the in vitro data. A unit impulse response (UIR) was assessed using the fastest ER formulation as a reference. The deconvolution of the in vivo concentration time data was performed using the UIR to estimate an in vivo drug release profile. A linear model with a time-scaling factor clarified the relationship between in vitro and in vivo data. The predictability of the final model was consistent based on internal validation. Average percent prediction errors for pharmacokinetic parameters were <10% and individual values for all formulations were <15%. Therefore, a Level A IVIVC was developed and validated for nevirapine extended release formulations providing robust predictions of in vivo profiles based on in vitro dissolution profiles.

Basilar Membrane Responses to Noise at a Basal Site of the Chinchilla Cochlea: Quasi-Linear Filtering

Basilar membrane responses to clicks and to white noise were recorded using laser velocimetry at basal sites of the chinchilla cochlea with characteristic frequencies near 10 kHz. Responses to noise grew at compressive rates and their instantaneous frequencies decreased with increasing stimulus level. First-order Wiener kernels were computed by cross-correlation of the noise stimuli and the responses. For linear systems, first-order Wiener kernels are identical to unit impulse responses. In the case of basilar membrane responses, first-order Wiener kernels and responses to clicks measured at the same sites were similar but not identical. Both consisted of transient oscillations with onset frequencies which increased rapidly, over about 0.5 ms, from 4-5 kHz to the characteristic frequency. Both first-order Wiener kernels and responses to clicks were more highly damped, exhibited slower frequency modulation, and grew at compressive rates with increasing stimulus levels. Responses to clicks had longer durations than the Wiener kernels. The statistical distribution of basilar membrane responses to Gaussian white noise is also Gaussian and the envelopes of the responses are Rayleigh distributed, as they should be for Gaussian noise passing through a linear band-pass filter. Accordingly, basilar membrane responses were accurately predicted by linear filters specified by the first-order Wiener kernels of responses to noise presented at the same level. Overall, the results indicate that cochlear nonlinearity is not instantaneous and resembles automatic gain control.

Dynamics of Practical Premixed Flames, Part I: Model Structure and Identification

For the analysis of thermoacoustic instabilities it is most important to determine the dynamic flame response to acoustic disturbances. Premixed flames are often modelled as single-input single-output system, where the “output” (the overall rate of heat release) responds to a single “input” variable (often the velocity at the exit of the burner nozzle). However, for practical premixed flames, where perturbations of pressure or velocity at the fuel injector will modulate the fuel equivalence ratio, the heat release rate will respond to fluctuations of equivalence ratio as well as nozzle mass flow rate. In this case, a multiple-input, single-output (MISO) model structure for the flame is appropriate. Such a model structure is developed in the present paper. Staged fuel injection as well as fuel line impedances can be taken into account, the integration with low-order or finite-element based models for stability analysis is straightforward. In order to determine unit impulse and frequency response functions for such a model structure, an identification scheme based on unsteady CFD calculation with broadband excitation followed by correlation analysis is proposed and validated successfully. Identification of MISO model coefficients is a challenging task, especially in the presence of noise. Therefore criteria are introduced which allow to ascertain a posteriori how well the identified model represents the true system dynamics. Using these criteria, it is investigated how excitation signal type, time series length and signal-to-noise ratio influence the results of the identification process. Consequences for passive design strategies based on multi-stage fuel injection and experimental work on practical premixed flame dynamics are discussed.

On the extended period of a frequency domain method to analyze transient responses

Transient response analysis can be conducted either in the time domain, or via the frequency domain. Sometimes a frequency domain method (FDM) has advantages over a time domain method. A practical issue in the FDM is to find out an appropriate extended period, which may be affected by several factors, such as the excitation duration, the system damping, the artificial damping, the period of interest, etc. In this report, the extended period of the FDM based on the Duhamel's integral is investigated. This Duhamel's integral based FDM does not involve the unit impulse response function (UIRF) beyond the period of interest. Due to this fact, the ever-lasting UIRF can be simply set as zero beyond the period of interest to shorten the extended period. As a result, the preferred extended period is the summation of the period of interest and the excitation duration. This conclusion is validated by numerical examples. If the extended period is too short, then the front portion of the period of interest is more prone to errors than the rear portion, but the free vibration segment is free of the wraparound error.

Structural damage detection with statistical analysis from support excitation

The unit impulse response (UIR) functions obtained from a structure under support excitation are used to identify local structural damages. The extraction of the UIR with the aid of discrete wavelet transform from the measured acceleration is described. The sensitivity matrix of these UIRs is then obtained based on the finite element model and the time-stepping integration method. Based on the computed sensitivity matrix of the UIRs from several accelerometers, a two-step model updating method is adopted for identifying the local damages. Statistical analysis is included into the damage identification procedure with the measurement noise taken as an independent random variable in the measured UIRs. A new damage localization index is proposed and the mean values of the identified parameters are taken as the damage severity. Finally, a nine-bay three-dimensional frame structure is analyzed numerically and experimentally using the proposed technique. The damage scenarios of multiple damages with different levels of noise are considered. The identified results are shown satisfactory, indicating the feasibility and effectiveness of the proposed technique.

Analysis and calculation of the electric drive for the extruder in a controllable electrical complex for production of synthetic yarns

The conditions that function We(z), which ensures the minimum acceptable time of the switching process, must satisfy can be formulated as follows: — function We(z) should have nulls equal to the poles of Wn(z) positioned in the unit circle or outside of plane z; — function W(z) or 1 — We(z) should have all nulls of Wn(z) located in the unit circle or outside of plane z as nulls; — function W(z) or 1 — We(z) should contain z−1 as cofactor z−1. If Wn(z) is an unstable unit impulse response, then F(z) should contain all poles of Wn(z) lying in the unit circle or outside of plane z as nulls (with the exception of the poles for z = 1). If Wn(z) has nulls located in or outside of the unit circle, these nulls must be nulls of W(z) = 1 − (1 − z−1). In these two cases, the briefest time of the switching process will exceed the value which can be obtained for a system with a stable unit impulse response Wn(z) which has no nulls in or outside of the unit circle of plane z.

Study of Occupant Chest Deceleration Analysis for Frontal Impact of Vehicle

This paper deals with investigation to determine optimum vehicle deceleration in order to reduce occupant injuries for frontal impact of vehicle at the primary design phase. The first purpose of this investigation is to calculate occupant chest deceleration under one degree of freedom spring and mass model by using convolution integral when various frontal crash impact of vehicle occurs. Next we developed and found out how to reduce maximum chest deceleration by analyzing unit impulse response and influence curve of chest deceleration sensitivity by changing small range of vehicle deceleration fluctuation. Further we also conducted three patterns of vehicle deceleration curve sled test for frontal impact under the Hybrid III dummy restrained by the rigid seat and seatbelt. As the result, this paper reports that this method can be estimated, as for the relative comparison of occupant chest deceleration when the frontal crash impact of vehicle changes even though the real world occupant safety restraint system of seatbelt is equipped.