Frequency Domain Calculations Research Articles

-Adaptive Stabilized Finite-Element Solver for Calculation of Generalized Aerodynamic Forces

A finite-element method based on the streamline-upwind/Petrov–Galerkin stabilization technique is developed to solve the compressible Euler equations. The method is extended for computing unsteady small disturbances in the flow domain as a result of boundary motion, which is applied through the linearized solid-wall boundary condition. The method is able to calculate the frequency-dependent generalized aerodynamic forces without any need for deformation of mesh inside the flow domain. Error estimates are used to drive automated -refinement of mesh for steady-state and frequency-domain calculations. Steady-state solutions and generalized aerodynamic forces are compared to benchmark data for transonic and supersonic Mach numbers. The results show that the presence of shocks in the flow makes it difficult to use a single mesh for all computations, including the linearized flow. The -refinement procedure is shown to be an effective way of ensuring reliable computations for the nonlinear and linearized solvers. Some associated challenges with the refinement procedure are also highlighted.

Accelerating the frequency-domain response calculation of complex grounding system using wavelet based MBPE technique

The transient response of grounding systems is essential for their designs and related electromagnetic compatibility problems in power systems. Although the method of moments (MoM) is a popular way to analyze the responses of grounding systems, it is prohibitively slow. In this paper, the model-based parameter estimation (MBPE) technique and wavelet matrix transform (WMT) are combined and applied to the method of moments (MOM). It is expanded on two-steps technique to accelerate solution of electric field integral equations (EFIE). In a first step, the WMT is used to get a sparse matrix equation in wavelet-domain. Then in a second step, the adaptive model-based parameter estimation is used to reduce the number of frequency-domain calculation points required for the evaluation of space–time current distribution along a grounding grid. The accuracy of the proposed model is confirmed by comparing the simulation results of several case studies with those obtained using the direct MoM method combined with inverse fast Fourier transform (IFFT). It is shown that considerable computation efficiency is achieved in terms of CPU time without losing accuracy.

Stroke and natural frequency estimation for linear compressor using phasor algorithm

The stroke and natural frequency are key controlling quantities for linear compressor controller. This paper proposes a phasor algorithm for the linear compress stroke and natural frequency calculation. The phasor algorithm employs vectors to express the voltage, current and velocity respectively for the calculation in frequency domain. A test bench has been installed, and the feasibility of phasor algorithm is verified with the linear compressor load changing. The relative deviation of the calculated stroke can be achieved within ± 3% compared to the experimental results at 0.4 MPa, 0.5 MPa and 0.6 MPa discharge pressure respectively. The calculated results shows that the computation time of phasor algorithm is one-third shorter than that of time domain algorithm, which means it is possible to employ relatively low energy consumption and low cost hard ware. The calculation natural frequency decreases with the stroke increases at 0.4 MPa, 0.5 MPa and 0.6 MPa discharge pressure respectively, and the tested motor efficiency reaches a maximum when the calculated natural frequency equals to the operating frequency.

Charged scalar field instability between the event and cosmological horizons

Recently, a new interesting instability of a charged scalar field in the Reissner-Nordstr\"om-de Sitter background has been found [Z. Zhu et al. Phys. Rev. D 90, 044042 (2014)] through the time-domain integration of the perturbation equation. We investigate further properties of this instability, confirm its existence by concordant frequency-domain and time-domain calculations, and show that it occurs at however small value of the coupling $eQ$, where $e$ and $Q$ are charges of a scalar field and black hole, respectively. We also investigate the parametric region of instability and show that the critical values of $eQ$ at which the stabilization happens strongly depends on the value of cosmological constant $\mathrm{\ensuremath{\Lambda}}$ and softly on $Q$. We show that all the unstable modes are superradiant, but not all the superradiant modes are unstable. We analytically prove that superradiance is a necessary (but not sufficient) condition for the instability in this case and, thereby, demonstrate the superradiant origin of the instability.

Partitioned Model Order Reduction of Partial Element Equivalent Circuit Models

As very large scale integration (VLSI) circuit speeds and density continue to increase, 3-D electromagnetic (EM) methods have become essential for the analysis, design and veri- fication of high-speed systems. Since such models are commonly used inside standard circuit simulators for time or frequency domain computations, it is imperative to make them as compact as possible without compromising accuracy. To this aim, model- order reduction techniques can be successfully adopted. In this paper, we describe a partitioned Krylov-subspace based method for deriving reduced-order models directly from 3-D quasistatic Partial Element Equivalent Circuit models. The multiscale block decomposition method is adopted to speed-up the computation of the projection matrix; furthermore, the adaptive cross approxi- mation and singular value decomposition techniques are used to achieve matrix compression exploiting the low-rank behavior of magnetic and electric field couplings. Results are presented on several examples to demonstrate the capabilities and speed of the proposed new method. Index Terms—acceleration techniques, adaptive cross approxi- mation, generalized minimal residual, model order reduction, modified nodal analysis, partial element equivalent circuit me- thod, singular value decomposition.

A Framework for the Calculation of Dynamic Crosstalk Cancellation Filters

Dynamic crosstalk cancellation (CTC) systems commonly find use in immersive virtual reality (VR) applications. Such dynamic setups require extremely high filter update rates, so filter calculation is usually performed in the frequency-domain for higher efficiency. This paper proposes a general framework for the calculation of dynamic CTC filters to be used in immersive VR applications. Within this framework, we introduce a causality constraint to the frequency-domain calculation to avoid undesirable wrap-around effects and echo artifacts. Furthermore, when regularization is applied to the CTC filter calculation, in order to limit the output levels at the loudspeakers, noncausal artifacts appear at the CTC filters and the resulting ear signals. We propose a global minimum-phase regularization to convert these anti-causal ringing artifacts into causal artifacts. Finally, an aspect that is especially critical for dynamic CTC systems is the filter switch between active loudspeakers distributed in a surround audio-visual display system with 360° of freedom of operator orientation. Within this framework we apply a weighted filter calculation to control the filter switch, which allows the loudspeakers' contribution to be windowed in space, resulting in a smooth filter transition.

A 2.5-D frequency-domain nonlinear computer model of coupled-cavity traveling wave tubes

Purpose – The purpose of this paper is to present a 2.5-dimensional (2.5-D) frequency-domain nonlinear computer model for the beam-wave interaction of coupled-cavity traveling wave tubes (CC-TWTs). Design/methodology/approach – MKK (proposed by Malykhin, Konnov, and Komarov) equivalent circuit model is used to describe the coupled-cavity slow-wave structure. And the losses are taken into account in the MKK equivalent circuit. Instead of one-dimensional (1-D) disk model, the electron beam is divided into a set of discrete rays and the electron dynamics are treated using the three-dimensional (3-D) Lorentz force equations. Findings – The simulated result obtained show that the computer model can give a good predict for CC-TWTs in V-band. Practical implications – The computer model is capable of treating nonlinear problems. Compared with particle-in-cell simulation, the 2.5-D frequency-domain computer model spends less time. Besides, the 3-D electron trajectory can be used to design high-efficiency collectors for CC-TWTs. Originality/value – The computer model is able to simulate nonlinear problems of coupled-cavity TWT faster.

A Hydrodynamic Analysis of Offshore Platform Based on the AQWA

With the drying up of land resources, the oil and natural gas,power generation equipment, and ocean data is in so great request that the research of the offshore platform is increasingly important.Offshore platform structure and the design of the mooring system both are influenced by the hydrodynamic performance.Then the floating type marine equipment carrier of hydrodynamic being analyzed after doing the frequency domain calculation for the model of floating body by the hydrodynamic software AQWA here.

Vortex algebra by multiply cascaded four-wave mixing of femtosecond optical beams.

Experiments performed with different vortex pump beams show for the first time the algebra of the vortex topological charge cascade, that evolves in the process of nonlinear wave mixing of optical vortex beams in Kerr media due to competition of four-wave mixing with self-and cross-phase modulation. This leads to the coherent generation of complex singular beams within a spectral bandwidth larger than 200nm. Our experimental results are in good agreement with frequency-domain numerical calculations that describe the newly generated spectral satellites.

Wakefield characterization in an asymmetric superconducting deflecting cavity

Nonaxially symmetric superconducting deflecting cavities are under development for a variety of purposes, but breaking the symmetry adds more complexity to wakefield characterization. This paper considers the influence of cavity asymmetry on the wakefield. Based on the TM-type deflecting cavity, we investigated how the polarized cell and the damper coupling affect the wake impedances. An improved method to characterize the wake impedances in asymmetric cavities is proposed by combining the time-domain and frequency-domain calculations. Compared to conventional methods, the new method decomposes the wake potential in terms of beam moments, which provides more information about the wakefield, and significantly reduces the simulation time with better accuracy. Using this method, we analyzed the wakefield of the deflecting cavity and the wakefield coupling between the cavities for the advanced photon source. Bench measurements were performed on a fabricated copper prototype; the measured mode field distributions and the ${Q}_{\text{ext}}$ values agreed well with calculations.

Heart Rate Variability for Preclinical Detection of Secondary Complications After Subarachnoid Hemorrhage

We sought to determine if monitoring heart rate variability (HRV) would enable preclinical detection of secondary complications after subarachnoid hemorrhage (SAH). We studied 236 SAH patients admitted within the first 48 h of bleed onset, discharged after SAH day 5, and had continuous electrocardiogram records available. The diagnosis and date of onset of infections and DCI events were prospectively adjudicated and documented by the clinical team. Continuous ECG was collected at 240 Hz using a high-resolution data acquisition system. The Tompkins-Hamilton algorithm was used to identify R-R intervals excluding ectopic and abnormal beats. Time, frequency, and regularity domain calculations of HRV were generated over the first 48 h of ICU admission and 24 h prior to the onset of each patient's first complication, or SAH day 6 for control patients. Clinical prediction rules to identify infection and DCI events were developed using bootstrap aggregation and cost-sensitive meta-classifiers. The combined infection and DCI model predicted events 24 h prior to clinical onset with high sensitivity (87 %) and moderate specificity (66 %), and was more sensitive than models that predicted either infection or DCI. Models including clinical and HRV variables together substantially improved diagnostic accuracy (AUC 0.83) compared to models with only HRV variables (AUC 0.61). Changes in HRV after SAH reflect both delayed ischemic and infectious complications. Incorporation of concurrent disease severity measures substantially improves prediction compared to using HRV alone. Further research is needed to refine and prospectively evaluate real-time bedside HRV monitoring after SAH.

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

The equations of motion of a point particle interacting with its own field are defined in terms of a certain regularized self-field. Two of the leading methods for computing this regularized field are the mode-sum and effective-source approaches. In this work we unite these two distinct regularization schemes by generalizing traditional frequency-domain mode-sum calculations to incorporate effective-source techniques. For a toy scalar-field model we analytically compute an appropriate puncture field from which the regularized residual field can be calculated. To demonstrate the method, we compute the self-force for a scalar particle on a circular orbit in Schwarzschild spacetime. We also demonstrate the relation between the worldtube and window function approaches to localizing the puncture field to the neighborhood of the worldline and show how the method reduces to the well-known mode-sum regularization scheme in a certain limit. This new computational scheme can be applied to cases where traditional mode-sum regularization is inadequate, such as in calculations at second perturbative order.

Feature selection on single-lead ECG for obstructive sleep apnea diagnosis

Many articles that appeared in the literature agreed upon the feasibility of diagnosing obstructive sleep apnea (OSA) with a single-lead electrocardiogram. Although high accuracies have been achieved in detection of apneic episodes and classication into apnea/hypopnea, there has not been a consensus on the best method of selecting the feature parameters. This study presents a classication scheme for OSA using common features belonging to the time domain, frequency domain, and nonlinear calculations of heart rate variability analysis, and then proposes a method of feature selection based on correlation matrices (CMs). The results show that the CMs can be utilized in minimizing the feature sets used for any type of diagnosis.

Estimation and Reduction of Motor Input Surge Voltage Using Frequency Response Analysis

SUMMARYThis paper presents a plain method of calculating the voltage waveform at motor input terminals from the inverter output terminal voltage and also a method of reducing the voltage. The voltage waveform at the motor input terminals is obtained by a frequency domain calculation using the transfer function of the cable and the frequency characteristics of the voltage waveform at the inverter output terminals. The analysis using this method clarifies the mechanism of the rise of the maximum voltage at the motor input terminals, that is, ringing at the motor input terminals, which occurs at specific frequencies at which the transfer gain increases, causing the rise of the maximum voltage. Therefore, to decrease the maximum voltage at the motor input terminals, it is effective to shorten the cable length or cancel the ringing voltage by preswitching. © 2013 Wiley Periodicals, Inc. Electr Eng Jpn, 186(4): 11–19, 2014; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/eej.22482

Structure–soil–structure interaction between underground structure and ground structure

The dynamic through-soil interaction between underground station and nearby pile supported structure on viscous–elastic soil layer, under vertically incident S wave, is numerically studied. To this end, a commercial software for finite element analysis, ANSYS, has been further developed and enhanced for calculation in frequency domain, in which damping of hysteretic type can be considered for both the soil and the structures, so that structure–soil–structure interaction (SSSI) can be investigated making use of a direct methodology. The influence of arrangement of structures, shaking direction of seismic wave, distances between structures, shear wave velocity, damping of soil, burial depth and number of spans of underground structure on SSSI, in terms of horizontal acceleration magnification factor of ground structure, is addressed. For ground structure, different lengths of pile, stiffnesses, styles, and numbers of storeys and structures are considered. Maximum acceleration responses are also presented for 12 seismic inputs. Arrangement and shaking direction are two of the most important factors. The system response can be either amplified or attenuated according to the distance between adjacent buildings, which has been related to dynamic properties of the overall system. Those neighboring low-slung buildings around underground structure are heavily affected.

Precision pointing estimator design for minimum absolute, window- and stability-time errors

Future satellite missions have high precision pointing performance requirements. This leads to the necessity of optimizing attitude estimators not only with respect to absolute, but also with respect to window- and stability-time errors. These errors are especially important for achieving unprecedented image quality. The standard attitude estimator is designed to be optimal with respect to its absolute knowledge error. In practice, such an estimator design is either assumed to be almost optimal for window- and stability-time errors or a cumbersome simulation-based optimization is conducted to achieve optimality in this respect. The first assumption is not sufficient for high precision pointing missions and simulation-based design is computationally expensive. In this article frequency-domain metrics are used to develop a continuous-time design approach for a fixed gain attitude estimator that is optimized with respect to window- or stability-time errors. Colored noise is explicitly taken into account in the optimization because its spectrum strongly affects the solution. However, that might lead to instability of the estimator. This issue is addressed and a solution is provided that guarantees stability with the burden of marginal performance losses only. The approach is applied to design a two-state attitude estimator and its results are verified in frequency-domain computations and time-domain simulations for the future ESA mission, Meteosat Third Generation. In this case, the computational time for optimizing the window-time error has been reduced from hours to a few seconds. The fast and high precision estimator design for window- and stability-time errors thus supports time-efficient design trade-offs.

Generalized Modes in Time-Domain Seakeeping Calculations

The concept of is to describe all degrees of freedom by mode shapes and not using any predefined shape, like rigid body modes. Generalized modes in seakeeping computations allow one to calculate the response of a single ship, springing, whipping, multibody interaction, etc., using a uniform approach. The generalized modes have already been used for frequency-domain seakeeping calculations by various authors. This article extents the generalized modes methodology to be used for time-domain seakeeping computations, which accounts for large-amplitude motions of the rigid-body modes. The time domain can be desirable for seakeeping computations because it is easy to include nonlinear load components and to compute transient response, like slamming and whipping. Results of multibody interaction, two barges connected by a hinge, whipping response of a ferry resulting from slamming loads, and the response of a flexible barge are presented to illustrate the theory.

Generalized Modes in Time-Domain Seakeeping Calculations

The concept of "generalized modes" is to describe all degrees of freedom by mode shapes and not using any predefined shape, like rigid body modes. Generalized modes in seakeeping computations allow one to calculate the response of a single ship, springing, whipping, multibody interaction, etc., using a uniform approach. The generalized modes have already been used for frequency-domain seakeeping calculations by various authors. This article extents the generalized modes methodology to be used for time-domain seakeeping computations, which accounts for large-amplitude motions of the rigid-body modes. The time domain can be desirable for seakeeping computations because it is easy to include nonlinear load components and to compute transient response, like slamming and whipping. Results of multibody interaction, two barges connected by a hinge, whipping response of a ferry resulting from slamming loads, and the response of a flexible barge are presented to illustrate the theory.

Validation of time-domain and frequency-domain calculations of acoustic propagation from breast models derived from magnetic resonance images

Magnetic resonance images with an isotropic resolution of 200 microns were collected for two human breast specimens. The images were interpolated to achieve a resolution of 50 microns and segmented to produce images of skin, fat, muscle, ductal structures, and connective tissues in consultation with a breast pathologist. The images were then mapped to acoustic parameters of sound speed, absorption and density. Calculations of acoustic propagation of fields radiated by point sources outside of the specimens were performed using the k-space finite-difference time-domain method and the frequency-domain fast multipole method. Time-domain k-space results were Fourier transformed and the 3-MHz component was compared to 3-MHz frequency-domain calculations. For the first model, measuring 1180 × 1190 × 290 voxels, the two methods were found to agree in a representative coronal slice to within 5.0% (RMS). The second specimen, comprising 1350 × 1170 × 790 voxels, yielded temporal and frequency-domain results that agreed to within 5.8% (RMS) in a representative coronal slice. Comparable results were obtained in other planes orthogonal to the representative slices. The close agreement establishes confidence in the accuracy of the methods when simulating propagation through large, complicated, realistic models of human tissue.

Time- and frequency-domain computations of broadband noise due to interaction between incident turbulence and rectilinear cascade of flat plates

Time-domain computational aeroacoustic (CAA) techniques are developed to investigate the broadband noise resulting from the interaction of a rectilinear cascade of flat plates with incident homogeneous, isotropic turbulence. The investigation is carried out by comparing the prediction results obtained by employing the time-domain CAA method with those using existing frequency-domain methods. A semi-analytic model (Wei & Cheong, 2010) and a full three-dimensional rectilinear cascade model (Lloyd & Peake, 2008; Lloyd, 2009) are adopted for the frequency-domain computations. By comparing these computation results, the three-dimensional characteristics of inflow turbulence noise are investigated; in particular, the effects of the wavenumber components of ingested turbulence in the spanwise direction are taken into consideration in the investigation. First, CAA results are compared with those from the semi-analytic model. The results for the acoustic modes of relatively low spanwise wavenumbers obtained using both methods show good agreement, but as the spanwise wavenumber increases, the results obtained by the two methods become increasingly different. To investigate in detail the reason for these differences, mode-decomposition analysis is performed by adopting a hybrid method as well as by employing the CAA and the semi-analytic method. The hybrid method involves the following two sequential computations: (i) the upwash velocities on the flat plate airfoils of the rectilinear cascade are first predicted using the frequency-domain method, and (ii) the acoustic wave propagation is subsequently analyzed using time-domain CAA techniques, with these upwash velocities applied as the boundary conditions on the flat plate. It is seen that the results of the time-domain CAA technique and the hybrid method show good agreement, irrespective of the wavenumber and frequency. However, comparisons of the acoustic solutions from three computations reveal that the prediction results of the semi-analytic model deviate more from the other two predictions as the spanwise wavenumber of the acoustic wave increases and the frequency decreases. On a basis of this observation, a formulation is derived for the error in the pressure jump across the flat-plate predicted by using the semi-analytic method. This formulation shows that the error is approximately inversely proportional to the sound speed in the spanwise direction of the concerned acoustic modes. This result quantitatively clarifies the limitations of applying the frequency-domain method of Wei & Cheong (2010) to the three-dimensional turbulence-cascade interaction problems. Secondly, the prediction results using the time-domain CAA method are compared with those from the full three-dimensional rectilinear model that is believed to be exact model for the cascade geometry considered in this paper. This comparison shows the good agreements between two predictions, which support the above arguments for the error and the successful application of the time-domain CAA methods. It is expected that these methods can be extended to the broadband noise problem in an annular cascade, including the nonlinear interaction of the real-airfoil cascade with the incident nonhomogeneous gust.