Frequency Range Of Interest Research Articles

The Rayleigh integral is always positive in steadily operated combustors

The Rayleigh index has been used for decades by a large number of researchers as an indicator to determine if a flame is driving or damping thermoacoustic interaction mechanisms. The use of the Rayleigh criterion has found applications in rocket combustors, gas turbine combustion technology and basic combustion research. The global Rayleigh index or integral is obtained by integrating the product of heat release rate and pressure fluctuations over space and time. Depending on the phase between pressure oscillations and heat release rate response, the oscillations can be enhanced or damped. It is commonly assumed in literature that the sign of the Rayleigh index from steady state data can be used to determine if the thermoacoustic feedback loop is stabilizing or destabilizing. However, we show in this paper that under fairly general conditions, a correctly measured Rayleigh index is always positive if evaluated from statistically stationary data. This proves to be true even if the heat release rate response to pressure fluctuations is in phase opposition to those pressure fluctuations. This is shown in a straightforward manner by substituting the wave equation with a heat release rate source term into the Rayleigh index. This was verified experimentally on a fully premixed combustion system by measuring the flame chemiluminescence using a photo multiplier and pressure fluctuations using a microphone placed sufficiently close to the flame to ensure acoustic compactness for the frequency range of interest. A large range of operating conditions have been tested, spanning linearly stable and unstable stationary thermoacoustic states, respectively corresponding to resonance or a limit cycle driven by the inherent stochastic forcing from the turbulent combustion noise. The experimental results corroborated the analytic finding: the Rayleigh index is found to be positive for all frequencies and all operating conditions.

Emergent Moiré Phonons Due to Zone Folding in WSe2-WS2 Van der Waals Heterostructures.

Bilayers of 2D materials offer opportunities for creating devices with tunable electronic, optical, and mechanical properties. In van der Waals heterostructures (vdWHs) where the constituent monolayers have different lattice constants, a moiré superlattice forms with a length scale larger than the lattice constant of either constituent material regardless of twist angle. Here, we report the appearance of moiré Raman modes from nearly aligned WSe2-WS2 vdWHs in the range of 240-260 cm-1, which are absent in both monolayers and homobilayers of WSe2 and WS2 and in largely misaligned WSe2-WS2 vdWHs. Using first-principles calculations and geometric arguments, we show that these moiré Raman modes are a consequence of the large moiré length scale, which results in zone-folded phonon modes that are Raman active. These modes are sensitive to changes in twist angle, but notably, they occur at identical frequencies for a given small twist angle away from either the 0-degree or 60-degree aligned heterostructure. Our measurements also show a strong Raman intensity modulation in the frequency range of interest, with near 0 and near 60-degree vdWHs exhibiting a markedly different dependence on excitation energy. In near 0-degree aligned WSe2-WS2 vdWHs, a nearly complete suppression of both the moiré Raman modes and the WSe2 A1g Raman mode (∼250 cm-1) is observed when exciting with a 532 nm CW laser at room temperature. Temperature-dependent reflectance contrast measurements demonstrate the significant Raman intensity modulation arises from resonant Raman effects.

Semi-active control of primary suspensions to improve ride quality in a high-speed railway vehicle

Semi-active suspensions are seen as a promising technology to improve ride quality in high-speed railway vehicles as they provide substantial benefits not achievable through an optimisation of the passive suspension while avoiding some drawbacks of full-active control. Previous works showed that semi-active primary suspensions (SAPS) offer significant improvement of ride quality. However, published implementations of this concept did not consider the coupling effects between car-body and bogie vibrations, which play a decisive role in the success of control strategies for SAPS. This paper provides a comprehensive analysis of SAPS to improve vehicle ride comfort, with special attention to the mitigation of car-body bending vibration. A vehicle model considering the coupling of car-body bending and bogie pitch is proposed and used for the synthesis of an LQG controller. The performance of the semi-active suspension and controller is assessed using a detailed flexible multibody system of a high-speed vehicle. The proposed control scheme is shown to be highly effective in reducing car-body vibration in the entire frequency range of interest from 1 Hz to20Hz. A remarkable ride comfort improvement is found in the entire speed range from 150 km/h to 350 km/h.

Multiband topologically protected states realized by elastic honeycomb structures based on fundamental mechanical elements

Topological metamaterials with novel properties of topologically protected states have been developed vigorously inspired by the recent emergence of topological phases in electronic systems. In mechanical community, however, the vast majority of such investigations have either focused on discrete mass–spring systems, whose operating bands are single and are difficult to be transformed into practice, or on various carefully designed continuous elastic systems, suffering from the drawbacks of structural complexity and the presence of modal hybridization. In this study, we design an elastic system composed of clamped-free Euler–Bernoulli (EB) beams coupled via slender rods capable of supporting multiband uni-mode topological states, which overcomes the shortcomings of both discrete lattice and continuous structure to a great extent. Through a theoretical beam-spring model together with finite element analysis, we reveal that the symmetry of the underlying lattice structure and the nature of multi-order natural modes of the beam ensure the deterministic existence of multiple Dirac points. Splittings of these degeneracies induced by the mirror symmetry breaking give rise to two kinds of valley Hall phases characterized by opposite valley Chern numbers. Numerical simulations demonstrate that the domain walls formed by structures with distinct topological indices support elastic interface waves at multiple frequency ranges, which are topologically protected manifested by the features of being immune to backscattering of sharp corners and defects. In addition, we show that the frequency range of interest can be easily tuned by varying strength of symmetry breaking. Our results provide an entirely novel paradigm to design topological mechanical systems with multiple working frequencies. Moreover, thanks to the structural simplicity, the proposed elastic system can be exploited in practice as an excellent and versatile platform to explore other intriguing wave phenomena brought about by richer topological notions.

A programmable digital metasurface structure designed using ANN technique

ABSTRACT In this paper, an 8-bit programmable digital metasurface is designed in the operating frequency range from 4 to 7 GHz. A monopole antenna operating at 5 GHz is characterised by using a metasurface structure as the ground plane. Various combinations of the metasurface structure are examined by altering the state of each unit-cell between ‘ON (1)’ and ‘OFF (0)’ in the coding matrix. The purpose of the 8-bit programmable digital metasurface is to control the electromagnetic wave effectively in the frequency range of interest. Dynamically controllable metasurface structure is utilised to change the unit cells’ configurations. In addition, the proposed adjustable metasurface is capable to control the monopole antenna directivity, gain and main lobe magnitude, efficiently. According to various via conditions, different metasurface-based antenna parameters such as return loss (S11), radiation pattern, directivity and surface current distribution are investigated by means of the commercial numerical simulation software, CST microwave studio. Employing the simulation results of the metasurface-based antennas, the artificial neural network (ANN) data set is obtained. 128 activation conditions are trained and tested by Levenberg Marquart learning algorithm. During the ANN procedure, MATLAB is used to obtain accurate results by changing the rate of test and training data set.

Graphite-oxide hybrid multi-degree of freedom resonator metamaterial for broadband sound absorption

Low frequency broadband sound absorption for thin structures is still a great challenge. A new concept of a stackable hybrid resonator metamaterial is proposed which exhibits super broadband low-frequency sound absorption. The proposed metamaterial is based on micrometric scale thickness Graphene Oxide (GO) embedded in a stacked structure or used as external skin in a designed honeycomb (HC) structure. The stackable nature of the proposed structure allows the GO-HC cores to be embedded within micro-perforated panels (MPP) providing enhanced stiffness/strength to the structure and high absorption characteristics. We demonstrate how the exploitation of the GO elastic and mass properties result in multiple hybrid structural–acoustic resonances. These resonances are tailored to occur in a frequency range of interest by the theoretical calculation of the sound absorption coefficient. The theoretical model combines the mutual interaction between the structural dynamic of the GO foil and acoustic higher modes of the HC core cell as well as stacked MPP-HC/GO-HC cores. The result is a multi-degree of freedom hybrid resonator which provides subwavelength scale broadband sound absorption in low frequency range between 300 and 2500 Hz.

Ultra-Low-Loss and Broadband All-Silicon Dielectric Waveguides for WR-1 Band (0.75–1.1 THz) Modules

This study presents ultra-low-loss and broadband all-silicon dielectric waveguides for the WR-1 band (0.75–1.1 THz). The waveguides are built in high-resistivity silicon (10 kΩ-cm) and integrated with supportive frames fabricated from the same silicon wafer in a single etch process to achieve a compact design. We pursued low-loss, broadband, substrateless, unclad and effective medium waveguides. Smaller propagation losses of 0.3 dB/cm and 0.1 dB/cm were achieved for the unclad and effective medium waveguides, respectively. The 3 dB bandwidth was not encountered in the frequency range of interest and was as broad as 350 GHz. An unclad waveguide was employed to devise a Y-junction to demonstrate its practical applications in terahertz imaging. An integrated circuit card was successfully scanned. In addition, we developed unclad waveguide, effective medium waveguide, and Y-junction modules. The modules incorporated an input/output interface compatible with a standard WR-1 flange (254 μm × 127 μm). Unlike the conventional hollow waveguide modules, the unclad waveguide and effective medium waveguide modules reported total loss improvements of 6 dB and 8 dB, respectively, across the operation band. Our results provided a systematic way of achieving low-loss, compact, and versatile modules in the WR-1 band based on all-dielectric-waveguide platforms.

Microstrip patch Antenna Array for Wireless Design Applications

The patch antenna multiple wireless applications recently indicate great prospects in the application of various areas of micro-engineering. The rapid development of wireless communication systems requires antenna researchers to develop miniature broadband antennas with communication systems with high radiation efficiency The paper relates to calculating the resonant frequency of a microstrip antenna, this article presents a low-profile, broadband microstrip printed circuit board (PCB) antenna with a double U-slot. Triple-Band resonant modes can be realized by precise slot placement on the board. Antenna performance is improved by installing a gap in the ground plane. The proposed antenna is made in the form of a rectangular patch. As a substrate, a widely used material is used – fib regals FR4 brand, in which, in the frequency range of interest to us, the relative permittivity εr = 4.3. The dimensions of the antenna are 30×40×0.8 mm, such an antenna is easily and simply powered using an unbalanced microstrip line with a wave impedance of 50 ohms.

Prior-Knowledge-Guided Deep-Learning-Enabled Synthesis for Broadband and Large Phase Shift Range Metacells in Metalens Antenna

A prior-knowledge-guided deep-learning-enabled (PK-DL) synthesis method is proposed for enhancing the transmission bandwidth and phase shift range of metacells used for the design of metalens antennas. The algorithm of conditional deep convolutional generative adversarial network (cDCGAN) is utilized in the proposed deep-learning (DL) method. Prior knowledge, including well-known fundamental electromagnetic theorems and experience in antenna design, is purposely applied at the early stage of the proposed method to strategically guide and speed up the synthesis. The proposed intelligent method provides the design of pixelated metacells with high degrees of freedom so that the key performance of the synthesized metacells exceeds the existing limit of conventional design methods by generating a rich profusion of cell patterns. For example, the synthesized triple-layer metacell achieves the −1 dB phase shift range of 330° breaking the limit of 308° derived by existing techniques. The proposed synthesis method also provides the additional capability to flexibly control the phase shift not only at the center frequency but also over a frequency range of interest. A Ku-band metalens antenna formed with the synthesized metacells demonstrates the achieved 1 and 3 dB gain bandwidths increase by 52.2% and 42.6%, respectively, compared to the metalens antenna using the well-known Jerusalem cross (JC) metacells. The proposed method extends the capability for the synthesis of metacells and metalens antennas with enhanced performance.

A 220–261 GHz Frequency Multiplier Chain (× 18) With 8-dBm Peak Output Power in 130-nm SiGe

This letter presents a 220–261-GHz frequency multiplier-by-18 chain in a 130-nm SiGe BiCMOS technology. It consists of three amplifiers (12–15, 110–135, and 216–270 GHz), two frequency triplers (36–45 and 108–135 GHz), a third-order Chebyshev bandpass filter (36–45 GHz), and a modified Gilbert-cell-based frequency doubler (216–270 GHz). The peak output power is about 8 dBm at 240 GHz with a 3-dB bandwidth of 41 GHz for an input power of −7 dBm. The unwanted harmonics are suppressed more than 25 dBc over the frequency range of interest. It consumes a dc current of 114.7 mA from a supply voltage of 3.3 V in the quiescent operation and drains a current of 130 mA for an input power of −7 dBm, which results in a drain efficiency of 1.47%. The total circuit occupies an area of 0.58 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> (1.35 mm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times0.43$ </tex-math></inline-formula> mm).

The application of tuned mass dampers to the Tire Pavement Test Apparatus to minimize the impact of rig resonance

The 'Tire Pavement Test Apparatus (TPTA)' at Purdue University is a device that was originally developed to study a tire's air-borne noise radiation in the frequency range 300 Hz to 2 kHz. However, in recent years, the device has mainly been used to quantify a tire's contribution to structure-borne noise by measuring the force and acceleration at the hub. Hence, the frequency range of interest has shifted to frequencies below 300 Hz. By comparing measurements and FE simulations, undesired vibration was observed around 160 Hz in the measurement data. It was speculated that the rig supporting the tire exhibits resonant vibration in a frequency range that hinders the measurement of tire force in that range. To identify the cause and mechanism of the rig resonance, Experimental Modal Analysis (EMA) was performed. In addition, to mitigate the influence of rig resonance, tuned mass dampers commonly used in vehicle suspension systems were applied to the connecting arms of the TPTA. Careful adjustments of their mass and stiffness were performed to maximize vibration suppression performance by matching the major natural frequency of the TPTA support arms. As a result, significant vibration attenuation could be achieved, which has made it possible to highlight the pure tire effects in the measurement data.

Elastic wave attenuation in a metaplate with periodic hollow shapes for vibration suppression

Low noise and vibrations are requirements for engineering structures to realize comfort and ensure systems are environmental friendly. Periodic structures such as phononic crystals have been investigated to suppress noise and vibrations because they inhibit elastic wave propagation in the frequency ranges referred to as the band gap. This study led to the proposal of a metaplate (MP) with periodic hollow structures to form band gaps by changing the surface shapes of the thin plates. MPs such as this can be beneficial from the viewpoints of manufacturing cost and structural reliability when applied to engineering structures. To evaluate the potential ability of the proposed MP to suppress vibrations, we obtained the band gap frequency of out-of-plane deformation waves using the wave finite element method. To efficiently evaluate the band gaps, the internal degrees of freedom in a unit cell are reduced based on the modal analysis technique. Furthermore, the out-of-plane and in-plane deformation waves are separated using kinematic energy to extract the band gap related to the out-of-plane deformation waves. Based on this numerical framework, we demonstrate that periodic hollow shapes can form a band gap in the frequency range of interest in engineering problems. Moreover, the potential of the MP was investigated using hollow shape design and material selection, and the results indicate that the proposed MP is beneficial for tuning the band gap frequency in a unit cell of which the size is acceptable for engineering structures. Finally, vibration suppression caused by band gap is experimentally demonstrated via impact testing of an MP fabricated using additive manufacturing.

Signal Estimation for Vehicle Body Accelerations Using Piecewise Linear System Identification in the Frequency Domain

Abstract In this work, we investigate a signal estimation problem which is common and critical for durability design of vehicle bodies. The relation between the frequency responses of accelerometers is the target to model so that the ones of easy-to-measure accelerometers can estimate the responses of hard-to-measure accelerometers. A piecewise linear frequency-domain identification method relying on finite impulse response (FIR) models is proposed and performed to tackle the nonlinearity issue in the signal estimation problems: first, the interesting frequency range is segmented into three subranges which are clearly identified by peak histograms of frequency signals. Then, FIR models which provide a satisfactory description of the system are constructed to estimate the frequency responses of the interesting signals at subranges, one for each. The performance of the proposed approach is validated by using real-world data under multiple working conditions. The results show that the proposed method has a good estimation accuracy, and it brings the benefit that the number of accelerometers can be significantly reduced during the durability design of vehicle bodies.

Design and Application of Double-Parallelograms-Based Tuned Mass Damper for Low-Frequency Vibration Absorption

Abstract Low-frequency vibration suppression is challenging in practical engineering problems due to the harsh requirement for vibration reduction devices, which requires constant low stiffness over a wide amplitude range. A passive tuned mass damper (TMD) composed of a positive stiffness module (PSM) in parallel with a negative stiffness module (NSM) is proposed, which are implemented by serial double-parallelograms (DP) and parallel-DP, respectively. The PSM has a large deflection range of constant stiffness for a given beam length, while the NSM offers negative stiffness within a certain deflection range when applied with axial load above the critical threshold. Based on the closed-form modeling of the stiffness modules using the beam constraint model (BCM), the design and analysis of the PSM and NSM are carried out considering the nonlinearity under large deflections. Afterward, with the structure of TMD implemented, its stiffness characteristics and low-frequency tunability are experimentally validated. Finally, the application on a suspension bridge model shows that a maximum of 29.8-dB vibration reduction of low-frequency mode is attained within the frequency range of interest. The proposed TMD well attenuates the vibrations excited by sweep sinusoidal and harmonic excitations under prespecified threshold levels of acceleration.

Relationship between age, sex and pupillary unrest.

Pupllary unrest in ambient light (PUAL) is the chaotic fluctuation of pupil diameter about its mean value. The degree of fluctuation is typically measured using a pupillometer and quantified using an index that sums fluctuations in a frequency range of interest. Indices that measure PUAL show promise in predicting effects of sedative or opioid drugs. Because PUAL is mediated by the parasympathetic nervous system, it would be expected to decline with age-related decreases in parasympathetic tone. In this study we measured average PUAL in 16 subjects from age 3 to 72, with at least 3 repeated measurements on each subject, each taken for approximately 10s, at approximately 30 frames per second, with illumination of the ipsilateral eye. PUAL was calculated using the algorithm of the Neurolight pupillometer. We found that average PUAL typically declines by approximately 1% per year, and that the standard deviation of repeated measurements is approximately 20%. We therefore conclude that repeated, averaged measurements of age-normed PUAL are likely to be much more clinically useful than single, uncorrected values.

A novel two-objective optimization computational framework for a two-body heaving wave energy converter

The present research introduces a novel two-objective optimization computational framework, named H∞ optimization criterion-constrained Two-Objective Optimization Genetic Algorithm (H∞ − cTOOGA). Within this optimization framework, the objectives considered are: (i) the sprung mass isolation, and (ii) the wave energy extraction of a heaving buoy coupled with a sprung (overlying) mass. In such a two-body heaving wave energy converter (WEC), the satisfaction of both objectives is pursued by proper selection of the buoy geometry (i.e., radius r and draft Dr) and the characteristics of the power take-off (PTO). The interrelation among potential buoy geometries and PTO characteristics is performed on the basis of a critical condition. The critical condition refers to the formation of equal double peaks in the sprung mass acceleration response curve and is imposed as the sprung mass acceleration limit that is not allowed to be exceeded throughout the entire frequency range of interest. The two-objective optimization is realized with r, Dr and the tuning ratio δOPT being the decision variables. The value of δOPT is constrained by the value of the critical tuning ratio δcrit, that is, δOPT <δcrit. This ensures the minimization of the maximum sprung mass acceleration response. Then, the Pareto-optimal solutions result to optimal wave energy extraction.

Improved Balance Technique for Common-Mode Noise Suppression of PCB-Based PFC

Symmetry and cancellation techniques are often employed to suppress common-mode (CM) noises in switching power converters. In addition, shielding technique (Fei <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">et al.</i> , 2018) has been demonstrated effectively to suppress CM noises for isolated converter with PCB-based transformer design. However, for nonisolated converters, such as power-factor correction circuits, none of the techniques mentioned above is deemed applicable or justifiable. Recently, the balance technique has been demonstrated effectively to suppress CM noises up to a point where the parasitic ringing between the inductor and its winding capacitor is observed (Wang <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">et al.</i> , 2007), (Yang, 2018). This article presents an improved balance technique in a PCB-based coupled inductor design that compensates the detrimental effect of the interwinding capacitors. In the given example, CM noise suppression is effective in the frequency range of interest up to 30 MHz. A CM noise model is established to substantiate the measurement results.

Switchable Terahertz Metasurfaces Based on Patterned Vanadium Dioxide and Graphene

This paper proposes a bi-functional switchable broadband terahertz metasurface ground on U-shaped vanadium dioxide (VO2) and graphene. The proposed design can effectively switch the current working state through a two-parameter regulation mechanism. Specifically, as we fix graphene’s Fermi level at 1 eV, and VO2 is in the form of insulating, the proposed design can be seen as a broadband terahertz absorber. Shifting the Fermi level of graphene can dynamically modulate the amplitude of the broadband absorption spectrum. In other words, by arbitrarily tailoring the Fermi level of graphene, the proposed design can freely switch states, i.e., mode switching from broadband absorption to broadband reflection, in the frequency range of interest. As graphene’s Fermi level is equal to 0.01 eV, and the vanadium dioxide in the structure is in the metallic state; the designed metasurface can be seen as a broadband terahertz linear polarization converter. It can convert the incident linearly polarized terahertz wave into its orthogonal polarization. By varying the conductivity of vanadium dioxide in the simulation, the proposed design can freely tune the current operating state over the operating frequency range, similar to “ON” and “OFF”. Metasurfaces can work efficiently in different frequency ranges by changing the geometric parameters. Therefore, the designed structure has switchable and tunable functions simultaneously, providing additional options for integrated, intelligent, and miniaturized devices.

Characterization of low levels of turbulence generated by grids in the settling chamber of a laminar wind tunnel

Wind tunnel investigations of how Natural Laminar Flow (NLF) airfoils respond to atmospheric turbulence require the generation of turbulence, whose relevant characteristics resemble those in the atmosphere. The lower, convective part of the atmospheric boundary layer is characterized by low to medium levels of turbulence. The current study focuses on the small scales of this turbulence. Detailed hot-wire measurements have been performed to characterize the properties of the turbulence generated by grids mounted in the settling chamber of the Laminar Wind Tunnel (LWT). In the test section, the very low base turbulence level of Tuu ≅ 0.02% (10 ≤ f ≤ 5000 Hz) is incrementally increased by the grids up to Tuu ≅ 0.5%. The turbulence spectrum in the u-direction shows the typical suppression of larger scales due to the contraction between grids and test section. Still, the generated turbulence provides a good mapping of the spectrum measured in flight for most of the frequency range 500 ≤ f ≤ 3000 Hz, where Tollmien-Schlichting (TS)-amplification occurs for typical NLF airfoils. The spectra in v and w-direction exhibit distinct inertial subranges with slopes being less steep compared to the − 5/3 slope of the Kolmogorov spectrum. The normalized spectra in u-direction collapse together well for all grids, whereas in v- and w-directions the inertial- and dissipative subranges are more clearly distinguished for the coarser grids. It is demonstrated that the dissipation rate ε is a suitable parameter for comparing the wind tunnel turbulence with the atmospheric turbulence in the frequency range of interest. By employing the grids, turbulence in the range 4.4 × 10–7 ≤ ε ≤ 0.40 m2/s3 at free-stream velocity U∞ = 40 m/s can be generated in the LWT, which covers representative dissipation rates of free flight NLF applications. In the x-direction, the spectra of the v and w-components develop progressively more pronounced inertial- and dissipative subranges, and the energy below f ≈ 400 Hz decreases. In contrast, the spectral energy of the u-component increases across the whole frequency range, when moving downstream. This behavior can be explained by the combination of energy transport along the Kolmogorov cascade and the incipient return to an isotropic state.Graphic

Flight Dynamics Simulation Modeling of a Large Flexible Tiltrotor Aircraft

A high-order tiltrotor mathematical model is developed and validated against flight-test data for the XV-15 and simulations of a large civil tiltrotor (LCTR) concept. Rigid body and inflow states, as well as flexible wing and blade states are used in the analysis. Wing flexibility is important when modeling large aircraft where structural modes effect the frequency range of interest for flight control, generally 1–20 rad/s. Details of the formulation of the mathematical model are given, including derivation of structural, aerodynamic, and inertial loads. A novel "quasi-multibody" approach, based on numerical kinematics but without equations of constraints, allows the modeling of complex, flexible aircraft configurations in an easy to set up, and computationally very efficient manner. Assessments of the effects of wing flexibility are given. Flexibility effects are also evaluated by looking at the nature of the couplings between rigid body modes and wing structural modes.