Time-dependent Waveform Research Articles

Lateral contact yields longitudinal cohesion in active undulatory systems.

Many animals and robots move using undulatory motion of their bodies. When the bodies are in close proximity undulatory motion can lead to novel collective behavior such as gait synchronization, spatial reconfiguration, and clustering. Here we study the role of contact interactions between model undulatory swimmers: three-link robots in experiment and multilink swimmers in simulation. The undulatory gait of each swimmer is generated through a time-dependent sinusoidal-like waveform which has a fixed phase offset, ϕ. By varying the phase relationship between neighboring swimmers we seek to study how contact forces and planar configurations are governed by the phase difference between neighboring swimmers. We find that undulatory actuation in close proximity drives neighboring swimmers into planar equilibrium configurations that depend on the actuation phase difference. We propose a model for stable planar configurations of nearest-neighbor undulatory swimmers which we call the gait compatibility condition, which is the set of planar and phase configurations in which no collisions occur. Robotic experiments with two, three, and four swimmers exhibit good agreement with the compatibility model. To study the contact forces and the time-averaged equilibrium between undulatory systems we perform simulations. To probe the interaction potential between undulatory swimmers we apply a small force to each swimmer longitudinally to separate them from the compatible configuration and we measure their steady-state displacement. These studies reveal that undulatory swimmers in close proximity exhibit attractive longitudinal interaction forces that drive the swimmers from incompatible to compatible configurations. This system of undulatory swimmers provides new insight into active-matter systems which move through body undulation. In addition to the importance of velocity and orientation coherence in active-matter swarms, we demonstrate that undulatory phase coherence is also important for generating stable, cohesive group configurations.

Steady state temperature rise in multilayered tissue due to arbitrary periodic SAR using finite difference FFT and transfer function method

Steady state (SS) and transient temperature-rise in tissue from radiofrequency exposure forms the underlying basis for limits in international exposure guidelines. Periodically pulsed or intermittent exposures form a special case of having both peak and average levels, producing temperature-rise oscillations in the SS. Presented here is a method for determining tissue temperature-rise for periodic specific absorption rate (SAR) modulation having arbitrary waveform. It involves the finite difference solution of a form of the Pennes Bioheat Transfer equation (BHTE) and uses the concept of the transfer function and the Fast Fourier Transform (FFT). The time-dependent BHTE is converted to a SS harmonic version by assuming that the time-dependent SAR waveform and tissue temperature can both be represented by Fourier series. The transfer function is obtained from solutions of the harmonic BHTE for an assumed SAR waveform consisting of periodic impulses. The temperature versus time response for an arbitrary periodic SAR waveform is obtained from the inverse FFT of the product of the transfer function and the FFT of the actual SAR waveform. This method takes advantage of existing FFT algorithms on most computational platforms and the ability to store the transfer function for later re-use. The transfer function varies slowly with harmonic number, allowing interpolation and extrapolation to reduce the computational effort. The method is highly efficient for the case where repeated temperature-rise calculations for parameter variations in the SAR waveform are sought. Examples are given for a narrow, circularly symmetric beam incident on a planar skin/fat/muscle model with rectangular, triangular and cosine-pulsed SAR modulation waveforms. Calculations of temperature-rise crest factor as a function of rectangular pulse duty factor and pulse repetition frequency for the same exposure/tissue model are also presented as an example of the versatility of the method.

2.6-GHz fundamental repetition rate, Q-switched mode-locking Nd[formula omitted]-doped single-mode silica fiber laser, fabricated by zeolite method

We successfully constructed a pulse laser whose fundamental repetition rate is 2.6-GHz in Q-switched mode-lock oscillation. To the best of our knowledge, this is the highest repetition rate among silica glass fiber lasers. The laser cavity was constructed by a 40-mm-long Nd3+-doped single-mode silica glass fiber (Nd-SMF) and a SESAM as a saturable absorber. The core glass of the Nd-SMF was fabricated by a zeolite method and its concentration of Nd2O3 was 1.25 wt%. The maximum laser power reached 27.2 mW, and its slope efficiency and lasing thresholds were 19.6% and 7.3 mW. The peak wavelength of the oscillation was 1062.7 nm. The RF spectrum and the time-dependent waveform from a 20-GHz-bandwidth oscilloscope show a 2.6-GHz repetition rate of an Nd-SMF laser. We also discuss the conditions for achieving a CW-mode-lock (CML) oscillation, which is critical laser output Pout of 180 mW. Such a simple multi-GHz repetition rate laser will be available to make a compact frequency comb or a laser oscillator that will achieve ablation-cooled material removal in laser processing.

The transient voltage response of ReBCO coated conductors exhibiting dynamic resistance

Dynamic resistance can be observed in a superconducting tape carrying a DC current which is exposed to an oscillating magnetic field. This effect is attributed to the interaction between the transport current and moving fluxons, and can occur in various superconducting components including high temperature superconducting (HTS) flux pumps, fast-ramping magnets and HTS rotating machines. Although conventionally expressed in terms of a DC ‘resistance,’ the phenomenon is inherently transient in nature, and the voltage drop across the superconductor follows a time-dependent periodic waveform. Here we present experimental measurements of the dynamic resistance of different REBCO tapes carrying a DC current and exposed to an oscillating perpendicular field. Measurements of both the transient voltage waveforms and the time-averaged DC resistances are compared with numerical finite element simulations obtained using the H-formulation. We observe clear variations between the voltage response from different tapes, which can be understood in terms of their differing Jc(B, θ) dependence. In particular, a key feature of the experimentally measured waveforms is the emergence of a split ‘double peak’ at higher applied fields. Graphical visualisations of the finite element data show that this coincides with a periodic increase in Jc(B, θ) throughout the tape. This occurs during each cycle at those times when the applied field falls below the shielding threshold of the tape (as the penetrating field within the tape then approaches zero). Our findings show that models which assume a constant Jc irrespective of local field strength cannot capture the full range of behaviour observed by experiment. This emphasises the importance of employing experimentally measured Jc(B, θ) data when simulating transient effects in HTS materials.

Visualization of Blood Flow in AAA Patient-Specific Geometry: 3-D Reconstruction and Simulation Procedures

Abdominal aortic aneurysm (AAA) is a life-threatening disease when the diameter exceeds its safety margin or the aortic wall reaches its mechanical strength. Both of these potential problems need to be clinically assessed. The geometrical influence on blood flow behavior and hemodynamic changes of AAA are not fully clinically understood. The study aimed to explore creating comprehensive and detailed models for use in reconstruction, modeling, and simulating three-dimensional (3D) patient-specific geometry based on two-dimensional (2D) computed tomography images. The patient information was extracted from computed tomography images and the AAA patient’s database. The 3D geometrical models were created using MIMICS software segmentation tools and exported in STL files to ANSYS Workbench. Computational fluid dynamics (CFD) and finite volume methods were used to solve Navier-Stokes equations for fluid flow in the 3D domain. Blood was treated as incompressible and Newtonian fluid, and a transient flow with a time-dependent velocity waveform assigned at the inlet boundary. The computational results were visualized using ANSYS Fluent post-processing. The CFD transient simulation results are presented using the hemodynamic parameters, including velocity vectors, flow patterns (streamlines), pressure distribution, and wall shear stress. The demonstrated results are part of the study aims and methods in order to provide detailed approaches of computational analysis. The procedures used in this study would be useful for understanding the biomechanical influence on blood flow and hemodynamics.

Tunable lenses: dynamic characterization and fine-tuned control for high-speed applications.

Tunable lenses are becoming ubiquitous, in applications including microscopy, optical coherence tomography, computer vision, quality control, and presbyopic corrections. Many applications require an accurate control of the optical power of the lens in response to a time-dependent input waveform. We present a fast focimeter (3.8 KHz) to characterize the dynamic response of tunable lenses, which was demonstrated on different lens models. We found that the temporal response is repetitive and linear, which allowed the development of a robust compensation strategy based on the optimization of the input wave, using a linear time-invariant model. To our knowledge, this work presents the first procedure for a direct characterization of the transient response of tunable lenses and for compensation of their temporal distortions, and broadens the potential of tunable lenses also in high-speed applications.

Improving Depth, Energy and Timing Estimation in PET Detectors with Deconvolution and Maximum Likelihood Pulse Shape Discrimination.

In a scintillation detector, the light generated in the scintillator by a gamma interaction is converted to photoelectrons by a photodetector and produces a time-dependent waveform, the shape of which depends on the scintillator properties and the photodetector response. Several depth-of-interaction (DOI) encoding strategies have been developed that manipulate the scintillator's temporal response along the crystal length and therefore require pulse shape discrimination techniques to differentiate waveform shapes. In this work, we demonstrate how maximum likelihood (ML) estimation methods can be applied to pulse shape discrimination to better estimate deposited energy, DOI and interaction time (for time-of-flight (TOF) PET) of a gamma ray in a scintillation detector. We developed likelihood models based on either the estimated detection times of individual photoelectrons or the number of photoelectrons in discrete time bins, and applied to two phosphor-coated crystals (LFS and LYSO) used in a previously developed TOF-DOI detector concept. Compared with conventional analytical methods, ML pulse shape discrimination improved DOI encoding by 27% for both crystals. Using the ML DOI estimate, we were able to counter depth-dependent changes in light collection inherent to long scintillator crystals and recover the energy resolution measured with fixed depth irradiation (~11.5% for both crystals). Lastly, we demonstrated how the Richardson-Lucy algorithm, an iterative, ML-based deconvolution technique, can be applied to the digitized waveforms to deconvolve the photodetector's single photoelectron response and produce waveforms with a faster rising edge. After deconvolution and applying DOI and time-walk corrections, we demonstrated a 13% improvement in coincidence timing resolution (from 290 to 254 ps) with the LFS crystal and an 8% improvement (323 to 297 ps) with the LYSO crystal.

Ambient Power Density and Electric Field From Broadband Wireless Emissions in a Reverberant Space

With broadband wireless communications networks currently being deployed in ships, aircraft, and other confined, reflective structures, it is necessary to assess the electromagnetic environment (EME) created by these radio-frequency emissions in a reverberant space. The resultant ambient power density and electric field may be identified as a cause of electromagnetic interference to co-located electronic equipment. In this study, we measure and characterize the EME created by emissions from a broadband IEEE 802.11n wireless local area network (WLAN) in a reverberation chamber mock-up of a shipboard below-deck space. The interaction between the wide-bandwidth power spectral density of the communications signal and the highly frequency-selective channel of the reverberant space leads to average ambient power density and average ambient electric field that exhibit very small spatial variation. This spatial uniformity in average field results from integration of the received signal power spectral density across the wide frequency bandwidth. The peak-to-average power ratio (PAPR) of the received wireless signal is derived from time-dependent power waveform statistics measured by a real-time spectrum analyzer. A maximum electric field is computed from the peak received power. Previously, the maximum electric field was assessed using continuous-wave (CW) signal transmission and mechanical mode stirring in the reverberant space. For a given total power radiated into the space, it is shown that maximum electric fields obtained from these two approaches are in good agreement.

Slot Harmonic Effect on Magnetic Forces and Vibration in Low-Speed Permanent-Magnet Machine With Concentrated Windings

In this paper, the influence of slot harmonics on magnetic forces and vibration is studied in a 120-slot/116-pole low-speed PM machine at no-load. It is shown how the lowest mode of vibration is produced at no-load due to slotting. Comparing the cases of open slots, semi-closed slots and magnetic wedges, the effect of slot closure on radial forces and torque production capability is discussed. Magnetic flux distribution in the airgap is computed using finite element analysis. Spatial harmonics due to slotting are investigated in different cases. Maxwell's stress tensor is employed to calculate radial and tangential components of the force density in the airgap. Spatial distribution of the total forces on the teeth and also time-dependent force waveform on one tooth are analyzed and discussed for different cases. It is shown how the magnitude of the lowest mode of vibration is reduced in the case of using semi-closed slots and magnetic wedges. Tangential force density distribution and torque production capability are also discussed. Structural analysis is presented to compute the maximum amplitude of the stator deformations due to the radial forces. Experimental results of the prototype generator are presented verifying the existence of the lowest mode of vibration at no-load because of the slot harmonics.

Time-Resolved Particle Image Velocimetry within the Nozzle of a Drop-on-Demand Printhead

Time-resolved particle image velocimetry (PIV) has been performed inside the nozzle of a commercially available inkjet print-head to obtain the time-dependent velocity waveform. A printhead with a single transparent nozzle 80 μm in orifice diameter was used to eject single droplets at a speed of 5 m/s. An optical microscope was used with an ultra-high-speed camera to capture the motion of particles suspended in a transparent liquid at the center of the nozzle and above the fluid meniscus at a rate of half a million frames per second. Time-resolved velocity fields were obtained from a fluid layer approximately 200 μm thick within the nozzle for a complete jetting cycle. A Lagrangian finite-element numerical model with experimental measurements as inputs was used to predict the meniscus movement. The model predictions showed good agreement with the experimental results. This work provides the first experimental verification of physical models and numerical simulations of flows within a drop-on-demand nozzle. © 2012 Society for Imaging Science and Technology.

Extrapolation of Time-Dependent Waveforms along their Path of Propagation

Summary Time-dependent waveforms are commonly extrapolated in space by means of rays and occasionally by means of diffraction integrals. It is possible to extrapolate time-dependent waves in space with a partial differential equation derived from the wave equation. There are stable numerical approximations. An example illustrates a mechanism for ' signal-generated noise ' which is consistent with observations. When a wave propagates in an inhomogeneous medium the waveform changes. Given that the wave has been observed at a suitable number of points in space we may attempt to solve two types of problems. First we may attempt to ascertain the nature of material inhomogeneity along the wave paths, and second, we may attempt to extrapolate the disturbance back to the source in an attempt to discover the nature of the source. With few exceptions, the methods used during the past decade for doing this kind of geophysical work may be summarized as follows: when wave equations are to be used, separability is achieved by considering cases in which the material inhomogeneity is a function of only one spatial co-ordinate. When higherdimensional inhomogeneity is so severe that it cannot be ignored, then the wave equations are almost always specialized to ray theory. Ray theory is especially useful when only the travel time is required. Although the amplitude may also be obtained by ray theory it is often of marginal utility because amplitude measurement is made ambiguous by changing waveforms. What we develop in this paper is a finite difference approach to the wave equation which tracks the time dependent waveform of a travelling wave in two-dimensionally inhomogeneous material. This is an extension of earlier work done by one of the authors in the frequency domain. Although the Fourier transform relates time-domain solutions to frequency-domain solutions, there are several compelling practical factors which give impetus to this study. When a waveform is small at certain times of interest and large at times which are not of interest, then a satisfactory approximation to the Fourier integral may be difficult to obtain even if values are obtained with good accuracy at many frequencies: an example is the head wave. Another example occurs in reflection seismology where the most interesting part of the waveform is the late-arriving weak echoes. Another example is when the time function is of long duration but only a small portion of it is of interest; this is usually the case with short-period earthquake seismograms where there is never any hope of interpreting more than the wave packets which come from identifiable phases.

Regional Analysis of Sinonasal Ciliary Beat Frequency

Mucociliary clearance, a primary host defense mechanism, depends on mucus production and its clearance by the coordinated beating of cilia lining the airways. Numerous investigations have analyzed ciliary activity in brushings from the inferior turbinate. To date, only one study has investigated whether there exists variation in ciliary beat frequency (CBF) within the sinonasal cavity. We analyzed CBF from the inferior turbinate, uncinate process, and sphenoethmoid recess in nonsinusitis patients to determine regional variability of ciliary activity within the sinonasal cavity. Explants of sinonasal epithelium were analyzed at 37 degrees C. Beating cilia were visualized with differential interference contrast microscopy. Images were captured using a high-speed digital camera with a sampling rate of 250 frames per second. A one-dimensional tracking algorithm analyzed individual pixel grayscale values within each frame of the video. The differences in grayscale were plotted as a time-dependent waveform, and frequency was calculated as the inverse of the peak-to-peak distance. A minimum of three areas of beating cilia were analyzed per regional sample. Statistical analysis was performed with repeated-measures analysis of variance. Complete sampling of all three sites was accomplished in 10 patients. No difference in CBF within the sinonasal cavity was identified (p < 0.05). The mean CBF for all sites in all patients was 12.6 +/- 2.9 Hz, in agreement with published values. This study shows no regional differences in CBF within the sinonasal cavity, supporting previous work and validating analysis of inferior turbinate cilia.

A miniaturized matrix-assisted laser desorption/ionization time of flight mass spectrometer with mass-correlated acceleration focusing.

Mass correlated acceleration (MCA) has now been integrated into a 4 in (10.2 cm) matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometer to achieve high resolving power across a broader mass range, without sacrificing detection of higher mass ions. The goal was to combine MCA with a custom-built miniaturized instrument such as those that might be used for field-portable applications. Unlike other pulsed extraction methods, MCA is not mass dependent and mass spectra can be achieved with a single tuning of instrument parameters. Additionally, the multi-channel recording advantage is better realized because ions of all masses can be brought into focus simultaneously. The MCA dual-stage ion source compensates dynamically for the mass dependence by incorporating an extraction pulse region followed by an acceleration region that contains a time-dependent waveform correlated with mass. The technique was validated with applications in peptide mixtures and protein digestions. Diagnostic studies for the instrument include m/z range and limits of detection.

Spatiotemporal characteristics of the collisionless rf sheath and the ion energy distributions arriving at rf-biased electrodes.

A self-consistent fluid model is proposed for describing collisionless radio-frequency (rf) sheaths driven by a sinusoidal current source. This model includes all time-dependent terms in ion fluid equations, which are commonly ignored in some analytical models and numerical simulations. Moreover, an equivalent circuit model is introduced to determine self-consistently the relationship between the instantaneous potential at a rf-biased electrode and the sheath thickness. The time-dependent voltage wave form, the sheath thickness and the ion flux at the electrode as well as the spatiotemporal variations of the potential, the electric field and the ion density inside the sheath are calculated for various rf powers and ratios of the rf frequency to the ion plasma frequency. The ion energy distributions ( IEDs) impinging on the rf-biased electrode are also calculated with the ion flux at the electrode. The numerical results show that the frequency ratio is a crucial parameter determining the spatiotemporal variations of the rf sheath and the shape of the IEDs.

Mass correlated acceleration in a reflectron MALDI TOF mass spectrometer: an approach for enhanced resolution over a broad mass range

Compared to continuous extraction, pulsed extraction (PE) of ions formed by matrix-assisted laser desorption/ionization (MALDI) in time-of-flight (TOF) mass spectrometers significantly improves mass resolution. Parameters such as extraction voltage, delay time, and correction pulse must be varied, however, to achieve optimum mass resolution over a broad mass range because the PE method is mass dependent. We previously reported a novel method, mass correlated acceleration (MCA), which we have now combined with a reflectron MALDI TOF mass spectrometer to further enhance mass resolution over a broader mass range. Unlike the PE method, MCA is not mass dependent and high resolution mass spectra can be achieved with a single tuning of instrument parameters. The ions may be brought into focus simultaneously, i.e., the multi-channel recording advantage can be more fully realized. The MCA dual-stage ion source design includes an extraction pulse region and an acceleration region that contains a time-dependent waveform correlated with mass. We demonstrate the validity of this novel technique with applications in peptide mixture analysis and protein digests of lysozyme and bovine serum albumin.

Optical waveguide sensors in analytical chemistry: today's instrumentation, applications and trends for future development

Current concepts for chemical and biochemical sensing based on detection with optical waveguides are reviewed. The goals are to provide a framework for classifying such sensors and to assist a designer in selecting the most suitable detection techniques and waveguide arrangements. Sensor designs are categorized on the basis of the five parameters that completely describe a light wave: its amplitude, wavelength, phase, polarization state and time-dependent waveform. In the fabrication of a successful sensor, the physical or chemical property of the determined species and the particular light wave parameter to detect it should be selected with care since they jointly dictate the sensitivity, stability, selectivity and accuracy of the eventual measurement. The principle of operation, the nature or the detected optical signal, instrumental requirements for practical applications, and associated problems are analyzed for each category of sensors. Two sorts of sensors are considered: those based on direct spectroscopic detection of the analyte, and those in which the analyte is determined indirectly through use of an analyte-sensitive reagent. Key areas of recent study, useful practical applications, and trends in future development of optical waveguide chemical and biochemical sensors are considered.

Numerical Studies of a Pulsing Burner-Stabilized Laminar Premixed Methane-Air Flame

Abstract The problem of NOx emission from flames is of considerable technological interest. It is known that this emission is influenced by flow heterogeneities and in particular by equivalence ratio fluctuations. This topic is studied in the present article for the case of a laminar flame fed by a stream of premixed reactants with a superimposed time-variable equivalence ratio. Experiments and numerical calculations are used to establish relations between the time-varying equivalence ratio, the flame temperature, and NOx emission. In the experiment, the equivalence ratio, is determined from light absorption measurements, the mean and fluctuating temperatures are obtained with thermocouples, and NOx emission is deduced from gas analysis. Time-dependent calculations are carried out with two reaction schemes and a set of models for the heat losses from the flame and burnt gases. Steady-state measurements and numerical calculations of temperature and NOx are in good agreement for lean flames. When the equivalence ratio takes values close to unity, agreement is obtained by including surface catalycity effects. The time-dependent temperature waveform calculations are in good agreement with measurements. In addition, the time-dependent response of the flame in terms of NOx emission is predicted. A numerical study of the frequency response of the flame shows that the cut-off frequency for the time varying NOx mole fraction is lower than the maximum temperature cut-off frequency, and lower than the spectral content of turbulent flames.

Effect of electromagnetic stimulation with different waveforms on cultured chick tendon fibroblasts

An energy efficient electromagnetic stimulator device for fracture healing was compared to a commercially available device in stimulating cell growth in tissue cultures. The energy efficient device, which conserves energy by using a bidirectional time-dependent magnetic wave form, and the commercially available stimulator, which uses a unidirectional time-dependent magnetic wave form, were tested on chick tendon fibroblasts in primary culture. Comparing non-stimulated control and cells electromagnetically stimulated with unidirectional and bidirectional waveforms showed that at the growth phase between days 2 and 3, both electrical stimulation techniques increased cell division as measured by DNA synthesis. When cells were dividing rapidly, collagen synthesis was reduced. When the cells reached the confluence there was no difference among the groups (control, unidirectionally stimulated, and bidirectionally stimulated) in terms of number of cells or collagen produced.

Coherence properties of some broadband highly coherent light fields

Coherence properties are studied for nonsinusoidal steady-state light fields, which have the property that they produce nonsinusoidal traveling waves with the same time-dependent waveform throughout the far field of their sources. It is found that many such deterministic light fields can have frequency bandwidths that more than span the visible spectrum and can produce interference fringes with unit visibility if light from any pair of points in the far field is combined. However, the complex degree of coherence (in the space–time domain) for such fields over the far field is not unimodular. Thus these fields are not perfectly coherent according to the usual definition within the theory of partial coherence.

Electron beam tester measurements of switching noise in VLSI circuits

Electron beam testers can be used to measure the noise generated on power supply voltages by the operation of VLSI circuits. The power supply busses on the circuit are probed by the electron beam, resulting in a time-dependent voltage waveform. This method of measurement assures that the noise is measured near the active circuit, where its impact on the circuit is most easily assessed. Examples are presented to illustrate the mechanism of noise generation and the use of an E-beam tester to measure the noise.