Frequency-dependent Radiation Research Articles

A QoS-Aware Underwater Optimization Framework for Inter-Vehicle Communication using Acoustic Directional Transducers

Underwater acoustic communications consume a significant amount of energy due to the high transmission power (10-50 W) and long data packet transmission times (0.1-1 s). Mobile Autonomous Underwater Vehicles (AUVs) can conserve energy by waiting for the `best' network topology configuration, e.g., a favorable alignment, before starting to communicate. Due to the frequency-selective underwater acoustic ambient noise and high medium power absorption - which increases exponentially with distance - a shorter distance between AUVs translates into a lower transmission loss and a higher available bandwidth. By leveraging the predictability of AUV trajectories, a novel solution is proposed that optimizes communications by delaying packet transmissions in order to wait for a favorable network topology (thus trading end-to-end delay for energy and/or throughput). In addition, the solution proposed - which is implemented and compared with geographic routing solutions and delay-tolerant networking solutions using an emulator that integrates underwater acoustic WHOI Micro-Modems - exploits the frequency-dependent radiation pattern of underwater acoustic transducers to reduce communication energy consumption by adjusting the transducer directivity on the fly.

Effect of frequency-dependent radiation pattern in the strong motion simulation of the 2011 Tohoku earthquake, Japan, using modified semi-empirical method

We perform a strong ground motion simulation using a modified semi-empirical technique (Midorikawa in Tectonophysics 218:287–295, 1993), with frequency-dependent radiation pattern model. Joshi et al. (Nat Hazards 71:587–609, 2014) have modified the semi-empirical technique to incorporate the modeling of strong motion generation areas (SMGAs). A frequency-dependent radiation pattern model is applied to simulate high-frequency ground motion more precisely. Identified SMGAs (Kurahashi and Irikura in Earth Planets Space 63:571–576, 2011) of the 2011 off the Pacific coast of Tohoku earthquake (M w = 9.0) were modeled using this modified technique. We analyzed the effect of changing seismic moment values of SMGAs on the simulated acceleration time series. Final selection of the moment values of SMGAs is based on the root-mean-square error (RMSE) of waveform comparison. Records are simulated for both frequency-dependent and constant radiation pattern function. Simulated records for both cases are compared with observed records in terms of peak ground acceleration, peak ground velocity and pseudo-acceleration response spectra at different stations. Comparison of simulated and observed records in terms of RMSE suggests that the method is capable of simulating record, which matches in a wide frequency range for this earthquake and bears realistic appearance in terms of shape and strong motion parameters. The results confirm the efficacy and suitability of rupture model defined by five SMGAs for the developed modified technique.

The reliability of approximate radiation transport methods for irradiated disk studies

Context: Dynamical studies of irradiated circumstellar disks require an accurate treatment of radiation transport to, for example, properly determine cooling and fragmentation properties. The radiation transport algorithm should be as fast as the (magneto-) hydrodynamics to allow for an efficient usage of computing resources. Methods: We use a setup of a central star and a slightly flared circumstellar disk. We perform simulations for a wide range of optical depths of the disk's midplane from tau(550nm) = 0.1 up to tau(810nm) = 1 million. We check the accuracy of the gray flux-limited diffusion (FLD) approximation and a gray and frequency-dependent ray-tracing plus FLD approximation. Results: 1. For moderate optical depths, a gray approximation of the stellar irradiation yields a slightly hotter inner rim and a slightly cooler midplane of the disk at larger radii, but is otherwise in agreement with the frequency-dependent treatment. 2. The gray FLD approximation fails to compute an appropriate temperature profile in all regimes of optical depth; the maximum deviations to the comparison runs are 50 percent in the optically thin and up to 280 percent in the optically thick limit. For low optical depth, the isotropic assumption within the FLD method yields a too steep decrease of the radial temperature slope. For higher optical depths, the FLD approximation does not reproduce the shadow behind the optically thick inner rim of the circumstellar disk, yielding artificial heating at larger disk radii. 3. The frequency-dependent RT + gray FLD approximation yields remarkable accuracy for the whole range of optical depths. Conclusions: The high accuracy of the frequency-dependent hybrid radiation transport algorithm makes this method ideally suited for (magneto-) hydrodynamical studies of irradiated circumstellar disks.

Compressive sensing of frequency-dependent seismic radiation from subduction zone megathrust ruptures

Megathrust earthquakes rupture a broad zone of the subducting plate interface in both along-strike and along-dip directions. The along-dip rupture characteristics of megathrust events, e.g., their slip and energy radiation distribution, reflect depth-varying frictional properties of the slab interface. Here, we report high-resolution frequency-dependent seismic radiation of the four largest megathrust earthquakes in the past 10 y using a compressive-sensing (sparse source recovery) technique, resolving generally low-frequency radiation closer to the trench at shallower depths and high-frequency radiation farther from the trench at greater depths. Together with coseismic slip models and early aftershock locations, our results suggest depth-varying frictional properties at the subducting plate interfaces. The shallower portion of the slab interface (above ∼15 km) is frictionally stable or conditionally stable and is the source region for tsunami earthquakes with large coseismic slip, deficient high-frequency radiation, and few early aftershocks. The slab interface at intermediate depths (∼15–35 km) is the main unstable seismogenic zone for the nucleation of megathrust quakes, typically with large coseismic slip, abundant early aftershocks, and intermediate- to high-frequency radiation. The deeper portion of the slab interface (∼35–45 km) is seismically unstable, however with small coseismic slip, dominant high-frequency radiation, and relatively fewer aftershocks.

Analysis of seismic magnitude differentials (m b−M w) across megathrust faults in the vicinity of recent great earthquakes

Spatial variations in underthrusting earthquake seismic magnitude differentials (mb − Mw) are examined for plate boundary megathrusts in the vicinity of the 26 December 2004 Sumatra-Andaman (Mw 9.2), 2010 Maule, Chile (Mw 8.8), and 11 March 2011 Tohoku, Japan (Mw 9.1) great earthquakes. The magnitude differentials, corrected for ω-squared source spectrum dependence on seismic moment, provide a first-order probe of spatial variations of frequency-dependent seismic radiation. This is motivated by observations that the three great earthquakes all have coherent short-period radiation from the down-dip portions of their ruptures as imaged through back-projections, but little coherent short-period energy from shallower regions where large coseismic slip occurred. While there is substantial scatter in the magnitude measures, all three regions display some increase in relative strength of short-period seismic waves with depth, with the pattern being strongest for Sumatra and Japan where the deeper portion of the seismogenic zone is below the overriding crust. Other regions such as the Kuril Islands, Aleutians, Peru, and Southern Sumatra/Sumba show little, if any, depth pattern in the magnitude differentials. Variation in material and frictional properties over particularly wide seismogenic megathrusts likely produce the depth-dependence observed in both mb−Mw residuals and great earthquake seismic radiation.

Radiation characteristics of multiple and single sound hole vihuelas and a classical guitar

Two recently built vihuelas, quasi-replicas of the Spanish Renaissance guitar, one with a small body and one sound hole and one with a large body with five sound holes, together with a classical guitar are investigated. Frequency dependent radiation strengths are measured using a 128 microphone array, back-propagating the frequency dependent sound field upon the body surface. All three instruments have a strong sound hole radiation within the low frequency range. Here the five tone holes vihuela has a much wider frequency region of strong sound hole radiation up to about 500 Hz, whereas the single hole instruments only have strong sound hole radiations up to about 300 Hz due to the enlarged radiation area of the sound holes. The strong broadband radiation of the five sound hole vihuela up to about 500 Hz is also caused by the sound hole phases, showing very consistent in-phase relations up to this frequency range. Also the radiation strength of the sound holes placed nearer to the center of the sound box are much stronger than those near the ribs, pointing to a strong position dependency of sound hole to radiation strength. The Helmholtz resonance frequency of the five sound hole vihuela is influenced by this difference in radiation strength but not by the rosettas, which only have a slight effect on the Helmholtz frequency.

BENCHMARK TESTS FOR STRONG GROUND MOTION PREDICTION METHODS: CASE FOR STOCHASTIC GREEN’S FUNCTION METHOD (PART2)

We performed a benchmark tests for the strong motion prediction methods using stochastic Green’s function method. In addition to the previous paper (Part 1), frequency dependent radiation coefficient of the source and oblique incidences of both SH and SV waves are newly considered. All the results calculated by four teams generally show good agreement to each other, although the analytical conditions are more complicated than the Part 1 paper. Since random numbers are used in generating time histories, synthesized amplitude shows variation in particular frequencies. When applying the stochastic Green’s function method, this variation should be in mind.

Frequency-dependent energy radiation and fault coupling for the 2010 Mw8.8 Maule, Chile, and 2011 Mw9.0 Tohoku, Japan, earthquakes

[1] We carried out back-projections of teleseismic data filtered in different frequency bands for the 2010 Maule, Chile and the 2011 Tohoku, Japan earthquakes. For the Maule earthquake, there were differences along strike of the fault, with the high-frequency energy mainly originating from an area 200 km northeast of the epicenter, whereas low-frequency energy came from a location closer to the epicenter. The Tohoku earthquake shows strong frequency dependence in the dip direction. High-frequency sources were located about 100 km west of the epicenter, while low-frequency sources were around epicenter, near the Japan Trench. We compare the spatial distributions of energy with estimates of seismic coupling before the earthquakes. Areas of high-frequency radiation seem correlated with regions that were strongly coupled before the earthquakes. Areas of high coupling, may be associated with fault properties that are more heterogeneous and/or have overall higher stress, producing higher frequency seismic waves.

Generalized Partial Element Equivalent Circuit (PEEC) Modeling With Radiation Effect

In this paper, a new frequency-domain formulation of the partial element equivalent circuit (PEEC) model incorporating the concept of generalized complex partial inductance and pure real capacitance is introduced for modeling of 3-D structures, to which the radiation effect is not negligible. Unlike conventional PEEC-based models, the proposed formulation accounts for the radiation effect by introducing physically meaningful complex-valued inductors and pure real-valued capacitors. In essence, the imaginary part of such an inductor represents a frequency-dependent radiation resistance. Having introduced the complex inductance, there is no inversion of the complex matrix of coefficients of potential, which is not physically meaningful and inevitably creates negative resistance. It is proven in this paper that the imaginary part of the generalized complex inductance for a short dipole exactly reflects the radiation resistance of the dipole. Several numerical examples are given to validate the proposed theory. The results obtained are in good agreement with those from commercial full-wave EM solvers, showing the potential of this technique for analyzing and designing high-frequency and high-speed electronic devices.

Fast and accurate frequency-dependent radiation transport for hydrodynamics simulations in massive star formation

Context: Radiative feedback plays a crucial role in the formation of massive stars. The implementation of a fast and accurate description of the proceeding thermodynamics in pre-stellar cores and evolving accretion disks is therefore a main effort in current hydrodynamics simulations. Aims: We introduce our newly implemented three-dimensional frequency dependent radiation transport algorithm for hydrodynamics simulations of spatial configurations with a dominant central source. Methods: The module combines the advantage of the speed of an approximate Flux Limited Diffusion (FLD) solver with the high accuracy of a frequency dependent first order ray-tracing routine. Results: We prove the viability of the scheme in a standard radiation benchmark test compared to a full frequency dependent Monte-Carlo based radiative transfer code. The setup includes a central star, a circumstellar flared disk, as well as an envelope. The test is performed for different optical depths. Considering the frequency dependence of the stellar irradiation, the temperature distributions can be described precisely in the optically thin, thick, and irradiated transition regions. Resulting radiative forces onto dust grains are reproduced with high accuracy. The achievable parallel speedup of the method imposes no restriction on further radiative (magneto-) hydrodynamics simulations. Conclusions: The proposed approximate radiation transport method enables frequency dependent radiation hydrodynamics studies of the evolution of pre-stellar cores and circumstellar accretion disks around an evolving massive star in a highly efficient and accurate manner.

Novel Compact Model for the Radiation Pattern of UWB Antennas Using Vector Spherical and Slepian Decomposition

A new compact model is described for the 3D radiation pattern of an ultrawideband antenna, using a vector spherical and Slepian decomposition. Vector spherical modes are known to provide a good basis for the angular dependency of the radiation pattern. This paper is the first to extend such a model to also incorporate the frequency dependency of a radiation pattern. This is achieved by using a Slepian mode expansion. It is shown that this model requires considerably less coefficients than traditional sampling to accurately describe a frequency-dependent 3D radiation pattern. Also, generating the Slepian modes is computationally more efficient than comparable techniques, such as the singularity expansion method (SEM). The coefficients can then directly be used to efficiently calculate performance measures such as the antenna fidelity factor for all angles (?,?) without reconstructing the radiation pattern, or to reduce the noise contribution.

COLLAPSE OF MOLECULAR CLOUD CORES WITH RADIATION TRANSFER: FORMATION OF MASSIVE STARS BY ACCRETION

Most early radiative transfer calculations of protostellar collapse have suggested an upper limit of ∼40 M☉ for the final stellar mass before radiation pressure can exceed the star's gravitational pull and halt the accretion. Here we perform further collapse calculations, using frequency-dependent radiation transfer coupled to a frequency-dependent dust model that includes amorphous carbon particles, silicates, and ice-coated silicates. The models start from pressure-bounded, logatropic spheres of mass between 5 M☉ and 150 M☉ with an initial nonsingular density profile. We find that in a logatrope the infall is never reversed by the radiative forces on the dust and that stars with masses ≳100 M☉ may form by continued accretion. Compared to previous models that start the collapse with a ρ ∝ r−2 density configuration, our calculations result in higher accretion times and lower average accretion rates with peak values of ∼5.8 × 10−5M☉ yr−1. The radii and bolometric luminosities of the produced massive stars (≳90 M☉) are in good agreement with the figures reported for detected stars with initial masses in excess of 100 M☉. The spectral energy distribution from the stellar photosphere reproduces the observed fluxes for hot molecular cores with peaks of emission from mid- to near-infrared.

Radiation of spin waves from the open end of a microscopic magnetic-film waveguide

We have studied experimentally the radiation of spin waves from a permalloy-film microwaveguide into a continuous permalloy film. We show that due to a strong mismatch of the spin-wave spectrum caused by a variation in the demagnetizing field at the interface between the waveguide and the film, a frequency interval exists, where spin waves experience total reflection from the junction penetrating into the permalloy film in a tunnelinglike manner. At frequencies above this interval, complex frequency-dependent radiation patterns were observed characterized by a preferential radiation direction appearing due to the intrinsic anisotropy of the spin-wave dispersion characteristics in the film.

Seismic-Wave Strain, Rotation, and Gradiometry for the 4 March 2008 TAIGER Explosions

Abstract Acceleration spatial gradients, horizontal strains, and horizontal rotation were computed using strong-motion array data from the 4 March 2008 TAIGER explosions in northeastern Taiwan and used in conjunction with the original three component acceleration data to perform a gradiometric analysis of the strong ground motion wave train. The analysis yields a complex, frequency-dependent view of the nature of seismic-wave propagation over short propagation distances that imply significant lateral velocity changes in structure. Areal strain and rotation about the vertical axis have equal amplitudes and suggest significant wave scattering within the confines of the river valley where the experiment was performed and/or significant departure from an axisymmetric explosion source. Gradiometry shows that the P wave arrives at the array 35° off-azimuth clockwise from the straight-line path and appears to have been refracted from the northern side of the valley. Large, slowly propagating secondary surface waves initially arrive 45° counterclockwise from the straight-line path but later arrivals are seen to propagate in all directions, including back toward the explosion source. A frequency-dependent radiation pattern for the triple-borehole explosion in comparison to the single-borehole explosion explains the differences in the maximum amplitudes between the sources seen in the acceleration data. The use of seismic strain and rotation with standard particle motion wave fields at a single location allows for a direct view of seismic-wave propagation that illuminates the true nature of the seismogram.

Performance Prediction of Carbon Nanotube Bundle Dipole Antennas

A theoretical investigation is carried out for predicting radiation characteristics of single-walled carbon nanotube (SWCNT) bundle dipole antennas based on the distributed circuit parameters and the model of an SWCNT, where the cross section of bundles can be in a circular and a rectangular geometry, respectively. The current distributions in such novel antennas are predicted to investigate the effects of bundle cross-sectional size, tube diameter, tube length, and operating frequency. Furthermore, comparative studies are performed to show the geometry- and frequency-dependent radiation resistance, far-field pattern, and radiation efficiency of some typical bundle dipole antennas, which are numerically confirmed to outperform an SWCNT antenna by 30-40 dB in radiation efficiency.

Stochastic Strong-Motion Simulation of the Mw 6 Umbria-Marche Earthquake of September 1997: Comparison of Different Approaches

Abstract We simulated strong-motion records from the Umbria–Marche, central Italy, earthquake ( M w 6) of September 1997 using a frequency-dependent S -wave radiation function. We compared the observed acceleration spectra, from strong-motion instruments located in the near field and at regional distances, with those simulated using the stochastic modeling technique of Beresnev and Atkinson (1997, 1998), and modified to account for a frequency-dependent radiation pattern correction. By using the frequency-dependent radiation function previously obtained by Castro et al. (2006), we reduced the overall fitting error of the acceleration spectra by about 9%. In general, we observed that the frequency-dependent radiation pattern correction has a small effect on the spectral amplitudes compared with site effects, which is an important factor controlling the strong-motion records generated by the 1997 Umbria–Marche earthquake. In addition, we modeled the observed ground-motion records using the dynamic corner frequency model of Motazedian and Atkinson (2005) to reproduce the directivity effects, reducing the average error of the spectral amplitudes by 24%. We concluded that although the frequency-dependent radiation pattern correction affects the frequency content of the spectral amplitudes simulated, site and directivity effects are more relevant.

The characteristics of a biconical antenna calculated via the partial-domain method

The characteristics of a biconical antenna with a flare angle of 1° to 80° and an electrical length of 1 to 60 are calculated via the partial-domain method. The maximum operating frequency band for such antennas is determined from the calculated frequency-dependent input impedance and radiation patterns.

The inner gaseous accretion disk around a Herbig Be star revealed by near- and mid-infrared spectro-interferometry

AbstractHerbig Ae/Be stars are pre-main-sequence stars of intermediate mass, which are still accreting material from their environment, probably via a disk composed of gas and dust. Here we present a recent study of the geometry of the inner (AU-scale) circumstellar region around the Herbig Be star MWC 147 using long-baseline interferometry. By combining for the first time near- and mid-infrared spectro-interferometry on a Herbig star, our VLTI/AMBER and VLTI/MIDI data constrain not only the geometry of the brightness distribution, but also the radial temperature distribution in the disk. The emission from MWC 147 is clearly resolved and has a characteristic physical size of ∼1.3 AU and ∼9 AU at 2.2 μm and 11 μm respectively. This increase in apparent size towards longer wavelengths is much steeper than predicted by analytic disk models assuming power-law radial temperature distributions. For a detailed modeling of the interferometric data and the spectral energy distribution of MWC 147, we employ 2-D frequency-dependent radiation transfer simulations. This analysis shows that passive irradiated Keplerian dust disks can easily fit the SED, but predict much lower visibilities than observed, so these models can clearly be ruled out. Models of a Keplerian disk with emission from an optically thick inner gaseous accretion disk (inside the dust sublimation zone), however, yield a good fit of the SED and simultaneously reproduce the observed near- and mid-infrared visibilities. We conclude that the near-infrared continuum emission from MWC 147 is dominated by accretion luminosity emerging from an optically thick inner gaseous disk, while the mid-infrared emission also contains strong contributions from the passive irradiated dust disk.

Winds of M- and S-type AGB stars: an unorthodox suggestion for the driving mechanism

Context: Current knowledge suggests that the dust-driven wind scenario provides a realistic framework for understanding mass loss from C-rich AGB stars. For M-type objects, however, recent detailed models demonstrate that radiation pressure on silicate grains is not sufficient to drive the observed winds, contrary to previous expectations. Aims: In this paper, we suggest an alternative mechanism for the mass loss of M-type AGB stars, involving the formation of both carbon and silicate grains due to non-equilibrium effects, and we study the viability of this scenario. Methods: We model the dynamical atmospheres and winds of AGB stars by solving the coupled system of frequency-dependent radiation hydrodynamics and time-dependent dust formation, using a parameterized description of non-equilibrium effects in the gas phase. This approach allows us to assess under which circumstances it is possible to drive winds with small amounts of carbon dust and to get silicate grains forming in these outflows at the same time. Results: The properties of the resulting wind models, such as mass-loss rates and outflow velocities, are well within the observed limits for M-type AGB stars. Furthermore, according to our results, it is quite unlikely that significant amounts of silicate grains will condense in a wind driven by a force totally unrelated to dust formation, as the conditions in the upper atmosphere and wind acceleration region put strong constraints on grain growth. Conclusions: .The proposed scenario provides a natural explanation for the observed similarities in wind properties of M-type and C-type AGB stars and implies a smooth transition for stars with increasing carbon abundance, from solar-composition to C-rich AGB stars, possibly solving the longstanding problem of the driving mechanism for stars with a C/O close to one.

SPECT3D – A multi-dimensional collisional-radiative code for generating diagnostic signatures based on hydrodynamics and PIC simulation output

SPECT3D is a multi-dimensional collisional-radiative code used to post-process the output from radiation-hydrodynamics (RH) and particle-in-cell (PIC) codes to generate diagnostic signatures (e.g. images, spectra) that can be compared directly with experimental measurements. This ability to post-process simulation code output plays a pivotal role in assessing the reliability of RH and PIC simulation codes and their physics models. SPECT3D has the capability to operate on plasmas in 1D, 2D, and 3D geometries. It computes a variety of diagnostic signatures that can be compared with experimental measurements, including: time-resolved and time-integrated spectra, space-resolved spectra and streaked spectra; filtered and monochromatic images; and X-ray diode signals. Simulated images and spectra can include the effects of backlighters, as well as the effects of instrumental broadening and time-gating. SPECT3D also includes a drilldown capability that shows where frequency-dependent radiation is emitted and absorbed as it propagates through the plasma towards the detector, thereby providing insights on where the radiation seen by a detector originates within the plasma. SPECT3D has the capability to model a variety of complex atomic and radiative processes that affect the radiation seen by imaging and spectral detectors in high energy density physics (HEDP) experiments. LTE (local thermodynamic equilibrium) or non-LTE atomic level populations can be computed for plasmas. Photoabsorption rates can be computed using either escape probability models or, for selected 1D and 2D geometries, multi-angle radiative transfer models. The effects of non-thermal (i.e. non-Maxwellian) electron distributions can also be included. To study the influence of energetic particles on spectra and images recorded in intense short-pulse laser experiments, the effects of both relativistic electrons and energetic proton beams can be simulated. SPECT3D is a user-friendly software package that runs on Windows, Linux, and Mac platforms. A parallel version of SPECT3D is supported for Linux clusters for large-scale calculations. We will discuss the major features of SPECT3D, and present example results from simulations and comparisons with experimental data.