Excitation Scenarios Research Articles

Hybrid Dielectric Resonator Antenna for Diversity Applications With Linear or Circular Polarization

This article presents a novel dielectric resonator antenna (DRA) with an eight-port feeding system offering gain and polarization diversity. The single-antenna unit is defined by a square arrangement of four aperture coupled slots (ACSs), hybridized with the radiation of the dielectric resonator. Basically, an atypical feeding layout, realized by two microstrip transmission lines driving each of the four ACSs (thus requiring a total of eight feed lines), allows for controlled excitation of the polarization. This resonator and feeder define the eight-port DRA, which, can offer linear polarization (LP), right-handed circular polarization (RHCP), or left-handed circular polarization (LHCP). Reflection coefficient and coupling values for different port excitation scenarios are also investigated for each polarization state of the L-band antenna. The maximum gain is about 5 dBic for LHCP or RCHP and 5 dBi when considering LP radiation. Such agility can be useful for the global navigation satellite system (GNSS), for example, and other RF challenged environments where the dominant polarization is not known a priori. Other applications include duplex systems, wireless communications, and other phased arrays where gain or polarization diversity is of interest.

The TESS-Keck Survey. III. A Stellar Obliquity Measurement of TOI-1726 c

Abstract We report the measurement of a spectroscopic transit of TOI-1726c, one of two planets transiting a G-type star with V = 6.9 in the Ursa Major Moving Group (∼400 Myr). With a precise age constraint from cluster membership, TOI-1726 provides a great opportunity to test various obliquity excitation scenarios that operate on different timescales. By modeling the Rossiter–McLaughlin (RM) effect, we derived a sky-projected obliquity of . This result rules out a polar/retrograde orbit and is consistent with an aligned orbit for planet c. Considering the previously reported, similarly prograde RM measurement of planet b and the transiting nature of both planets, TOI-1726 tentatively conforms to the overall picture that compact multitransiting planetary systems tend to have coplanar, likely aligned orbits. TOI-1726 is also a great atmospheric target for understanding differential atmospheric loss of sub-Neptune planets (planet b 2.2 R ⊕ and c 2.7 R ⊕ both likely underwent photoevaporation). The coplanar geometry points to a dynamically cold history of the system that simplifies any future modeling of atmospheric escape.

The effect of curvature angle of curved RC box-girder continuous bridges on their transient response and vertical pounding subjected to near-source earthquakes

Among highway bridges, curved ones are very common at urban intersections and, generally, in limited urban spaces. However, such bridges are often involved in multiple vibrational behaviors, making them more sensitive to vertical components of ground motion due to their irregular geometry. The primary objective of this study has been to develop a theoretical approach using the relationships of structural dynamics to evaluate the effect of central curvature angle on the seismic response of curved bridges with rubber bearings subjected to strong vertical ground motions of near field earthquakes. In this regard, the continuous flexural/torsional beam-spring-rod model, has been used to calculate vertical force and the effect of this force on the dynamic response of curved bridge components. The wave propagation in the form of harmonic functions is induced by changing the phase angle for piers, and the wave transfer functions are solved by using eigenfunctions. The results of numerical analyses show that the wave transient behavior leads to separation of the girder from the bearing along with the pounding of the girder returning back onto the bearing, and that in the curved bridges with different central angles, the phenomenon of vertical pounding would occur in the range of the bridge natural vibration period, and pounding intensity would be strongly dependent on seismic excitation scenarios. The results also show that with increase in the curvature angle of the girder, the girder's lateral displacement does not necessarily increase, while the increase in the girder’s torsion is related to its curvature angle.

Effectiveness of Friction Dampers in Seismic and Wind Response Control of Connected Adjacent Steel Buildings

Effectiveness of friction dampers (FDs) is investigated for connected dynamically similar and dissimilar steel buildings under uncorrelated seismic ground motion and wind excitations. The steel buildings involving moment-resisting frame (MRF) and braced frame (BF) are varied from five storeys to twenty storeys, which are connected by different configurations of the FDs. The steel buildings without and with bracing systems are modeled as plane frame structures with inertial masses lumped at each joint node. The FDs are modeled an element having yield force equal to slip load, with force-deformation behavior as elastic-perfectly plastic material. The dynamic responses of the unconnected and connected steel buildings are obtained in terms of top floor displacement and acceleration under the considered ground motion and wind excitations. It is concluded that the FDs help minimizing the gap between two adjacent buildings having utilized the space to connect the buildings. Moreover, the effectiveness of the FDs in terms of response reduction in dynamically dissimilar buildings is more than that in the similar buildings under the considered excitation scenarios. However, the effectiveness of the installed devices varies significantly under the multiple loading scenarios. Finally, the separation gap may be reduced by ∼30%, which would eventually minimize structural pounding as well as utilize the space for effective construction. Hence, important essential guidelines are outlined for structures installed with such passive control devices against such multiple scenario loadings.

Evaluation of dynamic response of connected and non-connected piled raft systems using shaking table tests

Regarding the inadequacy of quantitative measurements of geotechnical aspects for assessing the behavior of the piled raft with different head connections during seismic events, A series of shaking table tests modeling, including un-piled-raft foundation, connected and non-connected piled-raft systems with 4 piles and dry sand were conducted. The efficiency of pile head separation and the material of the interposed layer between pile and cap for reducing the bending moment along with the piles length under different excitation scenarios were investigated by imposing base shaking levels with PGAs that ranged between 0.1 to 0.4 g, simulating weak to strong ground motions. Moreover, a comprehensive study is conducted on seismic ground motion amplification patterns induced by piled-raft systems in the soil body and caps as well as lateral raft movements.Based on the shaking table results, the non-connectivity between piles and caps decreased the maximum bending moment of piles by 20–60%, based on input motion amplitudes and cushion layer material. The dependency of both mechanism and amplitude of amplification on the level of base shaking and pile-raft connection details are also revealed.

A High-Frequency Uniform Asymptotic Solution for Electromagnetic Field Scattering by a PEC Wedge Including Grazing Incidence Scenarios

An asymptotic solution for the vector problem of scattering by a perfectly electrically conducting (PEC) wedge is developed and formulated as a modification of that obtained by employing the uniform asymptotic theory (UAT) of diffraction approach. This modified UAT (MUAT) solution is validated via comparison with the eigenfunction series expansion of the exact solution of the problem for varying excitation and observation scenarios, therefore evidencing to correct the standard UAT results which fail for excitation at grazing incidence.

Development of a four-parameter phenomenological model for the nonlinear viscoelastic behaviour of magnetorheological gels

Magnetorheological gel (MRG) excels in the material properties in term of adjustability and sedimentation performance, which could upgrade the performances of the current magnetorheological fluid based adjustable devices for structural control and vibration mitigation. However, the characterization and modelling of the stress-strain hysteresis responses of MRG has not been reported in the past, which are fundamental steps towards engineering applications. In this study, the stress-strain hysteresis of polyurethane based MRG sample with 60% carbonyl iron particle weight fraction was characterized under sinusoidal shear excitations with broad ranges of strain amplitude (5%–100%), excitation frequency (0.1 Hz–2 Hz) and magnetic fields (0–0.91 T). Significant stress overshooting phenomenon were observed under the application of low magnetic fields (0.27 T). A structurally-simple and accurate phenomenological model has been established to capture this unique nonlinearity. By validating the experimental results, the proposed model accurately predicts the hysteretic behaviour and the overshoot of the MRG under the excitation scenarios and the magnetic fields considered. Finally, the support vector machine (SVM) was implemented to provide the solution to the model generalization. The SVM-assisted model showed good agreement with the experimental data and can benefit the efficiency and viability in developing controllable MRG-based devices and system.

Analysis and characterization of the friction of vehicle body vibration dampers

The friction at the contact surfaces of a vehicle body vibration damper, which are moved relatively to each other, influences its transmission behavior at the start of movement (breakaway force) as well as with excitation signals of higher velocity and thus has an impact on the comfort properties of the damper. According to Vibracoustic (Die wichtigsten Kriterien für deutsche Autofahrer beim Autokauf, Springer Fachmedien Wiesbaden GmbH, Wiesbaden, 2019), for most German drivers (63%) comfort (in addition to brand and appearance) before driving dynamics (53%) and environmental compatibility (48%) is the most important criteria when evaluating a new car, which explains the importance of this vehicle characteristics. Furthermore, the friction is present with any relative movement of the damper and is, therefore, relevant for the design of the damper and the associated vertical dynamics. The friction is generally determined in the fully assembled state of the damper, including oil filling and gas pressure at a very low movement velocity to eliminate the influence of the damping force. This measurement method allows no or only inadequate statements about the friction behavior at, e.g. more dynamic excitation scenarios. As a result, the aim should be to characterize the friction properties without the influence of hydraulic damping at the start of movement or reversal of movement, as well as at higher movement velocities. Another goal is to evaluate the influence of the internal pressure of the damper on its friction behavior. The test damper used here is a commercially available monotube damper that has been modified in accordance with the requirements for these tests. The results shown below can be used as starting variables for further investigations for the targeted optimization of the friction properties and thus for the improvement of driving comfort. The reduction in damper friction promises an increase in comfort due to the improved decoupling of the vehicle body from the road excitation. Furthermore, the data obtained enable the level of detail of simulation models to be increased and serve as a basis for comparing different friction pairings and contact surfaces in the damper. For the substitution of coatings (chrome-free piston rods → environmental protection) or tube materials (aluminum matrix composites → lightweight construction) as well as for changes in the surface structure and roughness, the results enable an evaluation of the friction properties compared to conventional dampers and the adjustment of the friction pairings in the sense of the best possible functionality.

(Invited) Local Electrolyte-Driven Redox Cycling within a Nanoporous Photoanode

Nanoporous photoanodes used in dye-sensitized solar energy conversion systems such as solar cells and solar hydrogen generators have been carefully studied to understand factors that influence their performance, as measured by photocurrents and open-circuit photovoltages. We have developed a chemically detailed, multiscale reaction-diffusion model that seeks to connect these observables to the detailed dye redox and electrolyte chemistry for the simpler dye-sensitized solar cell (DSC) system, incorporating dyes bound within a TiO2 nanoparticle-based anode, and an I-/I3 - redox shuttle connecting the anode to the cathode [1]. The model framework permits nanoscale characteristics of the interfacial and solution-phase redox processes, including charge trapping, to be connected to photocurrents and electron densities under various dye excitation scenarios, and is extendable to the more complex interactions occurring in hydrogen generators. In DSCs, it is generally assumed that the redox shuttle needs to make a full round trip between anode and cathode to convert I3 - to 3 I- at the cathode after dye reduction reactions in the anode. It is also thought that electron trapping by electrolyte components, in particular I2, is deleterious, and degrades performance. Detailed simulations assess these mechanistic elements. The results show that during operation, I- is deeply depleted in the photoanode pores, leading to significant populations of oxidized dyes and a shift in the I-/I3 - equilibrium with formation of I2. Electron trapping by I2, which forms I2 - at the photoanode-electrolyte interface, does not invariably affect photocurrent or photoanode charging. This is because the increased rate of I2 - disproportionation in the electrolyte enables local regeneration of I- within the pores and efficient dye reduction after photoexcitation, without requiring I3 - diffusion to the cathode. The possibility of local cycling, which results in photoanode redox activity without net current flow, has not been reported previously and suggest that causes of less-than-ideal performance may lie elsewhere. The simulations provide nanoscopic predictions for interfacial processes that control dye cycling. The timing of electrolyte anion diffusion toward the anode-dye interface within the pores, relative to charge injection and to dye neutralization, will be discussed. [1] F. A. Houle, J. Phys. Chem. C 123, 14459-14467 (2019).

Optimization-based design of a single-layer wideband reflectarray antenna in the terahertz regime

An optimization-based metasurface is proposed and employed to design a wideband single-layer reflectarray antenna for use in the terahertz (THz) regime. The random hill-climbing optimization method is utilized to obtain a wideband unit cell. A multiobjective fitness function is defined to consider three required design aims, whereas an established link between MATLAB and HFSS software is employed to simulate the required periodic metasurface. The final created unit cell has a remarkable bandwidth of 44.89% together with −0.44 dB average variation of the magnitude of the reflection coefficient. Moreover, the phase variation versus various scales of the metallic surface of the cell is more than 300° at all of the desired frequencies between 0.95 THz and 1.5 THz. Two different reflectarray antennas are proposed based on the optimized cell, and two excitation scenarios are implemented to investigate the performance of the designed reflectarray antennas. In the first scenario, the metasurface is designed to deflect a plane wave with angle of incidence of $$30^{^\circ }$$ to the normal direction. In the second one, a THz feeding horn illuminates a metasurface reflector including 201 elements. According to the simulation results, 3-dB and 1-dB gain bandwidths of 39.67% (0.99–1.48 THz) and 20.51% (1.05–1.29 THz), respectively, are achieved, whereas the maximum gain is 19.8 dBi at 1.2 THz.

A Probabilistic Study of Common-Mode and Differential-Mode Responses of a Plane-Wave Excited Twisted-Wire Pair Above a Perfect Ground Plane

This article presents a probabilistic approach for the immunity/susceptibility assessment of a twisted-wire transmission line in the presence of a perfect ground plane. A random electromagnetic field is assumed to excite the transmission line, which is essentially modeled as a plane wave with certain parameters (magnitude, polarization angle, direction-of-arrival angles) treated as independent random variables. Closed-form expressions are provided for the probability density function of the induced common-mode and differential-mode terminal voltages in certain cases. New formulas for the pertinent first and second moments (mean values and mean square values) are also obtained. A general Monte Carlo technique is employed to verify the aforementioned closed-form expressions, but also for treating excitation scenarios that are not analytically tractable.

A broadband electret-based vibrational energy harvester using soft magneto-sensitive elastomer with asymmetrical frequency response profile

This letter reports a broadband electret-based vibrational energy harvester (eVEH) targeting at applications in low frequency (4–10 Hz) and low excitation (0.1–0.2 g) scenarios. A soft magneto-sensitive elastomer (SMSE) cantilever with low resonance and strong softening nonlinear behaviour is utilized as the broadband oscillator. A spring suspended end-stopper attached to the electret converts low frequency excitation of the SMSE cantilever to high frequency vibrations of the eVEH for efficiency enhancement. An asymmetrical frequency response profile for the SMSE cantilever is intentionally designed to preserve its softening behaviour for broadband performance when the end-stopper is engaged. A proof-of-concept prototype is fabricated and the experiment shows that within a working frequency range of 4.0–8.2 Hz, the eVEH can harvest the average power output of 40.23 μW under a base excitation of 0.2 g.

New Investigations into Measurement Techniques Using the Pendulous Hammer to Assess Acoustic Insulation in Relation to User-Generated Noise in Residential Buildings

With the release of the revised version of the Swiss standard SIA 181:2006 "Protection Against Noise in Buildings", a measurement technique was introduced for simulating user-generated noise in bathtubs, shower cubicles, wash basins etc. employing a pendulous hammer. Despite the undoubted advantages of the measurement method, over the past few years questions have increasingly been raised regarding various issues involved in its use. The characteristics of the pendulous hammer are insufficiently well specified in the standard and a procedure specifying a periodic check of the instrument is lacking. The measurement method itself is described in inadequate detail, so that (for example) depending on the choice of the excitation, potentially very diff erent results may be obtained. In addition, no information is given regarding the measurement uncertainty. The standard gives level corrections for diff erent excitation scenarios. This allows a comparison of the diff erence between the level of the original noise and that generated by the pendulous hammer. The validity of these level corrections is now, to a certain extent, being challenged. In addition, it has been suggested that – depending on the constructional details - the airborne noise emitted by acoustic radiation from the component under investigation is excessive, thereby potentially falsifying the measured values. These issues have been investigated by Empa and, based on a comprehensive set of laboratory and field measurements, clarified. Measurement protocols are given for various components, and the measurement uncertainty has been evaluated. The eff ect of airborne noise has been shown to be acceptable for tests conducted in accordance with the SIA 181 standard. Sufficient measurement data is now available to allow the level corrections to be determined. Finally, specifications for the pendulous hammer itself have been formulated.

Backstepping adaptive control for real-time hybrid simulation including servo-hydraulic dynamics

Abstract A backstepping adaptive control method is proposed for on-line estimation of unknown servo-hydraulic dynamics and the compensation of time-varying lags in real-time hybrid simulation tests. The response tracking problem becomes a critical challenge when realistic experimental conditions are taken into consideration, such as control-structure interaction effects and sensor measurement noise. Unlike a conventional time-lag compensator, the proposed adaptive controller generates a command trajectory for the actuated system according to adaptive laws. Besides bringing response tracking error to zeros, the estimation of a first-principle actuator dynamic model is also facilitated in the proposed approach. Lyapunov stability analysis is systematically presented for designing the adaptive control law. Illustratively, a three-story seismically excited structure with different control strategies is utilized to demonstrate the efficiency and robustness of the proposed controller. A benchmark problem is then utilized for the verification of controller’s advancement. Four simulation cases with different damping/mass conditions and four ground excitation scenarios are selected for the application. As stated, favorable tracking performance has been observed with a remarkable improvement in performance evaluation.

Super- and subradiance of clock atoms in multimode optical waveguides

The transversely confined propagating modes of an optical fiber mediate virtually infinite range energy exchanges among atoms placed within their field, which adds to the inherent free space dipole–dipole coupling. Typically, the single atom free space decay rate largely surpasses the emission rate into the guided fiber modes. However, scaling up the atom number as well as the system size amounts to entering a collective emission regime, where fiber-induced superradiant spontaneous emission dominates over free space decay. We numerically study this super- and subradiant decay of highly excited atomic states for one or several transverse fiber modes as present in hollow core fibers. As particular excitation scenarios we compare the decay of a totally inverted state to the case of π/2 pulses applied transversely or along the fiber axis as in standard Ramsey or Rabi interferometry. While a mean field approach fails to correctly describe the initiation of superradiance, a second-order approximation accounting for pairwise atom–atom quantum correlations generally proves sufficient to reliably describe superradiance of ensembles from two to a few hundred particles. In contrast, a full account of subradiance requires the inclusion of all higher order quantum correlations. Considering multiple guided modes introduces a natural effective cut-off for the interaction range emerging from the dephasing of different fiber contributions.

Electromagnetic shunt damping with negative impedances: Optimization and analysis

Dynamic vibration absorbers (DVA) are classic and effective devices to reduce the amplitude of vibration sustained by structures. A promising alternative to the DVAs is the electromagnetic shunt damper (EMSD), in which the electrical shunt circuit serves as the absorbing oscillator. The objective of this paper is to carry out optimum designs of an EMSD connected to a resistive-inductive-capacitive (RLC) shunt circuit, by adopting three different calibration strategies: the fixed points theory, H2 optimization criterion and maximum damping criterion. The aforementioned optimization strategies are appropriate to different excitation scenarios: harmonic, random and transient vibration, respectively. Analytical expressions of optimal parameters are formulated in terms of the ratio of external inductance in the shunt and inherent inductance of electromagnetic transducer. Lower bounds of inductance ratio in the region of stability are also specified for each strategy, based on which the ultimate attainable performance of EMSDs can be predicted. Numerical investigation underlines that including a negative inductance in the shunt always contributes to reduce the frequency response magnitude, broaden the absorbing area around targeted vibration mode, increase the damping performance, and thereby accelerate the decay rate of transient vibration.

Frequency domain modeling of nonlinear end stop behavior in Tuned Mass Damper systems under single- and multi-harmonic excitations

Nonsmooth dynamics of a Tuned Mass Damper system with lateral stops are studied using an alternating frequency/time harmonic balancing (AFT-HB) method. To this end, an extremely stiff end stop nonlinearity is considered. The application range of AFT-HB is investigated by including up to 250 harmonics in the external force, as well as in the motion description. Numerical simulations are performed by making use of a Newmark time integration algorithm for numerical verification of the results. The results for single harmonic excitations are further verified with an existing pseudo-arclength path-following tool. Two excitation scenarios are considered: single harmonic- and a wide-spectrum excitation with uniform distribution and random phase correlation between the harmonics. The AFT-HB algorithm is found to accurately reproduce the time integration results, for all considered cases. Finally, insights are gained into the differences between the system responses to single- and multi-harmonic excitations.

Photoionization microscopy: Hydrogenic theory in semiparabolic coordinates and comparison with experimental results

Photoionization microscopy (PM) is an experimental method allowing for high-resolution measurements of the electron current probability density in the case of photoionization of an atom in an external uniform static electric field. PM is based on high-resolution velocity-map imaging and offers the unique opportunity to observe the quantum oscillatory spatial structure of the outgoing electron flux. We present the basic elements of the quantum-mechanical theoretical framework of PM for hydrogenic systems near threshold. Our development is based on the computationally more convenient semiparabolic coordinate system. Theoretical results are first subjected to a quantitative comparison with hydrogenic images corresponding to quasibound states and a qualitative comparison with nonresonant images of multielectron atoms. Subsequently, particular attention is paid on the structure of the electron's momentum distribution transversely to the static field (i.e., of the angularly integrated differential cross-section as a function of electron energy and radius of impact on the detector). Such 2D maps provide at a glance a complete picture of the peculiarities of the differential cross-section over the entire near-threshold energy range. Hydrogenic transverse momentum distributions are computed for the cases of the ground and excited initial states and single- and two-photon ionization schemes. Their characteristics of general nature are identified by comparing the hydrogenic distributions among themselves, as well as with a presently recorded experimental distribution concerning the magnesium atom. Finally, specificities attributed to different target atoms, initial states, and excitation scenarios are also discussed, along with directions of further work.

Theory and Ab Initio Computation of the Anisotropic Light Emission in Monolayer Transition Metal Dichalcogenides.

Monolayer transition metal dichalcogenides (TMDCs) are direct gap semiconductors with a unique potential for use in ultrathin light emitters. However, their photoluminescence (PL) is not completely understood. We develop an approach to compute the radiative recombination rate in monolayer TMDCs as a function of photon emission direction and polarization. Using exciton wavefunctions and energies obtained with the ab initio Bethe-Salpeter equation, we obtain polar plots of the PL for different scenarios. Our results can explain the PL anisotropy and polarization dependence measured in recent experiments and predict that light is emitted with a peak intensity normal to the exciton dipole in monolayer TMDCs. We show that excitons emit light anisotropically upon recombination when they are in any quantum superposition state of the K and K' inequivalent valleys. When averaged over the emission angle and exciton momentum, our new treatment recovers the temperature-dependent radiative lifetimes that we previously derived. Our work demonstrates a generally applicable first-principles approach to studying anisotropic light emission in two-dimensional materials.

An Analysis of Stochastic Jovian Oscillation Excitation by Moist Convection

Abstract Recent observations of Jupiter have suggested the existence of global oscillatory modes at millihertz frequencies, yet the source mechanism responsible for driving these modes is still unknown. However, the energies necessary to produce observable surface oscillations have been predicted. Here we investigate if moist convection in Jupiter’s upper atmosphere can be responsible for driving the global oscillations and what moist convective energy requirements are necessary to achieve these theoretical mode energies and surface amplitudes. We begin by creating a one-dimensional moist convective cloud model and find that the available kinetic energy of the rising cloud column falls below theoretical estimates of oscillation energies. That is, mode excitation cannot occur with a single storm eruption. We then explore stochastic excitation scenarios of the oscillations by moist convective storms. We find that mode energies and amplitudes can reach theoretical estimates if the storm energy available to the modes is more than just kinetic. In order for the modes to be excited, we find that they require 5 × 1027 to 1028 erg per day. However, even for a large storm eruption each day, the available kinetic energy from the storms falls two orders of magnitude short of the required driving energy. Although our models may oversimplify the true complexity of the coupling between Jovian storms and global oscillations, our findings reveal that enough thermal energy is associated with moist convection to drive the modes, should it be available to them.