Acoustic scattering by benthic and planktonic shelled animals

Acoustic backscattering measurements and associated scattering modeling were recently conducted on a type of benthic shelled animal that has a spiral form of shell (Littorina littorea). Benthic and planktonic shelled animals with this shape occur on the seafloor and in the water column, respectively, and can be a significant source of acoustic scattering in the ocean. Modeling of the scattering properties allows reverberation predictions to be made for sonar performance predictions as well as for detection and classification of animals for biological and ecological applications. The studies involved measurements over the frequency range 24 kHz to 1 MHz and all angles of orientation in as small as 1 degree increments. This substantial data set is quite revealing of the physics of the acoustic scattering by these complex shelled bodies and served as a basis for the modeling. Specifically, the resonance structure of the scattering was strongly dependent upon angle of orientation and could be traced to various types of rays (e.g., subsonic Lamb waves and rays entering the opercular opening). The data are analyzed in both the frequency and time domain (compressed pulse processing) so that dominant scattering mechanisms could be identified. Given the complexity of the animal body (irregular elastic shell with discontinuities), approximate scattering models are used with only the dominant scattering properties retained. Two models are applied to the data, both approximating the body as a deformed sphere: (1) an averaged form of the exact modal-series-based solution for the spherical shell, which is used to estimate the backscattering by a deformed shell averaged over all angles of orientation, and produces reasonably accurate predictions over all k1a(esr) (k1 is the acoustic wave number of the surrounding water and a(esr) is the equivalent spherical radius of the body), and (2) a ray-based formula which is used to estimate the scattering at fixed angle of orientation, but only for high k1a(esr). The ray-based model is an extension of a model recently developed for the shelled zooplankton Limacina retroversa that has a shape similar to that of the Littorina littorea but swims through the water [Stanton et al., J. Acoust. Soc. Am. 103, 236-253 (1998b)]. Applications of remote detection and classification of the seafloor and water column in the presence of shelled animals are discussed.     

Comparison between experiment and computer modeling for simple MILOconfigurations

The MILO is a crossed-field HPM, high power microwave source which uses its self-generated magnetic field to cut off electron flow to the anode. A detailed comparison of experimental results and a computer simulation has been made for a number of simple axisymmetric MILO structures designed to operate at 1 GHz. The structures were built from demountable components which enabled the number of cavities and their dimensions to be rapidly altered. Measurements were made of the fluctuating magnetic fields at the end of each cavity. The amplitude and depth of RF modulation of the magnetic fields, although repeatable, changed drastically from one configuration to the next; these parameters were compared with predictions from VIPER, a 2-D electromagnetic PIC code. Good quantitative agreement was obtained between experiments and the simulation in most situations, although, late in the current pulse, after about 100 ns, the level of RF began to decay; a phenomenon which became more pronounced as the applied voltage was increased. The decay was attributed to plasma formation on the cavity vanes and subsequent electron emission; this explanation was verified by computer modeling electron emission and by using vanes make from polished stainless steel in place of aluminum vanes     

Fattening complex manifolds: Curvature and Kodaira—Spencer maps

We present a calculus whereby the curvature of a geometry arising from any generalized twistor correspondence is related to an obstruction-theoretic classification of the infinitesimal neighborhoods of submanifolds of its twistor space. The crux of the argument involves a relation between Kodaira—Spencer maps and the Penrose transform.     

Marine vehicles and offshore installations: Laser diagnostics of machinery health

For reasons of both safety and economy it is desirable to avoid the breakdown at sea of heavy machinery in marine transport and offshore installations. Unscheduled loss of production or power during ‘downtime’ enhances vulnerability and often carries severe financial penalties. This has led to an increasing interest in the concept of monitoring machinery health to detect impending failure. This paper demonstrates how applied optics and laser technology can be used to address the problem of diagnosing failure in viscous shear torsional dampers which are normally fitted to the crankshaft of large marine propulsion diesels and generator sets. The method allows condition diagnosis to be achieved while the engine is running and avoids the need for ‘downtime’ associated with traditional fluid sampling methods.     

Synthesis and mesomorphic behaviour of novel light-emitting liquid crystals

We report the results of a systematic study of the structure-mesomorphic behaviour relationships of a diverse range of light-emitting liquid crystals, but especially nematic 2,7-disubstituted-9,9-dialkylfluorenes. The dependence of the mesomorphic behaviour and transition temperatures on the nature and length of the terminal chains, the nature, position and number of lateral substituents and the number and nature of aromatic rings with and without heteroatoms in the central core is studied. The results of these studies are used to design polymerizable, light-emitting crystals (reactive mesogens) with a nematic phase having a high clearing point and a melting point below room temperature for facile OLED fabrication.     

Infrared spectrometry field-method for identification of natural seep-oils

An infrared field-method has been developed which is capable of distinguishing between oils originating from natural seepage in the Santa Barbara (California) Channel region and closely similar oils from onshore drilling platforms. The technique involves a minimum of sample preparation and the use of simple infrared instrumentation which can be operated by non-technical personnel. Natural seep-oil samples were collected from the surface of the water, underwater, and from beaches in the area. The non-seep oils were obtained from production wells which were located in the same geographical areas as the seepage and were from several different well depths corresponding to different geological zones. Natural seep-oils are more aromatic than the production oils, and this difference is evidenced by observed differences in the spectra for both weathered and unweathered oils. These spectral differences between seep and non-seep oils have been found to persist after exposure to weathering for a week.