Biomedical subjects

Peter B Weichman

Publications and source records attributed to Peter B Weichman.

7 recordsLinked to original sources

PubMed2006

Sonar pulse wave form optimization in cluttered environments.

A theory of active sonar (or radar) pulse wave form design, for optimal target detection in cluttered environments, is presented. The received target signal is maximized via a cost function L that incorporates both the signal-to-noise ratio and a generalization of the Heisenberg uncertainty principle, which is used to balance bandwidth (or range resolution) against signal gain. The optimal pulse wave form is the ground state solution to a one-dimensional Schrödinger-type equation in frequency space, with an effective potential energy that tends to concentrate pulse energy in frequency bands where the target reflectivity dominates the clutter reflectivity.

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PubMed2006

Second-sound spectroscopy of a nonequilibrium superfluid-normal interface.

An experiment is proposed to test a previously developed theory of the hydrodynamics of a nonequilibrium heat current-induced superfluid-normal interface. It is shown that the interfacial "trapped" second-sound mode predicted by the theory leads to a sharp resonant dip in the reflected signal from an external second-sound pulse propagated toward the interface when its horizontal phase speed matches that of the interface mode. The influence of the interface on thermal fluctuations in the bulk superfluid is shown to lead to slow power dependence of the order parameter, and other quantities, on distance from it.

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PubMed2006

Equilibrium theory of coherent vortex and zonal jet formation in a system of nonlinear Rossby waves.

The problem of coherent vortex and zonal jet formation in a system of oceanic or planetary nonlinear Rossby waves is considered from the point of view of the late time steady state achieved by free decay of a given initial state. Statistical equilibrium equations respecting all conservation laws are constructed for a broad class of models, generalizing those derived previously for two-dimensional inviscid Euler flow. Jetlike solutions are ubiquitous, with large coherent vortices existing only when there is a background flow whose velocity locally cancels the beta effect.

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PubMed2005

Doppler effects in heterogeneous media with applications to ocean acoustic modeling.

Doppler shift corrections to ocean acoustic signals are complicated by the multi-spatial-scale structure of the ocean medium, resulting in a multi-time-scale structure of the acoustic Green function. Repeated reflections and refractions lead in general to an infinite number of acoustic paths or modes, with different times of flight, connecting source and receiver. The rate of change of these flight times with source or receiver motion gives rise to Doppler shift corrections, and each acoustic path or mode has a different correction. A clean Doppler correction (in the sense of an observable coherent motion-induced frequency shift for each path or mode) is shown to emerge only when the medium is homogeneous along the direction of source or receiver motion, even when it is highly inhomogeneous in directions orthogonal to the motion. A very general quantitative theory for this correction is developed, encompassing earlier results in the literature, and presented in a form amenable to efficient numerical implementation in data processing.

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PubMed2004

Surface modes and multipower-law structure in the early-time electromagnetic response of magnetic targets.

It was recently demonstrated [Phys. Rev. Lett. 91, 143908 (2003)] that the scattered electric field from highly conducting targets following a rapidly terminated electromagnetic pulse displays a universal t(-1/2) power law divergence at early time. It is now shown that for strongly permeable targets, micro(c)/micro(b)>>1, where micro(b) is the background magnetic permeability, the early-time regime separates into two distinct power law regimes, with the early-early time t(-1/2) behavior crossing over to t(-3/2) at late-early time, reflecting a spectrum of magnetic surface modes. The latter is confirmed by data from ferrous targets where micro(c)/micro(b)=O(10(2)), and for which the early-early-time regime is invisibly narrow.

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PubMed2004

Superfluid transitions in bosonic atom-molecule mixtures near a Feshbach resonance.

We study bosonic atoms near a Feshbach resonance and predict that, in addition to standard normal and atomic superfluid phases, this system generically exhibits a distinct phase of matter: a molecular superfluid, where molecules are superfluid while atoms are not. We explore zero- and finite-temperature properties of the molecular superfluid (a bosonic, strong-coupling analog of a BCS superconductor), and study quantum and classical phase transitions between the normal, molecular superfluid, and atomic superfluid states.

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PubMed2003

Universal early-time response in high-contrast electromagnetic scattering.

The time-domain response of highly conducting targets following a rapidly terminated electromagnetic pulse displays three distinct regimes: early, intermediate, and late time. The intermediate and late times are characterized by a superposition of exponentially decaying eigenmodes. At early time an ever increasing number of rapidly decaying modes contribute, with the result that the scattered electric field displays a universal t(-1/2) power law which emerges from the diffusive decay of a pattern of surface currents induced by the pulse. The power law amplitude reflects the surface geometry of the target, a property that may prove useful in buried target classification in geophysical remote sensing applications.

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