Undesired Waves Research Articles

Finite element modeling of electromagnetic properties of 3D printed materials in X‐band spectrum

AbstractMicrowave absorbers are distinctive class of materials, which are used to absorb undesired electromagnetic waves in the microwave spectrum. A finite element modeling approach was utilized in the present study to investigate the microwave absorption properties of three different materials such as polycaprolactone (PCL) polymer, polycaprolactone‐carbonyl iron particle (CIP) composite with random orientation of CIPs, and polycaprolactone‐carbonyl iron particle composite with oriented chain structures of CIPs, fabricated by extrusion‐based 3D printing process. A two‐dimensional model was formulated comprising the material under test and two rectangular waveguides attached to the vector network analyzer. A two‐port network model was used to emulate the real‐time experimental environment. It was found from the electric field distribution plot that the oriented PCL‐CIP model reflects and transmits marginal amplitude of electromagnetic waves when compared with nonoriented PCL‐CIP model followed by pure PCL model. The simulation results were validated with the experimental findings. A minimum loss of electromagnetic energy was observed for oriented model as compared to other models, supporting the experimental findings. Larger domain wall motions and micro capacitor behavior of oriented chains of carbonyl particles were responsible for such microwave absorption properties of oriented composites. The practical utility of the present modeling and simulation approach is multifold as it can be utilized to investigate and compare the electromagnetic shielding behavior of various polymeric composites without any material wastage, machine utilization, and physical testing.

Ultra-thin and broadband surface wave meta-absorber.

Perfect absorbers are highly desired in many engineering and military applications, including radar cross section (RCS) reduction, cloaking devices, and sensor detectors. However, most types of present absorbers can only absorb space propagation waves, but absorption for surface waves has not been researched intensively. Surface waves are easily excited on the interfaces between metal and dielectrics for electronic devices, which decreases their working performances due to the electromagnetic disturbances. Thus, it is of great significance to design appropriate absorbers to dissipate undesirable surface waves. Here, we propose the concept of a surface wave absorber, analyze its working principle, and prove its good performances experimentally. To demonstrate our concept, we design and fabricate a realistic surface wave absorber that is fixed on a metal surface. Experiments are performed to verify its electromagnetic characteristics. The results show that our designed meta-absorber can achieve an excellent surface wave absorption within a wide frequency window (5.8-11.2 GHz) and exhibit a very high efficiency over than 90%, but only with the thickness of 1 mm (0.028 λ). Our device can help to solve the issues of absorption at large angles, and it can find wide applications in large antenna array design and other communication systems.

3D-printed phononic crystal waveguide transducers for nonlinear ultrasonic damage detection

Nonlinear ultrasonic methods typically measure second harmonic waves for a rapid and sensitive detection of material micro-cracks. However, fictitious second harmonics generated by the ultrasonic equipment can interfere with signal measurements, causing poor damage detection. This paper aims at enhancing the damage sensitivity of nonlinear ultrasound by developing acrylic phononic crystal waveguide transducers using Polyjet additive manufacturing, which can filter out undesired nonlinear harmonic waves. These sensing devices consist of traditional piezoelectric sensors surface bonded on periodically corrugated structures. Such periodicity can induce stop band frequencies in which the propagation of ultrasonic waves is inhibited. Nonlinear ultrasonic tests confirmed the filtering capabilities of phononic crystal transducers to suppress harmonics created by the ultrasonic instrumentation, thus enabling second harmonic detection on a damage aluminium plate. These results pave the way for simpler, automatable and more accurate nonlinear ultrasonic inspection systems.

SCF-NiFe2O4/epoxy composites with high thermal conductivity and electromagnetic interference resistance

With the development of electronic components towards high power, high packaging density, and miniaturization of device size, heat dissipation and the electromagnetic interference problems between the electronic components are emerging. In order to solve the undesirable electromagnetic wave and heat emissions produced by electronic device simultaneously, the electronic packaging materials with high thermal conductivity and anti-electromagnetic interference are highly expected. In this work, NiFe2O4-modified short carbon fibers (SCF) were designed and prepared. NiFe2O4 were in situ grown on the surface of SCF by hydrothermal method. The chemical structure and morphology of SCF-NiFe2O4 were characterized by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The microwave absorbing performance test shows that SCF-NiFe2O4 possess superior microwave absorbing performance, where the minimum reflection loss is − 20.7 dB. Finally, SCF-NiFe2O4/epoxy resin composites were prepared by introducing SCF-NiFe2O4 as thermal conductive filler into epoxy resin. The surface-modified SCF with NiFe2O4 is more easily infiltrated by resin and exhibits strong interfacial interaction with the matrix. With the increase of the content of SCF-NiFe2O4, the thermal conductivity of the composites increases obviously. When the content of SCF-NiFe2O4 reaches 20 vol%, the composites show better thermal conductivity, and the thermal conductivity reaches 1.03 W/m K.

On the Boundary Control of a Boussinesq System

A boundary control problem is considered for determining a canal depth function optimally for a canal system modeled by a nonlinear Boussinesq equation. By determining the optimal canal depth function, it is aimed to damp out the undesired waves in canal system filled up water. For achieving this aim, the existence and uniqueness of the solutions to system and controllability properties of the system is investigated. Optimal canal depth control function is obtained by means of a maximum principle, which is an elegant tool for transferring the optimal boundary control problem to solving a system of equations including initial-terminal-boundary conditions. The reason making this paper is important that optimal control function is gained without linearization of nonlinear term in the system. In order to show the correctness of the obtained theoretical results, several numerical examples are presented by MATLAB in graphical and table forms. Observing these tables and graphics, it is concluded that introduced boundary control algorithm is effective and has the potential for extending to other nonlinear control systems.

Understanding the guided waves propagation behavior in timber utility poles

Guided stress waves are considered one of the most efficient and reliable techniques that provide sufficient quantitative and qualitative assessment. In this study, we focused on scrutinizing the propagation behavior of guided waves in western white pine timber poles, experimentally, and numerically using COMSOL Multiphysics. Macro fiber composites (MFCs), due to their flexibility and convenience to install on curved profiles, were used to actuate and sense guided waves along the tested specimens. Various solutions for wave mode tuning and characterization have been tested for traction free and embedded boundary conditions. The behavior of propagating wave modes was analyzed and compared in the two boundary conditions tested. Also, the excitation frequency, based on the dispersion curves generated for transversely isotropic timber, was selected to ensure the presence of favorable propagating (for instance longitudinal modes) modes with minimal dispersion. Undesirable wave modes—such as flexural modes (non-axisymmetric)—were eliminated by a ring design composed of multiple MFC actuators coupled around the pole’s circumference. The remaining propagating longitudinal modes and their reflections, such as modes L(0,1) and L(0,2) propagating at nearly 1000 m/s and 800 m/s respectively, were significantly enhanced by the actuation of the ring which could be effectively used for the assessment process. The results demonstrated the complexity of the propagating modes in circular timber structures and the importance of the ring design in the excitation of the selected modes of interest and damping unwanted ones.

Strategy for constructing electromagnetic interference shielding and flame retarding synergistic network in poly (butylene succinate) and thermoplastic polyurethane multilayered composites

Fabricating electromagnetic interference (EMI) shielding materials with excellent flame retarding properties is significant to attenuate undesirable electromagnetic waves caused by electronic devices and simultaneously improve the flame safety of the electronic components. Herein, a novel EMI shielding composite with multilayered structure was fabricated at a low particle loading via layer-multiplying coextrusion technology. Especially, within the multilayered composites, poly (butylene succinate) (PBS) filled with carbon nanotubes (CNTs) was used to construct conducting layer while thermoplastic polyurethane (TPU) filled with intumescent flame retardants (IFRs) and CNTs was used to construct synergistic flame-retarding layer. The content of CNTs and IFRs in the whole system were below 4 wt% and 20 wt%, respectively. Morphological observation demonstrated the alternating multilayered structure and the selective distribution of IFRs and CNTs. Besides, by tailoring the layer numbers, the composites showed tunable EMI shielding and flame retarding performances. With regard to EMI shielding performance, the absorption-dominated average EMI shielding effectiveness (SE) values of both 8- and 16-layer specimens exceeded 30 dB in the range of 8.2–12.4 GHz. Furthermore, superior flame retardancy with prompt self-extinguishment and anti-dripping was achieved in these specimens owning to the production of continuous and foaming carbonaceous structure among layers. Especially the 8-layer specimen, the UL-94 rating was categorized in V-0. Consequently, this work provided a promising and facile route to fabricate polymeric composites simultaneously possessing excellent EMI shielding and flame-retarding properties to fulfill the stringent requirements in practical application.

Reducing Undesired Wave Reflection at Domain Boundaries in 3D Finite Volume-Based Flow Simulations via Forcing Zones

This article reviews different types of forcing zones (sponge layers, damping zones, relaxation zones, etc.) as used in finite volume-based flow simulations to reduce undesired wave reflections at domain boundaries, with special focus on the case of strongly reflecting bodies subjected to long-crested incidence waves. Limitations and possible sources of errors are discussed. A novel forcing-zone arrangement is presented and validated via three-dimensional (3D) flow simulations. Furthermore, a recently published theory for predicting the forcing-zone behavior was investigated with regard to its relevance for practical 3D hydrodynamics problems. It was found that the theory can be used to optimally tune the case-dependent parameters of the forcing zones before running the simulations. 1. Introduction Wave reflections at the boundaries of the computational domain can cause significant errors in flow simulations, and must therefore be reduced. In contrast to boundary element codes, where much progress in this respect has been made decades ago (see e.g., Clement 1996; Grilli &Horillo 1997), for finite volume-based flow solvers, there are many unresolved questions, especially:How to reliably reduce reflections and disturbances from the domain boundaries?How to predict the amount of undesired wave reflection before running the simulation? This work aims to provide further insight to these questions for flow simulations based on Navier-Stokes-type equations (Reynolds-averaged Navier-Stokes, Euler equations, Large Eddy Simulations, etc.), when using forcing zones to reduce undesired reflections. The term "forcing zones" is used here to describe approaches that gradually force the solution in the vicinity of the boundary towards some reference solution, as described in Section 2; some examples are absorbing layers, sponge layers, damping zones, relaxation zones, or the Euler overlay method (Mayer et al. 1998; Park et al. 1999; Chen et al. 2006; Choi &Yoon 2009; Jacobsen et al. 2012; Kimet al. 2012; Schmitt & Elsaesser 2015; Perić & Abdel-Maksoud 2016a; Vukčević et al. 2016).

Multigrid dual-time-stepping lattice Boltzmann method.

The lattice Boltzmann method often involves small numerical time steps due to the acoustic scaling (i.e., scaling between time step and grid size) inherent to the method. In this work, a second-order dual-time-stepping lattice Boltzmann method is proposed in order to avoid any time-step restriction. The implementation of the dual time stepping is based on an external source in the lattice Boltzmann equation, related to the time derivatives of the macroscopic flow quantities. Each time step is treated as a pseudosteady problem. The convergence rate of the steady lattice Boltzmann solver is improved by implementing a multigrid method. The developed solver is based on a two-relaxation time model coupled to an immersed-boundary method. The reliability of the method is demonstrated for steady and unsteady laminar flows past a circular cylinder, either fixed or towed in the computational domain. In the steady-flow case, the multigrid method drastically increases the convergence rate of the lattice Boltzmann method. The dual-time-stepping method is able to accurately reproduce the unsteady flows. The physical time step can be freely adjusted; its effect on the simulation cost is linear, while its impact on the accuracy follows a second-order trend. Two major advantages arise from this feature. (i) Simulation speed-up can be achieved by increasing the time step while conserving a reasonable accuracy. A speed-up of 4 is achieved for the unsteady flow past a fixed cylinder, and higher speed-ups are expected for configurations involving slower flow variations. Significant additional speed-up can also be achieved by accelerating transients. (ii) The choice of the time step allows us to alter the range of simulated timescales. In particular, increasing the time step results in the filtering of undesired pressure waves induced by sharp geometries or rapid temporal variations, without altering the main flow dynamics. These features may be critical to improve the efficiency and range of applicability of the lattice Boltzmann method.

Evaluation of Undesired Radio Waves Below −170 dBm/Hz From Semiconductor Switching Devices for Impact on Wireless Communications

The spread of undesired radio waves and the development of semiconductor switching devices causes serious noise interference in wireless communication systems. We introduce a system to evaluate the noise interference caused by such switching devices with high resolution. Our evaluation system consists of two parts: a high-resolution noise measurement system and an evaluation of wireless communication performance by system level simulations. The integrated power of undesired radio waves is measured within a 5-MHz bandwidth and can be detected even at an ultra-low power level of -177 dBm/Hz. The lower detection limit of the undesired radio waves is enough to evaluate the impact on wireless communication performance. We evaluated undesired radio waves from a wireless power transfer (WPT) system for charging electric vehicles. In our simulation, at the 840-MHz band, we observed an undesired radio waves level of -143 dBm/Hz near the WPT system that degraded the long-term evolution (LTE) sensitivity to -74 dBm/5 MHz.

Parallel Architecture of Reconfigurable Hardware for Massive Output Active Noise Control

Filtered-x Least Mean Squares (FxLMS) is an algorithm commonly used for Active Noise Control (ANC) systems in order to cancel undesired acoustic waves from a sound source. There is a small number of hardware designs reported in the literature, that in turn only use one reference signal, one error signal and one output control signal. In this paper, it is proposed a 3-dimensional hardware-based version of the widely used FxLMS algorithm, using one reference microphone, 18 error microphones, one output and a FIR filter of 400[Formula: see text] order. The FxLMS algorithm was implemented in a Xilinx Artix 7 FPGA running at 25 MHz, which allowed to update the filter coefficients in 32.44[Formula: see text] s. The main idea behind this work is to propose a pipelined parallelized architecture to achieve processing times faster than real time for the filter coefficients update. The main contribution of this work is not the ANC technique itself, but rather the proposed hardware implementation that utilizes integer arithmetic, which provided an acceptable error when benchmarked with a software implementation. This parallel system allows a scalable implementation as an advantage of using FPGA without compromising the computational cost and, consequently, the latency.

Novel and Efficient Parasitic Decoupling Network for Closely Coupled Antennas

A novel and efficient parasitic decoupling network (PDN) is proposed for closely coupled antennas in this paper, which provides a new perspective and approach to design DNs based on the parasitic decoupling concept. The PDN is composed of sections of transmission lines (TLs) and reactive components, allowing additional power-traveling paths. With the TL lengths and reactive-component reactances being precisely determined, the traveling power from one antenna to another could cancel out the undesired power wave induced by mutual coupling, resulting in the high port isolation. The decoupling theory is rigorously derived with the design procedure of the PDN systematically described in connection with two examples of the two- and three-element monopole arrays. The measurement results show that good impedance matching for each antenna port, isolations of more than 25 dB, total efficiencies of more than 64%, and envelop correlation coefficients (ECCs) of less than 0.07 are attained simultaneously for both examples using the proposed PDN. The results verified the decoupling theory and proved the PDN concept.

Broadband transparent chiral mirrors: Design methodology and bandwidth analysis

Chiral mirrors are a class of metamaterials that reflect circularly polarized light of a certain helicity in a handedness-preserving manner, while absorbing circular polarization of the opposite handedness. However, most absorbing chiral mirrors operate only in a narrow frequency band, as limited by the causality principle. Instead of absorbing the undesired waveform, here we propose a transparent chiral mirror that allows undesired waves to pass through. In particular, the handedness-preserving band of the transparent chiral mirror is free of the causality limit, thus enabling broadband functionality. Furthermore, since electromagnetic waves outside the handedness-preserving band may transmit through the proposed chiral mirror, the reflected wave contains only circular polarization components of a certain handedness over a wide frequency range, which is favored in many applications. Moreover, the scheme is lossless and scalable. To realize the proposed transparent chiral mirror, we apply an array of helical microstructures in a two-dimensional square lattice. Traditionally, this kind of structure has been used as a circular polarizer but we apply it instead in a reflective mode. Our work provides a bandwidth analysis of chiral mirrors, and paves the way to new opportunities for creating broadband chiral metamaterials with handedness-preserving properties.

Measurements of acoustic impedance of porous materials using a parametric loudspeaker with phononic crystals and phase-cancellation method

When measuring the acoustic properties of architectural materials in a free field or in situ, reflections from the surroundings of the target material and diffraction from the sample edges often generate significant measurement errors. In this paper, a method for measuring acoustic properties by using a parametric loudspeaker, which is superdirective due to the nonlinearity of superhigh-pressure ultrasound, is proposed to overcome this problem: its strong directivity can reduce undesired waves by focusing the sound onto a narrow spatial range. However, such a superstrong ultrasound generates pseudo sound on the microphone surface and increases the measurement errors. Hence, two different strategies, acoustic filtering via phononic crystals and phase-cancellation excitation of the ultrasound, are experimentally investigated to reduce such induced errors. They are also used to evaluate the surface impedance of glass-wool boards at oblique incidence is measured in a free field and in situ. The results show that the proposed method can efficiently measure the surface impedance at frequencies higher than 800 Hz.

A review on electromagnetic band gap application in medical devices

Abstract There has been raising concerns about the healthcare field being in potential jeopardy due to the risks caused by the electromagnetic interference (EMI). Reports unfolding the robust electromagnetic energy emitted by the wireless devices has been responsible for most of the vital medical device malfunctions. Hence, this prompted the healthcare concerns to undertake continuous research to ascertain the theory of EMI shielding by means of different methodology that is proficient of mitigating interference in the localization of both critical and noncritical care medical equipment. This paper reviews research of electromagnetic band gap (EBG) application in medical devices in order to reduce EMI and improve device shielding. In recent times, there are countless publications presenting their vast applications of EBG to halt undesirable radiation at discontinuities. Therefore, it is capable of preventing undesired electromagnetic wave from the wireless communication devices and may be the key towards successive advancement in the healthcare domain.

Reducing Undesired Wave Reflection at Domain Boundaries in 3D Finite Volume–Based Flow Simulations via Forcing Zones

Reducing Undesired Wave Reflection at Domain Boundaries in 3D Finite Volume–Based Flow Simulations via Forcing Zones

Analysis of Automotive Cyber-Attacks on Highways Using Partial Differential Equation Models

This paper considers scenarios wherein a group of malicious vehicles on a highway perform a cooperative attack with the motive of creating undesirable wave effects among other vehicles on the highway. The two species of vehicles—malicious vehicles and normal vehicles, and their associated interaction effects—are modeled using partial differential equations. The malicious vehicles, which may be arbitrarily distributed on the highway, perform a sequence of velocity changes with the objective of making the density/velocity profile on the highway, track a reference profile. This reference profile (chosen by the malicious vehicles) has the property that once generated, it spontaneously evolves into a shock wave that propagates along the highway. Analytical expressions governing the velocity inputs of the malicious vehicles with which they can generate such waves are determined, for perfect as well as imperfect information scenarios. Simulation results are presented to validate the theory.

Experiments on helicons in DIII-D—investigation of the physics of a reactor-relevant non-inductive current drive technology**Paper based on contribution EX/P3-22 presented at IAEA FEC, Kyoto, October 2016.

Experiments have begun in DIII-D to evaluate a toroidally-directed spectrum of helicon waves (also known as ‘very high harmonic fast waves’, ‘fast waves in the lower hybrid range of frequencies’, or ‘whistlers’) that can generate efficient non-inductive current drive by Landau absorption in a reactor plasma. Modeling has shown (Prater et al 2014 Nucl. Fusion 54 083024) that non-inductive current drive at mid-radius (ρ ~ 0.5) should be achievable in DIII-D with fast waves at 0.5 GHz in high-beta conditions with an efficiency twice that of other non-inductive current drive tools currently available on DIII-D (neutral beams and electron cyclotron current drive). An innovative traveling wave antenna (TWA) of the ‘comb-line’ type with 12 radiating modules has been constructed, installed in DIII-D, and has been tested at very low power (<0.4 kW) to evaluate the antenna coupling in the linear regime, and to test technological aspects of such structures in the tokamak environment. Results show favorably strong antenna/plasma coupling in several plasma regimes, most significantly in the high-beta ELMing H-mode case with calculated full first-pass absorption of the helicon waves in the desired mid-radius region. No evidence of significant excitation of the undesirable slow wave was found when the total magnetic field adjacent to the structure was approximately aligned with the tilted Faraday screen of the antenna. The implications of these results for a high-power experiment currently under construction are discussed.

DEVELOPMENT OF A LOW PROFILE WIDE-BANDWIDTH CIRCULARLY POLARIZED MICROSTRIP ANTENNA FOR C-BAND AIRBORNE CP-SAR SENSOR

In this paper, a low-profile wide bandwidth circularly polarized microstrip antenna is proposed as element for a C-band airborne circularly polarized synthetic aperture radar sensor. Several bandwidth improvement techniques were proposed and implemented. In order to increase impedance bandwidth, the antenna is constructed using double-stacked substrate with low dielectric constant, modified radiating shape for multi-resonant frequency, and a circle-slotted parasitic patch. Generation of the circularly polarized wave employs a simple square patch with curve corner-truncation as radiating element. The asymmetric position of the feeding is attempted to improve the axial-ratio bandwidth. To avoid a complicated feed network, the antenna is fed by single-feed proximity-coupled microstrip line. The effect of copper-covering on the upper layer for decrease undesired radiation wave emitted by the feeding is also studied and presented. Measurement results show that the impedance bandwidth and axial ratio bandwidth are 20.9% (1,100 MHz) and 4.7% (250 MHz), respectively. Meanwhile the measured gain is 7 dBic at the frequency of 5.3 GHz.

Influence of Acousto-Optic Resonances in Electro-Optical Modulator on Fiber Optic Gyro Performance and Method for Its Compensation

The electro-optical modulator is one of the most important parts of the fiber optic gyroscope. Unfortunately, the integral implementation of the modulator has a number of undesirable effects such as parasitic amplitude modulation, phase drift, and acousto-optical resonance effects. Influence of the first two effects on the gyroscope operation has been described in a number of works, but at the moment, there are no published papers aimed at investigating the relationship between the presence of acousto-optic resonances in modulators and changes in the performance of a fiber optic gyroscope. There are several design solutions that provide damping of undesired acoustic waves in electro-optical modulators. However, the use of these solutions requires a method to estimate their effectiveness. The main purpose of the article is to describe such a method and to evaluate the correctness of its application. We also offer an additional way to reverse the effect of the mentioned resonant effects on the characteristics of the fiber gyroscope.