Variable Bandwidth Filter for Multibeam Echo-sounding Bottom Detection

The accuracy of a seafloor map derived from multibeam swath bathymetry depends first and foremost on the quality of the bottom detection process that yields estimates of the arrival time and angle of bottom echoes received in each beam. Filtering of each beam with a fixed bandwidth filter, with the bandwidth based on the length of the transmitted pulse, reduces the error associated with the time-angle estimates. However, filters of this type can not be optimal over the wide range of operational environments encountered. Better results are obtained with a processing scheme that varies the filter bandwidth across the swath width using detected time and angle information from the previous ping. This method is evaluated using sonar data obtained with a Reson SeaBat 8111ER and the results compared with those obtained using a fixed bandwidth filter

Improving the efficiency of thermoelectric generators by using solar heat concentrators

In this paper, we propose a method of improving the efficiency of thermoelectric generators (TEGs) by using a lens to concentrate heat on the heat source of a TEG. Initial experiments performed using discrete components show about 60mV increase in the amount of voltage generated when using a magnifying lens. Simulation results on the proposed TEG configuration exhibit up to 16% efficiency when the input heat flux is increased to 500 times that of the sun’s heat flux. The effects of varying the thermoelement length, width, and membrane diameter on the TEG’s performance are also characterized. Lastly, plans to fabricate the device on a SOI wafer in the future are presente

Design and modelling of SOI-based solar thermoelectric generators

In this work, solar micro-thermoelectric generators are designed with a lens concentrating solar radiation onto the membrane of a thermoelectric generator (TEG). By focusing solar radiation, the input heat flux increases; leading to an increase in the temperature gradient across the device. Consequently, a significant improvement in the device efficiency can be achieved. The TEG design involves the use of the SOI wafer's device layer as the first thermoelement and aluminum as the second thermoelement. Isolation trenches are also added to the design for electrical insulation. Heat transfer simulations in COMSOL are performed to verify the viability of the proposed system and an analytical model based on energy balance and heat transfer equations is developed to investigate the performance of solar TEGs with varying geometries, lens parameters, and external conditions. It is found that efficiency is improved by increasing both the concentration factor and the absorptance of the TEG membran

Seafloor Characterization from Spatial Variation of Multibeam Backscatter vs. Best Estimated Grazing Angle

Backscatter vs. grazing angle, which can be extracted from multibeam backscatter data, depends on characteristics of the multibeam system and the angular responses of backscatter that are characteristic of different seafloor properties, such as sediment hardness and roughness. Changes in backscatter vs. grazing angle that are contributed by the multibeam system normally remain fixed over both space and time. Therefore, they can readily be determined and removed from backscatter data. The component of backscatter vs. grazing angle due to the properties of sediments varies from location to location, as the sediment changes. The sediment component of variability can be inferred using the redundant observations from different grazing angles in several small sections of seafloor assuming that the sediment property is uniform in any given section of seafloor yet varies from one section of the seafloor to another. The multibeam data used in this research is from the ONR sponsored STRATAFORM project. The location of the study area was the mid-outer continental shelf off New Jersey. A small subset (11 x 17 km) of the NJ multibeam survey was selected and divided into 1380 equal working cells. The backscatter vs. grazing angle dependence for each cell was computed by averaging backscatter data by the corresponding grazing angles using all data with the same grazing angle from different survey lines. Taking into account the effects of local topographic variations of the seabed, the estimated grazing angle for each beam has been computed from available adjacent soundings within a 15-meter radius using a least squares fit with a Butterfly weighting function. A graphic interface was developed to ease evaluation of the spatial variation of backscatter vs. grazing angle. With a mouse click, images based on different subsets of the data can be compared throughout the survey area. The subsets were created from specific grazing angles. These images show significant variations between nadir and off-nadir beams. Variations apparent in the images may provide some indication of the sediment (or seafloor) characteristics, which can be compared to ground truth data (sediment grain size) and measured values such as velocity and density

The nullcone in the multi-vector representation of the symplectic group and related combinatorics

We study the nullcone in the multi-vector representation of the symplectic group with respect to a joint action of the general linear group and the symplectic group. By extracting an algebra over a distributive lattice structure from the coordinate ring of the nullcone, we describe a toric degeneration and standard monomial theory of the nullcone in terms of double tableaux and integral points in a convex polyhedral cone.Comment: 21 pages, v2: title changed, typos and errors correcte

VLT Observations of Turnoff stars in the Globular Cluster NGC 6397

VLT-UVES high resolution spectra of seven turnoff stars in the metal-poor globular cluster NGC 6397 have been obtained. Atmospheric parameters and abundances of several elements (Li, Na, Mg, Ca, Sc, Ti, Cr, Fe, Ni, Zn and Ba) were derived for program stars. The mean iron abundance is [Fe/H] = -2.02, with no star-to-star variation. The mean abundances of the alpha-elements (Ca, Ti) and of the iron-peak elements (Sc, Cr, Ni) are consistent with abundances derived for field stars of similar metallicity. Magnesium is also almost solar, consistent with the values found by Idiart & Th\'evenin (2000) when non-LTE effects (NLTE hereafter) are taken into account. The sodium abundance derived for five stars is essentially solar, but one object (A447) is clearly Na deficient. These results are compatible with the expected abundance range estimated from the stochastic evolutionary halo model by Argast et al. (2000) when at the epoch of [Fe/H] $\sim$ -2 the interstellar medium is supposed to become well-mixed.Comment: to appear in A&

Diffusion-based height analysis reveals robust microswimmer-wall separation

Microswimmers typically move near walls, which can strongly influence their motion. However, direct experimental measurements of swimmer-wall separation remain elusive to date. Here, we determine this separation for model catalytic microswimmers from the height dependence of the passive component of their mean-squared displacement. We find that swimmers exhibit "ypsotaxis", a tendency to assume a fixed height above the wall for a range of salt concentrations, swimmer surface charges, and swimmer sizes. Our findings indicate that ypsotaxis is activity-induced, posing restrictions on future modeling of their still debated propulsion mechanism

Thermal-Mechanical Properties of Polyurethane-Clay Shape Memory Polymer Nanocomposites

Shape memory nanocomposites of polyurethane (PU)-clay were fabricated by melt mixing of PU and nano-clay. Based on nano-indentation and microhardness tests, the strength of the nanocomposites increased dramatically as a function of clay content, which is attributed to the enhanced nanoclay–polymer interactions. Thermal mechanical experiments demonstrated good mechanical and shape memory effects of the nanocomposites. Full shape memory recovery was displayed by both the pure PU and PU-clay nanocomposites.

The diverse hot gas content and dynamics of optically similar low-mass elliptical galaxies

The presence of hot X-ray emitting gas is ubiquitous in massive early-type galaxies. However, much less is known about the content and physical status of the hot X-ray gas in low-mass ellipticals. In the present paper we study the X-ray gas content of four low-mass elliptical galaxies using archival Chandra X-ray observations. The sample galaxies, NGC821, NGC3379, NGC4278, and NGC4697, have approximately identical K-band luminosities, and hence stellar masses, yet their X-ray appearance is strikingly different. We conclude that the unresolved emission in NGC821 and NGC3379 is built up from a multitude of faint compact objects, such as coronally active binaries and cataclysmic variables. Despite the non-detection of X-ray gas, these galaxies may host low density, and hence low luminosity, X-ray gas components, which undergo a Type Ia supernova (SN Ia) driven outflow. We detect hot X-ray gas with a temperature of kT ~ 0.35 keV in NGC4278, the component of which has a steeper surface brightness distribution than the stellar light. Within the central 50 arcsec (~3.9 kpc) the estimated gas mass is ~3 x 10^7 M_sun, implying a gas mass fraction of ~0.06%. We demonstrate that the X-ray gas exhibits a bipolar morphology in the northeast-southwest direction, indicating that it may be outflowing from the galaxy. The mass and energy budget of the outflow can be maintained by evolved stars and SNe Ia, respectively. The X-ray gas in NGC4697 has an average temperature of kT ~ 0.3 keV, and a significantly broader distribution than the stellar light. The total gas mass within 90 arcsec (~5.1 kpc) is ~2.1 x 10^8 M_sun, hence the gas mass fraction is ~0.4%. Based on the distribution and physical parameters of the X-ray gas, we conclude that it is most likely in hydrostatic equilibrium, although a subsonic outflow may be present.Comment: 14 pages, 8 figures, 3 tables, accepted for publication in Ap

IRFM T_eff calibrations for cluster and field giants in the Vilnius, Geneva, RI(C) and DDO photometric systems

Based on a large sample of disk and halo giant stars, for which accurate effective temperatures derived through the InfraRed Flux Method (IRFM) exist, a calibration of the temperature scale in the Vilnius, Geneva, RI(C) and DDO photometric systems is performed. We provide calibration formulae for the metallicity dependent T_eff vs color relations as well as grids of intrinsic colors and compare them with other calibrations. Photometry, atmospheric parameters and reddening corrections for the stars of the sample have been updated with respect to the original sources in order to reduce the dispersion of the fits. Application of our results to Arcturus leads to an effective temperature in excellent agreement with the value derived from its angular diameter and integrated flux. The effects of gravity on these T_eff vs color relations are also explored by taking into account our previous results for dwarf stars.Comment: Accepted for publication in A&