9,203 research outputs found
Numerical thermo-elasto-plastic analysis of residual stresses on different scales during cooling of hot forming parts
In current research, more and more attention is paid to the understanding of residual stress states as well as the application of targeted residual stresses to extend e.g. life time or stiffness of a part. In course of that, the numerical simulation and analysis of the forming process of components, which goes along with the evolution of residual stresses, play an important role. In this contribution, we focus on the residual stresses arising from the austenite-to-martensite transformation at microscopic and mesoscopic level of a Cr-alloyed steel. A combination of a Multi-Phase-Field model and a two-scale Finite Element simulation is utilized for numerical analysis. A first microscopic simulation considers the lattice change, such that the results can be homogenized and applied on the mesoscale. Based on this result, a polycrystal consisting of a certain number of austenitic grains is built and the phase transformation from austenite to martensite is described with respect to the mesoscale. Afterwards, in a two-scale Finite Element simulation the plastic effects are considered and resulting residual stress states are computed
A bright nanowire single photon source based on SiV centers in diamond
The practical implementation of many quantum technologies relies on the
development of robust and bright single photon sources that operate at room
temperature. The negatively charged silicon-vacancy (SiV-) color center in
diamond is a possible candidate for such a single photon source. However, due
to the high refraction index mismatch to air, color centers in diamond
typically exhibit low photon out-coupling. An additional shortcoming is due to
the random localization of native defects in the diamond sample. Here we
demonstrate deterministic implantation of Si ions with high conversion
efficiency to single SiV- centers, targeted to fabricated nanowires. The
co-localization of single SiV- centers with the nanostructures yields a ten
times higher light coupling efficiency than for single SiV- centers in bulk
diamond. This enhanced photon out-coupling, together with the intrinsic
scalability of the SiV- creation method, enables a new class of devices for
integrated photonics and quantum science.Comment: 15 pages, 5 figure
Analysis, Test and Simulation of Landing System Touchdown Dynamics
Future exploration missions pose demanding requirements towards the access by vehicles to scientifically interesting sites on planetary surfaces. These stem particularly from the need of more flexibility in site selection, improved payload to vehicle mass ratios and higher mission success probabilities.
The Landing Technology group of the DLR Institute of Space Systems is focusing on the development and verification of experimental and analytical methods for the investigation of the touchdown dynamics of landing system, its capabilities the embedding into the landing site assessment.
Core element for the experimental investigation is the Landing & Mobility Test Facility (LAMA), which allows touchdown testing under Earth gravity and under a reduced gravitational environment using an active off-loading device. The test article for investigation of legged landing systems is a modular Lander Engineering Model (LEM) designed by the Astrium ST (Bremen), representing today's European mission scenarios to the Moon and Mars such as the ESA Lunar Lander or the ESA Mars Precision Lander. Another test object recently under retesting is the Rosetta lander Philae representing a touch down system concept developed for small body landings.
Usually not all relevant environmental properties of the target landing site can be provided in one single and complete test, any verification approach has to be supported by adequate numerical analyses. Thus, another key topic for the verification of the touchdown performance of a landing system is the accurate analytical and numerical representation of the flight system, its touchdown conditions and the landing site. In this area the research focuses on the development of high fidelity engineering simulations of the vehicle-to-terrain/soil interaction.
The landing site characterization and assessment focuses on the development of landing site assessment methods and tools and to provide terrain models for engineering simulations (both touchdown dynamics and/or hazard detection& avoidance simulations). In return landing system performance limits are mapped onto cartographic landing site representations to support the landing safety assessment.
This poster outlines the test facility, simulation and analysis tools developed by the working group and used in recent landing missions
Generic Modal Cut Elimination Applied to Conditional Logics
We develop a general criterion for cut elimination in sequent calculi for
propositional modal logics, which rests on absorption of cut, contraction,
weakening and inversion by the purely modal part of the rule system. Our
criterion applies also to a wide variety of logics outside the realm of normal
modal logic. We give extensive example instantiations of our framework to
various conditional logics. For these, we obtain fully internalised calculi
which are substantially simpler than those known in the literature, along with
leaner proofs of cut elimination and complexity. In one case, conditional logic
with modus ponens and conditional excluded middle, cut elimination and
complexity were explicitly stated as open in the literature
The Parkes HI Zone of Avoidance Survey
A blind HI survey of the extragalactic sky behind the southern Milky Way has
been conducted with the multibeam receiver on the 64-m Parkes radio telescope.
The survey covers the Galactic longitude range 212 < l < 36 and Galactic
latitudes |b| < 5, and yields 883 galaxies to a recessional velocity of 12,000
km/s. The survey covers the sky within the HIPASS area to greater sensitivity,
finding lower HI-mass galaxies at all distances, and probing more completely
the large-scale structures at and beyond the distance of the Great Attractor.
Fifty-one percent of the HI detections have an optical/NIR counterpart in the
literature. A further 27% have new counterparts found in existing, or newly
obtained, optical/NIR images. The counterpart rate drops in regions of high
foreground stellar crowding and extinction, and for low-HI mass objects. Only
8% of all counterparts have a previous optical redshift measurement. A notable
new galaxy is HIZOA J1353-58, a possible companion to the Circinus galaxy.
Merging this catalog with the similarly-conducted northern extension (Donley et
al. 2005), large-scale structures are delineated, including those within the
Puppis and Great Attractor regions, and the Local Void. Several
newly-identified structures are revealed here for the first time. Three new
galaxy concentrations (NW1, NW2 and NW3) are key in confirming the diagonal
crossing of the Great Attractor Wall between the Norma cluster and the CIZA
J1324.7-5736 cluster. Further contributors to the general mass overdensity in
that area are two new clusters (CW1 and CW2) in the nearer Centaurus Wall, one
of which forms part of the striking 180 deg (100/h Mpc) long filament that
dominates the southern sky at velocities of ~3000 km/s, and the suggestion of a
further Wall at the Great Attractor distance at slightly higher longitudes.Comment: Published in Astronomical Journal 9 February 2016 (accepted 26
September 2015); 42 pages, 7 tables, 18 figures, main figures data tables
only available in the on-line version of journa
Three-loop matching coefficients for hot QCD: Reduction and gauge independence
We perform an integral reduction for the 3-loop effective gauge coupling and
screening mass of QCD at high temperatures, defined as matching coefficients
appearing in the dimensionally reduced effective field theory (EQCD).
Expressing both parameters in terms of a set master (sum-) integrals, we show
explicit gauge parameter independence. The lack of suitable methods for solving
the comparatively large number of master integrals forbids the complete
evaluation at the moment. Taking one generic class of masters as an example, we
highlight the calculational techniques involved. The full result would allow to
improve on one of the classic probes for the convergence of the weak-coupling
expansion at high temperatures, namely the comparison of full and effective
theory determinations of the spatial string tension. Furthermore, the full
result would also allow to determine one new contribution of order O(g**7) to
the pressure of hot QCD.Comment: 19 pages, 2 figures. v2: new Section 6 discussing applications, to
match journal versio
Reasoning with global assumptions in arithmetic modal logics
We establish a generic upper bound ExpTime for reasoning with global assumptions in coalgebraic modal logics. Unlike earlier results of this kind, we do not require a tractable set of tableau rules for the in- stance logics, so that the result applies to wider classes of logics. Examples are Presburger modal logic, which extends graded modal logic with linear inequalities over numbers of successors, and probabilistic modal logic with polynomial inequalities over probabilities. We establish the theoretical upper bound using a type elimination algorithm. We also provide a global caching algorithm that offers potential for practical reasoning
Phase-dependent light propagation in atomic vapors
Light propagation in an atomic medium whose coupled electronic levels form a
diamond-configuration exhibits a critical dependence on the input conditions.
In particular, the relative phase of the input fields gives rise to
interference phenomena in the electronic excitation whose interplay with
relaxation processes determines the stationary state. We integrate numerically
the Maxwell-Bloch equations and observe two metastable behaviors for the
relative phase of the propagating fields corresponding to two possible
interference phenomena. These phenomena are associated to separate types of
response along propagation, minimize dissipation, and are due to atomic
coherence. These behaviors could be studied in gases of isotopes of
alkali-earth atoms with zero nuclear spin, and offer new perspectives in
control techniques in quantum electronics.Comment: 16 pages, 11 figures, v2: typos corrected, v3: final version, to
appear in Phys. Rev.
Second order analysis of geometric functionals of Boolean models
This paper presents asymptotic covariance formulae and central limit theorems
for geometric functionals, including volume, surface area, and all Minkowski
functionals and translation invariant Minkowski tensors as prominent examples,
of stationary Boolean models. Special focus is put on the anisotropic case. In
the (anisotropic) example of aligned rectangles, we provide explicit analytic
formulae and compare them with simulation results. We discuss which information
about the grain distribution second moments add to the mean values.Comment: Chapter of the forthcoming book "Tensor Valuations and their
Applications in Stochastic Geometry and Imaging" in Lecture Notes in
Mathematics edited by Markus Kiderlen and Eva B. Vedel Jensen. (The second
version mainly resolves minor LaTeX problems.
On the Response of an OST to a Point-like Heat Source
A new technique of superconducting cavity diagnostics has been introduced by
D. Hartrill at Cornell University, Ithaca, USA. Oscillating Superleak
Transducers (OST) detect the heat transferred from a cavity's quench point via
"Second Sound" through the superfluid He bath, needed to cool the
superconducting cavity. The observed response of an OST is a complex, but
reproducible pattern of oscillations. A small helium evaporation cryostat was
built which allows the investigation of the response of an OST in greater
detail. The distance between a point-like electrical heater and the OST can be
varied. The OST can be mounted either parallel or perpendicular to the plate,
housing the heat source. If the artificial quench-point releases an amount of
energy compatible to a real quench spot on a cavity's surface, the OST signal
starts with a negative pulse, which is usually strong enough to allow automatic
detection. Furthermore, the reflection of the Second Sound on the wall is
observed. A reflection coefficient R = 0.39 +- 0.05 of the glass wall is
measured. This excludes a strong influence of multiple reflections in the
complex OST response. Fourier analyses show three main frequencies, found in
all OST spectra. They can be interpreted as modes of an oscillating circular
membrane.Comment: 10 pages, 16 figure
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