Noise Production Mechanisms Research Articles

Slat noise measurements in open-jet, hard-wall and hybrid wind tunnel test sections

This paper assesses the comparability of aeroacoustic slat noise measurement in an open-jet, a hard-wall, and a hybrid test section configuration. The 30P30N common research model was tested in the Aeroacoustic Wind Tunnel of the University of Twente at a chord-based Reynolds number up to 1⋅106 and free-stream Mach number 0.15. Pressure distribution measurements were used to determine the aerodynamic similar condition of the flow in the slat cove. These conditions are subsequently validated by time-averaged 2D PIV measurements. The experiments show that general aspects of the flow in the slat cove are similar when the Cp distribution around the leading-edge slat element in each test section configuration is comparable. Overall, the shear layer path and reattachment point are in close agreement, showing that the aeroacoustic noise production mechanisms are comparable. Microphone phased array measurements were subsequently conducted to compare the far-field noise characteristics originating from the slat in each test section configuration. A framework is proposed to correct the acoustic measurements of each test section configuration to a standard free-field condition. Transmission loss corrections are required for the microphone measurements in the hard-wall and hybrid test section configurations, whereas a coherence loss model was validated and applied to correct the microphone measurements in the open-jet. The measured noise spectra in each test section configuration show differences up to approximately 5 dB. In general the differences were found to be smaller. The largest differences are seen for small angles of attack with noise levels in the open-jet test section showing considerably higher values than the hard-wall test section and hybrid test section results. In addition, the behavior of the vortex shedding hump in the high frequency range is different in the open-jet measurements, which could be related to the larger noise level differences at small angles of attack.

Coherence Analysis of the Noise from a Simulated Highly Heated Laboratory-Scale Jet

Measurements of full-scale high-performance military aircraft reveal phenomena that are not widely seen at laboratory scales. However, recent modifications to large eddy-simulation (LES) methods allow for simulations of jets operating at a high-temperature ratio in a similar regime as military aircraft operating at afterburner. This work applies coherence analyses that have been previously used to study the jet noise field produced by military aircraft to the LES of a highly heated laboratory-scale jet. The coherence of the complex pressures along a near-field line approximately parallel to the shear layer as well as along the nozzle lip line shows evidence of distinct noise production mechanisms that transfer information differently from the flow to the field. A phenomenological comparison between the LES and measurements of an afterburning F-35 aircraft is then made. Although the LES is not run at the exact same conditions as the aircraft and does not reproduce all of the phenomena present in the aircraft’s jet noise field, differences between noise production mechanisms observed in the LES may describe some of the spatiospectral lobe phenomena observed in the measurements of the F-35.

Modeling of Conducted Emissions for EMI Analysis of Power Converters: State-of-the-Art Review

Electromagnetic interference issues are associated with high-speed switching of power converters. EMI modeling is an essential tool to study and control the EMI emission, enabling more efficient solutions. A comprehensive review and comparison of different modeling approaches for conducted emissions are provided in this paper, which can be used as a design guideline for engineers. For a motor drive application, common mode and differential mode conducted emissions are studied, and dominant noise production mechanisms are identified. Moreover, a review of various modeling techniques is presented for the main parasitic components of the system. Finally, time domain and frequency domain analysis approaches are explored along with the equivalent circuits which enable fast prediction of EMI emissions. This paper intends to help the reader develop an organized understanding of conducted emission modeling to assist them with a more efficient and electromagnetically-compatible design.

Selection of a leading edge noise prediction method for PNoise

Wind energy expansion is worldwide followed by various limitations, i.e. land availability, the NIMBY (not in my backyard) attitude, interference on birds migration routes and so on. This undeniable expansion is pushing wind farms near populated areas throughout the years, where noise regulation is more stringent. That demands solutions for the wind turbine (WT) industry, in order to produce quieter WT units. Focusing in the subject of airfoil noise prediction, it can help the assessment and design of quieter wind turbine blades. Considering the airfoil noise as a composition of many sound sources, and in light of the fact that the main noise production mechanisms are the airfoil self-noise and the turbulent inflow (TI) noise, this work is concentrated on the latter. TI noise is classified as an interaction noise, produced by the turbulent inflow, incident on the airfoil leading edge (LE). Theoretical and semi-empirical methods for the TI noise prediction are already available, based on Amiet’s broadband noise theory. Analysis of many TI noise prediction methods is provided by this work in the literature review, as well as the turbulence energy spectrum modeling. This is then followed by comparison of the most reliable TI noise methodologies, qualitatively and quantitatively, with the error estimation, compared to the Ffowcs Williams-Hawkings solution for computational aeroacoustics. Basis for integration of airfoil inflow noise prediction into a wind turbine noise prediction code is the final goal of this work.

Large eddy simulations of launcher lift-off noise and comparisons to experiments on model flame trenches

During the lift-off phase of a space launcher, rocket motors generate harsh acoustic environment that is a concern for the payload and surrounding structures. Hot supersonic jets contribute to the emitted noise from both their own noise production mechanisms and their interactions with launch pad components, such as the launch table and flame trenches. The present work describes the results of computations performed by ONERA to predict the lift-off noise from reduced scale models of a flame trench. The results include both unsteady flow solution inside the flame trench and the computed noise on near and far field microphone arrays. Numerical computations involve two in-house codes: the flow solver CEDRE, used in LES mode to accurately predict the noise sources, and the acoustic code KIM to reconstruct the far field noise, thanks to an integral Ffowcs Williams and Hawkings porous surface approach. The computational model exactly reproduces the flame trench configurations used in a test campaign carried out by CNES at the MARTEL facility. Results are discussed and compared with experimental acoustic measurements on 48 microphones. Overall, the numerical results reproduce the acoustic measurements within 3 dB. To further improve these results, work is ongoing on acoustic nonlinear effects.

Experimental Study on Sound Characteristics Produced by DC Corona and Pulsed Discharges

The audible noise (AN) production mechanism caused by corona discharge in ultrahigh voltage transmission lines is less understood. The study on sound pressure characteristic is necessary to obtain the inherent relation between AN and corona discharge. In this paper, the experimental research on single-point direct current (dc) corona discharge was evolved and the time-domain sound pressure waveform was obtained using capacitor microphone. The experiment results show that the dc corona discharge comprises multiple subsequent discharge pulses. The discharge voltage and the electrode polarity have a direct effect on the rising time and pulsed width of the single current pulse. The waveform of sound pressure produced by single discharge is a bipolar microsecond pulse, which lags behind the current pulse in time domain. The time delay is propagation time of sound signal from the discharge spot to the measurement spot. Based on the corona discharge characteristic in dc voltage, the experiment platform for nanosecond pulsed discharge was built to obtain the relation between discharge parameter and sound parameter. The characteristics of sound pressure in different discharge conditions were analyzed. The study results indicate that the sound pressure produced by pulsed discharge is similar with that produced by corona discharge. The experiment platform for pulsed discharge can be used to research the AN production mechanism by dc corona discharge.

Noise Source Distribution of Coaxial Subsonic Jet-Short-Cowl Nozzle

The noise source distribution of a short-cowl coaxial jet operating at different velocity ratios is described in this work. This was motivated by an ongoing research about noise prediction of coaxial jets through Acoustic Analogy with purposes of industrial engine application. This research has been carried out between Universidade Federal de Uberlândia (UFU), Brazil and the Institute of Sound and Vibration Research (ISVR) at Southampton University, UK. The numerical approach employed is originally based on Lighthill Acoustic Analogy. This technique, although likely known, is associated with an improved energy transfer time-scale, used in the turbulence two-point correlation function, in order to enhance the source model. The source model is coupled with the aerodynamic calculation of flow through turbulence quantities evaluated by using a standard k-e turbulence modeling. The Computational Fluid Dynamics data have also been used to provide complementary information about the coaxial jet noise production mechanisms. Experimental data were used in order to corroborate the results from the current model. Good agreement has been found, showing that high and low frequency contributors to the radiated noise for low velocity ratio are aggregated in a region about seven to ten secondary diameters downstream, while at higher velocity ratios sources are continuously spread from about one up to ten secondary diameters from the jet exit.

High speed PIV applied to aerodynamic noise investigation

In this paper, we study the acoustic emissions of the flow over a rectangular cavity. Especially, we investigate the possibility of estimating the acoustic emission by analysis of PIV data. Such a possibility is appealing, since it would allow to directly relate the flow behavior to the aerodynamic noise production. This will help considerably in understanding the noise production mechanisms and to investigate the possible ways of reducing it. In this study, we consider an open cavity with an aspect ratio between its length and depth of 2 at a Reynolds number of 2.4 × 104 and 3.0 × 104 based on the cavity length. The study is carried out combining high speed two-dimensional PIV, wall pressure measurements and sound measurements. The pressure field is computed from the PIV data. Curle’s analogy is applied to obtain the acoustic pressure field. The pressure measurements on the wall of the cavity and the sound measurements are then used to validate the results obtained from PIV and check the range of validity of this approach. This study demonstrated that the technique is able to quantify the acoustic emissions from the cavity and is promising especially for capturing the tonal components on the sound emission.

Jet noise prediction using different turbulent scales

The turbulent energy dissipation rate time-scale and length-scale has been routinely used for the prediction of noise from turbulent flows, particularly jet streams. However, this is not the only possible choice. In general, scales evolving in a turbulent medium are threefold. First, those associated with the mean flow; second, those attributed to the turbulence and the mean flow interactions; and third, scales related to the turbulence-turbulence interactions. In this paper, special attention will be paid to further study of the underlying physics of aerodynamic noise by examining various time-scales. To do so, three time scales, namely, dissipation, production, and strain rate time scales, are defined and used in the source modelling to emphasis the effect of the turbulence structures at different jet regions on the jet noise production mechanism. The required mean value and turbulence parameters are obtained using a modified k − ∈ turbulence model, and Lighthill’s Acoustic Analogy is used for the prediction of the emanated noise.

Source location prediction of subsonic isothermal jet flows

The location of jet noise sources is a far from trivial problem that is of great importance for both understanding the noise production and radiation mechanisms and also for finding new jet noise reduction strategies. This paper presents comparisons of theoretical results with data for a number of jets. The theory used is based on the MGBK method but including a novel time scale based on the rate of energy transfer through the turbulent cascade. This new technique has been shown to give a number of advantages over existing models. The experimental results were obtained using the Polar Correlation Technique and were made at QinetiQ's Jet Noise Facility in the UK as part of the EU FP6 programme CoJeN. The high resolution jet noise images resulted from using a 64 microphone polar arc array set at two reference angles, namely 60 and 90 degrees to the jet axis. Comparisons with experimental data are made for coplanar and short cowl nozzles at different working conditions for predictions from different theoretical models. It is shown that the best agreement is obtained for the prediction methodology using the energy transfer rate timescale

Dynamic Anomalies in a Modular Bridge Expansion Joint

Environmental noise complaints from homeowners near bridges with modular bridge expansion joints (MBEJs) led to an engineering investigation into the noise production mechanism. The investigation identified modal vibration frequencies in the MBEJ coupling with acoustic resonances in the chamber cast into the bridge abutment below the MBEJ. This initial acoustic investigation was soon overtaken by observations of fatigue induced cracking in structural beams transverse to the direction of traffic. These beams are, in the English-speaking world, universally referred to as center beams. However, in Europe the term lamellae is equally common. A literature search revealed little to describe the structural dynamics behavior of MBEJs but showed that there was an accepted belief dating from around 1973 that the loading was dynamic. In spite of this knowledge many bridge design codes used throughout the world specify a static or quasi-static load case with no mention of the dynamic behavior. This paper identifies the natural modes and operational response modes of vibration of the MBEJ installed into Sydney’s Anzac Bridge. In addition, the paper will introduce the dynamic range factor (DRF) and report a DRF of 4.6 obtained after extensive static and dynamic strain gage measurements. The studies indicated that the Anzac Bridge MBEJ was very lightly damped (<2% of critical) and a reduction in the measured DRF through the introduction of additional damping was an option.

Mach Number Effects on Jet Noise Sources and Radiation to Shallow Angles

LL jets have noise generation due to turbulent mixing; suchmixingleadstotheproductionofReynoldsstresses,thesourceterm within the Lighthill acoustic analogy [1]. In addition to thissource,ajetthatpossessescoherent,large-scalestructuresthattravelwith velocities exceeding the local speed of sound will emit Machwave radiation, a highly efficient noise production mechanism. Theamplitudeofjetnoisescaleswiththejetareaandtheexitvelocitytoanangle-dependentpowern.Thereisdisagreement,however,inthevalueofn.Basedondimensionalanalysis,Lighthill[1]foundntobe8. In anextensive examination of jet noise scaling, Viswanathan [2]observed that n 9:8 for the measurement location (30 deg) andambient to total temperature ratio of unity examined in this work.Venkatesh et al. [3] presented an extensive set of subsonic jet noisemeasurements that show the region of maximum noise generationcoincides withthecollapseoftheuniform, irrotationaljetcore.Thistechnical note examines the impact of Mach number on themechanism of jet noise generation, the scaling of jet noise, and theregion of maximum noise generation. It does this throughexamination of the time and frequency domain properties of theshallow angle jet noise from Mach 0.9, and ideally expandedMach 1.3 and 2.0 jets.A recent set of experiments showed that a Mach 1.3 jet possesseslarge amplitude acoustic peaks within the time-domain data andthesepeakscouldbeusedtorelatethegenerationoffarfieldradiatedsound to the evolution of the mixing layer [4–7]. These acousticpeaks are examined in this technical note to characterize the noisegeneration mechanism and source region of the three jets. For allMachnumbersstudied,thelargeamplitudeacousticpeaksoriginatefrom an ellipsoid-shaped region that is centered past the end of thepotentialcoreandthenoisesourceregionnarrowsinthecross-streamwith increasing Mach numbers. Unlike at Mach 0.9 and 1.3, theMach2.0acousticpeakshavetemporalasymmetry,whichisrelatedtothechangeinnoisegenerationmechanismfromturbulencemixingto Mach wave emission.

Estimating traffic noise for inclined roads with freely flowing traffic

Traffic noise measurements on the kerbs of 19 independent inclined trunk roads with freely flowing traffic within the residential areas of Hong Kong are carried out in the present investigation. The performance of the existing noise prediction models in predicting traffic noise from inclined roads is evaluated. By regression analysis and simple physical consideration of the traffic noise production mechanisms, formulae for the prediction of the L A10, L A50, L A90 and L Aeq are developed or re-calibrated. Results suggest tyre noise has the major contribution to the overall noise environment when the source is an inclined trunk road. Also, the road gradient is found to have a higher contribution to the traffic noise than assumed in the existing models, but becomes unimportant when the background noise level L A90 is concerned.

Acoustic Measurements in High-speed Subsonic Jets

ACOUSTIC measurements have been made in the near (/•/£>< 60, JC/ZX60) field of high Reynolds number (184,000-262,000) axisymmetric cold-air jets exhausting at atmospheric pressure. These measurements were in conjunction with an investigation that characterized the largescale coherent structure in the flowfield of Mach number 0.60.8 jets. Natural jets as well as artificially excited jets were studied. Directivity plots were made for all jets at various frequencies. Overall noise radiated by the jets was found to reach a maximum value around 30 deg from the jet axis. However, individual frequencies emitted maximum sound pressure level (SPL) at different angles from the jet axis. As the angle from the jet axis increased, the spectra of the noise shifted to higher frequencies. It was observed that the coherent structure's axial wave speed decreased with decreasing frequency, much in the same way the angle of peak noise emission decreased with decreasing frequency. We propose that it was this slowing of the wave speed that caused the decrease in the peak noise emission angle. The noise production mechanism in the jet was found to be more responsive to midband excitation frequencies. Excitation at these frequencies caused a small increase in SPL at the frequency of excitation, but a much larger increase in full spectrum noise. Contents The present research was conducted to determine the Mach number dependence of large-scale coherent structures in highspeed, subsonic jet flowfields.1 Acoustic measurements were made in conjunction with the flowfield measurements. This synoptic will present the results of these acoustic measure