Basic principles of aerodynamic noise generation (original) (raw)
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Aerodynamic sound generation by turbulence in shear flows
2009
The nonlinear aerodynamic sound generation by turbulence has been long analyzed since the foundation of the theory of aerodynamic sound in pioneering paper by Lighthill [1]. Also, it was Lighthill [2] who noted that velocity shear can increase the acoustic wave emission in the aerodynamic situation due to the existence of linear terms in the inhomogeneous part of the analogy equations (second derivative of the Reynolds stress). In [3] it was disclosed and described a linear aerodynamic sound generation mechanism. Specifically, it was shown that the flow non-normality induced linear phenomenon of the conversion of vortex mode into the acoustic wave mode is the only contributor to the acoustic wave production of the unbounded shear flows in the linear regime. From the physical point of view the potential vorticity was identified as the linear source of acoustic waves in shear flows.
Evaluation of noise radiation mechanisms in a turbulent jet
1998
Data from the direct numerical simulation (DNS) of a turbulent, compressible (Mach=1.92) jet has been analyzed to investigate the process of sound generation. The overall goals are to understand how the di erent scales of turbulence contribute to the acoustic eld and to understand the role that linear instability waves play in the noise produced by supersonic turbulent jets. Lighthill's acoustic analogy was used to predict the radiate sound from turbulent source terms computed from the DNS data. Preliminary computations (for the axisymmetric mode of the acoustic eld) show good agreement between the acoustic eld determined from DNS and acoustic analogy. Further work is needed to re ne the calculations and investigate the source terms. Work was also begun to test the validity of linear stability wave models of sound generation in supersonic jets. An adjoint-based method was developed to project the DNS data onto the most unstable linear stability mode at di erent streamwise positions. This will allow the evolution of the wave and its radiated acoustic eld, determined by solving the linear equations, to be compared directly with the evolution of the near-and far-eld uctuations in the DNS.
Large Eddy Simulation of Sound Generation by Turbulent Reacting and Nonreacting Shear Flows
The objective of the present study was to investigate the mechanisms of sound generation by subsonic jets. Large eddy simulations were performed along with bandpass filtering of the flow and sound in order to gain further insight into the role of coherent structures in subsonic jet noise generation. A sixth-order compact scheme was used for spatial discretization of the fully compressible Navier-Stokes equations. Time integration was performed through the use of the standard fourth-order, explicit Runge-Kutta scheme. An implicit low dispersion, low dissipation Runge-Kutta (ILDDRK) method was developed and implemented for simulations involving sources of stiffness such as flows near solid boundaries, or combustion. A surface integral acoustic analogy formulation, called Formulation 1C, was developed for farfield sound pressure calculations. Formulation 1C was derived based on the convective wave equation in order to take into account the presence of a mean flow. The formulation was derived to be easy to implement as a numerical post-processing tool for CFD codes. Sound radiation from an unheated, Mach 0.9 jet at ReD = 400, 000 was considered. The effect of mesh size on the accuracy of the nearfield flow and farfield sound results was studied. It was observed that insufficient grid resolution in the shear layer results in unphysical laminar vortex pairing, and increased sound pressure levels in the farfield. Careful examination of the bandpass filtered pressure field suggested that there are two mechanisms of sound radiation in unheated subsonic jets that can occur in all scales of turbulence. The first mechanism is the stretching and the distortion of coherent vortical structures, especially close to the termination of the potential core. As eddies are bent or stretched, a portion of their kinetic energy is radiated. This mechanism is quadrupolar in nature, and is responsible for strong sound radiation at aft angles. The second sound generation mechanism appears to be associated with the transverse vibration of the shear-layer interface within the ambient quiescent flow, and has dipolar characteristics. This mechanism is believed to be responsible for sound radiation along the sideline directions. Jet noise suppression through the use of microjets was studied. The microjet injection induced secondary instabilities in the shear layer which triggered the transition to turbulence, and suppressed laminar vortex pairing. This in turn resulted in a reduction of OASPL at almost all observer locations. In all cases, the bandpass filtering of the nearfield flow and the associated sound provides revealing details of the sound radiation process. The results suggest that circumferential modes are significant and need to be included in future wavepacket models for jet noise prediction. Numerical simulations of sound radiation from nonpremixed flames were also performed. The simulations featured the solution of the fully compressible Navier-Stokes equations. Therefore, sound generation and radiation were directly captured in the simulations. A thickened flamelet model was proposed for nonpremixed flames. The model yields artificially thickened flames which can be better resolved on the computational grid, while retaining the physically currect values of the total heat released into the flow. Combustion noise has monopolar characteristics for low frequencies. For high frequencies, the sound field is no longer omni-directional. Major sources of sound appear to be located in the jet shear layer within one potential core length from the jet nozzle.
Acoustics of turbulent flows: a report on Euromech 142
Journal of Fluid …, 1982
The European Mechanics Colloquium, Euromech 142, ww held at the Ecole Centrale de Lyon from 23 to 26 September 1981 and WM attended by 70 participants, from 9 countries, active in the fields of (i) sound production by turbulent flows and (ii) the effects of flow and turbulence on the propagation of acoustic waves. For topic (i), attention was mostly paid to shear-layer and jet instabilities and to the flow-surface interaction of flexible boundaries, vibrating blades and rigid thick airfoils. Applications were concerned in particular with propeller noise and airframe self-noise of large aircraft. Impinging shear layers were also considered for single and multiple cavities in which self-sustained oscillations can occur. Another subject discussed at the meeting w a the noise from inhomogeneities, with applications to flames. For topic (ii), theoretical formulations were presented for the far field of moving multipole sources in the presence of flow and for sound propagation in ducts of variable cross section, with flow, for frequencies around the cutoff. The effect of turbulence was investigated in terms of the space-time coherence of the transmitted pressure fields. Broadband active sound control in the presence of flow was also considered, with emphasis on the progress made possible by use of digital filters. Finally, new experimental techniques, such = acoustic intensity measurements, were presented and large anechoic wind tunnels and other acoustic facilities were described.
Direct Numerical Simulation of Aerodynamic Noise
1989
The program includes~study of the physics of compressible turbulence, shock-turbulence interactions, reacting flows with heat release, and aerodynamic sound generation in shear flows. The objective of the work in aerodynamic sound generation is to use direct numerical simulations as a tool to study the noise generation processes directlyj 'pecifically,..-wier-t answer the following questions:
Sound generation by turbulence
European Journal of Mechanics - B/Fluids, 2004
Sound is a weak by-product of a subsonic turbulent flow. The main convective elements of the turbulence are silent and it is only spectral components with supersonic phase speeds that couple to the far-field sound. This paper reviews recent work on sound generation by turbulence. Just as there is a hierarchy of numerical models for turbulence (scaling, RANS, LES and DNS), there are different approaches for relating the near-field turbulence to the far-field sound. Kirchhoff approaches give the far-field sound in a straightforward way, but provide little insight into the sources of sound. Acoustic analogies can be used with different base flows to describe the propagation effects and to highlight the major noise producing regions.
Mechanisms and Active Control of Jet-Induced Noise
Notes on Numerical Fluid Mechanics and Multidisciplinary Design, 2009
Fundamental mechanisms of jet noise are investigated by means of direct numerical simulation. In the mixing layer, subharmonics of the respective vortex pairing are found to be responsible for the main part of the generated noise which is directed in downstream direction. By modifying the phase shift between introduced disturbances it is possible to diminish or enhance relevant portions of the emitted sound. Optimal control has been applied successfully to a plane mixing layer. In the far field, the mean noise level could be reduced. Depending on the measurement line, some distributed control or anti-noise is generated by the control. A more realistic configuration is achieved by adding a splitter plate representing the nozzle end. Rectangular serrations lead to a breakdown of the large coherent spanwise vortical structures and thus provide a noise reduction of 9dB. mixing behind the trailing edge of the nozzle. However the underlying physical mechanisms are not yet fully understood.
Comparative Study of the Propagation of Jet Noise in Static and Flow Environments
Sound&Vibration, 2019
In order to analyze the effect of the background flow on the sound prediction of fine-scale turbulence noise, the sound spectra from static and flow environments are compared. It turns out that, the two methods can obtain similar predictions not only at 90 deg to the jet axis but also at mid-and high frequencies in other directions. The discrepancies of predictions from the two environments show that the effect of the jet flow on the sound propagation is related to low frequencies in the downstream and upstream directions. It is noted that there is an obvious advantage of computational efficiency for calculating in static environment, compared with that in flow environment. A good agreement is also observed to some extent between the predictions in static environment and measurements of subsonic to supersonic. It is believed that the predictions in static environment could be an effective method to study the propagation of the sound in jet flow and to predict the fine scale turbulence noise accurately in a way as well.
Journal of Fluid Mechanics, 2012
Five isothermal round jets at Mach number M = 0.9 and Reynolds number Re D = 10 5 originating from a pipe nozzle are computed by large-eddy simulations to investigate the effects of initial turbulence on flow development and noise generation. In the pipe, the boundary layers are untripped in the first case and tripped numerically in the four others in order to obtain, at the exit, mean velocity profiles similar to a Blasius laminar profile of momentum thickness equal to 1.8 % of the jet radius, yielding Reynolds number Re θ = 900, and peak turbulence levels u ′ e around 0, 3 %, 6 %, 9 % or 12 % of the jet velocity u j . As the initial turbulence intensity increases, the shear layers develop more slowly with much lower root-mean-square (r.m.s.) fluctuating velocities, and the jet potential cores are longer. Velocity disturbances downstream of the nozzle exit also exhibit different structural characteristics. For low u ′ e /u j , they are dominated by the first azimuthal modes n θ = 0, 1 and 2, and show significant skewness and intermittency. The growth of linear instability waves and a first stage of vortex pairings occur in the shear layers for u ′ e /u j 6 %. For higher u ′ e /u j , threedimensional features and high azimuthal modes prevail, in particular close to the nozzle exit where the wavenumbers naturally found in turbulent wall-bounded flows clearly appear. Concerning the sound fields, strong broadband components mainly associated with mode n θ = 1 are noticed around the pairing frequency for the untripped jet. With rising u ′ e /u j , however, they become weaker, and the noise levels decrease asymptotically down to those measured for jets at Re D 5 × 10 5 , which are likely to be initially turbulent and to emit negligible vortex-pairing noise. These results correspond well to experimental observations, made separately for either mixing layers, jet flow or sound fields.