Sound radiation from two semi‐infinite dissimilar plates subject to a harmonic line force excitation in mean flow. I. Theory

1995 ◽  
Vol 97 (5) ◽  
pp. 2709-2723 ◽  
Author(s):  
Sean F. Wu ◽  
Jinshuo Zhu
Keyword(s):  
Sound Radiation ◽  
Mean Flow ◽  
Line Force ◽  
Force Excitation

Sound radiation from a double-leaf elastic plate with a point force excitation: effect of an interior panel on the structure-borne sound radiation

2002 ◽  
Vol 63 (7) ◽  
pp. 737-757 ◽  
Author(s):  
Motoki Yairi ◽  
Kimihiro Sakagami ◽  
Eiji Sakagami ◽  
Masayuki Morimoto ◽  
Atsuo Minemura ◽  
...  
Keyword(s):  
Elastic Plate ◽  
Sound Radiation ◽  
Point Force ◽  
Force Excitation

On the causal behaviour of flow over an elastic wall

1999 ◽  
Vol 396 ◽  
pp. 319-344 ◽  
Author(s):  
R. J. LINGWOOD ◽  
N. PEAKE
Keyword(s):  
Shear Layer ◽  
Finite Time ◽  
Radiation Condition ◽  
Flow Speed ◽  
Uniform Flow ◽  
Dispersion Function ◽  
Mean Flow ◽  
Wide Range ◽  
Line Force ◽  
Layer Flow

In this paper we consider the causal response of the inviscid shear-layer flow over an elastic surface to excitation by a time-harmonic line force. In the case of uniform flow, Brazier-Smith & Scott (1984) and Crighton & Oswell (1991) have analysed the long-time limit of the response. They find that the system is absolutely unstable for sufficiently high flow speeds, and that at lower speeds there exist certain anomalous neutral modes with group velocity directed towards the driver (in contradiction of the usual radiation condition of out-going disturbances). Our aim in this paper is to repeat their analysis for more realistic shear profiles, and in particular to determine whether or not the uniform-flow results can be regained in the limit in which the shear-layer thickness on a length scale based on the fluid loading, denoted ε, becomes small. For a simple broken-line linear shear profile we find that the results are qualitatively similar to those for uniform flow. However, for the more realistic Blasius profile very significant differences arise, essentially due to the presence of the critical layer. In particular, we find that as ε → 0 the minimum flow speed required for absolute instability is pushed to considerably higher values than was found for uniform flow, leading us to conclude that the uniform-flow problem is an unattainable singular limit of our more general problem. In contrast, we find that the uniform-flow anomalous modes (written as exp (ikx − iωt), say) do persist for non-zero shear over a wide range of ε, although now becoming non-neutral. Unlike the case of uniform flow, however, the k-loci of these modes can now change direction more than once as the imaginary part of ω is increased, and we describe the connection between this behaviour and local properties of the dispersion function. Finally, in order to investigate whether or not these anomalous modes might be realizable at a finite time after the driver is switched on, we evaluate the double Fourier inversion integrals for the unsteady flow numerically. We find that the anomalous mode is indeed present at finite time, once initial transients have propagated away, not only for impulsive start-up but also when the forcing amplitude is allowed to grow slowly from a small value at some initial instant. This behaviour has significant implications for the application of standard radiation conditions in wave problems with mean flow.

External Mean Flow on Sound Radiation of Active Mechanical Metamaterials

AIAA Journal ◽  
2020 ◽  
Vol 58 (11) ◽  
pp. 4751-4763
Author(s):  
Zhi-Hua He ◽  
Yi-Ze Wang ◽  
Yue-Sheng Wang
Keyword(s):  
Sound Radiation ◽  
Mean Flow ◽  
Mechanical Metamaterials

Coherence decay and its impact on sound radiation by wavepackets

2014 ◽  
Vol 748 ◽  
pp. 399-415 ◽  
Author(s):  
André V. G. Cavalieri ◽  
Anurag Agarwal
Keyword(s):  
Coherence Function ◽  
Sound Radiation ◽  
Turbulent Jets ◽  
Sufficient Condition ◽  
Far Field ◽  
Mean Flow ◽  
Spectral Densities ◽  
Flow Fluctuations ◽  
Order Of Magnitude ◽  
Field Sound

AbstractWavepackets obtained by a linear stability analysis of the turbulent mean flow were shown in recent works to agree closely with some relevant statistics of turbulent jets, such as power spectral densities and averaged phases of flow fluctuations. However, when such wavepacket models were used to calculate the far-field sound, satisfactory agreement was only obtained for flows that were supersonic relative to the ambient speed of sound; attempts with subsonic flows led to errors of more than an order of magnitude. We investigate here the reasons for such discrepancies by developing the integral solution of the Helmholtz equation in terms of the cross-spectral densities of turbulent quantities. It is shown that agreement of a statistical source, such as would be obtained by the above-mentioned wavepacket models, in averaged amplitudes and phases in the near field is not a sufficient condition for exact agreement of the far-field sound. The sufficient condition is that, in addition to the amplitudes and phases, the statistical source should also match the coherence function of the flow fluctuations. This is exemplified in a model problem, where we show that the effect of coherence decay on sound radiation is more prominent for subsonic convection velocities, and its neglect leads to discrepancies of more than an order of magnitude in the far-field sound. For supersonic flows errors are reduced for the peak noise direction, but for other angles the coherence decay is also seen to have a significant effect. Coherence decay in the model source is seen to lead to similar decays in the coherence of two points in the far acoustic field, these decays being significantly faster for higher Mach numbers. The limitations of linear wavepacket models are illustrated with another simplified problem, showing that superposition of time-periodic solutions can lead to a correlation decay between two points. However, the coherence between any pair of points in such models remains unity, and cannot thus represent the behaviour observed in turbulent flows.

Sign in / Sign up

Export Citation Format

Share Document