Numerical predictions of diffraction of broadband noise by an engine nacelle. I - Calculation of a stator cascade's near- and far-field radiations from the ingestion of rapidly distorting turbulence

1997 ◽  
Author(s):  
R. Martinez ◽  
J. Rudzinsky ◽  
R. Martinez ◽  
J. Rudzinsky
Keyword(s):  
Broadband Noise ◽  
Far Field ◽  
Numerical Predictions ◽  
Engine Nacelle

Wake-Airfoil Interaction as Broadband Noise Source: A Large-Eddy Simulation Study

2005 ◽  
Vol 4 (1-2) ◽  
pp. 93-115 ◽  
Author(s):  
Jérôme Boudet ◽  
Nathalie Grosjean ◽  
Marc C. Jacob
Keyword(s):  
Large Eddy Simulation ◽  
Particle Image ◽  
Broadband Noise ◽  
Far Field ◽  
Acoustic Analogy ◽  
Eddy Simulation ◽  
Naca0012 Airfoil ◽  
Image Velocimetry ◽  
Large Eddy ◽  
Proper Orthogonal

A large-eddy simulation is carried out on a rod-airfoil configuration and compared to an accompanying experiment as well as to a RANS computation. A NACA0012 airfoil (chord c = 0.1 m) is located one chord downstream of a circular rod (diameter d = c/10, Red = 48 000). The computed interaction of the resulting sub-critical vortex street with the airfoil is assessed using averaged quantities, aerodynamic spectra and proper orthogonal decomposition (POD) of the instantaneous flow fields. Snapshots of the flow field are compared to particle image velocimetry (PIV) data. The acoustic far field is predicted using the Ffowcs Williams & Hawkings acoustic analogy, and compared to the experimental far field spectra. The large-eddy simulation is shown to accurately represent the deterministic pattern of the vortex shedding that is described by POD modes 1 & 2 and the resulting tonal noise also compares favourably to measurements. Furthermore higher order POD modes that are found in the PIV data are well predicted by the computation. The broadband content of the aerodynamic and the acoustic fields is consequently well predicted over a large range of frequencies ([0 kHz; 10 kHz]).

Acoustic and flow fields of an excited high Reynolds number axisymmetric supersonic jet

2010 ◽  
Vol 656 ◽  
pp. 507-529 ◽  
Author(s):  
M. SAMIMY ◽  
J.-H. KIM ◽  
M. KEARNEY-FISCHER ◽  
A. SINHA
Keyword(s):  
Reynolds Number ◽  
Nozzle Exit ◽  
Noise Suppression ◽  
Polar Angle ◽  
Broadband Noise ◽  
Far Field ◽  
Initial Growth ◽  
Wide Range ◽  
Field Noise ◽  
Noise Amplification

An axisymmetric perfectly expanded Mach 1.3 jet, with a Reynolds number based on the nozzle exit diameter (ReD) of 1.1 × 106 and turbulent boundary layer at the nozzle exit, was excited using localized arc filament plasma actuators over a wide range of forcing Strouhal numbers (StDF). Eight actuators distributed azimuthally were used to excite azimuthal modes m = 0–3. Far-field acoustic, flow velocity and irrotational near-field pressure were probed with a three-fold objective: (i) to investigate the broadband far-field noise amplification reported in the literature at lower speeds and ReD using excitation of m = 0 at low StDF; (ii) to explore broadband far-field noise suppression using excitation of m = 3 at higher StDF; and (iii) to shed some light on the connection between the flow field and the far-field noise. The broadband far-field noise amplification observed is not as extensive in amplitude or frequency range, but still sufficiently large to be of concern in practical applications. Broadband far-field noise suppression of 4–5 dB at 30° polar angle peak frequency, resulting in approximately 2 dB attenuation in the overall sound pressure level, is achieved with excitation of m = 3 at StDF ~ 0.9. Some of the noteworthy observations and inferences are (a) there is a strong correlation between the far-field broadband noise amplification and the turbulence amplification; (b) far-field noise suppression is achieved when the jet is forced with the maximum jet initial growth rate frequency thus limiting significant dynamics of structures to a shorter region close to the nozzle exit; (c) structure breakdown and dynamic interaction seem to be the dominant source of noise; and (d) coherent structures dominate the forced jet over a wide range of StDF (up to ~ 1.31) with the largest and most organized structures observed around the jet preferred mode StDF.

scholarly journals Experimental Characterisation of the Far-Field Noise in Axial Fans Fitted with Shaped Tip End-Plates

2012 ◽  
Vol 2012 ◽  
pp. 1-9 ◽  
Author(s):  
S. Bianchi ◽  
A. Corsini ◽  
A. G. Sheard
Keyword(s):  
Vortex Breakdown ◽  
Vortex Formation ◽  
Broadband Noise ◽  
Sound Power ◽  
Far Field ◽  
Tip Leakage Flow ◽  
Noise Sources ◽  
End Plate ◽  
Field Noise ◽  
Flow Vortex

The authors investigate the far-field noise emissions of a datum fan blade fitted with tip end-plate geometries, originally designed to control the leakage vortex swirl level. The end-plate geometries influence the tip-leakage flow, vortex formation, and swirl level. In doing so, the end-plate geometries influence the sound-power levels. After an evaluation of fan rotors' aerodynamic performance, the study compares the rotors' far-field noise signature characterised in terms of sound-power and pressure-level spectra to enable and assess the end-plate acoustic pay-off. The investigation attempts to establish a cause-and-effect relationship between the tip-flow dynamics and the radiated sound fields, exploring the diverse directivity patterns. The authors found a tonal reduction, due to the enhanced blade-tip end-plates and clarified the relevance of the tip features influencing the radial distribution of the noise sources using coherence analysis. The modified multiple-vortex breakdown end-plate design was effective in reducing the broadband noise, giving an improvement in the frequency range of the turbulent noise.

scholarly journals Some Analytic Results for the Study of Broadband Noise Radiation from Wings, Propellers and Jets in Uniform Motion

2003 ◽  
Vol 2 (3) ◽  
pp. 335-350 ◽  
Author(s):  
F. Farassat ◽  
J. Casper
Keyword(s):  
Correlation Function ◽  
Reference Frame ◽  
Cross Correlation ◽  
Near Field ◽  
Space Time ◽  
Broadband Noise ◽  
Uniform Motion ◽  
Far Field ◽  
Forward Motion ◽  
Radiated Noise

Alan Powell has made significant contributions to the understanding of many aeroacoustic problems, in particular, the problems of broadband noise from jets and boundary layers. In this paper, some analytic results are presented for the calculation of the correlation function of the broadband noise radiated from a wing, a propeller, and a jet in uniform forward motion. It is shown that, when the observer (or microphone) motion is suitably chosen, the geometric terms of the radiation formula become time independent. The time independence of these terms leads to a significant simplification of the statistical analysis of the radiated noise, even when the near field terms are included. For a wing in forward motion, if the observer is in the moving reference frame, then the correlation function of the near and far field noise can be related to a space-time cross-correlation function of the pressure on the wing surface. A similar result holds for a propeller in forward flight if the observer is in a reference frame that is attached to the propeller and rotates at the shaft speed. For a jet in motion, it is shown that the correlation function of the radiated noise can be related to the space-time cross-correlation of the Lighthill stress tensor in the jet. Exact analytical results are derived for all three cases. For the cases under present consideration, the inclusion of the near field terms does not introduce additional complexity, as compared to existing formulations that are limited to the far field.

scholarly journals On a uniformly valid analytical rectilinear cascade response function

2010 ◽  
Vol 663 ◽  
pp. 22-52 ◽  
Author(s):  
HELENE POSSON ◽  
M. ROGER ◽  
S. MOREAU
Keyword(s):  
Three Dimensional ◽  
Good Alternative ◽  
Broadband Noise ◽  
Pressure Jump ◽  
Theory Approach ◽  
Acoustic Analogy ◽  
Future Assessment ◽  
Numerical Predictions ◽  
Strip Theory ◽  
Whole Space

This paper extends an existing analytical model of the aeroacoustic response of a rectilinear cascade of flat-plate blades to three-dimensional incident vortical gusts, by providing closed-form expressions for the acoustic field inside the inter-blade channels, as well as for the pressure jump over the blades in subsonic flows. The extended formulation is dedicated to future implementation in a fan-broadband-noise-prediction tool. The intended applications include the modern turbofan engines, for which analytical modelling is believed to be a good alternative to more expensive numerical techniques. The initial model taken as a reference is based on the Wiener–Hopf technique. An analytical solution valid over the whole space is first derived by making an extensive use of the residue theorem. The accuracy of the model is shown by comparing with numerical predictions of benchmark configurations available in the literature. This full exact solution could be used as a reference for future assessment of numerical solvers, of linearized Euler equations for instance, in rectilinear or narrow-annulus configurations. In addition, the pressure jump is a key piece of information because it can be used as a source term in an acoustic analogy when the rectilinear-cascade model is applied to three-dimensional blade rows by resorting to a strip-theory approach. When used as such in a true rectilinear-cascade configuration, it reproduces the exact radiated field that can be derived directly. The solution is also compared to a classical single-airfoil formulation to highlight the cascade effect. This effect is found important when the blades of the cascade overlap significantly, but the cascade solution tends to the single-airfoil one as the overlap goes to zero. This suggests that both models can be used as the continuation of each other if needed.

scholarly journals An analytic solution for the noise generated by gust–aerofoil interaction for plates with serrated leading edges

2018 ◽  
Vol 853 ◽  
pp. 515-536 ◽  
Author(s):  
Lorna J. Ayton ◽  
Jae Wook Kim
Keyword(s):  
Analytic Solution ◽  
Separation Of Variables ◽  
Leading Edge ◽  
Acoustic Energy ◽  
Destructive Interference ◽  
Far Field ◽  
Numerical Predictions ◽  
Leading Edges ◽  
Field Noise ◽  
Good Agreement

This paper presents an analytic solution for the sound generated by an unsteady gust interacting with a semi-infinite flat plate with a serrated leading edge in a background steady uniform flow. Viscous and nonlinear effects are neglected. The Wiener–Hopf method is used in conjunction with a non-orthogonal coordinate transformation and separation of variables to permit analytical progress. The solution is obtained in terms of a modal expansion in the spanwise coordinate; however, for low- and mid-range incident frequencies only the zeroth-order mode is seen to contribute to the far-field acoustics, therefore the far-field noise can be quickly evaluated. The solution gives insight into the potential mechanisms behind the reduction of noise for plates with serrated leading edges compared to those with straight edges, and predicts a logarithmic dependence between the tip-to-root serration height and the decrease of far-field noise. The two mechanisms behind the noise reduction are proposed to be an increased destructive interference in the far field, and a redistribution of acoustic energy from low cut-on modes to higher cut-off modes as the tip-to-root serration height is increased. The analytic results show good agreement in comparison with experimental measurements. The results are also compared against nonlinear numerical predictions where good agreement is also seen between the two results as frequency and tip-to-root ratio are varied.

Possible applications of fraunhofer holography in high resolution electron microscopy

1978 ◽  
Vol 36 (1) ◽  
pp. 222-223 ◽  
Author(s):  
N. Bonnet ◽  
M. Troyon ◽  
P. Gallion
Keyword(s):  
Electron Microscopy ◽  
High Resolution ◽  
Transfer Function ◽  
Radiation Damage ◽  
High Resolution Electron Microscopy ◽  
Far Field ◽  
Field Region ◽  
Crystalline Materials ◽  
Resolution Electron ◽  
The Ideal

Two main problems in high resolution electron microscopy are first, the existence of gaps in the transfer function, and then the difficulty to find complex amplitude of the diffracted wawe from registered intensity. The solution of this second problem is in most cases only intended by the realization of several micrographs in different conditions (defocusing distance, illuminating angle, complementary objective apertures…) which can lead to severe problems of contamination or radiation damage for certain specimens.Fraunhofer holography can in principle solve both problems stated above (1,2). The microscope objective is strongly defocused (far-field region) so that the two diffracted beams do not interfere. The ideal transfer function after reconstruction is then unity and the twin image do not overlap on the reconstructed one.We show some applications of the method and results of preliminary tests.Possible application to the study of cavitiesSmall voids (or gas-filled bubbles) created by irradiation in crystalline materials can be observed near the Scherzer focus, but it is then difficult to extract other informations than the approximated size.

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