2,038 research outputs found
Regenerator seal design
A rotary regenerator disc matrix has a face seal with a cross arm and arcuate rim segments joined by prestress clamps to prestrain the arcuate rim seals so as to compensate seal rim twisting or coning and resultant disc face seal leakage as produced by operating thermal gradients across the seal
Wind-driven mixed layer and coastal upwelling processes off the south coast of South Africa
Most coastal mixed layer and upwelling investigations have concentrated on the more important fisheries regions situated on the west coasts of the continents, while this paper analyzes conditions on the zonally-orientated south coast of South Africa. Most of the data originate from a short measurement program in the southern summer of 1995, involving both CTD measurements from a small boat and data from a temperature array and coastal sea surface temperature (SST). Limited mixed layer statistics are derived and yield results similar to those of Lentz (1992), although a lack of current measurements precludes any assessment of the Ekman drift. It is shown that the nature of the summer wind forcing is very different to the situation off the South African west coast, with wind variability occurring on much shorter time scales from 2 to 6 days: this has important consequences for plankton blooms and is a likely reason why production is not as efficient off this coast. Past results (Tilney et al., 1996) have shown the importance of coastal trapped waves at depth, but it is apparent that very different conditions occur near the surface. It is found that the coastal SST is a very good monitor of wind-forced coastal upwelling conditions, probably because of the abrupt topography. Substantial baroclinic tidal signals are also apparent at times in the temperature measurements
Rheological and Mechanical Gradient Properties of Polyurethane Elastomers for 3D-Printing with Reactive Additives
Polyurethane (PU) elastomers with their broad range of strength and elasticity are ideal materials for additive manufacturing of shapes with gradients of mechanical properties. By adjusting the mixing ratio of different polyurethane reactants during 3D-printing it is possible to change the mechanical properties. However, to guarantee intra- and inter-layer adhesion, it is essential to know the reaction kinetics of the polyurethane reaction, and to be able to influence the reaction speed in a wide range. In this study, the effect of adding three different catalysts and two inhibitors to the reaction of polyurethane elastomers were studied by comparing the time of crossover points between storage and loss modulus GâČ and GâČâČ from time sweep tests of small amplitude oscillatory shear at 30°C. The time of crossover points is reduced with the increasing amount of catalysts, but only the reaction time with one inhibitor is significantly delayed. The reaction time of 90% NCO group conversion calculated from the FTIR-spectrum also demonstrates the kinetics of samples with different catalysts. In addition, the relation between the conversion as determined from FTIR spectroscopy and the mechanical properties of the materials was established. Based on these results, it is possible to select optimized catalysts and inhibitors for polyurethane 3D-printing of materials with gradients of mechanical properties.DFG, 414044773, Open Access Publizieren 2019 - 2020 / Technische UniversitĂ€t Berli
An overnational cereal circuit for developing locally adapted organic seeds of wheat
On several locations in Germany and Switzerland, new and local varieties of winter wheat were compared to the variety "Bussard" in trial plots with 2-4 replications. Among other parameters, baking quality and gluten content were analised and discussed
Simultaneous NIR/sub-mm observation of flare emission from SgrA*
We report on a successful, simultaneous observation and modeling of the
sub-millimeter to near-infrared flare emission of the Sgr A* counterpart
associated with the super-massive black hole at the Galactic center. Our
modeling is based on simultaneous observations that have been carried out on 03
June, 2008 using the NACO adaptive optics (AO) instrument at the ESO VLT and
the LABOCA bolometer at the APEX telescope. Inspection and modeling of the
light curves show that the sub-mm follows the NIR emission with a delay of
1.5+/-0.5 hours. We explain the flare emission delay by an adiabatic expansion
of the source components.Comment: 12 pages, 9 figures, 3 tables, in press with A&
Coordinated NIR/mm observations of flare emission from Sagittarius A*
We report on a successful, simultaneous observation and modelling of the
millimeter (mm) to near-infrared (NIR) flare emission of the Sgr A* counterpart
associated with the supermassive black hole at the Galactic centre (GC). We
present a mm/sub-mm light curve of Sgr A* with one of the highest quality
continuous time coverages and study and model the physical processes giving
rise to the variable emission of Sgr A*.Comment: 14 pages, 16 figure
Hybrid Thermal-Nonthermal Synchrotron Emission from Hot Accretion Flows
We investigate the effect of a hybrid electron population, consisting of both
thermal and non-thermal particles, on the synchrotron spectrum, image size, and
image shape of a hot accretion flow onto a supermassive black hole. We find two
universal features in the emitted synchrotron spectrum: (i) a prominent
shoulder at low (< 10^11 Hz) frequencies that is weakly dependent on the shape
of the electron energy distribution, and (ii) an extended tail of emission at
high (> 10^13 Hz) frequencies whose spectral slope depends on the slope of the
power-law energy distribution of the electrons. In the low-frequency shoulder,
the luminosity can be up to two orders of magnitude greater than with a purely
thermal plasma even if only a small fraction (< 1%) of the steady-state
electron energy is in the non-thermal electrons. We apply the hybrid model to
the Galactic center source, Sgr A*. The observed radio and IR spectra imply
that at most 1% of the steady-state electron energy is present in a power-law
tail in this source. This corresponds to no more than 10% of the electron
energy injected into the non-thermal electrons and hence 90% into the thermal
electrons. We show that such a hybrid distribution can be sustained in the flow
because thermalization via Coulomb collisions and synchrotron self-absorption
are both inefficient. The presence of non-thermal electrons enlarges the size
of the radio image at low frequencies and alters the frequency dependence of
the brightness temperature. A purely thermal electron distributions produces a
sharp-edged image while a hybrid distribution causes strong limb brightening.
These effects can be seen up to frequencies ~10^11 Hz and are accessible to
radio interferometers.Comment: 33 pages with figures, to appear in the Astrophysical Journa
ALFA: First Operational Experience of the MPE/MPIA Laser Guide Star System for Adaptive Optics
The sodium laser guide star adaptive optics system ALFA has been constructed
at the Calar Alto 3.5-m telescope. Following the first detection of the laser
beacon on the wavefront sensor in 1997 the system is now being optimized for
best performance. In this contribution we discuss the current status of the
launch beam and the planned improvements and upgrades. We report on the
performance level achieved when it is used with the adaptive optics system, and
relate various aspects of our experience during operation of the system. We
have begun to produce scientific results and mention two of these.Comment: 9 pages, 6 figures, LaTeX (spie.sty). SPIE conf proc 3353, Adaptive
Optical System Technologies, March 199
Chemical and Thermal Effects of Trace Components in Hydrogen Rich Gases on Combustion
The production of carbon-neutral fuels through clean energy has been defined as a target by the European Union and by several international institutions. If the concepts are available, hydrogen, in particular, is considered to be one of the most target-oriented, ecologically and economically realizable approaches. In terms of safety, long-time storage and long-distance transport of hydrogen are still under development. However, pipeline systems similar to those for natural gas are being considered. Gas quality criteria will have to be developed for this case. The effects of trace components in the hydrogen on chemical and thermal aspects are not yet sufficiently understood and need to be characterized more precisely. For these reasons, this work presents a detailed analysis for a more complete understanding of the phenomena involved. More specifically, the flame structure, temperature profile and overall reactivity were first determined for gas mixtures analyzed under four varying dilutions of carbon dioxide, carbon oxide, nitrogen and methane in hydrogen. The characterization of the total reactivity and the laminar burning velocity offers an appealing solution to quantify the effects of dilution. The most distinctive effects of the operating conditions on the ignition phenomena have been worked out numerically for the lower and upper boundaries and have been discussed. The results collected in this work provide a robust feature for a detailed evaluation of normal operation as well as the accidental release of the hydrogen-rich fuels
Polarized mm And sub-mm Emission From Sgr A* At The Galactic Center
The recent detection of significant linear polarization at mm and sub-mm
wavelengths in the spectrum of Sgr A* (if confirmed) will be a useful probe of
the conditions within several Schwarzschild radii () of the event horizon
at the Galactic Center. Hydrodynamic simulations of gas flowing in the vicinity
of this object suggest that the infalling gas circularizes when it approaches
within of the black hole. We suggest that the sub-mm ``excess'' of
emission seen in the spectrum of Sgr A* may be associated with radiation
produced within the inner Keplerian region and that the observed polarization
characteristics provide direct evidence for this phenomenon. The overall
spectrum from this region, including the high-energy component due to
bremsstrahlung and inverse Compton scattering processes, is at or below the
recent {\it Chandra} measurement, and may account for the X-ray source if it
turns out to be the actual counterpart to Sgr A*.Comment: 12 pages, 2 figures. published in APJ Letter
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