Although zooplanktonic organisms nearly have the acoustical impedance of water, they backscatter the sound. Strong acoustical echoes from salps were recorded in the Weddell sea at 150 kHz (1992), in the Indian ocean at 12 kHz (1994) and in the Mediterranean at 120 and 38 kHz (1996). Sphere and cylinder models were applied to estimate the salp target strength. The salp is modelled by a cylindrical gelatinous body (length L) and a spherical nucleus (radius a). Backscattering from each part was estimated individually. With a detection threshold of-100 dB, the minimum body length and the minimum radius of nucleus to be theoretically detected were estimated for each of the following frequencies: 12, 38, 120 and 150 kHz. A small salp (L = 14 mm and a = 1 mm) would be only detected at 120 and 150 kHz whereas a big salp (L = 100 mm and a = 7.5 mm) would be also detected at 12 and 38 kHz. The nucleus of the salp can reveal by itself the salp's presence in the medium by concentrating a maximum weight into a minimum volume. The effects of variations in the input parameters on the estimated acoustical backscattering are discussed.
The boundary-layer flow, for instance along a sonar dome, gives rise to hydrodynamic noise due to the pressure fluctuations. The prediction of the resulting self-noise received by the sonar antenna is based on models, which in general take only partially into account the flexibility of the dome wall. The present work readdresses the problem of hydrodynamic noise, considering the geometrically simplified model of a two-dimensional unstable boundary-layer flow along an elastic plate with clamped ends. The incompressible Navier-Stokes equations are fully coupled to the elastic plate model and the system is numerically solved for various plate materials. The unstable flow dynamics is analyzed with respect to the wall properties. The Fourier-transformed stress tensor is then used in the framework of Lighthill's analogy to determine the generated radiative sound, emphasizing the effect of wall-flexibility. This work is supported by Thales Underwater System and DCNS.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.