Ali R. Kolaini

Air entrapment by a falling water mass

The impact of a nearly cylindrical water mass on a water surface is studied both experimentally and theoretically. The experiments consist of the rapid release of water from the bottom of a cylindrical container suspended above a large water tank and of the recording of the free-surface shape of the resulting crater with a high-speed camera. A bubble with a diameter of about twice that of the initial cylinder remains entrapped at the bottom of the crater when the aspect ratio and the energy of the falling water mass are sufficiently large. Many of the salient features of the phenomenon are explained on the basis of simple physical arguments. Boundary-integral potential-flow simulations of the process are also described. These numerical results are in fair to good agreement with the observations.

Observations of underwater sound from laboratory breaking waves and the implications concerning ambient noise in the ocean

The results of an experiment to characterize the underwater sound field radiated by various breaking waves intensities in fresh water in the range from 0.1 to 20 kHz are described. Waves are generated by a computer‐controlled plunging‐type wave maker and propagate along a 12.7‐m‐long channel where they are made to break at the mid‐surface of a 3‐×3‐×2.5‐m anechoic water tank. The individual bubbles and bubble clouds entrained by the breaking wave provide a mechanism for sound production. Using high‐speed cinephotography, correlations were established between the hydrodynamic evolution of the cloud and the radiated acoustic emissions. The bubble size distributions inside the cloud were measured with the aid of a high‐speed video camera and a fiber optic cable. These measurements indicate that single bubbles with radii as large as 7–8 mm may be entrained in this fresh‐water system by moderate spilling breakers. Detailed measurements of the bubble size distribution of the bubble cloud enabled us also to obta...

Sound radiation by various types of laboratory breaking waves in fresh and salt water

A recent article [A. R. Kolaini and L. A. Crum, J. Acoust. Soc. Am. 96, 1755–1765 (1994)] reported the measurements of the ambient sound generated by laboratory breaking waves over the range 100–20 000 Hz in fresh water. Those observations from both spilling- and plunging-type breakers have been repeated in the same manner and wavemaker/anechoic tank with water that had 25‰ salt in its content. The observations in salt water, just like those in fresh water, reveal that the sources of sound in laboratory spilling breakers are due primarily to single bubble oscillations that can have frequencies lower than a few hundred Hertz. In the case of weak spilling breakers, the sound spectra level in fresh water was due primarily to single bubble oscillation, while the same breakers in salt water have introduced smaller size bubbles with large density. The relatively high-density populated bubble cloud generated by weak breakers shows the evidence of the onset of collective oscillation that was absent for the same b...

An investigation of the acoustic emissions from a bubble plume

This letter presents some preliminary results of an experimental study of the underwater sound field emitted by a bubble plume. The densely populated bubble plumes were generated by dropping a fixed volume of water, held in a cylindrical container, onto a still water surface. The characteristics of plumes were varied by changing the container volume and height above the surface. Acoustic emissions from these plumes appear to depend on the volume of the injected water, with the emitted frequency band decreasing with increasing plume volume. In addition, large-amplitude, low-frequency emissions correlate well with the observed detachment of “substructures” with the plume. The frequencies of the acoustic signals associated with the formation of these structures range as low as a few tens of Hertz.

Low‐frequency underwater sound generation by impacting transient cylindrical water jets

The impact of a jet of water onto a still‐water surface results in the entrainment of large amounts of air and the eventual formation of a bubble plume. Results from an experimental study of the noise produced by this process is presented. Preliminary results of this study were reported previously by Kolaini et al. [J. Acoust. Soc. Am. 89, 2452–2455 (1991)]. The densely populated bubble plumes were generated by dropping a fixed volume of water, held in a cylindrical container, onto a still‐water surface. High‐speed video images reveal the formation of a cylindrical bubble plume with a very high void fraction which grows in size until all the water is injected into the tank. As the leading end of the plume advances, a section of the plume separates near the crater region formed by the jet. After detachment, the separated plume, which is roughly spherical in shape, undergoes volume pulsations, and radiates relatively large‐amplitude, low‐frequency sound. The nature of the acoustic emissions from bubble plum...

Bubble production by capillary‐gravity waves

In the absence of whitecapping, other physical mechanisms may contribute to the generation of high‐frequency ambient noise. It has been suggested [Longuet‐Higgins, in NATO Advanced Research Workshop on Sound Generation Mechanisms at the Open Surface (NATO, Geneva, 1987)] that capillary waves, with surface profiles that are peaked downward in the troughs and are relatively flat at the crests, can inject acoustically active bubbles into the ocean, and thus contribute to the ambient noise background. It has been demonstrated in the laboratory that bubble injection can be generated at the trough of capillary‐gravity, short‐fetched waves by blowing air over water contained in a long, narrow tank. Simultaneous in situ acoustic and high‐speed video monitoring of the capillary‐gravity waves demonstrate that these waves can produce acoustically active bubbles. The generation of capillary waves depends principally upon the surface tension, which can be changed by adding surface‐active agents to the water. The bubbl...

Low‐frequency acoustic emissions in fresh and salt water

The impact of a jet of water onto a still water surface results in the entrainment of large amounts of air and the eventual formation of a bubble plume. Densely populated bubble plumes are generated by dropping a fixed volume of water, held in a cylindrical container, onto a still‐water surface. The detached bubble plume, which is roughly spherical in shape, then undergoes volume pulsation and radiates relatively large‐amplitude, low‐frequency sound. The results of a laboratory study of the noise produced by this process were reported previously by Kolaini et al. [J. Acoust. Soc. Am. 94, 2809–2820 (1993)]. In this paper we report the results of a field study of noise produced by this process in both fresh water (Lake Washington, WA) and salt water (Puget Sound, WA). Studies of acoustic emissions from transient bubble plumes as a function of cylinder parameters are described, with specific attention devoted to a comparison of the results obtained in salt and fresh water. The measurements, which exhibit goo...

Effects of salt on bubble acoustic radiation in water

Recent measurements of laboratory-generated noise by breaking waves exhibit an increase in sound-pressure levels for salt water compared to fresh water over a broader range of frequencies [A. R. Kolaini, J. Acoust. Soc. Am. 103, 300–308 (1998)]. The surprise increase in sound-pressure levels inspired the study of the effects of salt on the sound radiation by single bubbles released from various size needles. A needle assembly and a false tank (2×2×4 ft), with acoustically transparent walls placed in the middle of an anechoic tank (12×12×8 ft), were used to study the acoustical characteristics of bubbles released from needles. The false tank was filled with water that contained various percentages of sodium chloride (NaCl). In this paper, the results of experiments to examine the variations in radiated sound pressure and the change in the damping coefficient of bubbles as the salinity level increased are reported. The effects of a surfactant agent such as Triton™ 100-X and other mineral acids such as HCl o...

The Production of High-Frequency Ambient Noise by Capillary Waves

The quantitative characterization of noise sources in the ocean is a task which has attracted growing attention in recent years. In the absence of breaking gravity waves, capillary waves may contribute to the generation of high-frequency ambient noise. Surface profiles of capillary water waves are peaked downward in the troughs and relatively flat at the crests. At a limiting amplitude given by H/λ ≈ 0.73 (where H and λ are the wave height and wavelength, respectively) the free surface in the trough may fold onto itself, encapsulating an air bubble. By blowing air over a water-filled, plexiglass tank, bubble-producing capillary waves can be generated in the laboratory. Underwater acoustic emissions from these bubbles were monitored and a visual record of their formation was established via high-speed video tape. The initial findings of an experimental study of the generation, size distribution, and acoustic emissions of bubbles produced by capillary water waves is presented.

An improved method of structural-borne random vibration mass attenuation

In 2009, the NASA Engineering and Safety Center (NESC) Vibroacoustics Working Group identified vibroacoustic environment predictions as one of the highest risk areas for new launch vehicle programs. Specifically, the working group identified the need for improved random vibration mass attenuation prediction methods as key to mitigating this risk. This paper derives a multi drive point, multi axis, interface sized equation between the unloaded and loaded drive point random vibration accelerations and cross correlations. The methods development and associated acoustic test validation program were a collaborative effort between ASD, NESC, and NASA/JPL. The derivation utilizes the methods of modal synthesis and random vibration averaging and precludes the use of any simplifying structural interaction assumptions besides the interfaces behaving linearly (no interface deadbands). Key to the methods improved accuracy is its built-in mechanism for combining the contributions of the different unloaded drive-poin...