Joseph Jankovsky

Acoustic scattering from partially voided compliant and fluid spheres

The presence of bubbles has been shown to change the compressibility and complex sound speed in a liquid. In the ocean, acoustically compact bubbly mixtures manifest themselves as highly compressible regions that effectively scatter low‐frequency sound. To study low‐frequency sound scattering, multifrequency backscattering experiments have been performed in a tank using partially voided three‐quarter‐inch diameter polyurethane spheres as targets. Target strengths (2‐20 kHz) were measured for four spheres with void fractions of 0%, 3.4%, 4.2%, and 6%. Measured target strengths for the voided spheres were on the order of −40 to −60 dB (re 1 m). The frequency response exhibited modal structure, with peaks shifting to lower frequencies for higher void fractions. No backscatter signal was detected for the solid polyurethane sphere. Target strength was also measured for a hollow polyurethane sphere containing a suspension of bubbles in polymer gel. The void fraction was determined by fitting the scattering theo...

The effects of oceanic surfactants on acoustic propagation in bubbly liquids

The chemical composition of the top layer of the ocean is known to contain surface‐active substances that are readily adsorbed to an air‐water interface. The presence of surfactants in a liquid induces a viscoelastic stress along a two‐dimensional interface. These surfactants can coat bubbles that alter their individual dynamics. A model is presented that incorporates the effects of surface viscoelasticity for acoustic propagation in bubbly fluids. The effects of both surface dilatational viscosity and surface elasticity on the phase speed and attenuation are considered. Surface viscosity is found to decrease the attenuation near bubble resonance frequencies, yet increase damping below resonance. Surface viscosity also diminishes the resonant effects for the phase speed in bubbly fluids. Both effects become significant for bubble sizes below 100 microns. The addition of surface elasticity is found to decrease the mean oscillation bubble radius, and thus shift bubble resonance to a higher frequency. The ef...

The effects of interfacial viscosity on the attenuation and phase speed of acoustic waves in bubbly liquids

The attenuation and phase speed of bubbly fluids have been shown to be highly dispersive due in part to the resonance response of individual bubbles. Surface‐active materials, or surfactants, are known to dramatically alter the viscoelastic dynamics of an interface without affecting the bulk fluid properties. In this theoretical model the effects of surface viscosity on the propagation of linear pressure waves through a bubbly fluid are examined. The attenuation and phase speed are calculated for varying radii, void fraction, and interfacial viscosity for air bubbles in water. The results show an increase in attenuation for frequencies far removed from the bubble resonance for radii less than 100 microns. There is no significant increase in attenuation for larger bubbles (order 1 mm). Interestingly, near resonance, the interfacial viscous case has a lower attenuation than the clean interface, most likely due to the reduced radiation damping associated with smaller radial excursions. Likewise, resonance ef...

Drop dynamics in microgravity

The dynamics of liquid drops in microgravity were observed in the Drop Physics Module flown aboard the second United States Microgravity Laboratory (USML‐2) in 1995. Water drops with varying concentrations of the surfactants Triton‐X 100 and Bovine Serum Albumin were suspended in air and deformed by an acoustic standing wave. When the deforming force was removed, the drops oscillated freely about a spherical equilibrium shape, as opposed to the oblate equilibrium shape of a drop levitated in 1‐g. Drop diameters between 1 and 3 cm were observed. Frequencies and decays for the dynamics have been extracted from the data. Results are compared to theoretical models and experiments in 1‐g. Results for frequency shifts due to large amplitude oscillations are also presented. [Work supported by NASA through JPL, contract 958722.]

Feasibility of low‐frequency single‐bubble sonolumin‐ escence

The potential to perform single‐bubble sonoluminescence (SBSL) at low frequencies is motivated by the payoff of greatly enhanced energy concentration during collapse. Yet it is also known that bubbles undergoing such catastrophic collapse tend to be unstable. Experimental apparatus has been designed and computer simulations have been performed to test the feasibility of low‐frequency, single‐bubble sonoluminescence. The experimental apparatus consists of a cylindrical cell that is driven by an aluminum, half‐wavelength resonator with fundamental resonance of less than 15 kHz. The cell is designed to be pressurized up to 5 atmospheres to allow levitation without significant spurious cavitation in the liquid. To complement this experimental work, our computer simulations of this phenomena are continuing [T. Shi and R. Apfel, 3253 (1994)] in order to follow the shape distortion of collapsing bubbles for varying parameters, including acoustic frequency. The basic characteristics of our low‐frequency resonator...

Exploring the parameter space of low frequency of sonoluminescence

An experimental apparatus has been designed to measure the physical properties of low‐frequency sonoluminescence (SL) (15 kHz). Radial motion of the bubble has been measured by Mie scattering and also by a new method, which images the bubble through a magnifying CCD camera illuminated by a strobing LED lamp. The light emission of SL is measured by a photomultiplier tube. An automated gas handling system has been constructed in order to measure the effects of various gas mixtures and concentrations on SL. The system consists of voltage actuated valves, a mixing vessel, and pressure sensors, which are interfaced to a computer and controlled using the LabVIEW programming environment. With the lower frequencies and the resulting larger bubbles, it is hoped that the parameter space for this region of SL can be mapped out. [Work supported by NASA through JPL Contract No. 958722.]

Experimental study of streaming in acoustic resonators

Anomalous rotations of solid and liquid drops have been observed in resonant chambers in both 1g and in the microgravity environment of space in the Drop Physics Module (DPM) aboard the United States Microgravity Laboratory (USML‐1) mission in 1992. The observed torques are produced by acoustic streaming, typically in the nondegenerate plane where rotations are undesired, and have been attributed to an imbalance in the paired driver amplitudes. To quantify the torque exerted on the inclusion, measurements were made using suspended and levitated spherical samples with single and paired driver configurations. Experiments were conducted in both degenerate and nondegenerate planes to study the effects of orthogonal coupling. Absorption effects of active and passive drivers are presented, along with asymmetrical driver configurations. The results will be used to control and eliminate unwanted tumbling rotations in the USML‐2 DPM experiments to be flown in 1995. [Work supported by NASA through JPL Contract No. ...