Moshe Strauss

An extended Rayleigh model of bubble evolution

An extended Rayleigh model for laser generated bubbles in water and soft tissue is presented. This model includes surface tension, viscosity, a realistic equation of state, material strength and failure, stress wave emission, and linear growth of interface instabilities. The model is validated by comparison to detailed compressible hydrodynamic simulations using the LATIS computer program. The purpose of this study is to investigate the use of the extended Rayleigh model as a much faster and simpler substitute for the detailed hydrodynamic simulations when only limited information is needed. It is also meant to benchmark the hydrosimulations and highlight the relevant physics. The extended Rayleigh model and the hydrosimulations are compared using both a 1D spherical geometry with a bubble in the center and a 2D cylindrical geometry of a laser fiber immersed in water with a bubble formed at the end of the fiber. Studies are done to test the validity of the material strength and failure, stress wave emissi...

Two-dimensional Rayleigh model of vapor bubble evolution

The understanding of vapor bubble generation in an aqueous tissue near a fiber tip has required advanced two dimensional (2D) hydrodynamic simulations. For 1D spherical bubble expansion a simplified and useful Rayleigh-type model can be applied. For 2D bubble evolution, such a model does not exist. The present work proposes a Rayleigh-type model for 2D bubble expansion that is faster and simpler than the 2D hydrodynamic simulations. The model is based on a flow potential representation of the hydrodynamic motion controlled by a Laplace equation and a moving boundary condition. We show that the 1D Rayleigh equation is a specific case of our model. The Laplace equation is solved for each time step by a finite element solver using a triangulation of the outside bubble region by a fast unstructured mesh generator. Two problems of vapor bubbles generated by short-pulse lasers near a fiber tip are considered: (1) the outside region has no boundaries except the fiber, (2) the fiber and the bubble are confined in a long channel, which simulates a fiber in a vessel wall. Our simulations for problems of type (1) include features of bubble evolution as seen in experiments, including a collapse away from the fiber tip. A different behavior was obtained for problems of type (2) when the channel boundary is close to the fiber. In this case the bubbles expansion and collapse are both extremely slow in the direction normal to this boundary and distortion of the bubble is observed.

Two-dimensional Rayleigh model for bubble evolution in soft tissue

The understanding of vapor bubble generation in a soft tissue near a fiber-optic tip has in the past required two-dimensional (2D) hydrodynamic simulations. For 1D spherical bubble expansions a simplified and useful Rayleigh-type model can be applied. For 2D bubble evolution, such a model has not been developed. In this work we develop a Rayleigh-type model for 2D bubble expansion that is much faster and simpler than 2D hydrodynamic simulations and can be applied toward the design and understanding of fiber-based medical therapies. The model is based on a flow potential representation of the hydrodynamic motion and is described by a Laplace equation with a moving boundary condition at the bubble surface. In order for the Rayleigh-type 2D model to approximate bubble evolution in soft tissue, we include viscosity and surface tension in the fluid description. We show that the 1D Rayleigh equation is a special case of our model. The Laplace equation is solved for each time step by a finite-element solver usin...

Geologic lithofacies identification using the multiscale character of seismic reflections

A forward acoustic model shows that geologic lithofacies groups can be identified by the character of the wavelet transform of their seismic reflection response even for incident signals with a wavelength much larger than the dominant bed thickness. The same model shows that multiple interbed reflections can be neglected. This allows the use of an analytical relation of the linear reflection response expressed as a convolution between the incident signal and the scaled derivative of the acoustic impedance. The relation is applied to solve the inverse problem for the acoustic impedance, using orthogonal discrete wavelet transform (DWT) and Fourier transform methods; good agreement is obtained between the well log waveletspectrum and both the forward modeled seismic data and the real seismic data. It is found that the DWT approach is superior, having a better signal-to-noise ratio and more localized deconvolution artifacts. A population of well logs containing a wide range of lithologies and bed thicknesses, which are categorized into lithofacies groups, is used to define the conditional probability of a wavelet transform response given a lithofacies group. These conditional probabilities are used to estimate the lithofacies probability given a seismicwavelet response via a Bayesian inversion.

Self-consistent coupling of cavitation bubbles in aqueous systems

The dynamics of an ensemble of cavitation voids initiated by laser-produced stress waves in aqueous systems is considered. Aqueous systems have large similarity to soft tissues. Laser-initiated stress waves are reflected from tissue boundaries, thereby inducing a tensile stress that is responsible for tissue damage. The early stage of damage is represented by an ensemble of voids or bubbles that nucleate and grow around impurities under stress wave tension. For impurity densities larger than 105 cm−3 the bubbles growth reduces the tensile wave component and causes the pressure to oscillate between tension and compression. For impurity densities below 108 cm−3 the bubbles grow on a long time scale (∼10 μs) relative to the wave interaction time (∼100 ns). For bubble densities above 108 cm−3 the bubble lifetime is greatly shortened because of the reduced tensile component. On a long time scale the growing bubbles cause a significant reduction in the liquid average compression pressure below the ambient atmos...

Extended Rayleigh model of bubble evolution with material strength compared to detailed dynamic simulations

The validity of an extended Rayleigh model for laser generated bubbles in soft tissue is examined. This model includes surface tension, viscosity, a realistic water equation of state, material strength and failure, stress wave emission and linear growth of interface instabilities. It is compared to detailed dynamic simulations using the computer program LATIS. These simulations include stress wave propagation, a realistic water equation of state, material strength and failure, and viscosity. The extended Rayleigh model and the detailed dynamic simulations are compared using a 1D spherical geometry with a bubble in the center and using a 2D cylindrical geometry of a laser fiber immersed in water with a bubble formed at the end of the fiber. Studies are done to test the validity of the material strength and failure, stress wave emission, and the interface instability terms in the extended Rayleigh model. The resulting bubble radii, material damage radii, the emitted stress wave energies, and the size of the interface distortions are compared. Conclusions are made on the validity of the extended Rayleigh model and on possible improvements to this model. The purpose of this study is to investigate the use of the extended Rayleigh model as a substitute for the detailed dynamic simulations when only limited information is needed. It is also meant to benchmark the detailed dynamic simulations when only limited information is needed. It is also meant to benchmark the detailed dynamic simulations and highlight the relevant physics. It is shown that the extended Rayleigh model executes over 300 times faster on a computer than the detailed dynamic simulations.

Geologic lithofacies identification using seismic wavelet transformation

A forward acoustic model shows that geologic lithofacies groups can be identified by the character of the wavelet transform of their seismic response even for incident signals with wavelength much larger than the dominant bed thickness. The same model shows that multiple interbed reflections can be neglected. This allows the use of a simple analytical relation of the linear reflection response expressed as a convolution between the incident signal and the scaled derivative of the acoustic impedance. The relation is applied to solve the inverse problem for the acoustic impedance, using orthogonal discrete wavelet transform (DWT) and Fourier transform (FT) methods; good agreement is obtained between the well log wavelet spectrum and both the forward modeled seismic data and the real seismic data. It is found that the DWT approach is superior, having a better signal-to-noise ratio and more localized deconvolution artifacts. A population of well logs containing the lithofacies groupings is used to define the conditional probability of a wavelet transform response given a lithofacies group. These conditional probabilities are used to estimate the lithofacies probability given a seismic wavelet response via a Bayesian inversion.

Simulation studies of vapor bubble generation by short-pulse lasers

Formation of vapor bubbles is characteristic of many applications of short-pulse lasers in medicine. An understanding of the dynamics of vapor bubble generation is useful for developing and optimizing laser-based medical therapies. To this end, experiments in vapor bubble generation with laser light deposited in an aqueous dye solution near a fiber-optic tip have been performed. Numerical hydrodynamic simulations have been developed to understand and extrapolate results from these experiments. Comparison of two-dimensional simulations with the experiment shows excellent agreement in tracking the bubble evolution. Another regime of vapor bubble generation is short-pulse laser interactions with melanosomes. Strong shock generation and vapor bubble generation are common physical features of this interaction. A novel effect of discrete absorption by melanin granules within a melanosome is studied as a possible role in previously reported high Mach number shocks [Lin and Kelly, SPIE 2391, 294 (1995)].

Self-consistent evolution of tissue damage under stress wave propagation

Laser-initiated stress waves are reflected from tissue boundaries, thereby inducing tensile stresses, which are responsible for tissue damage. A self-consistent model of tissue failure evolution induced by stress wave propagation is considered. The failed tissue is represented by an ensemble of spherical voids and includes the effect of nucleation, growth and coalescence of voids under stress wave tension. Voids nucleate around impurities and grow according to an extended Rayleigh model that includes the effects of surface tension, viscosity and acoustic emission at void collapse. The damage model is coupled self-consistently to a one-dimensional planar hydrodynamic model of stress waves generated by a short pulse laser. We considered the problem of a bipolar wave generated by a short pulse laser absorbed on a free boundary of an aqueous system. The propagating wave includes a tensile component, which interacts with the impurities of exponential distribution in dimension, and an ensemble of voids is generated. For moderate impurity density (approximately 108 cm-3) void growth reduces the tensile wave component and causes the pressure to oscillate between tension and compression. For low impurity density (approximately 106 cm-3) the bubbles grown on a long time scale (5 - 10 microseconds) relative to the wave interaction time (approximately 100 nsec). At later times the growing bubbles interact with each other causing pressure oscillations between tension and compression, with an average compression pressure below 1 bar. This effect increases considerably the bubble lifetime consistent with experiments. At the collapse stage small bubbles collapse earlier and induce pressures, which reduce the collapse time of the larger bubbles.