Index of content:
Volume 129, Issue 3, March 2011
- AEROACOUSTICS, ATMOSPHERIC SOUND 
A coupled modal-finite element method for the wave propagation modeling in irregular open waveguides129(2011); http://dx.doi.org/10.1121/1.3531928View Description Hide Description
In modeling the wave propagation within a street canyon, particular attention must be paid to the description of both the multiple reflections of the wave on the building facades and the radiation in the free space above the street. The street canyon being considered as an open waveguide with a discontinuously varying cross-section, a coupled modal-finite element formulation is proposed to solve the three-dimensional wave equation within. The originally open configuration—the street canyon open in the sky above—is artificially turned into a close waveguiding structure by using perfectly matched layers that truncate the infinite sky without introducing numerical reflection. Then the eigenmodes of the resulting waveguide are determined by a finite element method computation in the cross-section. The eigensolutions can finally be used in a multimodal formulation of the wave propagation along the canyon, given its geometry and the end conditions at its extremities: initial field condition at the entrance and radiation condition at the output.
129(2011); http://dx.doi.org/10.1121/1.3543984View Description Hide Description
The diffraction of a sonic boom around a building of finite dimensions yields amplification of the front shock and a positive spike that follows the tail shock in the pressure waveform recorded at the incident side of the building’s exterior surface. This physical phenomenon is consistently found both in the data obtained from a 2006 NASA flight test and field experiment, and in the finite-difference time-domain simulation that models this particular experiment, and the authors call it the “building spiking” effect. This paper presents an analysis of the numerical and the accompanying experimental results used to investigate the cause of this effect. The simulation assumes linear acoustics only, which sufficiently describes the physics of interest. Separating the low and high frequency components of boom recordings using optimal finite impulse response filters with complementary magnitude responses shows that the building spiking effect can be attributed to the frequency dependent nature of diffraction. A comparison of the building spiking effect of a conventional N-wave and a low-amplitude sonic boom shows that a longer shock rise time leads to less pronounced amplification of the exterior pressure loading on buildings, and thus reveals an advantage of shaping a boom to elongate its rise time.
129(2011); http://dx.doi.org/10.1121/1.3543987View Description Hide Description
At high acoustic level, non-linear losses at the end of a tube are usually interpreted as the consequence of a jet formation at the tube end resulting in annular vortices dissipating part of the acoustic energy. Previous work has shown that two different regimes may occur. The present work, using particle image velocimetry visualization, lattice Boltzmann method simulation in 2D, and an analytical model, shows that the two different regimes correspond to situations for which the annular vortices remain attached to the tube (low acoustic particle velocity) or detached (high acoustic particle velocity).