Round jets originating from a pipe nozzle are computed by large-eddy simulations (LES) to investigate the effects of the nozzle-exit conditions on the flow and sound fields of initially laminar jets. The jets are at Mach number 0.9 and Reynolds number 105, and exhibit exit boundary layers characterized by Blasius velocity profiles, maximum root-mean-square (r.m.s.) axial velocity fluctuations between 0.2 and 1.9% of the jet velocity, and momentum thicknesses varying from 0.003 to 0.023 times the jet radius. The far-field noise is determined from the LES data on a cylindrical surface by solving the acoustic equations. Jets with a thinner boundary layer develop earlier but at a slower rate, yielding longer potential cores and lower centreline turbulent intensities. Adding random pressure disturbances of low magnitude in the nozzle also increases the potential core length and reduces peak r.m.s. radial velocity fluctuations in the shear layer. In all the jets, the shear-layer transition is dominated by vortex rolling-ups and pairings, which generate strong additional acoustic components, but also amplify the downstream-dominant low-frequency noise component when the exit boundary layer is thick. The introduction of inlet noise however results in weaker pairings, thus spectacularly reducing their contributions to the sound field. This high sensitivity to the initial conditions is in good agreement with experimental observations.

/pdf/influence-of-nozzle-exit-boundary-layer-conditions-on-the-497wfykzms.pdf

Influence of nozzle-exit boundary-layer conditions on the flow and acoustic fields of initially laminar jets

The Theory of Sound, Two Volumes In One

There is an indispensable need for fluid flow rate and direction sensors in various medical, industrial and environmental applications. Besides the critical demands on sensing range of flow parameters (such as rate, velocity, direction and temperature), the properties of different target gases or liquids to be sensed pose challenges to the development of reliable, inexpensive and low powered sensors. This paper presents an overview of the work done on design and development of Microelectromechanical system (MEMS)-based flow sensors in recent years. In spite of using some similar principles, diverse production methods, analysis strategies, and different sensing materials, MEMS flow sensors can be broadly categorized into three main types, namely thermal sensors, piezoresistive sensors and piezoelectric sensors. Additionally, some key challenges and future prospects for the use of the MEMS flow sensors are discussed briefly.

/pdf/design-and-applications-of-mems-flow-sensors-a-review-515rj4x2ar.pdf

Design and applications of MEMS flow sensors: A review

Large-eddy simulations (LESs) of isothermal round jets at a Mach number of 09 and a diameter-based Reynolds number ReD of 105 originating from a pipe are performed using low-dissipation schemes in combination with relaxation filtering The aim is to carefully examine the capability of LES to compute the flow and acoustic fields of initially nominally turbulent jets As in experiments on laboratory-scale jets, the boundary layers inside the pipe are tripped in order to obtain laminar mean exit velocity profiles with high perturbation levels At the pipe outlet, their momentum thickness is δθ(0)=0018 times the jet radius, yielding a Reynolds number Reθ=900, and peak turbulence intensities are around 9% of the jet velocity Two methods of boundary-layer tripping and five grids are considered The results are found to vary negligibly with the tripping procedure but appreciably with the grid resolution Based on analyses of the LES quality and on comparisons with measurements at high Reynolds numbers, fine d

/pdf/large-eddy-simulation-of-the-flow-and-acoustic-fields-of-a-7t2v4wl777.pdf

Large-eddy simulation of the flow and acoustic fields of a Reynolds number 10 5 subsonic jet with tripped exit boundary layers

Jittering wave-packet models for subsonic jet noise

half-velocity diameter in reference to length in the shear layer and on the jet axis, respectively. From these results, a discussion on the modeling of turbulence in jets is addressed. The self-similarity of some space correlation functions in the shear layer and on the jet axis is shown. Furthermore, far enough downstream in the shear layer, some of the ratios between the space and time scales are relatively close to the values expected in homogeneous and isotropic turbulence. It is also found that the ratio between the integral length and the time scales in the fixed frame is of the order of the local mean flow velocity. In the convected frame, the appropriate scaling factor is the rms velocity.

/pdf/space-time-correlations-in-two-subsonic-jets-using-dual-er5zq5s6kg.pdf

Space-Time Correlations in Two Subsonic Jets Using Dual Particle Image Velocimetry Measurements

Airfoil broadband trailing edge noise is reduced by modification of the trailing edge geometry. A brush made of a single row of flexible polypropylene fibers is integrated in the trailing edge of a cambered airfoil. Far field acoustic measurements show a noise reduction potential reaching 3 dB on a wide frequency range. Due to high curvature of the incident flow, a secondary acoustic source partly masks the trailing edge noise reduction. Hot wire correlation measurements in the very near wake of the airfoil show that longitudinal as well as transversal length scales are affected by the brush. Span wise coherence length of boundary layer eddies falls off by 25 % in the presence of a brush in the adequate frequency range, possibly explaining a 1.3 dB contribution to the noise reduction mechanism. Boundary layer turbulence exhibits a preferred coherence length l y v on a wide frequency range. l y v /d ≈ 2, is considered a proper brush design law, d being the diameter of the brush.

Broadband Noise Reduction With Trailing Edge Brushes

The goal of this experimental study is to investigate the wall pressure wavenumberfrequency spectra induced by a turbulent boundary layer in the presence of a mean pressure gradient. The mean pressure gradient is achieved by changing the ceiling angle of a rectangular channel flow. Wall pressure spectra are measured for zero-, adverse- and favorablepressure-gradient boundary layers by using a pinhole microphone in conjunction with a high-frequency-calibration procedure. A linear antenna based on a non-uniform distribution of remote microphones mounted on a rotating disk is also developed to obtain a direct measurement of both aerodynamic and acoustic components of wavenumber-frequency spectra. First results, comparisons and analyses are then discussed.

/pdf/an-experimental-characterisation-of-wall-pressure-wavevector-2zs8p1f4pq.pdf

An experimental characterisation of wall pressure wavevector-frequency spectra in the presence of pressure gradients

Dual Particule Image Velocimetry (dual-PIV) measurements have been performed to investigate the space-time correlations in two subsonic isothermal round jets at Mach numbers 0.6 and 0.9. Measurements are obtained along the centerline and the shear-layer region which are closely connected with the noise generation. Integral scales have been calculated with as much as 2000 samples which provides high-quality data. The correlation scales are given in function of appropriate references, namely the local momentum thickness (respectively the jet diameter) for the shear-layer (respectively the jet axis) and the mean or rms local velocity. Far enough downstream in the shear-layer, some of the ratios between the space scales and between the time scales are relatively close to the values expected in isotropic an homogeneous turbulence. Furthermore, the relation between the time and space scales follows well the Taylor’s assumption in the shear-layer.

/pdf/space-time-correlations-in-two-subsonic-jets-using-dual-piv-8s4zbrloeo.pdf

Space-time correlations in two subsonic jets using dual-PIV measurements

https://acoustique.ec-lyon.fr/publi/marsden_jsv12.pdf

Emmanuel Jondeau

Papers

Space-Time Correlations in Two Subsonic Jets Using Dual Particle Image Velocimetry Measurements

Broadband Noise Reduction With Trailing Edge Brushes

An experimental characterisation of wall pressure wavevector-frequency spectra in the presence of pressure gradients

Space-time correlations in two subsonic jets using dual-PIV measurements

Investigation of flow features and acoustic radiation of a round cavity