The optimization of stealth (response to an active sonar) and discretion (noise radiated from the vehicule) performance for underwater vehicules at very low frequencies is currently an important research topic in underwater acoustics. One classical way to improve acoustic performance is to cover the hull with specific acoustic coatings. This talk focuses on another approach, which is to introduce slight modifications of the hull internal structure. The results that are presented have been obtained in the frame of a project with the ANR French agency, supported by the french DGA. Classically, in order to withstand hydrostatic pressure, the hulls of underwater vehicles support periodically distributed rib stiffeners, which give rise to typical phenomena of acoustic diffraction at low frequencies and thus allow the detection and identification of these vehicles (Bloch-Floquet waves and Bragg diffractions). To overcome these problems, we propose to attach to a part of the stiffeners resonant systems whose characteristics vary randomly from one stiffener to the next. The frequency signature created by the network of stiffeners equipped with resonators is thus jammed and attenuated, improving stealth at low and very low frequencies This paper first presents analytical, numerical and experimental results obtained on a plate equipped with simple rib stiffeners, associated or not with resonators. Stealth performance diagrams highlight the usefulness of the proposed rib attachments. Then, optimization algorithms are used to choose the parameters of the resonators and their distribution in order to minimize a given objective function. The extension of this concept to the discretion problem is finally discussed.
The bladder type water muffler is a new type of liquid pipe muffler, by which the pulsating pressure and the acoustic noise of the fluid can be well absorbed. The theory here is the changed pressure generated by the built-in airbag when it meets the pulsating liquid. Howev-er, the performance of the muffler depends enormously on the relationship between the infla-tion pressure of the airbag within the muffler and the pressure of the piping system. In this paper we aim to design a test bench with continuously variable pressure of the piping system to approve their relevance to the performance of the muffler. In the experiment, the optimal state of the muffler is found by adjusting the inflation pressure of the muffler airbag and the pressure of the system pipeline. Meanwhile, the relevance of the inflating pressure in the bladder and working pressure of the pipe system to the performance of the muffler is further verified, which provides a useful guide for the application of the muffler on ships and submarines.
The distribution of underwater acoustic field is traditionally measured by a scanning hydrophone or a hydrophone array, which would both perturb the acoustic field. Optical technique provides an alternative way to measure underwater acoustic noninvasive. As such, an optical measurement system for underwater acoustic field is implemented in an anechoic water tank. This system is based on the so-called acousto-optic interaction between an optical beam and the acoustic field, and it uses a laser Doppler vibrometer (LDV) to measure the rate of change of optical path length caused by the presence of the acoustic field. Then processed by a tomographic technique, the underwater acoustic field could be reconstructed without perturbation. Three piston transducers covering the frequency range 100 to 500 kHz are utilized to validate the optical measurement system. The reconstructed acoustic distributions obtained by the LDV are also compared with the scanning results of a hydrophone at the same plane, and good agreements are observed in the frequency range of interest.
After cavitation, the noise of propeller will become the main component of ship radiation noise. It is necessary to detect propeller cavitation in real time to reflect a ship's acoustic performance. In order to detect propeller cavitation accurately, various characteristics of cavitation noise are inves-tigated. In this research, propeller noise under different rotating speeds of a trial ship is collected by a hydrophone fixed at the aft hull. The original noise signal is processed by wavelet packet re-construction to obtain cavitation characteristic signal. The envelope of cavitation characteristic signal is demodulated and its features changes in time domain and frequency domain are analyzed. The results indicate that the structure of DEMON spectrum changes and the modulation degree of cavitation characteristic signal increases with the development of cavitation. In addition, it is more effective to detect propeller cavitation by monitoring the amplitude of modulation frequency line spectrum in DEMON spectrum than sound pressure level of noise signal.
Propeller cavitation can produce strong noise, and serious cavitation will also lead to the damage of propeller blades. Therefore, it is necessary to conduct real-time detection of propeller cavitation. Acoustic method is the main research orientation of propeller cavitation detection. In this study, an experiment is carried out in cavitation tunnel to obtain noise of a propeller model under differ-ent cavitation states. A number of typical parameters related to energy intensity, pulse character-istics, spectrum structure of the noise signal are calculated and their changes with the develop-ment of cavitation are analyzed. The effective parameters are chosen to comprise feature vectors, and a classify based on support vector machine (SVM) is designed to identify the propeller cavita-tion state. The result shows that the method is effective, and the recognition rate of the classifier reaches 95.8% using 240 test samples.
Detecting low-speed targets such as underwater unmanned vehicles(UUVs) by using active sonar is becoming attractive. Recently, binary phase shift keying (BPSK) pulse is taken as active waveform because it has both high doppler and range resolution. However, its doppler sensitive property limit the application of BPSK heavily. In this paper, we focus on discussing the application area which is suitable for BPSK waveform. It is proved by simulation that shorter time duration of waveform could avoid time distortion of pulse and energy lose of matched filter. BPSK is more suitable for short range detection on high frequency sonar than low frequency sonar. The small doppler tolerance of BPSK could help to suppress clutters which arise from physical objects in the ocean (e.g. rocks, shipwreck, or fish). It is effective to detect low-speed target by BPSK in clutter-filled environment such as harbor and littoral area. Compared with linearly frequency modulation(LFM) waveform, the experiment results testify the benefit of applying BPSK to detect low-speed target in clutter-filled area on HFAS.
