Frequency Domain Calculations Research Articles

Detecting of Barely Visible Impact Damage on Carbon Fiber Reinforced Polymer Using Diffusion Ultrasonic Improved by Time-Frequency Domain Disturbance Sensitive Zone.

Based on the decorrelation calculation of diffusion ultrasound in time-frequency domain, this paper discusses the repeatability and potential significance of Disturbance Sensitive Zone (DSZ) in time-frequency domain. The experimental study of Barely Visible Impact Damage (BVID) on Carbon Fiber Reinforced Polymer (CFRP) is carried out. The decorrelation coefficients of time, frequency, and time-frequency domains and DSZ are calculated and compared. It has been observed that the sensitivity of the scattered wave disturbance caused by impact damage is non-uniformly distributed in both the time and frequency domains. This is evident from the non-uniform distribution of the decorrelation coefficient in time-domain and frequency-domain decorrelation calculations. Further, the decorrelation calculation in the time-frequency domain can show the distribution of the sensitivity of the scattered wave disturbance in the time domain and frequency domain. The decorrelation coefficients in time, frequency, and time-frequency domains increase monotonically with the number of impacts. In addition, in the time-frequency domain decorrelation calculation results, stable and repetitive DSZ are observed, which means that the specific frequency component of the scattered wave is extremely sensitive to the damage evolution of the impact region at a specific time. Finally, the DSZ obtained from the first 15 impacts is used to improve the decorrelation calculation in the 16-th to 20-th impact. The results show that the increment rate of the improved decorrelation coefficient is 10.22%. This study reveals that the diffusion ultrasonic decorrelation calculation improved by DSZ makes it feasible to evaluate early-stage damage caused by BVID.

Matrix-based integral transformations for Stokes imaging with partially polarized and partially coherent light

With the aid of the matrix-based integral transforms called matrix convolution and matrix direct correlation, we provide a simplified expression for the space- and frequency-domain calculations of polarization imaging with partially polarized and partially coherent light. As an example of practical interest, a formula for Stokes imaging, based on the generalized Stokes parameters, is presented, in which a hypermatrix-based transmission cross-coefficient matrix is introduced to represent the combined effects of diffraction and aberrations of a polarization-dependent imaging system and a partially polarized, partially coherent illumination system. The coherent and incoherent limits in Stokes imaging are discussed with the optical transfer matrix, along with its frequency response for a diffraction-limited incoherent polarization imaging system. A generalized concept of the apparent transfer matrix is introduced to deal with the nonlinearity inherent in the polarization imaging system under partially coherent illumination.

Rigorous coupled-wave analysis of unilateral Smith-Purcell radiation from asymmetric resonators.

The Smith-Purcell radiation produced by electrons moving closely to a grating can be enhanced by resonances. Here, we show a method to manipulate the directionality of the resonance-enhanced radiation. Using the rigorous coupled-wave analysis method, we compare the radiation from symmetric and asymmetric gratings, showing that the enhanced Smith-Purcell radiation can become unilateral with a perturbation that breaks the structural symmetry. Our work provides an effective method for frequency-domain calculation of Smith-Purcell radiation and also an approach to realize more efficient use of the radiation.

Spectrum Extension of a Real-Aperture Microwave Radiometer Using a Spectrum Extension Convolutional Neural Network for Spatial Resolution Enhancement

Enhancing the spatial resolution of real-aperture microwave radiometers is an essential research topic. The accuracy of the numerical values of brightness temperatures (BTs) observed using microwave radiometers directly affects the precision of the retrieval of marine environmental parameters. Hence, ensuring the accuracy of the enhanced brightness temperature values is of paramount importance when striving to enhance spatial resolution. A spectrum extension (SE) method is proposed in this paper, which restores the suppressed high-frequency components in the scene BT spectrum through frequency domain transformation and calculations, specifically, dividing the observed BT spectrum by the conjugate of the antenna pattern spectrum and applying a Taylor approximation to suppress error amplification, thereby extending the observed BT spectrum. By using a convolutional neural network to correct errors in the calculated spectrum and then reconstructing the BT through inverse fast Fourier transform (IFFT), the enhanced BTs are obtained. Since the extended BT spectrum contains more high-frequency components, namely, the spectrum is closer to that of the original scene BT, the reconstructed BT not only achieves an enhancement in spatial resolution, but also an improvement in the accuracy of BT values. Both the results from simulated data and satellite-measured data processing illustrate that the SE method is able to enhance the spatial resolution of real-aperture microwave radiometers and concurrently improve the accuracy of BT values.

FEMFFUSION and its verification using the C5G7 benchmark

FEMFFUSION is an open source code that solves the multigroup neutron transport equation using the diffusion and the SPN approximations. This code uses the continuous Galerkin finite element method to be able to deal with any type of geometry, structured or unstructured, and problem dimension. To verify the program, a research study of the two-dimensional C5G7 benchmark has been performed, including the static, time-dependent and neutron noise computations. Numerical results include the effective multiplication factor, keff, the spatial neutron fluxes, and neutron power distributions for the static computations. The total neutron power evolution of a given transient and the neutron flux distribution evolution for the different energy groups. In addition, neutron noise frequency domain calculation were completed for perturbations of some cross-section in the reactor core. All computation have been performed using different SPN approximations. FEMFFUSION results are verified against the official OECD/NEA benchmark results.

Applying the effective-source approach to frequency-domain self-force calculations for eccentric orbits

Applying the effective-source approach to frequency-domain self-force calculations for eccentric orbits

A new method for measurement of impulsive peak level attenuation of hearing protectors

ASA/ANSI S12.42-2010 contains the first standardized test method for measuring the performance of hearing protection devices (HPDs) with high-level impulsive noise. This standard defines the impulsive peak insertion loss (IPIL) as a time-domain metric that quantifies the reduction in peak sound pressure level provided by an HPD for impulsive noises. However, IPIL as defined in S12.42-2010 is dependent on the spectrum of the impulsive noise source used for measurements of HPDs. Recent studies of HPDs with impulsive noise have led to the investigation of frequency-domain metrics and calculation methods. Using frequency-domain calculations allows the impulsive peak level attenuation to be computed for a wide range of impulsive noises, not only those used for the measurement. Consequently, significant revisions to S12.42 are under consideration and a new standard, ISO 4869-7, is being developed in parallel with the ANSI revision. This presentation highlights key proposals and substantive changes including: measurement and calculation of the frequency-dependent impulsive insertion loss (IIL), incorporation of frequency-domain aspects such as real-ear attenuation at threshold (REAT) limits to attenuation, and a new impulsive peak level attenuation metric (IPLA) to be calculated from the REAT-bounded IIL.

Upsampling wave temporal resolution: Investigating wave parameters and the influence on WEC power performance

Power production of wave energy converters (WEC) predicted in the time domain utilize wave resource parameters and time-domain hydrodynamic model simulations of the WEC. While the hydrodynamic model provides high temporal resolution of power production (10’s of Hz), the wave resource parameters are often based on frequency-domain calculations with temporal resolution of 30 minutes to an hour. However, real ocean wave conditions vary on much shorter time scales. Relying on frequency-domain calculations will not be sufficient to capture the short-term variability and accurately predict WEC power production for a standardized methodology that follows power system requirements. These requirements need forecasted data with high sampling frequency for accurate energy predictions. Looking specifically at resource characterization, high temporal resolution datasets are not publicly available or do not exist for many coastal locations. Due to data availability, low temporal resolution datasets are being used in a majority of studies to generate representative wave conditions as inputs to numerical simulations. Representative wave conditions are used to generate wave spectrums. The issue with this practice is spectrums are then used to predict the efficiency of systems that will not accurately capture the variability of waves in short timeframes. Creating a standardized methodology to increase the temporal resolution of metaocean conditions to inform model development can provide better forecasting of power production. Random amplitude, Fourier coefficient methods have been suggested for WEC simulations of finite durations to improve the observed variability in wave heights and power production. Variability using this method does increase for finite durations compared to the commonly used deterministic amplitude method. In this paper we will investigate the influence of wave parameters (significant wave height, maximum wave height, and energy period) on the prediction of WEC power production. A better understanding of the influence of these parameters will provide a path towards future standardization methodology for resource inputs for time-domain modeling.

A novel approach to detecting pulse onset in photoplethysmographic signal using an automatic non assisted method

An automatic method for determining the location of the pulse onset on arterial pulse waves is presented. The aim is to develop an automatic method for enhance the accuracy and precision of pulse onset detection without ECG assistance (R peaks as reference). The approach is composed by two stages: pulse delineator and onset decision. Pulse delineator is based on amplitude and time thresholds computation in time and frequency domain respectively, for locating each pulse wave. Decision stage is based on triangle area algorithm that computes on each pulse the triangle area successive with two fix points and one mobile. The mobile point that corresponds to higher area is the pulse onset. Pulse delineator stage of proposed method was evaluated with CSL database, and their results were comparable to other pulse detection algorithm. Simultaneous ECG and PPG records were used to evaluate the repeatability of proposed method for several signal-noise ratios, as well as the concordance respect to trained observers. The performance of proposed method was compared with ECG assisted methods, such as tangents intersection, diastolic point and second derivative. The approach obtained acceptable values of sensitivity (> 97.99 %), positive predictivity (> 97.91 %), failed rate detection (< 4.11 %) and error (< 5.78 ± 6.64 ms). This proposal found the repeatability condition for each signal-noise ratios. This method could be used on medical systems that need the pulse onset to compute diagnostic markers.

Ultrasonic tomography using sparse antenna arrays and frequency domain post-processing

Currently, an important problem for industrial ultrasonic tomography using post-processing of matrix phased arrays signals is the improving restoration speed of results in the form of images of flaws in inspected objects. One of the approaches aimed at solving this issue is the application of sparse matrix antenna arrays. In this paper, we consider the use of the sparse matrix probes operating in a separate mode of signals acquisition together with the post-processing algorithm using the calculations in frequency-domain. Considered algorithm is based on the application of non-uniform fast Fourier transform. The efficiency of the considered approach is verified via in-situ experiments.

Mach–Zehnder Modulator Output in Time and Frequency Domain—Calculation and Experimental Confirmation

The Mach–Zehnder intensity Modulator (MZM), named after Ludwig Mach and Ludwig Zehnder, is based on the corresponding interferometer. It splits light into two counter-rotating partial beams, which are later recombined with a controlled phase difference. The output of the MZM depends on the phase difference of the interferometer paths. This phase difference is usually adjusted by an electrical voltage applied to a Phase Shifter (PS) placed in one of the interferometer arms. For MZM applications in which the wavelength is changing, the applied voltage must be adjusted accordingly. We derived the equations describing the MZM output in the frequency domain for the case of a triangular PS voltage (necessary for a sinusoidal output) and compared the analytical results with measurements. Our setup uses an Optical Parametric Oscillator (OPO) with a tunable wavelength from 3.2–3.5 μm as the light source and a Lithium Tantalate (LT)-PS for the MZM’s phase modulation. The novel insights enable new control methods for MZMs particularly suited for spectroscopic applications where the wavelength is scanned or otherwise altered.

Nonsinusoidal in situ electric field caused by magnetic deactivator device for EAS labels—assessment of field strength inside a detailed anatomical hand model

In order to evaluate the localised magnetic field (MF) exposure of the cashier’s hand due to a particular demagnetization device (deactivator) for single-use labels of an acoustomagnetic (AM) electronic article surveillance (EAS) system, comprehensive measurements of the MF near the surface of the deactivator, and numerical computations of the induced electric field strength E i, were performed in high-resolution anatomical hand models of different postures and positions with respect to the deactivator. The measurement results for magnetic induction B were assessed with respect to the action levels (AL) for limb exposure, and the computational results for E i were evaluated with respect to the exposure limit values (ELV) for health effects according to European Union (EU) directive 2013/35/EU. For the ELV-based assessment, a maximum of the 2 × 2 × 2 mm3 averaged E i (maxE i,avg) and the respective 99.9th, 99.5th, and 99.0th percentiles were used. As the MF impulse emitted by the deactivator for demagnetization of the AM-EAS labels was highly nonsinusoidal, measurement results were assessed based on the weighted peak method in the time domain (WPM-TD). A newly developed scaling technique was proposed to also apply the WPM-TD to the assessment of the (nonsinusoidal) E i regarding the ELV. It was used to calculate the resulting WPM-TD-based exposure index (EI) from frequency domain computations. The assessment regarding the AL for limbs yielded peak values of magnetic induction of up to 97 mT (measured with a 3 cm2 MF probe on top of the deactivator surface) corresponding to an EI of 443%. However, this was considered an overestimation of the actual exposure in terms of E i as the AL were intentionally defined conservatively. A WPM-TD-based assessment of E i finally led to the worst case EI of up to 135%, 93%, 78%, and 72% when using the maxE i,avg, 99.9th, 99.5th, and 99.0th percentiles, respectively.

Nonlinear Aeroelastic Characteristics of a Supersonic All-Movable Fin with Single or Multiple Freeplay Nonlinearities

Establishing the aeroelastic characteristic of all-movable fins with freeplay nonlinearities is one of the most common problems in the design of supersonic flight vehicles. In this context, this study provided novel points of view on the nonlinear aeroelastic characteristics of an all-movable fin with freeplay nonlinearities in its root. The unsteady aerodynamic model that was employed uses the second-order piston theory considering thickness effects. For a system with multiple freeplay nonlinearities, a discrete scanning method based on the describing function method was established to solve the limit-cycle oscillations (LCOs) and avoid the loss of solutions. Combining this with the time-domain integration method, the influences of the support stiffness at the root of the fin and the freeplay size ratio of the bending and torsional degrees of freedom on the dynamical response of the system were analyzed. The results demonstrate that systems with a single freeplay nonlinearity exhibit two completely different types of LCO, while systems with multiple freeplay nonlinearities exhibit complex dynamical behaviors such as LCO and quasi-periodic and chaotic motions. The path of a quasi-periodic torus breaking into chaos was observed. Furthermore, a harmonic initial condition for the time-domain integration is proposed; this can be used for a quick check of the frequency-domain calculation results.

Study on Characteristics and Optimal Layout of Components in Shallow Water Mooring System of Floating Wind Turbine

Offshore wind energy resources far exceed those on land. However, the increase in water depth of the continental shelf in China’s sea area slows down as the distance from the shore increases. In view of the characteristics of China’s sea, how to optimize the design of mooring systems to adapt to the water depth conditions in China’s waters and to resist the harsh sea conditions is one of the major problems encountered in the development of floating wind turbines. In this paper, the 5MW-OC4 semi-submersible floating wind turbine is taken as the research object, and the frequency domain and time domain calculation of the floating wind turbine are carried out by using SESAM software under the water depth of 40 m in the Bohai Sea. By comparing the motion response and tension of catenary and tension mooring floating wind turbine. The mooring system is optimized by combining the buoy and clump weight. The results show that the catenary mooring mode is more suitable for the floating wind turbine under shallow water conditions. Mooring accessories used in combination have the same effect on the overall response of the floating wind turbine as changing the same mooring parameters when used alone, but the effect of changing the mooring parameters on the performance optimization of the mooring system is more obvious when used in combination, and a reasonable combination of accessories can significantly change the overall characteristics of the floating system and affect the safety and cost of the system.

Proposed updates for ANSI/ASA S12.42 impulsive noise test methods

ANSI/ASA S12.42-2010 contains the first standardized test method for measuring the performance of hearing protection devices (HPDs) with high-level impulsive noise. This standard defines the impulsive peak insertion loss (IPIL) as a time-domain metric that quantifies the reduction in peak sound pressure level provided by an HPD for impulsive noises. However, IPIL as defined in S12.42-2010 is dependent on the spectrum of the impulsive noise source used for measurements of HPDs. Recent studies of HPDs with impulsive noise have led to the investigation of frequency-domain metrics and calculation methods. Using frequency-domain calculations allows IPIL to be computed for a wide range of impulsive noises, not only those used for the measurement. Consequently, significant revisions to S12.42 are under consideration. This presentation highlights select proposals including: definition and calculation of the frequency-dependent impulsive insertion loss, incorporation of frequency-domain aspects such as bone-conduction limits to attenuation, and redefinition of IPIL to be calculated from the impulsive insertion loss.

Digital Coherent Signal Processing with Calculations in Frequency Domain for Solving Ultrasound Tomography Problems Using Matrix Antenna Arrays with Nonequidistant Arrangement of Elements

Digital Coherent Signal Processing with Calculations in Frequency Domain for Solving Ultrasound Tomography Problems Using Matrix Antenna Arrays with Nonequidistant Arrangement of Elements

Comparative Study on Predicting Ship Maneuvering in Waves Using a Quasi-Steady Method and a Time Domain Approach

_ Numerical prediction of ship maneuvering in waves was considered in this article. The wave drift loads, computed using the potential flow theory, were added into the mathematical modeling group (MMG) equations to account for the effect of waves on the ship maneuvering. Two numerical methods were tested for dealing with the coupled maneuvering-seakeeping problem, namely a time domain approach and a quasi-steady method. For the former approach, a time domain seakeeping computation was conducted that parallels to the maneuvering simulation. For the later one, it is assumed that at each time of the maneuvering process, the wave-ship interaction is in a time harmonic status and, therefore, the wave drift loads could be evaluated using a frequency domain computation. Turning maneuvers of the S-175 container ship in regular waves were numerically tested. The results of the quasi-steady method and the time domain approach show good agreements, which proved the validity of the quasi-steady assumption. The wave drift loads during the turning process were also presented, demonstrating the significant effect of the added resistance on the maneuvering prediction, in contrast to the less remarkable effects of the lateral wave drift force and the wave drift yaw moment. Introduction Ship maneuverability is typically predicted under calm water condition. This provides valuable information at the ship design stage. However, an actual seagoing ship usually maneuvers in waves. To reliably assess a ship’s navigation safety and total performance in a seaway, it is deemed important to understand the maneuvering behavior of a ship in waves. Indeed, ship maneuverability in waves has been increasingly investigated in recent years. Although the physical experiment is still regarded as the most reliable way to investigate ship maneuverability in waves, there are more and more studies providing practical mathematical models of ship maneuvering prediction. Generally, a mathematical model that is suitable for investigating the maneuvering of a ship in waves has to encapsulate the traditional theories of calm water maneuvering and forward-speed seakeeping.

Finite time stable inversion in discrete frequency domain: Accuracy analysis, improvement and application to wafer stage

Stable inversion represents a classic approach to achieve the exact inverse input for non-minimum-phase (NMP) systems. Solutions deduced based on state–space equations and transfer functions have been frequently proposed, however they are basically developed in the infinite-time horizon and are inherently time-domain computation methods. Considering the practical finite-time tracking tasks, this paper investigates the finite-time stable inversion problem. In particular, the discrete-frequency-domain solution which enables frequency-domain computation is studied. As the main contribution of the paper, the accuracy issues of the discrete-frequency-domain solution are revealed and an easy-to-use procedure is provided to improve the inversion accuracy by utilizing pre-actuation and post-actuation methods. Simulation and experiment both verify the effectiveness of the developed discrete-frequency-domain stable inversion technique.

Band gap analysis of composite fluid-filled pipe with periodically axial support or dynamic vibration absorbers

Two kinds of periodic composite pipes with support or dynamic vibration absorber are designed based on the theory of phononic crystals. Axial vibration and band gaps of composite fluid-filled pipe are calculated by using transfer matrix method and Bloch wave theory. The present fluid–structure interaction model and frequency domain calculation method are validated by comparing the velocity of fluid from a FEM model. The results show that the stop bands frequency of velocity responses are in good agreement with band gap. Longer length of a single cell decreases the beginning frequency and width of both two kinds’ band gaps.

Aircraft Flutter and Aerodynamic Work

This paper is about an application of an energy approach to computational aeroelasticity. A frequency-domain calculation of aerodynamic work is presented in a form that has not been previously discussed. In large aeroelastic systems such as aircraft flutter models, the obtained expression allows us to quantify the roles played in flutter by the generalized coordinates, the phases between them, and the generalized aerodynamic forces. This is exemplified in a body-freedom flutter analysis of a flying-wing aircraft: the X-56A. Another interesting feature that is proposed is a diagram of the aerodynamic work as a function of airspeed. Such functions allow an observance of an evolution with the airspeed of the terms that are most reflective of the aeroelastic stability changes: for example, a phase between the two dominant modes. These diagrams can complement typical flutter trends in analysis documentation and aid in flutter suppression. The approach also permits a perspective on flutter as an interaction of aircraft surfaces rather than vibrational modes. Once a sensitivity of an aeroelastic eigenvalue to a surface area is measured with the presented approach, it can be used in the aircraft design to mitigate flutter and other undesirable aeroelastic responses associated with lightly damped eigenvalues. An illustration of this idea is provided. Finally, energy-based computations allow posing energy-efficient active flutter suppression problems. This has been presented before in the literature. Examples of this aspect are made here with aircraft models (for the first time, as far as the author knows).