Low-amplitude Broadband Research Articles

Impact of previous work in the conditions of industrial noise on the development of sensorineural hearing loss in the outcome of a military injury.

The results of a survey of militia members - industrial workers aged 35-39 years who had previously worked in the conditions of industrial noise are presented. As a result of a military trauma (combined head injury, acutrauma, barotrauma) they developed sensorineural hearing loss. The level of hearing loss and the degree of involvement in the pathological process of the sound-perceiving apparatus is related to the length of work experience in the conditions of industrial noise. For persons who worked in conditions of constant low-amplitude broadband noise (n=30), hearing loss is more pronounced compared to persons (n=30) who worked in conditions of pulsed high-frequency high-amplitude noise. In the control group (35-39 years, n=30), who previously worked outside of industrial noise, the level of hearing loss is determined by the frequency of traumatic brain injury and is associated with the formation of post-traumatic osteochondrosis and vertebrobasilar insufficiency.

Weakly-nonlinear interactions of broadband Tollmien–Schlichting waves in a non-self-similar boundary layer on an airfoil

Abstract The present investigation is devoted to initial nonlinear stages of laminar–turbulent transition initiated by broadband disturbances in an essentially non-self-similar boundary layer, developing on a two-dimensional airfoil. The measurements are performed at fully controlled disturbance conditions with respect to the distribution of the initial spectra of excited 3D (in general) Tollmien–Schlichting (TS) waves. The main attention is devoted to the investigation of resonant (and non-resonant) interactions and amplification of broadband low-amplitude 3D instability waves in the presence (and in absence) of one or two primary harmonic 2D TS-waves. The broadband, “noise-like” TS-waves are composed of controlled perturbations, pseudo-random in time and space, with a continuous frequency–wavenumber spectrum. These perturbations simulate typical background disturbances usually observed at so-called “natural” disturbance conditions. The harmonic primary TS-wave simulates a primary instability mode, which has been formed by the linear instability mechanism. In fact, this paper represents the third step in a set of detailed investigations (Wurz et al., 2012 [1] ) and Wurz et al., 2012 [2] ) of weakly-nonlinear stages of transition performed in the same base flow (on an airfoil) but at increased complexity of initial disturbance conditions. In contrast to the studies of periodic resonant regimes described in refs. Wurz et al. (2012) [1] and Wurz et al. (2012) [2] , no specific (preinstalled) disturbance frequencies or/and spanwise wavenumbers were excited in the boundary layer (except for the primary wave). So, the flow was free to amplify almost any mode of the frequency–wavenumber spectrum by means of any available mechanisms of TS-wave interaction. Such disturbance conditions are the closest to the so-called “natural” ones, i.e. to those observed typically for uncontrolled external perturbations. Four main groups of regimes of disturbance excitation were studied: (i) two harmonic fundamental waves and a pair of frequency detuned subharmonics, (ii) the low-amplitude broadband disturbances only and (iii) the low-amplitude broadband disturbances together with harmonic 2D primary TS-waves, and (iv) the high-amplitude broadband disturbances only. It is found that in case (i) the superposition of the resonances takes place in a nonlinear way and joint efforts of the two fundamental waves are similar to a fundamental-mode amplitude enhancement, which always leads to the increase of the subharmonic amplification rates. In case (ii) the perturbations develop downstream in agreement with the linear stability mechanism. In case (iii) strong resonant interactions between the broadband perturbations and the harmonic primary TS-wave is observed in a wide frequency range. In case (iv) very similar strong resonant interactions are observed between 2D and 3D spectral modes starting already at significantly lower amplitudes of the 2D modes in the frequency range of the primary fundamental modes. In cases (i), (iii) and (iv) the amplification is double-exponential (exponent in exponent), similar to that found in isolated tuned and detuned resonances of deterministic modes in papers Wurz et al. (2012) [1] and Wurz et al. (2012) [2] . For all resonantly interacting disturbances the phase synchronism conditions are found to be satisfied. Based on the cumulative results obtained in the framework of this intensive investigation, the most important observation is the predominant role of resonant TS-wave interactions in the non-self-similar boundary layer developing on a natural laminar flow airfoil section.

Analytical modeling and validation of multi-mode piezoelectric energy harvester

Abstract Energy consumption of electronic devices continues to reduce dramatically, enabling the ambient energy harvesting devices as a solution for powering the wireless sensor nodes (WSNs), micro-electromechanical systems and portable electronics. Over the years, vibration-based piezoelectric harvesting has been emerged as a potential technology to recharge/replace chemical batteries in WSNs. A conventional linear piezoelectric energy harvester (PEH) cannot meet the requirement of broadband vibration resources as it generates useful power only in a narrow bandwidth near the resonant frequency. Several techniques such as passive/active resonance tuning, array of harvester beams, nonlinear designs and multi-degree-of-freedom energy harvesters have been investigated. This paper proposes a generalized multi-mode PEH (MPEH) accompanied with analytical modeling, which generates multiple close peaks of voltage output from low amplitude broadband and low frequency ambient vibration sources. The proposed MPEH comprises of a main cantilevered beam bonded with a patch of piezoelectric layer and attached with multiple branches with tip masses at their free ends. The useful bandwidth and the number of peaks for the MPEH could be tuned conveniently by varying multiple branches and tip masses. The proposed MPEH offers high design flexibility in tuning geometric parameters for achieving target frequencies and generates high power density. The mathematical modeling of generalized MPEH under transverse harmonic base vibrations is presented and the dynamic equations of the multi-modal harvester system are obtained following the Lagrangian method. The derived distributed parametric model of MPEH is more accurate than lumped parametric models of PEHS presented in the literature. The accuracy of the proposed analytical model is validated by two examples with numerical simulation and experimental results. It is shown that the multi-mode PEH has potential to generate sufficient power output from broadband vibration sources to sustain low-power electronic devices.

A NICER Discovery of a Low-frequency Quasi-periodic Oscillation in the Soft-intermediate State of MAXI J1535–571

Abstract We present the discovery of a low-frequency ≈5.7 Hz quasi-periodic oscillation (QPO) feature in observations of the black hole X-ray binary MAXI J1535–571 in its soft-intermediate state, obtained in 2017 September–October by the Neutron Star Interior Composition Explorer. The feature is relatively broad (compared to other low-frequency QPOs; quality factor Q ≈ 2) and weak (1.9% rms in 3–10 keV), and is accompanied by a weak harmonic and low-amplitude broadband noise. These characteristics identify it as a weak Type A/B QPO, similar to ones previously identified in the soft-intermediate state of the transient black hole X-ray binary XTE J1550–564. The lag-energy spectrum of the QPO shows increasing soft lags toward lower energies, approaching 50 ms at 1 keV (with respect to a 3–10 keV continuum). This large phase shift has similar amplitude but opposite sign to that seen in Rossi X-Ray Timing Explorer data for a Type B QPO from the transient black hole X-ray binary GX 339–4. Previous phase-resolved spectroscopy analysis of the Type B QPO in GX 339–4 pointed toward a precessing jet-like corona illuminating the accretion disk as the origin of the QPO signal. We suggest that this QPO in MAXI J1535–571 may have the same origin, with the different lag sign depending on the scale height of the emitting region and the observer inclination angle.

Underwater acoustic emission monitoring – Experimental investigations and acoustic signature recognition of synthetic mooring ropes

Mooring ropes are essential components of offshore installations, and synthetic ropes are increasingly preferred because of their favourable cost to weight ratios. In-service condition of these materials is traditionally monitored through costly visual inspection, which adds to the operating costs of these structures. Acoustic Emissions (AE) are widely used for condition-monitoring in air, and show great potential underwater. This paper investigates the AE signatures of synthetic mooring ropes subjected to sinusoidal tension-tension loading in a controlled environment, using a large-scale dynamic tensile test rig. With a linear array of 3 broadband (20Hz to 50kHz) hydrophones, four main signatures are identified: low-to high frequency, low-amplitude signals (50Hz to 10kHz), low-amplitude broadband signals (10kHz to 20kHz), high amplitude signals (10Hz to 48kHz) and medium-amplitude signals (500Hz to 48kHz). These AE types are related to different stages of rope behaviour, from bedding-in to degradation and failure. The main findings are that the failure location and breaking load can be identified through the detection of AE. The occurrence of high amplitude AE bursts in relation to the applied tensile load allows the detection of an imminent failure, i.e. prior to the failure event. These initial results indicate that AE analyses can enable the integrity of synthetic mooring ropes to be monitored.

Intermittent Burst Oscillations: Signature Prior to Flame Blowout in a Turbulent Swirl-Stabilized Combustor

This experimental study focuses on characterizing the nonlinear combustion dynamics prior to blowout in a partially premixed combustor with a swirl-stabilized burner. Experiments were performed by varying the global equivalence ratio from 0.92 to 0.33. The acquired time-series data of pressure fluctuations were analyzed using nonlinear time-series analysis. When the combustor was operated at an equivalence ratio of 0.92, broadband low-amplitude pressure oscillations called combustion noise were observed. As the equivalence ratio is decreased to 0.62, the oscillations change their characteristics from low-amplitude broadband oscillations to high-amplitude discrete tones, and the combustor undergoes limit-cycle oscillations. A sharp peak is observed in the amplitude spectra of the acoustic pressure oscillations at this stage. On reducing the equivalence ratio to 0.42 and lower, the periodic motion is interrupted by occasional irregular bursts and the system switches back and forth between the noisy (periodic) and quiet (aperiodic) zones. In the dynamical systems theory parlance, this irregular switching between periodic and aperiodic behaviors is termed intermittency. On reducing the equivalence ratio further, the system reaches blowout condition. Intermittency can be used as the signature prior to flame blowout. The dynamical patterns and inherent nonlinearity during the transition from chaotic behavior to intermittent burst oscillations prior to blowout are shown qualitatively by drawing phase portraits, return maps, and recurrence plots.

Forced-Vibration Testing and Experimental Modal Analysis of a Steel Footbridge for Structural Identification

Abstract Dynamic characteristic identification provides an important insight about structural behavior of steel structures. The experimental modal analysis is used to detect dynamic characteristics and consists of several phases. First, forced-vibration tests are carried out, spectral functions are produced, dynamic characteristics are determined by analyzing processed spectral functions, and, finally, analytical models are calibrated or updated depending on experimental analysis results. In this study, forced-vibration testing is conducted on the steel footbridge by using an impact hammer. Low-amplitude broadband excitations are applied using an impact hammer to excite the bridge. The excitation force and measured responses are processed by passing them through a band-pass filter to obtain frequency-response functions, cross-power spectra, auto-power spectra, power spectral densities, and spectrograms. The system-identification procedure is based on input–output measurements. The obtained system-identification results are compared with the analytical results, which were obtained in a different previous study. A very good agreement is observed, thus providing a reliable set of identified modal properties (natural frequencies, damping ratios, and mode shapes) of the structure. The good correlations between analytical and experimental analyses show that no anomalies are detected along the structure. The experimental mode shapes are similar to what is expected from this type of structure. There is not any unexpected mechanism that changes the dynamic characteristics of the system.

Forced Vibration Testing of a Four-Story Reinforced Concrete Building Utilizing thenees@UCLA Mobile Field Laboratory

The nees@UCLA mobile field laboratory was utilized to collect forced and ambient vibration data from a four-story reinforced concrete (RC) building damaged in the 1994 Northridge earthquake. Both low amplitude broadband and moderate amplitude harmonic excitation were applied using a linear shaker and two eccentric mass shakers, respectively. Floor accelerations, interstory displacements, and column and slab curvature distributions were monitored during the tests using accelerometers, linear variable differential transformers (LVDTs) and concrete strain gauges. The use of dense instrumentation enabled verification of common modeling assumptions related to rigid diaphragms and soil-structure-interaction. The first six or seven natural frequencies, mode shapes, and damping ratios were identified. Significant decreases in frequency corresponded to increases in shaking amplitude, most notably in the N-S direction of the building, most likely due to preexisting diagonal joint cracks that formed during the Northridge earthquake.

Experimental Investigation of the Acoustic Field in a Bluff-Body Combustor

Experimental results on the longitudinal acoustic field generated by a methane-air turbulent flame stabilized downstream of a bluff body (V-gutter) confined in a rectangular duct are presented. The V-gutter runs straight across the width of the rectangular cross-section of the duct. The fuel is injected at some distance upstream of the bluff body, as in practical combustors. Varying the distance between the fuel injection and the bluff body alters the degree of fuel-air mixing and hence influences the flame structure, and consequently the nature of the generated acoustic field. Other geometrical variations considered are the length of the combustor duct, the axial location of the bluff body, and the width of the bluff body. The flow rates of air and fuel are varied widely to traverse conditions of generation of low-amplitude broadband noise to those of excitation of high-amplitude discrete tones. The observed dominant frequencies are expressed in terms of non-dimensional numbers such as the Helmholtz number and the Strouhal number, representing the natural acoustic modes and the free-field vortex shedding process, respectively. It is seen that, as the flow conditions are varied, the amplitude of the dominant frequency steeply rises, accompanying an abrupt jump in the dominant frequency from a constant Helmholtz number regime to a constant Strouhal number regime. High-speed images of CH* chemiluminescence of the flame zone exhibit periodic shedding of vortical structures at the same dominant frequencies as in the observed acoustic spectra. These results show that the natural acoustic modes shift to follow the vortex shedding trend with the Reynolds number of the air-flow, under conditions of excitation of loud discrete tones. Higher fuel flow rates are observed to be capable of exciting higher acoustic modes to exhibit the above behaviour. A larger location of the bluff body away from the fuel injection location, and a lesser width of the bluff body, tend to excite acoustic oscillations of lower amplitude.

철근콘크리트조 4층 골조건물의 강제진동실험

노스리지 지진에 의해 손상을 받은4층 철근콘크리트조 골조건물을 대상으로 선형가진기 및 대용량의 편심가진기를 이용한 강제진동실험과 상시미진동 측정을 실시하였다. 미진동 가속도데이터 및 선형가진기에 의한 백색잡음 실험시의 가속도데이터로부터 구조물식별을 수행하여 7차모드까지의 고유진동수 및 모드감쇠비를 얻었다. 두대의 대용량 편심가진기를 사용하여 얻은 큰 진폭의 조화 진동하에서는 가속도데이터를 사용하여 각 방향 1차모드를 식별하였으며 변위계와 변형게이지를 이용하여 층간변위각, 기둥과 슬래브와 같은 구조부재의 곡률분포를 측정하였다. 각 경우 고유진동수는 진동의 크기가 클수록 낮아졌다. 즉, 편심가진기가력시 고유진동수는 상시미진동시에 비해 <TEX>$70{\sim}75%$</TEX>, 선형가진시가력시에 비해 <TEX>$92{\sim}93%$</TEX> 정도로 낮게 나타났다. 이러한 진동수의 감소폭은 지진에 의해 큰 손상을 받았던 건물의 남북방향에서 크게 나타났다. A series of forced vibration tests and ambient vibration measurement was conducted on a four-story reinforced concrete building damaged in the 1994 Northridge earthquake. Both low amplitude broadband and moderate amplitude harmonic excitation were applied using a linear shaker and two eccentric mass shakers, respectively, and ambient vibrations were measured before and after each forced vibration test. Accelerations, interstory displacements, and curvature distributions were monitored using accelerometers, LVDTs and concrete strain gauges. Natural frequencies and the associated mode shapes fur the first 7 modes were identified. Fundamental frequencies determined from the eccentric mass shaker tests were 70% to 75% of the values determined using ambient vibration data, and 92% to 93% of the values determined using the linear shaker test data. Larger frequency drops were observed in the NS direction of the building, apparently due to damage that was induced during the Northridge earthquake.

Three-dimensional mode growth in boundary layers with tuned and detuned subharmonic resonance

This work involves the use of controlled periodic disturbances to excite a plane Tollmien-Schlichting (TS) wave at one frequency ( f 2D) along with pairs of oblique waves with equal but opposite wave angles at a different frequency ( f 3D) in order to study the resonant growth of 3D modes in a Blasius boundary layer. In our earlier work (Corke &amp; Mangano 1989; Corke 1990), the frequency of the oblique modes was exactly the subharmonic of the plane Tollmien-Schlichting (TS) mode. These modes were also phase-speed locked so that in terms of their streamwise wave numbers, α2D=1/2 α3D. This so-called ‘tuned’ subharmonic resonance leads to the enhanced growth of the otherwise linearly damped oblique waves, as well as the growth of higher harmonic 3D modes with frequencies and wave numbers: (3/2 f2d, 3/2 α2D, ±β3D), (5/2 f2d, 5/2 α2D, ±β3D), (f2d, α2D, ±2β3D) and (0, 0, ±2β3D). Even when the initial 3D oblique waves have frequencies which are close to the TS subharmonic frequency, a ‘detuned’ subharmonic resonance leads to the enhanced growth of the 3D mode. In addition, it promotes the growth of numerous discrete modes produced by successive sum and difference interactions. These interacted modes are also three dimensional, with higher amplification rates that increase with the interaction order. The growth of these modes accounts for the rapid spectral filling, and low-frequency modulation commonly observed in natural subharmonic transition. Starting from a ‘tuned' resonance, this scenario then provides a mechanism for the generation of a broad spectrum at the later stages of subharmonic mode transition. However, the results also suggest that with ‘natural’ transition, starting from low-amplitude broadband disturbances, the most likely 2D/3D resonance will be ‘detuned’.