34 research outputs found
Local heat flux and energy loss in a 2D vibrated granular gas
We performed event-driven simulations of a two-dimensional granular gas
between two vibrating walls and directly measured the local heat flux and
energy dissipation rate in the stationary state. Describing the local heat flux
as a function of the coordinate x in the direction perpendicular to the driving
walls, we use a generalization of Fourier's law, q_x(x) = kappa d_x T(x) + mu
d_x rho(x), to relate the local heat flux to the local gradients of the
temperature and density. This ansatz accounts for the fact that density
gradients also generate heat flux, not only temperature gradients. The
transport coefficients kappa and mu are assumed to be independent of x, and we
check the validity of this assumption in the simulations. Both kappa and mu are
determined for different system parameters, in particular, for a wide range of
coefficients of restitution. We also compare our numerical results to existing
hydrodynamic theories. Agreement is found for kappa for very small
inelasticities only. Beyond this region, kappa and mu exhibit a striking
non-monotonic behavior.Comment: 8 pages, 5 figure
A driven two-dimensional granular gas with Coulomb friction
We study a homogeneously driven granular gas of inelastic hard particles with
rough surfaces subject to Coulomb friction. The stationary state as well as the
full dynamic evolution of the translational and rotational granular
temperatures are investigated as a function of the three parameters of the
friction model. Four levels of approximation to the (velocity-dependent)
tangential restitution are introduced and used to calculate translational and
rotational temperatures in a mean field theory. When comparing these
theoretical results to numerical simulations of a randomly driven mono-layer of
particles subject to Coulomb friction, we find that already the simplest model
leads to qualitative agreement, but only the full Coulomb friction model is
able to reproduce/predict the simulation results quantitatively for all
magnitudes of friction. In addition, the theory predicts two relaxation times
for the decay to the stationary state. One of them corresponds to the
equilibration between the translational and rotational degrees of freedom. The
other one, which is slower in most cases, is the inverse of the common
relaxation rate of translational and rotational temperatures.Comment: 23 pages, 17 figure
Local Equation of State and Velocity Distributions of a Driven Granular Gas
We present event-driven simulations of a granular gas of inelastic hard disks
with incomplete normal restitution in two dimensions between vibrating walls
(without gravity). We measure hydrodynamic quantities such as the stress
tensor, density and temperature profiles, as well as velocity distributions.
Relating the local pressure to the local temperature and local density, we
construct a local constitutive equation. For strong inelasticities the local
constitutive relation depends on global system parameters, like the volume
fraction and the aspect ratio. For moderate inelasticities the constitutive
relation is approximately independent of the system parameters and can hence be
regarded as a local equation of state, even though the system is highly
inhomogeneous with heterogeneous temperature and density profiles arising as a
consequence of the energy injection. Concerning the local velocity
distributions we find that they do not scale with the square root of the local
granular temperature. Moreover the high-velocity tails are different for the
distribution of the x- and the y-component of the velocity, and even depend on
the position in the sample, the global volume fraction, and the coefficient of
restitution.Comment: 14 pages, 14 figures of which Figs. 13a-f and Fig. 14 are archived as
separate .gif files due to upload-size limitations. A version of the paper
including all figures in better quality can be downloaded at
http://www.theorie.physik.uni-goettingen.de/~herbst/download/LocEqSt.ps.gz
(3.8 MB, ps.gz) or at
http://www.theorie.physik.uni-goettingen.de/~herbst/download/LocEqSt.pdf (4.9
MB, pdf
Concerns about anti-angiogenic treatment in patients with glioblastoma multiforme
BACKGROUND: The relevance of angiogenesis inhibition in the treatment of glioblastoma multiforme (GBM) should be considered in the unique context of malignant brain tumours. Although patients benefit greatly from reduced cerebral oedema and intracranial pressure, this important clinical improvement on its own may not be considered as an anti-tumour effect. DISCUSSION: GBM can be roughly separated into an angiogenic component, and an invasive or migratory component. Although this latter component seems inert to anti-angiogenic therapy, it is of major importance for disease progression and survival. We reviewed all relevant literature. Published data support that clinical symptoms are tempered by anti-angiogenic treatment, but that tumour invasion continues. Unfortunately, current imaging modalities are affected by anti-angiogenic treatment too, making it even harder to define tumour margins. To illustrate this we present MRI, biopsy and autopsy specimens from bevacizumab-treated patients. Moreover, while treatment of other tumour types may be improved by combining chemotherapy with anti-angiogenic drugs, inhibiting angiogenesis in GBM may antagonise the efficacy of chemotherapeutic drugs by normalising the blood-brain barrier function. SUMMARY: Although angiogenesis inhibition is of considerable value for symptom reduction in GBM patients, lack of proof of a true anti-tumour effect raises concerns about the place of this type of therapy in the treatment of GBM
Towards long-term standardised carbon and greenhouse gas observations for monitoring Europe's terrestrial ecosystems : a review
Research infrastructures play a key role in launching a new generation of integrated long-term, geographically distributed observation programmes designed to monitor climate change, better understand its impacts on global ecosystems, and evaluate possible mitigation and adaptation strategies. The pan-European Integrated Carbon Observation System combines carbon and greenhouse gas (GHG; CO2, CH4, N2O, H2O) observations within the atmosphere, terrestrial ecosystems and oceans. High-precision measurements are obtained using standardised methodologies, are centrally processed and openly available in a traceable and verifiable fashion in combination with detailed metadata. The Integrated Carbon Observation System ecosystem station network aims to sample climate and land-cover variability across Europe. In addition to GHG flux measurements, a large set of complementary data (including management practices, vegetation and soil characteristics) is collected to support the interpretation, spatial upscaling and modelling of observed ecosystem carbon and GHG dynamics. The applied sampling design was developed and formulated in protocols by the scientific community, representing a trade-off between an ideal dataset and practical feasibility. The use of open-access, high-quality and multi-level data products by different user communities is crucial for the Integrated Carbon Observation System in order to achieve its scientific potential and societal value.Peer reviewe
The kinase inhibitor D11 induces caspase-mediated cell death in cancer cells resistant to chemotherapeutic treatment
Eigenschaften stationärer getriebener granularer Gase
In der vorliegenden Dissertation geht es um getriebene granulare Gase in zwei Dimensionen. Zunächst werden Systeme glatter Kugeln zwischen zwei vibrierenden Wänden betrachtet. Solche Systeme sind immer inhomogen. Mit Hilfe ereignisgesteuerter Molekulardynamik-Simulationen messen wir alle hydrodynamischen Felder für einen großen Bereich von Systemparametern. Durch Verknüpfen des lokal gemessenen Drucks mit der lokalen Dichte und Temperatur erhalten wir eine konstitutive Gleichung. Diese kann für Systeme, die nicht allzu inelastisch sind, als Zustandgleichung aufgefasst werden, während sie für stärker inelastische Systeme von den globalen Systemparametern abhängt. Außerdem überprüfen wir eine Verallgemeinerung des Fourier'schen Wärmeleitungsgesetzes, das den lokalen Wärmestrom als Funktion der lokalen Dichte- und Temperaturgradienten ausdrückt. Dies funktioniert gut für mittlere Inelastizitäten, solange die Systeme nicht allzu inhomogen sind. Außerdem zeigen wir, dass die räumlich aufgelöste Geschwindigkeitsverteilung der Teilchen nicht durch ein Skalengesetz beschrieben werden kann, sondern dass das Verhalten nicht-universell ist. Insbesondere unterscheidet sich die Verteilungsfunktion für die Geschwindigkeitskomponente senkrecht zur Wand von der für die Komponente parallel zur Wand, und hohe Geschwindigkeiten werden viel häufiger beobachtet, als es eine Maxwell-Boltzmann-Verteilung voraussagen würde. Zudem finden sich charakteristische Merkmale des Treibens in der Wahrscheinlichkeitsverteilung für hohe Geschwindigkeiten in der Mitte des Systems wieder. Zum Schluss werden homogen getriebene Systeme rauer Kugeln mit Coulomb'scher Reibung präsentiert. Für diese Systeme führen wir eine analytische Berechnung der zeitlichen Entwicklung der granularen Translations- und Rotationstemperaturen in Molekularfeldnäherung durch und beobachten eine gute Übereinstimmung mit Simulationen
Modelling particulate self-healing materials and application to uni-axial compression
Using an advanced history dependent contact model for DEM simulations, including elasto-plasticity, viscosity, adhesion, and friction, pressure-sintered tablets are formed from primary particles. These tablets are subjected to unconfined uni-axial compression until and beyond failure. For fast and slow deformation we observe ductile-like and brittle softening, respectively. We propose a model for local self-healing that allows damage to heal during loading such that the material strength of the sample increases and failure/softening is delayed to larger strains. Local healing is achieved by increasing the (attractive) contact adhesion forces for those particles involved in a potentially breaking contact. We examine the dependence of the strength of the material on (a) the damage detection sensitivity, (b) the damage detection rate, and (c) the (increased) adhesion between healed contacts. The material strength is enhanced, i.e., the material fails at larger strains and reaches larger maximal stress values, when any of the parameters (a)–(c) is increased. For very large adhesion between the healed contacts an interesting instability with strong (brittle) fluctuations of the healed material’s strength is observed