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This paper is devoted to the study of acoustic vibrations induced by a flow upon an air-filled cylindrical tube vertically placed in water. A water pump with adapted piping generates a turbulent flow horizontally canalized in a large laboratory tank (6 m × 4 m × 3 m). The tube is located across this flow and an accelerometer measures vibrations. The signal processing performed on the recorded signals brings out resonance modes of the tube excited by the flow. A theoretical study (tube in air) and complementary experiments (tube in air and in water) are conducted to identify these modes.

An important challenge for current naval research is the modernization of battleships. Their target detection system must be increasingly efficient and they must be increasingly undetectable. Due to turbulent flows and bubbles, ship wakes are a detectable acoustic signature and ship bow waves disturb sonar detection. In this work, we study sound propagation through bubble clouds in water. We have developed an experimental set-up which permits us to acquire, in synchronization, acoustical signals and optical images. The phenomenon of bubble monopole resonance in very low frequency, related to bubble size, provokes effects of strong sound damping and sound speed dispersion. These experimental results, related to theoretical results, permit to estimated sizes and concentrations of bubbles. The acquired bubble images permit to know the real bubble sizes and concentrations, in order to correlate with the experimental acoustical results. Air bubbles are generated with a high pressure water jet introduced into the host liquid medium. A hydrodynamic study is done to characterize the bubble jet. We present in this work, theoretical results establishing a complex effective wave number characterizing the sound propagation in an effective medium. All results are discussed and compared with results of others papers on this subject.

Commander and Prosperetti works [J. Acoust. Soc. Am. 85, 732–746 (1989)] show that an acoustic wave, propagating in a bubbly water, is very strongly damped and its phase speed is disturbed for bubble resonance frequency neighboring. In relation to our experimental results, numerical results obtained with these analytical works are presented. Experiments are realized in a large water-filled tank. A frequency modulated burst, generated with broadband transducers of central frequency 100, 200, and 500 kHz, propagates through the bubble cloud obtained by a high pressure water jet. Our measurements show simultaneously, after the stop of jet, a strong damping of acoustic signal and a small change of its phase speed. After some minutes, this signal gets back progressively its initial amplitude and phase speed values. One can notice that the acoustic signal amplitude and phase speed are not identical in all emission frequency domain. To explain this phenomenon, we assume the existence of frequency band gaps related to the multiple scattering from the bubble cloud. [Work supported by Bassin d’Essais des Carènes (France).]

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