5,450 research outputs found
Gas Bubbles Emerging from a Submerged Granular Bed
This fluid dynamics video was submitted to the Gallery of Fluid Motion for
the 2009 APS Division of Fluid Dynamics Meeting in Minneapolis, Minnesota. In
this video we show some results from a simple experiment where air was injected
by a single nozzle at known constant flow rates in the bottom of a granular bed
submerged in water. The injected air propagates through the granular bed in one
of two modes. Mode 1 emergence involves small discrete bubbles taking tortuous
paths through the interstitial space of the bed. Multiple small bubbles can be
emitted from the bed in an array of locations at the same time during Mode 1
emergence. Mode 2 emergence involves large discrete bubbles locally fluidizing
the granular bed and exiting the bed approximately above the injection site.
Bead diameter, bead density, and air flow rate were varied to investigate the
change in bubble release behavior at the top of the granular bed.
This system is a useful model for methane seeps in lakes. Methane bubbles are
released from the decomposition of organic matter in the lake bed. The initial
size of the bubble determines how much of the gas is absorbed into the lake and
how much of the gas reaches the surface and is released into the atmosphere.
The size and behavior of the emerging bubbles may also affect the amount of
vertical mixing occurring in the lake, as well as the mixing from the lake bed
into the benthic layer.Comment: 2009 APS DFD Gallery of Fluid Motion Submissio
Thermophysical Phenomena in Metal Additive Manufacturing by Selective Laser Melting: Fundamentals, Modeling, Simulation and Experimentation
Among the many additive manufacturing (AM) processes for metallic materials,
selective laser melting (SLM) is arguably the most versatile in terms of its
potential to realize complex geometries along with tailored microstructure.
However, the complexity of the SLM process, and the need for predictive
relation of powder and process parameters to the part properties, demands
further development of computational and experimental methods. This review
addresses the fundamental physical phenomena of SLM, with a special emphasis on
the associated thermal behavior. Simulation and experimental methods are
discussed according to three primary categories. First, macroscopic approaches
aim to answer questions at the component level and consider for example the
determination of residual stresses or dimensional distortion effects prevalent
in SLM. Second, mesoscopic approaches focus on the detection of defects such as
excessive surface roughness, residual porosity or inclusions that occur at the
mesoscopic length scale of individual powder particles. Third, microscopic
approaches investigate the metallurgical microstructure evolution resulting
from the high temperature gradients and extreme heating and cooling rates
induced by the SLM process. Consideration of physical phenomena on all of these
three length scales is mandatory to establish the understanding needed to
realize high part quality in many applications, and to fully exploit the
potential of SLM and related metal AM processes
Managing in the regulatory thicket: regulation legitimacy and expertise
While the influence of government regulation on organizations is undeniable, empirical research in this field is scarce. This study investigates how the understanding of and attitudes towards government regulation among public, nonprofit, and for-profit managers affect organizational performance, using U.S. nursing homes as the empirical setting. Our findings suggest that managers’ perceptions of regulation legitimacy – views of regulation fairness, inspectors’ effectiveness, and internal utility of the mandates – positively affect service quality. Sub-group analysis suggests that the managers’ views of regulation matter in nonprofit and for-profit, but not public organizations. In nonprofit homes, performance declines when managers report higher regulatory expertise – better knowledge of the regulatory standards. In for-profit facilities, frequent communication with regulators lowers quality. These findings suggest that the regulated entities’ views of government regulation are central to their success, which necessitates improvements in the regulatory process
A novel smoothed particle hydrodynamics formulation for thermo-capillary phase change problems with focus on metal additive manufacturing melt pool modeling
Laser-based metal processing including welding and three dimensional
printing, involves localized melting of solid or granular raw material, surface
tension-driven melt flow and significant evaporation of melt due to the applied
very high energy densities. The present work proposes a weakly compressible
smoothed particle hydrodynamics formulation for thermo-capillary phase change
problems involving solid, liquid and gaseous phases with special focus on
selective laser melting, an emerging metal additive manufacturing technique.
Evaporation-induced recoil pressure, temperature-dependent surface tension and
wetting forces are considered as mechanical interface fluxes, while a Gaussian
laser beam heat source and evaporation-induced heat losses are considered as
thermal interface fluxes. A novel interface stabilization scheme is proposed,
which is shown to allow for a stable and smooth liquid-gas interface by
effectively damping spurious interface flows as typically occurring in
continuum surface force approaches. Moreover, discretization strategies for the
tangential projection of the temperature gradient, as required for the discrete
Marangoni forces, are critically reviewed. The proposed formulation is deemed
especially suitable for modeling of the melt pool dynamics in metal additive
manufacturing because the full range of relevant interface forces is considered
and the explicit resolution of the atmospheric gas phase enables a consistent
description of pore formation by gas inclusion. The accuracy and robustness of
the individual model and method building blocks is verified by means of several
selected examples in the context of the selective laser melting process
Novel Simulation-Inspired Roller Spreading Strategies for Fine and Highly Cohesive Metal Powders
When fine powders are to be used in powder bed metal additive manufacturing
(AM), a roller is typically utilized for spreading. However, the cohesive
nature of fine metal powder still presents challenges, resulting in low density
and/or inconsistent layers under sub-standard spreading conditions. Here,
through computational parameter studies with an integrated discrete
element-finite element (DEM-FEM) framework, we explore roller-based strategies
that are predicted to achieve highly cohesive powder layers. The exemplary
feedstock is a Ti-6Al-4V 0-20 um powder, that is emulated using a
self-similarity approach based on experimental calibration. The computational
studies explore novel roller kinematics including counter-rotation as well as
angular and transverse oscillation applied to standard rigid rollers as well as
coated rollers with compliant or non-adhesive surfaces. The results indicate
that most of these approaches allow to successfully spread highly cohesive
powders with high packing fraction (between 50%-60% in a single layer) and
layer uniformity provided that the angular/oscillatory, relative to the
transverse velocity, as well as the surface friction of the roller are
sufficiently high. Critically, these spreading approaches are shown to be very
robust with respect to varying substrate conditions (simulated by means of a
decrease in surface energy), which are likely to occur in LBPF or BJ, where
substrate characteristics are the result of a complex multi-physics (i.e.,
powder melting or binder infiltration) process. In particular, the combination
of the identified roller kinematics with compliant surface coatings, which are
known to reduce the risk of tool damage and particle streaking in the layers,
is recommended for future experimental investigation
Laboratory Scale Testing of Ignition Overpressure for Space Vehicle Launch Pad Environments
A scale model of a NASA representative space vehicle is used to develop a refined estimate of the transient pressure loads that are expected to form at the base of the vehicle in the event of a vapor cloud explosion. Flammable vapor clouds are known to form prior to engine startup due to the significant amount of unburned hydrogen that is ejected from the combustion chamber. In the event of a vapor cloud explosion, the vehicle and payload must be able to withstand the resulting overpressure waves. The study comprises an array of pressure sensors located along the base heat shield of the scale model space vehicle as well as the interior wall and throat plug plane of the solid rocket booster. A spark source generator is used to simulate the overpressure wave produced by a vapor cloud explosion while measurements are acquired with and without the effect of a mobile launcher. Time- resolved schlieren images of the simulated vapor cloud explosion reveal the path and impact of both the initial wave and several reflected waves on the various components at the base of the space vehicle. In some instances, the reflected waves superpose to create waves that are higher in amplitude than the initial overpressure wave. A time frequency analysis of the pressure waveforms measured inside the solid rocket booster reveal a ring down tone corresponding to a standing wave that is four times the length of the nozzle
The integral cohomology of the group of loops
Let PSigma_n denote the group that can be thought of either as the group of
motions of the trivial n-component link or the group of symmetric automorphisms
of a free group of rank n. The integral cohomology ring of PSigma_n is
determined, establishing a conjecture of Brownstein and Lee.Comment: This is the version published by Geometry & Topology on 11 July 200
Leaf Litter Decomposition and Nutrient Dynamics in Four Southern Forested Floodplain Communities
Decomposition of site-specific litter mixtures was monitored for 100 wk in four Roodplaht communities: (i) a mixed oak community along the Cache River in central Arkansas, (ii) s sweetgum (Liquidambar styracijlua L.)-cherrybark oak (Quercus falcata var. pagodaefolia ELI.) community along Iatt Creek in central Louisiana, (iii) a sweetgum-swamp tupelo [Nyssa sylvatica var. biflora (Walt.) Sarg.] community, and (iv) a laurel oak (Quercus laurifolia Michx.) commnnityalong the Coosawhatchie River in southeastern South Carolina. Soil temperature, hydroperiod, and litter quality (C:N, C:P, N:P, fignin: N) were used to interpret differences in the rates of mass loss and nutrient dynamics. After 100 wk, litter mixtures retained 33, 18, 8, and 5% of original mass on the Cache, Coosawhatchie (laurel oak community), Coosawhatchie (sweetgum-swamp tupelo community), and Iatt floodplains, respectively, and these differences appeared related to hydroperiod. Decay rates were comparable to rates reportedin similar floodplain environments. Net mineralization of both N and P was observed after 100 wk, but both elements accumulated in litter mixtures periodically. Differences in hydroperiod were observed among the four floodplain communities and decomposition of and nutrient mineralization from litter among them appeared to be inversely related to the number and duration of flood events. Litterbags containing leaf litter of a single-species (i.e., cherrybark oak) were also monitored on three of the four sites to compare decay rates and nutrient dynamics with the litter mixtures. On the Cache River floodplain, slower decay of poorer quality cherrybark oak litter suggested that titter quality drove decomposition under similar edaphic conditions
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