Free Decay Time Research Articles

Novel approach for precise identification of vibration frequencies and damping ratios from free vibration decay time histories data of nonlinear single degree of freedom models

The significance of Single Degree of Freedom (SDOF) systems lies in their ability to serve as foundational elements for modeling more complex dynamic problems. By capturing essential dynamic behavior with simplicity, SDOF models enable efficient analysis and comprehension of complex systems, justifying the investigation of these simplified models. In nonlinear scenarios, SDOF models result in time series data wherein vibration frequencies vary over time. Classically, time–frequency or Hilbert transforms applied to temporal responses are frequently used to identify the evolution of frequencies and damping ratio over time. These techniques provide results that reflect the spectrum composition achieved for the analyzed time window and present difficulties in precisely determining the magnitude and the exact instant of an effective frequency or damping ratio variation. In this sense, this work introduces a new methodology capable of accurately identifying the vibration frequency as a function of time, i.e., the instantaneous frequency, along with the instantaneous damping ratio. At this initial stage, the focus is on validating the methodology by comparing its performance with the classical approach based on time–frequency transforms. The initial results obtained from synthetic free vibration decay responses of SDOF nonlinear models highlight the accuracy of our findings compared to those obtained from time–frequency transforms. The presented methodology holds promise for further advancement, with potential impacts including structural damage identification, modal identification and nonlinear dynamic analysis.

Ablation of TiO[formula omitted] surface with a double-pulse femtosecond laser

Ablation of TiO2 surface using a double-pulse femtosecond laser was studied applying plasma model calculations and experimental work. The free electron decay time measured by using the double-pulse breakdown threshold method was 550 fs for TiO2. A plasma model is applied with the experimentally derived decay time constant for TiO2 surface ablation. The transient electron density evolution during the TiO2 surface ablation with a double-pulse femtosecond laser was calculated. The ablation experiment using a double-pulse femtosecond laser indicated that the ablation threshold depended on calculated electron density. Furthermore, ablated crater diameter near the threshold was found to depend on the laser fluence with a constant electron density.

Excitonic light emission decay time measurements in moderatelyδ-doped GaAs/AlAs multiple quantum wells

The radiative recombination rate of moderately dopedn-type andp-type GaAs/AlAs multiple quantum wells using a timecorrelated single photon counting system is presented. The experimental study has been obtained within a wide temperature range from liquid helium to room temperature and the work has focused on identifying photoluminescence decay rates based on freeexciton recombinations. It was found that the free exciton decay time was reduced in doped multiple GaAs/AlAs quantum wells, and that the reduction rate depends on both the concentration and doping type.

A LOWER INITIAL ABUNDANCE OF SHORT-LIVED41Ca IN THE EARLY SOLAR SYSTEM AND ITS IMPLICATIONS FOR SOLAR SYSTEM FORMATION

The short-lived radionuclide Ca-41 plays an important role in constraining the immediate astrophysical environment and the formation timescale of the nascent solar system due to its extremely short half-life (0.1 Myr). Nearly 20 years ago, the initial ratio of Ca-41/Ca-40 in the solar system was determined to be (1.41 +/- 0.14) x 10(-8), primarily based on two Ca-Al-rich Inclusions (CAIs) from the CV chondrite Efremovka. With an advanced analytical technique for isotopic measurements, we reanalyzed the potassium isotopic compositions of the two Efremovka CAIs and inferred the initial ratios of Ca-41/Ca-40 to be (2.6 +/- 0.9) x 10(-9) and (1.4 +/- 0.6) x 10(-9) (2 sigma), a factor of 7-10 lower than the previously inferred value. Considering possible thermal processing that led to lower Al-26/Al-27 ratios in the two CAIs, we propose that the true solar system initial value of Ca-41/Ca-40 should have been similar to 4.2 x 10(-9). Synchronicity could have existed between Al-26 and Ca-41, indicating a uniform distribution of the two radionuclides at the time of CAI formation. The new initial Ca-41 abundance is 4-16 times lower than the calculated value for steady-state galactic nucleosynthesis. Therefore, Ca-41 could have originated as part of molecular cloud materials with a free decay time of 0.2-0.4 Myr. Alternative possibilities, such as a last-minute input from a stellar source and early solar system irradiation, could not be definitively ruled out. This underscores the need for more data from diverse CAIs to determine the true astrophysical origin of Ca-41.

Numerical and experimental investigation on cable vibration mitigation using shape memory alloy damper

This paper estimates the effectiveness of shape memory alloy (SMA) damper on cable vibration mitigation by numerical simulation and experimental investigation. First, a simplified constitutive law of superelastic SMA and the structure of the SMA damper developed by superelastic SMA wires are presented. Second, based on Hamilton principle and Galerkin method, the vibration equations for a cable–SMA damper system are established, and numerical solutions of which are adopted to obtain the dynamic responses of the system without and with SMA damper using the Newmark-β method. Finally, numerical simulation associated with a practical cable and experiments on a scaled test cable are, respectively, carried out to investigate the effectiveness of SMA damper on the vibration mitigation of the cable subjected to the free and forced vibration. Numerical simulations indicate that the SMA damper developed can clearly shorten the free vibration decay time of the cable and increase the equivalent damping ratios of the cable with the reasonable parameter and location of SMA damper, and that it can decrease the displacement response amplitude when the cable is subjected to white-noise excitations. The experimental results show that the SMA damper can reduce the acceleration response of the test cable under free vibration and forced vibration induced by the deck motion. As a result, the SMA damper developed is very effective for mitigating cable vibration. Copyright © 2009 John Wiley & Sons, Ltd.

Observing system simulation experiments in geomagnetic data assimilation

Observing System Simulation Experiments (OSSEs) have been carried out for a geomagnetic data assimilation system. The purpose of these experiments is to demonstrate how new algorithms for assimilating geomagnetic observations into a geodynamo model can be evaluated. A “nature” run is carried out to establish a “true” evolution of the Earth's core state, from which synthetic surface geomagnetic field observations are created. These observations are then assimilated into the geodynamo model using an optimal interpolation (OI) scheme. Model error is simulated by using a different Rayleigh number in the nature and model runs. Because the true core state evolution is known completely, the assimilation results can be evaluated in terms of any state variable and at any point in the computational domain. In this work we focus on the poloidal (observed) and toroidal (unobserved) components of the magnetic field throughout the outer core. Experiments are carried out using observations of different degrees and varying forecast error correlation length scales. We also investigate the impact of model error on the assimilation and on the accuracy of geomagnetic forecasts. Assimilation runs lasting about 90% of the magnetic free decay time show a positive impact on both components of magnetic field deep within the outer core. Forecasts of the surface magnetic field show much lower error growth, indicating that the initial condition used for the forecasts has been substantially improved through assimilation.

Mechanisms for Lagged Atmospheric Response to ENSO SST Forcing*

Abstract The mechanism and sensitivity of the lagged response of tropical tropospheric temperature to El Niño–Southern Oscillation (ENSO) SST forcing are examined using the Quasi-Equilibrium Tropical Circulation Model (QTCM) coupled to a slab mixed layer ocean model, along with a simple analytical model. It is found that the lag and amplitude of tropospheric temperature response depend on mixed layer depth (MLD), ENSO SST forcing period, areal fraction of the mixed layer ocean, and the strength of Tropics to midlatitude transports. The phase lag is not a monotonic function of mixed layer depth. It maximizes at moderate MLD and, thus, is not very sensitive to MLD in the realistic range. The phase lag asymptotes to values determined by free-atmospheric time scales, between 1 and 2 months, for small or large values of MLD. The amplitude of the tropospheric temperature response decreases with increasing MLD. The phase lag and amplitude of tropospheric temperature both increase as a specified ENSO SST forcing period increases and they appear to be rather insensitive to the seasonal cycle of SST. On the other hand, the phase lag and amplitude of mixed layer ocean SST change monotonically with MLD and ENSO forcing period, with a deeper mixed layer producing longer lag and smaller amplitude of SST anomalies. Longer ENSO SST forcing periods correspond to longer lag and larger amplitude of mixed layer ocean SST anomalies. While the ENSO region convective heating (precipitation) anomalies are closely tied to SST anomalies, the tropical mean precipitation seems best viewed as a complex by-product of the response rather than as a driver. One useful parameter determining the lag of tropospheric temperature to ENSO SST is the freedecay time scale of the coupled system. This parameter combines the effects of surface flux exchanges, heat loss at the top of the atmosphere and from the Tropics to midlatitudes, and finite ocean heat capacity. It is indicative of the extent to which the lagged response of tropical tropospheric temperature to ENSO SST is a coupled phenomenon. Overall, the contribution of coupling to SST outside the ENSO region substantially increases the amplitude and lag of the tropospheric temperature response to ENSO.

DECAY PROCESS OF PHOTOELECTRONS RELATED TO SHALLOW ELECTRON TRAP IN SILVER HALIDE MICROCRYSTAL

Combining the microwave absorption dielectric spectrum experiment with computer simulation, the decay process of free photoelectrons related to shallow electron trap in silver halide microcystal is investigated. It is found that when shallow electron traps exist in silver halide, the free photoelectron decay will be sped up at the beginning, and then slowed down in the subsequent process. Shallow electron traps like a reservoir can store up photoelectrons temporarily, and then release them tardily? which results in lengthening free photoelectron lifetime or decay time and enhancing the efficiency of latent image formation.

The influence of K 4 Fe(CN) 6 on the photosensitivity of cubic AgCl microcrystal

The influence of K4Fe(CN)6 at various doping concentrations on the photosensitivity of cubic AgCl microcrystals has been investigated by using the microwave absorption and phase-sensitive measurement technique. The time behaviour of free photoelectrons in AgCl microcrystals is analysed by using computer simulation as a function of three parameters of shallow electron trap (SET) including doping concentration, trap depth and capture cross-section (CCS). It is found that the three parameters of SET play different roles on the free photoelectron decay time (FDT). After considering the threshold effects of the three parameters and their collective effects, the trap depth value, the CCS value and the optimal doping concentration of the SET introduced by the dopant K4Fe(CN)6 in cubic AgCl microcrystals can also be determined and the best photosensitivity of cubic AgCl can be obtained.

Hydrogeological interpretation and potential of the new magnetic resonance sounding (MRS) method

This study reviews the up-to-date hydrogeological contribution of the new, geophysical magnetic resonance sounding (MRS) method and presents new interpretation ideas and potential regarding various hydrogeological applications. The main advantage of the MRS method as compared to other geophysical methods is in its water selectivity. MRS is commercially available but is also the subject of R & D to minimize a number of limitations related not only to data acquisition but also to hydrogeological data interpretation. The two main MRS output data types are free water content ( Φ MRS) and decay time constant ( T d). Relations between Φ MRS and effective porosity, specific yield and specific storage are discussed in the framework of an original groundwater storage concept. T d is correlated empirically with hydraulic conductivity, however such correlation is not yet quantified for various rock types so T d has to be calibrated by borehole data. As an improvement of the data interpretation, a proposal of a new, multi-decay approach is presented. The advantage of such analysis is that it combines storage and flow property characteristics in assessing water content at various pore size fractions, and the disadvantage is that it is applicable only at large signal to noise ratios. Five case examples are used to show that combined interpretation of Φ MRS and T d can be used to evaluate subsurface hydrostratigraphy although such analysis is vulnerable to equivalence error and its resolution decreases with depth. MRS with large volume-averaging schema and current limitation to 1D approach is already suitable for groundwater modelling applications and for evaluation of groundwater potential in the MRS measurement scale (∼100×100 m 2) but is not yet optimized for well siting. MRS can detect water in the unsaturated zone, which can be potentially used in real time subsurface flux monitoring. Field experiments indicate that MRS is not appropriate for groundwater salinity detection. No field experiments have yet been made to detect hydrogenated compounds like hydrocarbons and tracers in the subsurface.

Temperature dependence of localized and free exciton lifetime in CdZnSSe/ZnSSe single quantum wells

Time-integrated and time-resolved photoluminescence of Cd0.3Zn0.7S0.06Se0.94/ ZnS0.06Se0.94single quantum wells (QW) have been studied. The radiative recombination process is dominated by localized excitons which have a constant lifetime of ∼ 300 ps when the temperature is less than 130 K. As the temperature increases, localized excitons are thermally activated to form free excitons which are stable up to room temperature because of the large exciton binding energy and strong confinement of electrons and holes in these QWs. Free exciton radiative recombination is thus dominated when temperature is in the 130 K to room temperature (295 K) range. The free exciton decay time of ∼ 300 ps at 130 K increased linearly to ∼ 1ns at 295 K. This linear increase in exciton lifetime with temperature agrees with the theoretical prediction by considering the conservation of momentum requirement for radiative recombination for excitons in QW.

Adriatic seiche decay and energy loss to the Mediterranean

A salient feature of sea level records from the Adriatic Sea is the frequent occurrence of energetic seiches of period about 21 h. Once excited by a sudden wind event, such seiches often persist for days. They lose energy either to friction within the Adriatic, or by radiation through Otranto Strait into the Mediterranean. The free decay time of the dominant (lowest mode) seiche was determined from envelopes of handpassed sea level residuals from three locations (Bakar, Split and Dubrovnik) along the Croatian coast during twelve seiche episodes between 1963 and 1986 by taking into consideration only time intervals when the envelopes decreased exponentially in time, when the modelled effects of along-basin winds were smaller than the error of estimation of decay time from the envelopes and when across-basin winds were small. The free decay time thus obtained was 3.2±0.5 d. This value is consonant with the observed width of the spectral peak. The decay caused by both bottom friction and radiation was included in a one dimensional variable cross section shallow water model of the Adriatic. Bottom friction is parameterized by the coefficient k appearing in the linearized bottom stress term ρ 0u (where u is the along-basin velocity and ρ 0 the fluid density). The coefficient k is constrained by values obtained from linearization of the quadratic bottom stress law using estimates of near bottom currents associated with the seiche, with wind driven currents, with tides and with wind waves. Radiation is parameterized by the coefficient f appearing in the open strait boundary condition ζ = auh/ c (where ζ is sea level, h is depth and c is phase speed). This parameterization of radiation provides results comparable to allowing the Adriatic to radiate into an unbounded half plane ocean. Repeated runs of the model delineate the dependence of model free seiche decay time on k and a, and these plus the estimates of k allow estimation of a. The principle conclusions of this work are as follows. 1. (1) Exponential decay of seiche amplitude with time does not necessarily guarantee that the observed decay is free of wind influence. 2. (2) Winds blowing across the Adriatic may be of comparable importance to winds blowing along the Adriatic in influencing apparent decay of seiches; across-basin winds are probably coupled to the longitudinal seiche on account of the strong along-basin variability of across-basin winds forced by Croatian coastal orography. 3. (3) The free decay time of the 21.2 h Adriatic seiche is 3.2±0.5 d. 4. (4) A one dimensional shallow water model of the seiche damped by bottom stress represented by Godin's (1988) approximation to the quadratic bottom friction law ρ 0 C D u| u| using the commonly accepted drag coefficient C D = 0.0015 and quantitative estimates of bottom currents associated with wind driven currents, tides and wind waves, as well as with the seiche itself with no radiation gives a damping time of 9.46 d; radiation sufficient to give the observed damping time must then account for 66% of the energy loss per period. But independent estimates of bottom friction for Adriatic wind driven currents and inertial oscillations, as well as comparisons between quadratic law bottom stress and directly measured bottom stress, all suggest that the quadratic law with C D=0.0015 substantially underestimates the bottom stress. Based on these studies, a more appropriate value of the drag coefficient is at least C D=0. In this case, bottom friction with no radiation leads to a damping time of 4.73 d, radiation sufficient to give the observed damping time then accounts for 32% of the energy loss per period.

Luminescence Dynamics of Exciton Replicas in Homoepitaxial GaN Layers

Photoluminescence of excitons and their phonon replicas in homoepitaxial MOCVD-grown gallium nitride (GaN) layers have been studied by picosecond (ps) time-resolved photoluminescence spectroscopy. The time-resolved photoluminescence spectroscopy has shown that the free excitons and their replicas have the fastest dynamics (decay time of about 100 ps). Then, the excitons-bound-to-donors emission rises (with the rise time similar to the free excitons decay time) and decays with t = 300 ps. The excitons-bound-to-acceptors has the slowest decay (about 500 ps). It has been found that the ratio of excitons-bound-to-acceptors and excitons-bound-to-donors amplitudes and their decay times are different for 1-LO replicas and then for zero-phonon lines, whereas the ratio of amplitudes and the decay time of the 2-LO replicas are similar to the ones of the zero-phonon lines.

Comparison of Force Reconstruction Methods for a Lumped Mass Beam

Two extensions of the force reconstruction method, the sum of weighted accelerations technique (SWAT), are presented in this article. SWAT requires the use of the structure’s elastic mode shapes for reconstruction of the applied force. Although based on the same theory, the two new techniques do not rely on mode shapes to reconstruct the applied force and may be applied to structures whose mode shapes are not available. One technique uses the measured force and acceleration responses with the rigid body mode shapes to calculate the scalar weighting vector, so the technique is called SWAT-CAL (SWAT using a calibrated force input). The second technique uses the free-decay time response of the structure with the rigid body mode shapes to calculate the scalar weighting vector and is called SWAT-TEEM (SWAT using time eliminated elastic modes). All three methods are used to reconstruct forces for a simple structure.

Comparison of Force Reconstruction Methods for a Lumped Mass Beam

Two extensions of the force reconstruction method, the sum of weighted accelerations tech­ nique (SWAT), are presented in this article. SWAT requires the use of the structure's elastic mode shapes for reconstruction of the applied force. Although based on the same the­ ory, the two new techniques do not rely on mode shapes to reconstruct the applied force and may be applied to structures whose mode shapes are not available. One technique uses the measured force and acceleration responses with the rigid body mode shapes to calculate the scalar weighting vector, so the technique is called SWAT-CAL (SWAT using a calibrated force input). The second technique uses the free-decay time response of the structure with the rigid body mode shapes to calculate the scalar weighting vector and is called SWAT-TEEM (SWAT using time eliminated elastic modes). All three methods are used to reconstruct forces for a simple structure.

Irreversibility of Adiabatic Demagnetization in the Rotating Frame in Double and Single Quantum NMR in Solids

We have clarified the origin of the irreversibility involved in adiabatic demagnetization in the rotating frame (ADRF). The irreversibility is very high (≃50%) in single and double quantum ADRF for 27 Al nuclei in Al 2 O 3 , whereas it is very low (≃15%) in single quantum ADRF for 19 F in CaF 2 , 7 Li and 19 F in LiF. Using an analysis based on the Provotorov's theory we conclude that the irreversibility is due to the thermal mixing between Zeeman and dipole systems in the demagnetization process. The conclusion is also supported by the experimental result that the reversibility becomes higher (or lower) when the dipole system is prepared at a positive (or negative) low temperature before ADRF. It is also shown that the characteristics of ADRF can be specified by a parameter T 2 D , where T 2 is the free decay time and D the local field in frequency units.

Dynamics of the Nitrogen Bound Excitons in 6H and 3C SiC

AbstractWe have measured the photoluminescence decay time of the bound excitons at the neutral nitrogen donors in the 6H and 3C polytypes of SiC. At 2K the decay times are 8.0 ns, 1.8 ns and 1.5 ns, for the P, R and S bound excitons in 6H SiC. For the nitrogen exciton in 3C, we find a decay time of 160 ns. These values are faster than previously reported for shallow donors in other indirect bandgap materials such as Si or GaP. Each of the observed decay times is found to be independent of the doping level in the sample, is temperature independent at low temperatures but decrease when the bound excitons are thermally ionised. The decay time related to different donor levels in 6H exhibits a strong dependence on the donor binding energy. We suggest that the dominant mechanism responsible for the observed decay time is a phonon-less Auger process. In high-purity 6H samples we have also measured the free exciton decay time at low temperatures to be 12 ns.

Exciton relaxation in MOVPE grown ZnSe xTe 1- x epilayers

Exciton dynamics in ZnSe x Te 1- x epilayers (0 < x < 0.4) is studied through the emission decay kinetics at liquid helium temperature. At low concentration ( x < 0.1) the emission contains free exciton lines and several impurity-related bands. The free exciton decay times in these samples are less than 100 ps due to the fast energy transfer to defects and the efficient migration of excitons into the interface region. The increase of Se content up to x ≈ 0.2 causes gradual increase of the exciton luminescence decay times (up to 1.5ns) which exhibits strong variation across the exciton emission band contour. This indicates the onset exciton localization by the compositional fluctuations of the mixed crystal.

Resonant decay of global MHD modes at “thick” interfaces

A global surface mode can exist on a “surface” which is not a true discontinuity, but without sufficient dissipation it is not a normal mode and it decays in time via resonance absorption. The decay rate can be calculated analytically when the “surface” is thin. The goal of this paper is to numerically estimate the decay rate when the surface is not thin. We consider a cold plasma in a uniform magnetic field, and we take the density to vary linearly across the “surface.” In our linearized calculation, the global surface mode is driven in steady state by an antenna located in one of the uniform regions external to the surface. The frequency is a free parameter, and resonance curves are computed numerically without approximations. The widths of the resonance curves are used to estimate the free decay times of undriven surface modes, via the uncertainty principle, equation (1). This procedure agrees with the analytical results for a thin surface. The thin surface results are found to break down when kT ≈ 0.3, where T is the surface thickness and k is the wave number along the surface. When the angle between k and B0 exceeds about 40 degrees, the decay rates show distinct maxima at kT ≈ 0.5–1.0. When applied to the active solar corona, the decay rates are large enough to account for the coronal heating, but it should be kept in mind that the role of nonlinearity in resonance absorption is still undetermined.

Damping identification from non-linear random responses using a multi-triggering random decrement technique

The concept of multi-triggering random decrement technique is introduced. Like the single triggering technique, it reduces multi-mode multi-measurement stationary random responses to free decay responses but has the advantage of increasing the apparent number of the resulting free decay time response functions. The maximum number of these free decay responses is equal to the square of the number of random measurements. These free decay responses are then used in a linear time domain modal identification algorithm to extract frequencies, damping factors and mode shapes of a structure under test. The quasi-linear modal approach is used to deal with non-linearities by repeating the linear identification process at different levels of inputs/responses. The procedure is applied to rectangular panels subjected to acoustic random input ranging from 130 to 157 dB. The changes in frequencies and damping factors with input level are reported. This application is part of a sonic fatigue research program.