Estimated flow noise levels due to a thin line digital towed array

Abstract

Flow noise levels for a digital thin line towed array for normal operating speeds of an AUV were estimated using empirical models and towing experiments. The wavenumber filtering associated with the finite size and distribution of hydrophones in acoustic element of DTLA was applied to the empirical models of turbulent wall pressure spectra to estimate the expected flow noise levels. The empirical results were then compared with the noise spectra measured during towing experiments conducted in a quiet lake. The results showed rapid loss of flow noise energy with increase in frequency arising due to the very nature of turbulent noise spectra and flow noise averaging introduced by the finite size of the hydrophone.

Numerical prediction of flow noise levels on towed sonar array

“…The flow noise spectra, as defined in equation (8), obtained from LES, are compared with experimental 20 spectra and semi-empirical model 35 in Figure 2 for U = 2–5 knots. From these figures, it is seen that the computed spectra compare quite well with the experimental spectra reported by Unnikrishnan et al 20 It can be seen that the TBL wall pressure fluctuations are predominant in the lower frequency regime, which is approximately 0 to 50 Hz. At frequencies higher than roughly 50 Hz, the results show a significant decrease in energy.…”

“…Experimental analysis of flow noise levels for the array was reported in Unnikrishnan et al, 20 as described in Section 2. The pressure fluctuations were measured at a point on the array surface located at x = 11 m (see Figure 1) when the array has attained a steady speed.…”

“…The array radius is a = D /2 = 5 mm. The experimental flow noise levels for this array were reported in Unnikrishnan et al 20,22 The experiments were carried out in a quiet lake by towing the array behind a dinghy with an onboard data acquisition system. It was ensured that no undue snaking motion of the array was present at the chosen tow speeds and the array stayed aligned with the direction of tow.…”

“…[15][16][17] Since the towing experiments were conducted in open water (lakes/seas), the TBL parameters such as d/a could not be measured unlike in wind or water tunnel. 10,[18][19][20] These papers offer baseline experimental data to validate CFD models for wall pressure spectra (Table 1).…”

Improved techniques for detection and localization of underwater acoustic sources using acoustic vector sensors

“…This includes flow noise due to the turbulent flow of water around the sensor which affects the particle velocity measurements at the sensors. This noise can severely affect these measurements as the flow velocity increases [196]. However, if this noise can be suitably modeled, the distortion caused due to this noise at the velocity sensors can be reduced.…”