Primary acoustic signal structure during free falling drop collision with a water surface
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TICAL, NONLINEAR, AND SOFT MATTER PHYSICS
Primary Acoustic Signal Structure during Free Falling Drop Collision with a Water Surface Yu. D. Chashechkin* and V. E. Prokhorov Ishlinskii Institute for Problems in Mechanics, Russian Academy of Sciences, Moscow, 119526 Russia *e-mail: [email protected]; [email protected] Received July 2, 2015
Abstract—Consistent optical and acoustic techniques have been used to study the structure of hydrodynamic disturbances and acoustic signals generated as a free falling drop penetrates water. The relationship between the structures of hydrodynamic and acoustic perturbations arising as a result of a falling drop contacting with the water surface and subsequent immersion into water is traced. The primary acoustic signal is characterized, in addition to stably reproduced features (steep leading edge followed by long decay with local pressure maxima), by irregular high-frequency packets, which are studied for the first time. Reproducible experimental data are used to recognize constant and variable components of the primary acoustic signal. DOI: 10.1134/S1063776116020175
1. INTRODUCTION Short-lived hydrodynamic processes (the formation of splashes, caverns, crowns, streamers, and capillary waves) that arise when a liquid drop falls onto a free resting liquid surface have studied since the second half of the 19th century [1, 2], which have been complemented by acoustic measurements since the beginning of the 20th century. As experimental techniques have been developed, the scope of studied problems has expanded, because related phenomena widely encountered in nature and used in numerous technical applications still attract the attention of researchers. Moreover, the growing variety of problems under consideration and the increasing number of publications are evidence of the topical character and complexity of this phenomenon. Already the first experiments that measured acoustic signals in air revealed the tonal character of sound emission [3]. The development of highly sensitive broadband hydrophones allowed underwater acoustic signals to be detected and emission sources to be identified as gas cavities separating from the main cavern. A formula for the sound frequency generated by a bubble was found in the 1930s [4] and is still used in practice. Laboratory investigations of the hydroacoustics of individual drops were later supplemented by marine measurements, which made it possible to resolve spectral peaks generated by intense rainfall in the 0.5– 25 kHz frequency range [5–7]. Marine measurements have stimulated interest in more detailed laboratory investigations of splash hydroacoustics [8]. Based on experimental data, several mechanisms of splash-induced sound generation have been proposed,
including (i) impact during primary contact of a falling drop and a liquid surface, (ii) resonance caused by oscillations of gas voids, and (iii) the “horn effect” of resonance oscillations of cavern walls generated by an immersing drop [9]. Subsequent investigations confirmed the validity of th
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