Analysis of scattering from an acoustic cloak in a moving fluid

Abstract

This work develops a theoretical framework for acoustic cloak scattering analysis in a low speed non-stationary fluid that is simply described as a potential flow. The equivalent sound source induced by the moving fluid local to the cloak is analytically constructed and is then estimated using Born approximation. The far-field scattering can thereafter be obtained using the associated Green's function of the convected wave equation. The results demonstrate that the proposed analytical approach, which might be helpful in the design and evaluation of cloaking systems, effectively elucidates key characteristics of the relevant physics. In addition, it can be seen that, in a moving fluid, the so-called convected cloaking design achieves better cloaking performance than the classical cloaking design.

Theoretical and Numerical Modeling of Acoustic Metamaterials for Aeroacoustic Applications

“…This will result in an imperfect cloaking of the object even in the presence of the convective metamaterial design (see Section 5). On the other hand, Huang et al [14] shows that the use of a convective cloak design produces an improvement of the masking capability even if the simplistic assumption of a uniform stream is used (see Figure 1 of [14]) and that the introduction of a more realistic external flow pattern recovers the complete cloaking property of the metamaterial layer. As already mentioned, one of the objectives of the present paper is the development and the preliminary validation of an efficient numerical solution of the aeroacoustic cloaking based on the integral formulation of the problem, and for this reason, the analysis is limited to the uniform stream assumption.…”

“…As already mentioned in the Introduction, the problem has been already addressed in the literature by several authors. In particular, Huang et al [14,15] have developed an approach that is, in the author's opinion, the most suitable for a direct application to real aeroacoustic problems. In particular, in [14], the convective design of the cloaking material is obtained by imposing the matching of the wave vector at the cloak/host interface, taking into account the effect of a uniform mean flow in Ω h on the propagation speed of an incoming plane wave.…”

On the Use of the Analogue Transformation Acoustics in Aeroacoustics

“…To the best of the authors' knowledge, the ATA has never been applied to such a problem, where a blunt obstacle is submerged by a moving compressible fluid. Actually, the analysis of the performance deterioration of blunt anisotropic cloaking devices in presence of a mean flow has been already addressed by several authors (see, e.g., [6][7][8]) who have also proposed strategies to recover, at least partially, the cloaking efficiency lost. All those contributions are based on a correction of the design obtained by STA to compensate the lack of formal invariance of the convective equations.…”

“…In their work, the authors make use of the correction proposed by Huang et al [6] to adapt the static acoustic cloaking design for moving fluid conditions and consider an impinging planar wave on a cylindrical object. The designed cloaking device is divided into zones and the corrective term is then optimized for each zone, giving rise to an equivalent Mach number of the part.…”

“…A detailed bibliographic review is beyond the scope of this paper; however it is worth mentioning some attempts to address the aeroacoustic cloaking problem, that is, in the presence of flow or motion of scatterers and sources, which can be found in literature. In Huang et al [6] a convected cloaking design is proposed, based upon a simple Doppler-like correction, valid for an object at rest surrounded by a moving medium impinged by a planar wave. However, flow Mach numbers that have been used, up to 0.06, are too far from realistic aeroacoustic applications of some interest in aeronautics.…”