The influence of a pressure wavepacket's characteristics on its acoustic radiation

Abstract

Noise generation by flows is modeled using a pressure wavepacket to excite the acoustic medium via a boundary condition of the homogeneous wave equation. The pressure wavepacket is a generic representation of the flow unsteadiness, and is characterized by a space envelope of pseudo-Gaussian shape and by a subsonic phase velocity. The space modulation yields energy in the supersonic range of the wavenumber spectrum, which is directly responsible for sound radiation and directivity. The influence of the envelope's shape on the noise emission is studied analytically and numerically, using an acoustic efficiency defined as the ratio of the acoustic power generated by the wavepacket to that involved in the modeled flow. The methodology is also extended to the case of acoustic propagation in a uniformly moving medium, broadening possibilities toward practical flows where organized structures play a major role, such as co-flow around cruising jet, cavity, and turbulent boundary layer flows. The results of the acoustic efficiency show significant sound pressure levels, especially for asymmetric wavepackets radiating in a moving medium.

Acoustic, hydrodynamic and thermal modes in a supersonic cold jet

“…Semeraro, Lesshafft & Sandberg (2015) optimally forced a linear wavepacket model in a subsonic jet at St = 0.6 to recreate downstream-direction response and compared it with the Fourier modes obtained from direct numerical simulation. Likewise, Serré et al (2015) modelled the jittering process to recreate radiated sound patterns. Since the current decomposition highlights the internal dynamics of the wavepacket, it offers an opportunity to further investigate factors responsible for this amplification and spatial modulation.…”

Input-output analysis of Mach 0.9 jet noise

“…In wavenumber space, acoustic radiation comes instead from the supersonic "tail" of the wavepacket [17]. This depends on the modulation and asymmetry of the wavepacket envelope in physical space [38]. The result is that, in isolation, wavepackets seem to underpredict the sound generated by subsonic jets.…”

“…For a given a base flow, wavepackets can be efficiently computed using reduced-order methods based on the parabolized stability equations (PSE) [17,[26][27][28][29], global mode analysis [30], and optimal forcing approaches [31][32][33][34]. In addition, several studies have developed theoretical models of wavepackets based on experimental and numerical data [23,24,[35][36][37][38]. The theoretical models use analytic envelope functions to modulate an underlying instability wave.…”

“…The theoretical models use analytic envelope functions to modulate an underlying instability wave. Both the compactness and the asymmetry of the wavepacket envelope affect its acoustic efficiency and directivity [38].…”

Analysis of hydrodynamic and acoustic events in a turbulent boundary layer using a direct noise simulation database

“…close to the wall with respect to the wavelength (see figure 2), and (iii) the hydrodynamic events are extended, thus it is not defined whether the rising or the height or the setting of the hydro event may be considered as the source location, while the envelope of a convected structure has a key influence on both its acoustic efficiency and directivity. 15 There is very few chance that the contribution of an hydrodynamic event prints the acoustic field downstream from it, because convected propagation effects leading to enhance the amplitude of a spherical wave toward the upstream. Moreover, figure 10 shows that most of the source events lay in the downstream region of the domain, while the acoustic events lay in the upstream region.…”