14,739 research outputs found
Boundary-induced inhomogeneity of particle layers in the solidification of suspensions
When a suspension freezes, a compacted particle layer builds up at the
solidification front with noticeable implications on the freezing process. In a
directional solidification experiment of monodispersed suspensions in thin
samples, we evidence a link between the thickness of this layer and the sample
depth. We attribute it to an inhomogeneity of particle density induced by the
sample plates. A mechanical model enables us to relate it to the layer
thickness with a dependency on the sample depth and to select the distribution
of particle density that yields the best fit to our data. This distribution
involves an influence length of sample plates of about nine particle diameters.
These results clarify the implications of boundaries on suspension freezing.
They may be useful to model polydispersed suspensions since large particles
could play the role of smooth boundaries with respect to small ones.Comment: 16 pages, 13 figure
Modeling of asphalt durability and self-healing with discrete particles method
Asphalt is an important road paving material. Besides an acceptable price, durability, surface conditions (like
roughening and evenness), age-, weather- and traffic-induced failures and degradation are relevant aspects. In the
professional road-engineering branch empirical models are used to describe the mechanical behaviour of the material
and to address large-scale problems for road distress phenomena like rutting, ravelling, cracking and roughness. The
mesoscopic granular nature of asphalt and the mechanics of the bitumen layer between the particles are only partly
involved in this kind of approach. The discrete particle method is a modern tool that allows for arbitrary (self-
)organization of the asphalt meso-structure and for rearrangements due to compaction and cyclic loading. This is of
utmost importance for asphalt during the construction phase and the usage period, in forecasting the relevant distress
phenomena and understand their origin on the grain-, contact-, or molecular scales. Contact models that involve viscoelasticity,
plasticity, friction and roughness are state-of-the art in fields like particle technology and can now be
modified for asphalt and validated experimentally on small samples. The ultimate goal is then to derive micro- and
meso-based constitutive models that can be applied to model behaviour of asphalt pavements on the larger macroscale.
Using the new contact models, damage and crack formation in asphalt and their propagation can be modelled, as
well as compaction. Furthermore, the possibility to trigger self-healing in the material can be investigated from a
micro-mechanical point of view
Thermal evolution and sintering of chondritic planetesimals III. Modelling the heat conductivity of porous chondrite material
The construction of models for the internal constitution and the temporal
evolution of large planetesimals, the parent bodies of chondrites, requires
information on the heat conductivity of the complex mixture of minerals and
iron metal found in chondrites. It is attempted to evaluate the heat
conductivity of a multi-component mineral mixture and granular medium from the
heat conductivities of its mixture components. Random mixtures of solids with
chondritic composition and packings of spheres are numerically generated. The
heat conduction equation is solved in high spatial resolution for a test cube
filled with such matter. From the heat flux through the cube the heat
conductivity of the mixture is derived. The model results for porous material
are consistent with data for compacted sandstone, but are at odds with
measurements for H and L chondrites. The discrepancy is traced back to shock
modification of the currently available meteoritic material by impacts on the
parent body over the last 4.5 Ga. This causes numerous micro-cracks that act as
additional barriers for heat transfer. The void structure in meteorites is
different from that which probably existed in the pristine material of the
parent bodies. The results obtained for the heat conductivity of the pristine
material are used for calculating models for the evolution of the H chondrite
parent body which are fitted to the cooling data of a number of H chondrites.
The fit to the data good.Comment: 19 pages, 8 figures, accepted by Astronomy & Astrophysic
Bio-Based Renewable Additives for Anti-Icing Applications (Phase II)
The performance and impacts of several agro-based anti-icers along with a traditional chloride-based anti-icer (salt brine) were evaluated. A statistical design of experiments (central composite design) was employed for developing anti-icing liquids consisting of cost-competitive chemicals such as agro-based compounds (e.g., Concord grape extract and glycerin), sodium chloride, sodium metasilicate, and sodium formate. The following experimentally obtained parameters were examined as a function of the formulation design: ice-melting capacity at 25°F (â3.9°C), splitting strength of Portland cement mortar samples after 10 freeze-thaw/deicer cycles, corrosion rate of C1010 carbon steel after 24-hour immersion, and impact on asphalt binder stiffness and m-value. One viable formula (âbest performerâ) was tested for thermal properties by measuring its differential scanning calorimetry (DSC) thermograms, the friction coefficient of asphalt pavement treated by this anti-icing formulation (vs. 23 wt.% NaCl and beet juice blend) at 25°F after being applied at 30 gallons per lane mile (1 hour after simulated trafficking and plowing), and other properties (pH, oxygen demand in COD). Laboratory data shed light on the selection and formulation of innovative agro-based snow- and ice-control chemicals that can significantly reduce the costs of winter maintenance operations
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The influences of environmental conditions on source localisation using a single vertical array and their exploitation through ground effect inversion
The performance of microphone arrays outdoors is influenced by the environmental conditions. Numerical simulations indicate that, while horizontal arrays are hardly affected, direction-of-arrival (DOA) estimation with vertical arrays becomes biased in presence of ground reflections and sound speed gradients. Turbulence leads to a huge variability in the estimates by reducing the ground effect. Ground effect can be exploited by combining classical source localization with an appropriate propagation model (ground effect inversion). Not only does this allow the source elevation and range to be determined with a single vertical array but also it allows separation of sources which can no longer be distinguished by far field localization methods. Furthermore, simulations provide detail of the achievable spatial resolution depending on frequency range, array size and localization algorithm and show a clear advantage of broadband processing. Outdoor measurements with one or two sources confirm the results of the numerical simulations
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