Efficient, wide-band coupled structural-acoustic computations combining time and frequency domain finite elements/equivalent sources

Time domain simulations of an offshore floating wind turbine have been performed. Hydrodynamic impulse responses of the floating platform are calculated with linear hydrodynamic simulation tool ACHIL3D. A user defined module for the wind turbine design code FAST has been developed to calculate hydrodynamic and mooring loads on the structure. Resolution of the movements of the system is done with FAST. Simulation results in time domain are compared with frequency domain results. In the frequency domain model, the whole system is linearized. In the time domain model, the wind turbine model is not linearized. A good agreement between time and frequency domain calculations is observed, even for the pitch motion. Furthermore we observe a non linearity in the response of sway, roll and yaw degrees of freedom around 0.3 rad.s-1. The effect of viscous damping on the movements of the floating wind turbine system has been studied with the time domain model, and a non linear hydrostatic and Froude-Krylov load model has been developed. Effects of these non linear terms are shown.

The finite-difference time domain (FDTD) method is a numerical technique based on the finite difference concept. It is employed to solve Maxwell's equations for the electric and magnetic field distributions in both the time and spatial domains. The FDTD method utilizes the central difference approximation to discretize two of Maxwell's curl equations, namely, Faraday's and Ampere's laws, in both the time and spatial domains, and then solve the resulting equations numerically to derive the electric and magnetic field distributions at each time step and spatial point using the explicit leap-frog scheme. The FDTD solution, thus derived, is second-order accurate, although the difference formulation is first order, and is stable if the time step size is chosen to satisfy the special criterion.

This paper describes the procedure for calculating low frequency magnetic fields radiated by high voltage (HV) powertrain systems of electric vehicles (EV). Simulations or measurements of common mode currents on HV cables are done in time domain. The spectrum of such signals contains many harmonics and 3D radiation calculations in frequency domain and might be computationally very costly. We propose a special procedure to automatically capture the main harmonics of the signal and perform calculations only for these harmonics. It is shown that this procedure is very effective at low frequencies, where car body resonances do not appear yet. Application examples are also demonstrated.

BackgroundIn investigations of the human motor system, two approaches are generally employed toward the identification of common modulating drives from motor unit recordings. One is a frequency domain method and uses the coherence function to determine the degree of linear correlation between each frequency component of the signals. The other is a time domain method that has been developed to determine the strength of low frequency common modulations between motor unit spike trains, often referred to in the literature as 'common drive'.MethodsThe relationships between these methods are systematically explored using both mathematical and experimental procedures. A mathematical derivation is presented that shows the theoretical relationship between both time and frequency domain techniques. Multiple recordings from concurrent activities of pairs of motor units are studied and linear regressions are performed between time and frequency domain estimates (for different time domain window sizes) to assess their equivalence.ResultsAnalytically, it may be demonstrated that under the theoretical condition of a narrowband point frequency, the two relations are equivalent. However practical situations deviate from this ideal condition. The correlation between the two techniques varies with time domain moving average window length and for window lengths of 200 ms, 400 ms and 800 ms, the r2 regression statistics (p < 0.05) are 0.56, 0.81 and 0.80 respectively.ConclusionsAlthough theoretically equivalent and experimentally well correlated there are a number of minor discrepancies between the two techniques that are explored. The time domain technique is preferred for short data segments and is better able to quantify the strength of a broad band drive into a single index. The frequency domain measures are more encompassing, providing a complete description of all oscillatory inputs and are better suited to quantifying narrow ranges of descending input into a single index. In general the physiological question at hand should dictate which technique is best suited.

New version of FDTD with GFM program in Schwarzschild space–time

Time-domain prediction models have been developed for auditorium reflectors and room acoustic diffusers. The models are time-domain equivalents of the single-frequency formulations that exploit the Kirchhoff boundary conditions. Consequently, they are approximate, wave-based solutions to the Kirchhoff integral equation using surface meshes. The new time-domain formulations are validated by comparison to their frequency-domain equivalents for three different surfaces: a plane surface, a curved reflector, and a Schroeder diffuser. In terms of computation time and accuracy, the new models lie between the finite difference time domain and geometric room models.

Near to Far zone transformation can be performed by integration of the equivalent electric and magnetic currents originating from scattered electric and magnetic fields on a surface enclosing the object. This can be done with finite difference time domain and frequency domain. In this way, near zone fields can be computed by finite difference time domain and conversion of near zone to far zone can be done in frequency domain. This method combines the accuracy of time domain in near zone and speed of computation in frequency domain to convert near zone to far zone.

We present two approaches to solving the airborne electromagnetic (AEM) problems given the induced polarization (IP) effect: with calculating the field in a polarizable medium directly in the time domain (TD) and with calculating the EM+IP field in the frequency domain (FD) followed by a transition to the time domain. The first approach is based on calculating the field at each time step given the sources that depend on the currents excited in the medium at previous time steps. This approach allows us to use any IP decay functions. The frequency domain approach is based on the Fourier series expansion of a non-stationary source and the Cole-Cole model. In order to reduce the computational cost, we use the Hermite spline interpolation. Both approaches allow modeling EM + IP processes in complex media with topography and the curved boundaries of layers containing 3D heterogeneities. 3D modeling is performed on non-conforming hexahedral meshes generated fully automatically. The analysis of computational efficiency and verification of the developed approaches are presented in comparison with the results from other researchers. Moreover, we compare the results obtained in the time and frequency domains. The results of 3D modeling the IP effects, which are obtained for some geoelectrical models typical for AEM exploration problems, are demonstrated.

This article investigates unstable tiltrotor in hover system identification from flight test data. The aircraft dynamics was described by a linear model defined in Body-Fixed-Coordinate System. Output Error Method was selected in order to obtain stability and control derivatives in lateral motion. For estimating model parameters both time and frequency domain formulations were applied. To improve the system identification performed in the time domain, a stabilization matrix was included for evaluating the states. In the end, estimates obtained from various Output Error Method formulations were compared in terms of parameters accuracy and time histories. Evaluations were performed in MATLAB R2009b environment.

Since PWM signal is hard to express with formulation in time domain while it is simple in the frequency domain and taking the frequency dependence of the system into consideration, this paper establishes a new way to the reconstruction of PWM signal from frequency domain to time domain. It uses double Fourier transform to decompose the PWM signal in frequency domain, then gets terminal waveforms by using the model of different frequency viewing from motor. Finally, it refactors phase information based on principle of minimum phase system to recovery time domain waveform and then to compare with the waveform of reality. The recovery of the signal is nearly consistent to the real waveform. It can be laid the theoretical foundation of long-line transmission systems of oil exploration in our country.

Within the FaUSt (Fast Unmanned Scout) project, an existing UAV with intermeshing rotors was enhanced by two electric pusher propellers to extend the speed range for future manned-unmanned teaming research. The modeling of such a compound UAV has not yet been covered in the literature. To extract models for simulation and flight control design, dedicated flight tests were performed using a multi-axis binary noise signal for system identification with activated base flight control system. Two different system identification methods are applied to estimate bare-airframe models of the modified UAV at hover condition: one in the time domain, the other in the frequency domain. The time domain method can inherently be applied to correlated closed-loop data, while input correlations have to be accounted for when using the frequency domain method. The resulting models are analyzed in both time and frequency domain and the model eigenvalues and modes are compared in detail. The results indicate that the time-domain model matches low frequency modes of the vehicle more accurately, but both models show good model performance.

In order to study the effects of large HVDC power converters on the feeding AC networks, it is of importance to explain and to calculate harmonic phenomena which are a result of power converter operation. During commissioning of real HVDC power converters, it could be seen that harmonics resulting from asymmetry in the system voltages or from asymmetry in power converter operation led to significant difficulties concerning the system design. For this reason, not only the effects of characteristics but also the effects of noncharacteristic converter harmonics must be taken into account. The aim is to describe the steady state harmonic behavior of the converter. The harmonic spectra are not determined by time domain analysis but instead the solution is found by frequency domain calculations. This can result in reduced calculation time in comparison to conventional Fourier analysis of the time functions. The power converter is interpreted as an amplitude modulator with voltage and current converter functions which describe the coupling of the DC circuit and the AC network through the power converter. To verify the theory, comparisons of frequency domain with time domain calculations were carried out. >

We describe the main properties of the EQUATOR airborne geophysical system and how the system can be employed to solve some engineering and environmental problems. To date, the EQUATOR system has been used for groundwater search in 2014 and 2015 in Siberia. The name “EQUATOR” originates from the intention to EQUATE, or to balance time and frequency domain approaches in airborne electromagnetics, which is why the system was originally designed. To the best of our knowledge, EQUATOR is the only system to collect electromagnetic data both in time and in frequency domains. We further explain the difference between the two methods on some numerical and field examples. Also we describe some advantages of their coupling. In particular, by virtue of our approach, a numerically more robust solution of the one-dimensional electromagnetic inverse problem may be achieved. The recorded full-time measurements are further converted to in-phase, quadrature and off-time responses, as opposed to standard frequency or time domain solutions, which have been widely adapted within the industry. When inverting the data, our main focus is within the top 100 m of the Earth, which are the most relevant for engineering and environmental studies. We emphasize that a combined use of frequency and time domain data gives more detailed information about geological structures in the near-surface. As an example, we present some case studies obtained over a platform (Siberia) and a crystalline shield (Western Africa) setting. We further suggest using these results in such near-surface studies as sediment thickness estimation, soil state analysis, groundwater search, and geological structural analysis.

Computation of dynamic stress intensity factors by the time domain boundary integral equation method—II. Examples

Influence of water activity on acoustic emission of flat extruded bread