Acoustics in Bubbly Media

Increased understanding of sound's interaction with the real ocean environment is sought via theoretical and experimental pursuits. Specifically, we seek to further understand the effects of bubbles and bubble clouds on sound propagation, with practical applications in shallow-water sonar and minehunting. Bubbles and bubble clouds are generated in the near surface layer of the ocean due to both natural processes and human activities. Common examples are breaking waves, biologics and ship wakes.

Faculty:
Ron Roy, William Carey

Students:
Preston Wilson, Ryan McCormick

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Quantatative Ultrasound Imaging

The goal of this project is to employ tomographic inversion of ultrasound data to detect HIFU lesions.  In general tomographic reconstruction of ultrasound data in the body is not practical because the limited acoustic windows into the body normally mean only limited-view backscatter data is available.  The resulting inversion problem is therefore poorly posed.  In the case of HIFU however the individual lesions are approximately ellipsoidal in shape and the approximate position and size is known a priori.  Second, the process of necrosis results in change in sound speed and attenuation inside the lesion from their nominal values. Therefore the lesion can be described using shape-based methods where it is necessary to estimate only a small number of parameters to describe the geometry of the lesion rather than determine all the acoustics properties over all space.  This makes the inversion problem tractable even with the limited view backscatter data of an ultrasound probe.

Faculty:  R.O. Cleveland,  E.L. Miller (ECE-Tufts), B. Durning (BU/ECE-Tufts).

Recent Students:    B. Ulker-Karbeyaz (Northeasten PhD 2005), A. Draudt (PhD), E. Guven (Northeastern PhD)

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Detection of Buried Mines Using Laser-Acoustic Sensors

The humanitarian search for buried unexploded mines is a problem of grave importance. In this project, we are working in conjunction with Northeastern University to develop a technique for acoustic mine detection using a high-powered CO2 laser as a sound source. When the laser impacts the soil surface, rapid heating occurs, resulting in thermal expansion and the generation of an acoustic wave that propagates into the soil. This wave scatters off of the buried object and generates a scattered field that can be remotely detected using a scanning laser Doppler vibrometer. BU's effort is focused on the physics of opto-acoustic sound generation, and on the possibility that Biot waves may be created by this novel source. The work is supported by the Army Research Office via a subcontract from Northeastern University.

Faculty:
Ron Roy, Robin Cleveland, Chuck DiMarzio (Northeastern Univ.)

Students:
Wen Li (Northeastern Univ.)

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The Physics of Acoustic Hyperthermia and Acoustic Hemostasis

The primary cause of death on the battlefield is blood loss. If one could come up with a way to arrest internal bleeding long enough to get an injured soldier into a field surgical unit (or a blunt trauma victim into the hospital emergency room), then lives will be saved. Work is under way that focuses on a study of the underlying physical mechanisms behind acoustic hemostasis, which is the process in which bleeding can be controlled using high-intensity focused ultrasound (HIFU). A primary mechanism for hemostasis is the acoustically-induced rapid heating of tissue and blood leading to coagulative necrosis and the subsequent stoppage of bleeding. It is believed that both radiation stress on flowing blood and cavitation activity can contribute to this effect. The research is focused on in vitro experimentation using tissue-mimicking phantoms and numerical simulations using finite-difference time domain techniques. The work is funded by the Defense Advanced Projects Agency through a subcontract from the Unviersity of Washington.

Faculty:
Ron Roy, R. Glynn Holt

Students:
Patrick Edson and Jinlan Huang

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Ocean Wave Energy

Only two thousandths of the ocean's power would supply the entire world's power (Jouanne, 2007). We are creating and researching technologies for harvesting this power in a cost-effective way.

Faculty:
J. Gregory McDaniel

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Steering and Mixing of Waves in Composite Structures

Composite structures support waves with strong dependences on propagation angle, enabling acoustic optimization that is not possible with isotropic materials. This research is examining the vibrational and structural acoustic implications by developing models for the steering and mixing of waves.

Faculty:
J. Gregory McDaniel, Paul Barbone, Allan Pierce

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Automotive Brake Squeal

Automotive companies in North America spend approximately one billion dollars every year on warranty costs associated with brake squeal. This estimate does not include other vehicles, such as trucks and buses, nor does it include out-of-warranty costs. Recognizing that significant design changes to brake systems are not economically feasible, this research develops and interrogates models of advanced damping treatments that mitigate brake squeal.

Faculty:
J. Gregory McDaniel

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Snapshots from a 3D computer simulation of the passage of a shock wave through a kidney stone.

Lithotripsy

Shock Wave Lithotripsy (SWL) is a non-invasive medical technique for treating kidney stones.  Shock waves generated outside the body are focused onto the kidney stones resulting in fragmentation into pieces small enough to be passed naturally.   The goal of the research is to understand the process by which the shock waves fragment the stones and to understand the mechanisms by which shock waves can damage the surrounding soft tissue.  This work is carried out in collaboration with colleagues at Indiana University Medical School, University of Washington at Seattle, Caltech, University of Illinois and the BU Medical School.   This work is primarily funded by the National Institutes of Health and has also been supported by the Whitaker Foundation.

Faculty:  R.O. Cleveland
Recent Students: P Chitnis (PhD 2006), H. Luo (PhD), J. Kracht (PhD)

To read more about this project visit the Lithotripsy Research page on the Physical Acoustics Laboratory web site.

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Targeted Ultrasound Contrast Agents for Drug Delivery

This is project is a collaboration with Prof. Joyce Wong of the Department of Biomedical Engineering (BME) at BU.  Prof. Wong is developing molecules that target and bind to diseased tissue.   One barrier to her work is finding appropriate contrast agents that these molecules can be attached to so that the diseased tissue can be imaged.  One candidate is the ultrasound contrast agent, which typically consists of a gas microbubble, 1 to 5 µm in diameter, encapsulated with a thin shell.  We are investigating the use of polymer-shelled contrast agents.  The ultrasound imaging systems and scanning acoustic microscope are used to characterize the properties of the microbubbles developed in the Wong Lab.  This work is funded by the National Institutes of Health and the NSF Centre for Subsurface Sensing and Imaging Systems.

Faculty:          R.O. Cleveland,  J. Y. Wong (BME-BU).
Recent Students:          W. Duncanson (PhD)

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Rheology of Foam

Foams are extremely important in a variety of industrial applications. They are widely used in firefighting applications, including the Fire Suppression Systems aboard the Space Shuttle and Spacelab module. The petroleum industry utilizes foams in flow applications such as enhanced oil recovery and as drilling fluids. They are used in various industries as trapping, transport, and separation agents. Arguably the most important quality of a foam in many of these industrial processes is its response to imposed strain, or its rheological behavior. There exists almost no experimental data on the rheological properties of real 3D foams, even though such knowledge would likely enhance the efficacy of current applications and suggest other unique applications. The lack of 3D data is due in large part to the earth-based requirements for contact containment, and to the fact that gravity-induced drainage quickly destroy all but the "driest" foams, those with a very high gas volume fraction. Our goal in this project is to develop and refine a unique acoustic levitation method to provide non-contact control and manipulation of foam samples. The development of this technique, together with experimentation in Og, will provide the ability to carry out a set of benchmark experiments which will allow determination of a foam's yield stress, bulk shear, and dilatational moduli and viscosities as a continuous function of gas volume (or 'void') fraction from the dry limit through the order-disorder phase transition to the wet limit of a bubbly liquid. In addition to providing the first measurements of such quantities as functions of void fraction to compare with theory, the knowledge gained will have practical application to the myriad of actual uses of foams on the earth, and to space-based systems. By closely interfacing these data with emerging theoretical results from other groups (including our own proposed work) the fundamental understanding of foam rheology will be advanced. We will perform experiments and simulations in our laboratory, with selected tests to be flown aboard NASA's KC-135 research aircraft.

Faculty:
R. Glynn Holt, J. Gregory McDaniel

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Sonoluminescence in Space

Sonoluminescence is the term to describe the emission of light from a violently collapsing bubble which is acoustically levitated in water. Sonoluminescence ("light from sound") is the result of extremely nonlinear pulsations of gas/vapor bubbles in liquids when subject to sufficiently high amplitude acoustic pressures. In a single collapse, a bubble's volume can be compressed more than a thousand-fold in the span of less than a microsecond. Even the simplest consideration of the thermodynamics yields pressures on the order of 10,000 ATM, and temperatures of at least 10,000K. On the face of things, it is not surprising that light should be emitted from such an extreme process. Single Bubble Sonoluminescence (SBSL) has been intensively investigated both experimentally and theoretically in the past 5 years. Despite such recent attention, there remain (at least!) 3 unexplained phenomena associated with SBSL:

1. The light emission mechanism itself

2. The disappearance of the bubble at some critical acoustic pressure, and

3. The appearance of quasiperiodic and chaotic oscillations in flash timing.

Gravity, in the context of time-varying buoyancy, is implicated in these unexplained phenomena, which have all been observed in 1g experiments. SBSL bubbles experience a time-varying buoyancy which reaches maximal excursions precisely where sonoluminescence is observed. This results in a strong nonlinear coupling between volume and translatory motions. Removing the acceleration of gravity from the system will eliminate buoyancy-driven translatory oscillations of the bubble. Our goal in this project is to perform careful experiments coupled with relevant numerical modeling in order to understand bubble dynamics and light emission in variable acceleration environments. We will perform experiments and simulations in our laboratory, with selected tests to be flown aboard NASA's KC-135 research aircraft.

Faculty:
R. Glynn Holt, Ron Roy

Students:
Sean Wyatt, Charles Thomas

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