Broadband Content Research Articles

Optimized compact finite volume formulation with dispersion relation preserving characteristics in structured space discretizations

Computationally extensive fluid flow simulations usually require numerical schemes with a high degree of numerical precision, where high wavenumber and frequency capturing capability is desired in solutions with high physical fidelity. Numerical dispersion and diffusion errors must be reduced to a minimum in order to caputre low amplitude and high frequency flow aspects, such as flow induced sound waves and minor instabilities. Such schemes also must be capable of dealing with flow discontinuities, such as shockwaves and expansion waves in cases where the flow velocity stands near or above sound velocity. In this work the Dispersion Relation Preserving (DRP) optimization is formally extended to the compact Finite Volume (FV) framework where the error is minimized along a given spectral interval. This is done by equaling the resulting algebraic compact spectral optimized Finite Difference (FD) operators with the Finite Volume formulation with the desired numeric characteristics. Numerical tests are carried over linear and nonlinear governing equations models for one and three-dimensional cases, where the travelling Gaussian wave, the transient expansion wave and the smooth to nonsmooth step wave flows are considered. The proposed compact DRP FV schemes surpasses its counterparts in some numerical tests while maintaining comparable capabilities with other more basic schemes in the remaining numerical tests. Excellent broadband content capturing capability is observed, while possessing reduced numerical instabilities generation.

Subharmonics are not the full story—Searching for acoustic signatures of cavitation and other fantastic beasts

Cavitation induced bio-effects are being exploiting in a wide range of applications from physiotherapy to brain surgery. The acoustic emissions generated by bubble activity are extremely useful in enabling real time treatment monitoring. The relationship between the spectral characteristics of these emissions and bubble dynamics is, however, complex. Here we report data from experiments with simultaneous ultra-high-speed optical imaging and passive acoustic mapping of individual microbubbles exposed to 50 cycles of ultrasound at 0.5 MHz with varying peak negative pressures. The spectral content of the acoustic emissions from individual microbubble was compared to the bubble dynamics observed by the imaging. As expected from prior work, both the number of discrete harmonics and broadband content in the emissions increased with increasing amplitude of bubble oscillation. There was no clear correlation, however, between the presence of ultra and sub-harmonic components and bubble behaviour. Indeed, these components were frequently absent. Moreover, phenomena such as microjetting, fragmentation and coalescence, that could produce very different effects in tissue, were indistinguishable acoustically. The results thus indicate that the definition of cavitation thresholds or doses should be very carefully considered depending on the therapeutic effect (or avoidance of unwanted bioeffects) required for a particular application.

Active noise cancellation with FPGA - practical considerations

Active noise cancellation (ANC) has been successfully applied to mitigate noise in many applications, such as in headphones. Besides algorithm development and computer simulation, developing an ANC system involves many practical considerations. From hardware selection to fixed-point implementation, many practical constraints can prevent a good algorithm from functioning properly on the hardware. This paper discusses the development of an ANC prototype system using Field-Programmable Gate Array (FPGA) hardware. The paper also reviews several practical issues that were encountered in the development of the prototype, the symptom and root cause of each issue, and the solutions to mitigate them. The final ANC prototype system successfully cancels noise of both tonal and broadband contents.

Correlating high-speed optical imaging and passive acoustic mapping of cavitation dynamics

The biological effects of acoustic cavitation are mediated by a range of phenomena associated with different types of bubble activity, e.g., micro-streaming, micro-jetting, and shockwave generation. The acoustic emissions generated by cavitation are also correlated to bubble dynamics. Hence, monitoring these emissions during ultrasound therapy is desirable, to maximise treatment safety and efficacy. The precise relationship between the spectral content of acoustic emissions and bubble dynamics is, however, less well understood. The aim of this study was to use simultaneous ultra-high-speed optical imaging (1–10 MHz) and passive acoustic mapping to characterise the behaviour of individual and clusters of microbubbles over a range of ultrasound exposures. As expected from the literature and numerical modelling, both the number of discrete harmonic components and amplitude of broadband content in the emissions increased with increasing amplitude of bubble oscillation. This suggests that passive acoustic mapping provides a useful indicator of spherical bubble behaviour. Frequently, however, complex bubble behaviour, such as fragmentation and coalescence was observed, which could produce substantially different effects in tissue compared with spherical bubble collapse. Future work will focus on determining whether these differences can be adequately captured in defining a future cavitation “dose” based on acoustic emissions for different applications.

Aerodynamic simulation of a high-pressure compressor stage using the lattice Boltzmann method

PurposeThe present paper aims at evaluating the lattice Boltzmann method (LBM) on a high-subsonic high-pressure compressor stage at nominal regime.Design/methodology/approachThe studied configuration corresponds to the H25 compressor operated in a closed-loop test rig at the von Karman Institute. Several operating points are simulated with LBM for two grids of successive refinements. A detailed analysis is performed on the time-averaged flow predicted by LBM, using a comparison with experimental and existing RANS data.FindingsThe finest grid is found to correctly predict the mean flow across the machine, as well as the influence of the rotor tip gap size. Going beyond time-averaged data, some flow analysis is performed to show the relevance of such a high-fidelity method applied to a compressor configuration. In particular, vortical structures and their evolution with the operating points are clearly highlighted. Spectral analyses finally hint at a proper prediction of tonal and broadband contents by LBM.Originality/valueThe application of LBM to high-speed turbomachinery flows is very recent. This paper validates one of the first LBM simulations of a high-subsonic high-pressure compressor stage.

Vortex Formation Times in the Glottal Jet, Measured in a Scaled-Up Model.

In this paper, the timing of vortex formation on the glottal jet is studied using previously published velocity measurements of flow through a scaled-up model of the human vocal folds. The relative timing of the pulsatile glottal jet and the instability vortices are acoustically important since they determine the harmonic and broadband content of the voice signal. Glottis exit jet velocity time series were extracted from time-resolved planar DPIV measurements. These measurements were acquired at four glottal flow speeds (uSS = 16.1–38 cm/s) and four glottis open times (To = 5.67–23.7 s), providing a Reynolds number range Re = 4100–9700 and reduced vibration frequency f* = 0.01–0.06. Exit velocity waveforms showed temporal behavior on two time scales, one that correlates to the period of vibration and another characterized by short, sharp velocity peaks (which correlate to the passage of instability vortices through the glottis exit plane). The vortex formation time, estimated by computing the time difference between subsequent peaks, was shown to be not well-correlated from one vibration cycle to the next. The principal finding is that vortex formation time depends not only on cycle phase, but varies strongly with reduced frequency of vibration. In all cases, a strong high-frequency burst of vortex motion occurs near the end of the cycle, consistent with perceptual studies using synthesized speech.

Research on the rotor/stator interaction noise through vane unsteady pressure test based on polyvinylidene fluoride sensor array

The rotor/stator interaction noise is the primary noise source of modern turbofan engines. Vane unsteady pressure test is essential to understand the generation mechanism and propagation characteristics of the noise, as well as examine the passive/active noise control technologies. However, limited by the Vane’s thickness as well as the requirement for high spatial and temporal resolutions of the sensor, traditional transducers still have considerable limitations and difficulties carrying out the measurement. In this paper, a polyvinylidene fluoride (PVDF) piezoelectric-film sensor array was developed for the vane unsteady pressure test. The dynamic characteristics of the sensor were obtained by the shock tube test. The calibration results further confirm the effectiveness of the self-developed PVDF sensor and also show the advantage in frequency response over traditional pressure transducer. Time histories of the vane fluctuating pressures were obtained by the PVDF sensor. The blade passing frequency (BPF) harmonic component and the broadband component of the unsteady pressure were analyzed individually by time-domain averaging and spectral analysis. The distributions of BPF content and broadband content on both sides of the vane at different fan tip speeds and operating conditions were investigated in detail. The results indicate that the operating conditions have little effect on both of them. The broadband component yielded by the rotor-wake turbulence seems to be quasi-isotropic. The average broadband level increases linearly with the fan tip speed. The developed PVDF sensor shows potential in noise evaluation and noise source diagnosis, which can properly guide further noise reduction technology development.

Angular dependence of the acoustic signal of a microbubble cloud.

Microbubble-mediated ultrasound therapies have a common need for methods that can noninvasively monitor the treatment. One approach is to use the bubbles' acoustic emissions as feedback to the operator or a control unit. Current methods interpret the emissions' frequency content to infer the microbubble activities and predict therapeutic outcomes. However, different studies placed their sensors at different angles relative to the emitter and bubble cloud. Here, it is evaluated whether such angles influence the captured emissions such as the frequency content. In computer simulations, 128 coupled bubbles were sonicated with a 0.5-MHz, 0.35-MPa pulse, and the acoustic emissions generated by the bubbles were captured with two sensors placed at different angles. The simulation was replicated in experiments using a microbubble-filled gel channel (0.5-MHz, 0.19-0.75-MPa pulses). A hydrophone captured the emissions at two different angles. In both the simulation and the experiments, one angle captured periodic time-domain signals, which had high contributions from the first three harmonics. In contrast, the other angle captured visually aperiodic time-domain features, which had much higher harmonic and broadband content. Thus, by placing acoustic sensors at different positions, substantially different acoustic emissions were captured, potentially leading to very different conclusions about the treatment outcome.

Scale-Resolving Simulations of a Civil Aircraft Wing Transonic Shock-Buffet Experiment

The flow physics governing the shock-buffet phenomenon on swept wings remain disputed without an unequivocal description. Herein is a contribution to the ongoing discussion that addresses the inherent dynamics near the onset of unsteadiness, complementing experimental data of a large aircraft wing geometry with scale-resolving simulation. Specifically, delayed detached-eddy simulations are performed to reproduce the experiments with a Reynolds number of and a Mach number of 0.801, and they are analyzed using modal methods. Motivated by the recognized difficulty in simulating separating and reattaching shallow shear layers employing such techniques, the impact of two subgrid length-scale definitions are scrutinized. The simulations, using both the standard definition of maximum local spacing and a more recent vorticity-sensitive variant, are validated against experimental data from conventional steady and unsteady instrumentation and dynamic pressure-sensitive paint. Remarkable agreement is found between experiment and simulation when using the vorticity-sensitive definition, whereby dominant modal features extracted from surface pressures show striking similarity in describing localized pockets of shear-layer pulsation synchronized with an outboard-propagating shock oscillation. Identified modes from simulation cover broadband content centered at a Strouhal number of approximately 0.27, which is within the experimental frequency range characteristic for swept-wing shock buffet. Modes capturing an inboard-running lower-frequency shock unsteadiness centered at a Strouhal number of approximately 0.07 are exclusive to wind-tunnel conditions. Nevertheless, modal analysis succeeds in extracting key characteristics from such practical unsteady-flow datasets. It is anticipated that the current findings with help clarify these edge-of-the-envelope flow phenomena and ultimately inform shock-buffet delay and control strategies.

Turbulent flow interaction with a circular cylinder

This paper presents a comprehensive experimental study on the unsteady pressure exerted on the surface of a round cylinder in smooth and turbulent flows. A highly instrumented cylinder with several static pressure taps and dynamic pressure transducers at different spanwise and peripheral locations was used, enabling extensive dynamic surface pressure, coherence, and turbulence length-scale analysis. The effects of the free-stream turbulence and turbulent length scale are investigated by placing the turbulent-generating grids within the wind tunnel duct. For both the laminar and turbulent incident flows, the surface pressure results show the emergence of the fundamental, first and second harmonics at most peripheral angles, while at the cylinder base, the surface pressure spectra are dominated by the first harmonic. It has also been observed that an increase in the level of the turbulence intensity results in an increase in the energy level of unsteady pressure acting on the cylinder. An increase in the length scale of the incoming flow structures is shown to result in an increase in the energy level of the tonal frequencies and the broadband content of the surface pressure spectra. The spanwise coherence results have also shown that an increase in the length scale of the flow structures increases the spanwise correlation length of the flow structures at the vortex shedding frequency at the stagnation point, while at the cylinder base, the spanwise correlation length decreases at the vortex shedding frequency.

Synchronization Mechanisms for Multi-User and Multi-Device Hybrid Broadcast and Broadband Distributed Scenarios

Traditionally, co-located TV content consumption has been a social habit (e.g., family members in the living room watching TV). It cannot be always possible to enjoy these shared experiences in the current global society, in which relatives or friends might live apart. In this context, the emergence of new technologies together with the advent of social media and conferencing services brings the possibility of recreating them when users are geographically distributed. When remote users are involved in a networked shared TV consumption experience (Social TV), inter-destination media synchronization (IDMS) is a key requirement to provide them with a satisfactory experience. Moreover, enriching the broadcasted TV content with alternative (or complementary) broadband content is gaining momentum. When several co-located consumption devices are involved in the TV viewing experience, inter-device synchronization (IDES) mechanisms are also needed to allow successful multi-screen experiences. This paper presents a combined synchronization mechanism (including both IDMS and IDES solutions) for distributed scenarios involving remote users and hybrid (broadcast and broadband) contents, to be consumed in one or several devices. The mechanism provides multi- and cross-technology support and achieves accurate synchronization levels even when different versions or formats of the media contents are being played out. It has been integrated into a hybrid broadcast broadband TV standard-based platform, and the achieved synchronization accuracy and the performance of the solution for the Social TV use case have been evaluated, obtaining satisfactory and promising results.

Aeroacoustic analysis of automotive roof crossbars through on-track acoustic measurements

This work concerns the noise radiated by crossbars installed on the roof of automotive vehicles. The Reynolds numbers analyzed range between 98,000 and 147,000 based on the crossbar chord. Through a comparison of results obtained in an aeroacoustic wind-tunnel, we demonstrate that on-track sound pressure measurements from a single in-flow microphone are suitable to study the aeroacoustics of crossbars under realistic operational conditions. The current investigation presents noise measurements by reference profiles such as elliptic and circular crossbars, as well as a NACA 0012 airfoil. Results from the elliptical crossbar show both tonal and broadband components in the noise spectra, combining features observed for the circular bar and the airfoil. For higher Reynolds numbers, the tones observed in the elliptical profile are broad and free of harmonics, similar to those observed for the NACA 0012. The influences of the leading and trailing-edge shapes of the elliptical crossbar on the radiated noise are presented together with the effects of the angle of attack. A thinner trailing edge leads to a reduction in the tonal noise followed by an increase in the broadband content. On the other hand, a blunter leading edge reduces the broadband noise, especially for higher frequencies. Positive angles of attack provide a reduction in both tonal and broadband noise components for the elliptical crossbar, particularly at lower Reynolds numbers.

A two-step elastic full-waveform inversion applied to reflection seismic data for shallow hazard identification

We apply a two-step elastic full-waveform inversion (FWI) to well-site survey (WSS) marine seismic data to estimate high-resolution P-wave (Vp) and S-wave (Vs) velocity models. Our approach combines a first global, genetic-algorithm optimization and a subsequent gradient-based inversion. The broad-band frequency content of the available seismic data makes it possible to extend the frequency range considered in the inversion up to 70 Hz and thus to derive a high-resolution elastic characterization of the shallowest part of the subsurface. The lack of low frequencies and the limited maximum source-to-receiver offset of the WSS acquisition, make the GA inversion particularly crucial as it provides a starting model for the gradient-based FWI that contains the large-medium wavelengths of the seismic velocity field. The following gradient-based FWI yields Vp and Vs models characterized by an improved resolution with respect to the outcomes of GA-FWI. The match between the observed and the predicted seismic data proves the reliability of our predictions.

Coexistence of Digital Terrestrial Television and 4G LTE Mobile Network Utilizing Supplemental Downlink Concept: A Real Case Study

This paper contributes to the studies for long-term evolution (LTE) deployment into the ultrahigh-frequency (UHF) broadcasting spectrum (470–790 MHz) utilizing the supplemental downlink (SDL) concept and its coexistence with the digital terrestrial Television (DTT) service. The performed real and practical coexistence scenario studies were based on the modeling and simulation of the LTE and DTT systems in order to assess potential interference between systems and the availability of the spectrum for the LTE SDL services. Simulations were aligned with the scenarios and assumptions agreed to in the European regulation complemented with real case study parameters from Finland: the early adopter of the digital dividend II. The results lead to some conclusions about the LTE SDL-DTT coexistence and indicated availability of the complementary LTE SDL services on the lower UHF spectrum. Flexible use of the UHF band aims to increase the efficiency of spectrum use in delivering fast growing and converging mobile broadband, media, and TV content to meet the changing consumer needs. Simulated scenarios can be utilized by regulation and standardization forums for studying the future of UHF spectrum and LTE broadcasting evolution in mobile communications toward fifth-generation (5G).

Performance Evaluation of Service Delivery (SDP) as Hub Model of Broadband Content and Application

Performance Evaluation of Service Delivery (SDP) as Hub Model of Broadband Content and Application

Characterization of the supersonic wake of a generic space launcher

The wake flow of a generic axisymmetric space-launcher model is investigated experimentally for flow cases with and without propulsive jet to gain insight into the wake-flow phenomena at a supersonic stage of the flight trajectory which is especially critical with respect to dynamic loads on the structure. Measurements are performed at Mach 2.9 and a Reynolds number Re D = 1.3 × 106 based on model diameter D. The nozzle exit velocity of the jet is at Mach 2.5, and the flow is moderately underexpanded (p e/p ∞ = 5.7). The flow topology is described based on velocity measurements in the wake by means of particle image velocimetry and schlieren visualizations. Mean and fluctuating mass-flux profiles are obtained from hot-wire measurements, and unsteady wall-pressure measurements on the main-body base are performed simultaneously. This way, the evolution of the wake flow and its spectral content can be observed along with the footprint of this highly dynamic flow on the launcher main-body base. For the case without propulsive jet, a large separated zone is forming downstream of the main body shoulder, and the flow is reattaching further downstream on the afterbody. The afterexpanding propulsive jet (air) causes a displacement of the shear layer away from the wall, preventing the reattachment of the flow. In the spectral analysis of the baseline case, a dominant frequency around St D = 0.25 is found in the pressure-fluctuation signal at the main-body base of the launcher. This frequency is related to the shedding of the separation bubble and is less pronounced in the presence of the propulsive jet. In the shear layer itself, the spectra obtained from the hot-wire signal have a more broadband low-frequency content, which also reflects the characteristic frequency of turbulent structures convected in the shear layer, a swinging motion (St D = 0.6), as well as the radial flapping motion of the shear layer (St D = 0.85), respectively. Moving downstream along the shear layer, spectral content at slightly higher frequencies (St D < 4) gets more pronounced and can be related to the shear layer instability process and the signature of smaller turbulent structures that appear in the wake.

Trends in Cable Network Economics: Implications for the Open Internet

Over the past 50 years the network economics of cable networks have had a profound impact upon the diversity of information available to modern society in the form of video and broadband content. Cost-effective one-way transport increased the diversity of television channels to several hundred from a handful; cost-effective two-way transport established societal reliance upon always-on access to Internet content at broadband speeds. Incremental technological innovation over time has been the key driver establishing favorable network economics to push this evolution of expanding services and information carried on cable networks. This paper develops a framework to describe how the network economics of cable services has evolved from the use of analog television and digital television to new IP transport technologies such as Cable Service Interface Specification version 3.1 (DOCSIS 3.1) and network management technologies of software defined networking (SDN) and network virtual function (NFV), and the implications for public policy. This analysis shows that as video becomes the dominant application carried by the Internet, the Open Internet framework will need to address the network access issues specific to IP video including the wholesale migration of video to IP transport, definition of services using quality of service (QoS) and reasonable approaches for managing public Internet congestion. The analysis also shows how the economics of new channel deployment over cable will become easier in the IP environment, but more complex with regard to the network operations and traffic management associated with quality.

Free Basics: Is it Really Free?

The United Nations Broadband Commission’s most recent study on the Global State of Internet Connectivity reported approximately fifty-seven percent of the world population, more than four billion people, suffer from inconsistent access to the internet. Free Basics offers a new solution through the provision of free, yet limited, mobile broadband content to impoverished vulnerable populations in developing countries. However, the initiative is extremely controversial and has been at the center of policy debates due to a myriad of purported ethical violations. The research proposed seeks to analyze the economic impact on the impoverished vulnerable users exploited by Free Basics, who are tempted to buy data plans they cannot afford. The study will include an investigation of the impact on household budgets and uncover opportunity costs, the trade-offs made to obtain the service. The findings will contribute to building a larger policy framework that responsibly advances connecting the world.

Investigation of the Unsteady Flow and Noise Generation in a Slat Cove

This study presents hybrid Reynolds-averaged Navier–Stokes/large-eddy simulations of the unsteady flow and noise-generation phenomena in the slat cove of a high-lift wing profile. These computations are part of a joint numerical/experimental aeroacoustics collaborative program dedicated to slat-flow analysis. A dedicated two-element wing profile (slat plus main body) has been designed to isolate slat noise from other possible sources (e.g., the flap), while minimizing mean flow deflection effects, to improve the fidelity of open-jet wind-tunnel measurements. The design of this two-element airfoil has been performed numerically, using an optimization process based on steady Reynolds-averaged Navier–Stokes calculations. This airfoil has been investigated experimentally at the École Centrale de Lyon open jet facility. Unsteady zonal hybrid Reynolds-averaged Navier–Stokes/large-eddy simulations have been performed to provide a comprehensive description of the unsteady flow inside the slat cove, focusing on the noise-generation processes. A detailed analysis of the physics of the unsteady flow inside the slat cove is presented as well as a comparison of numerical results with available experimental data. The pressure spectra associated with the slat-cove flow are characterized by several tonal peaks emerging from the underlying broadband content. The existence of such peaks is attributed to a feedback loop involving the main shear layer inside the slat cove. A theoretical law is proposed to predict the associated tonal frequencies and assessed at the end of the paper.

On the relation between tonal and broadband content of hull pressure spectra due to cavitating ship propellers

Cavitating propellers generate pressure fluctuations on the hull of the ship. These pressure fluctuations are usually analyzed in the frequency domain using FFTs and the spectrum is composed of tonals at multiples of the blade passage frequency and a broadband part. The two are often considered separately but a relation between the two exists which has been investigated by theoretical signal analysis. It will be shown that the broadband part is related to the variability of the signal between blade passages and a simple procedure is proposed to quantify the variability in terms of amplitude and phase angle. The procedure has been applied to a data set obtained at sea trials.