479 research outputs found
An evaluation of the ATM man/machine interface. Phase 3: Analysis of SL-3 and SL-4 data
The functional adequacy of human factored crew operated systems under operational zero-gravity conditions is considered. Skylab ATM experiment operations generated sufficient telemetry and voice transcript data to support such an assessment effort. Discussions are presented pertaining to the methodology and procedures used to evaluate the hardware, training and directive aspects of Skylab 3 and Skylab 4 manned ATM experiment operations
Ratcheting of granular materials
We investigate the quasi-static mechanical response of soils under cyclic
loading using a discrete model of randomly generated convex polygons. This
response exhibits a sequence of regimes, each one characterized by a linear
accumulation of plastic deformation with the number of cycles. At the grain
level, a quasi-periodic ratchet-like behavior is observed at the contacts,
which excludes the existence of an elastic regime. The study of this slow
dynamics allows to explore the role of friction in the permanent deformation of
unbound granular materials supporting railroads and streets.Comment: Changed content Submitted to Physical Review Letter
Influence of Model Accuracy on Load and Resistance Factor
Risk and Reliability in Geotechnical Engineerin
Bounds on the shear load of cohesionless granular matter
We characterize the force state of shear-loaded granular matter by relating
the macroscopic stress to statistical properties of the force network. The
purely repulsive nature of the interaction between grains naturally provides an
upper bound for the sustainable shear stress, which we analyze using an
optimization procedure inspired by the so-called force network ensemble. We
establish a relation between the maximum possible shear resistance and the
friction coefficient between individual grains, and find that anisotropies of
the contact network (or the fabric tensor) only have a subdominant effect.
These results can be considered the hyperstatic limit of the force network
ensemble and we discuss possible implications for real systems. Finally, we
argue how force anisotropies can be related quantitatively to experimental
measurements of the effective elastic constants.Comment: 17 pages, 6 figures. v2: slightly rearranged, introduction and
discussion rewritte
Stress and Strain in Flat Piling of Disks
We have created a flat piling of disks in a numerical experiment using the
Distinct Element Method (DEM) by depositing them under gravity. In the
resulting pile, we then measured increments in stress and strain that were
associated with a small decrease in gravity. We first describe the stress in
terms of the strain using isotropic elasticity theory. Then, from a
micro-mechanical view point, we calculate the relation between the stress and
strain using the mean strain assumption. We compare the predicted values of
Young's modulus and Poisson's ratio with those that were measured in the
numerical experiment.Comment: 9 pages, 1 table, 8 figures, and 2 pages for captions of figure
Force transmission in a packing of pentagonal particles
We perform a detailed analysis of the contact force network in a dense
confined packing of pentagonal particles simulated by means of the contact
dynamics method. The effect of particle shape is evidenced by comparing the
data from pentagon packing and from a packing with identical characteristics
except for the circular shape of the particles. A counterintuitive finding of
this work is that, under steady shearing, the pentagon packing develops a lower
structural anisotropy than the disk packing. We show that this weakness is
compensated by a higher force anisotropy, leading to enhanced shear strength of
the pentagon packing. We revisit "strong" and "weak" force networks in the
pentagon packing, but our simulation data provide also evidence for a large
class of "very weak" forces carried mainly by vertex-to-edge contacts. The
strong force chains are mostly composed of edge-to-edge contacts with a marked
zig-zag aspect and a decreasing exponential probability distribution as in a
disk packing
Solid behavior of anisotropic rigid frictionless bead assemblies
We investigate the structure and mechanical behavior of assemblies of
frictionless, nearly rigid equal-sized beads, in the quasistatic limit, by
numerical simulation. Three different loading paths are explored: triaxial
compression, triaxial extension and simple shear. Generalizing recent results
[1], we show that the material, despite rather strong finite sample size
effects, is able to sustain a finite deviator stress in the macroscopic limit,
along all three paths, without dilatancy. The shape of the yield surface is
adequately described by a Lade-Duncan (rather than Mohr-Coulomb) criterion.
While scalar state variables keep the same values as in isotropic systems,
fabric and force anisotropies are each characterized by one parameter and are
in one-to-one correspondence with principal stress ratio along all three
loading paths.The anisotropy of the pair correlation function extends to a
distance between bead surfaces on the order of 10% of the diameter. The tensor
of elastic moduli is shown to possess a nearly singular, uniaxial structure
related to stress anisotropy. Possible stress-strain relations in monotonic
loading paths are also discussed
Memory of the Unjamming Transition during Cyclic Tiltings of a Granular Pile
Discrete numerical simulations are performed to study the evolution of the
micro-structure and the response of a granular packing during successive
loading-unloading cycles, consisting of quasi-static rotations in the gravity
field between opposite inclination angles. We show that internal variables,
e.g., stress and fabric of the pile, exhibit hysteresis during these cycles due
to the exploration of different metastable configurations. Interestingly, the
hysteretic behaviour of the pile strongly depends on the maximal inclination of
the cycles, giving evidence of the irreversible modifications of the pile state
occurring close to the unjamming transition. More specifically, we show that
for cycles with maximal inclination larger than the repose angle, the weak
contact network carries the memory of the unjamming transition. These results
demonstrate the relevance of a two-phases description -strong and weak contact
networks- for a granular system, as soon as it has approached the unjamming
transition.Comment: 13 pages, 15 figures, soumis \`{a} Phys. Rev.
Rise and shine: The use of polychromatic short-wavelength-enriched light to mitigate sleep inertia at night following awakening from slow-wave sleep
Sleep inertia is the brief period of performance impairment and reduced alertness experienced after waking, especially from slow-wave sleep. We assessed the efficacy of polychromatic short-wavelength-enriched light to improve vigilant attention, alertness and mood immediately after waking from slow-wave sleep at night. Twelve participants (six female, 23.3 ± 4.2 years) maintained an actigraphy-confirmed sleep schedule of 8.5 hr for 5 nights, and 5 hr for 1 night prior to an overnight laboratory visit. In the laboratory, participants were awakened from slow-wave sleep, and immediately exposed to either dim, red ambient light (control) or polychromatic short-wavelength-enriched light (light) for 1 hr in a randomized crossover design. They completed a 5-min Psychomotor Vigilance Task, the Karolinska Sleepiness Scale, and Visual Analogue Scales of mood at 2, 17, 32 and 47 min after waking. Following this testing period, lights were turned off and participants returned to sleep. They were awakened from their subsequent slow-wave sleep period and received the opposite condition. Compared with the control condition, participants exposed to light had fewer Psychomotor Vigilance Task lapses (χ2[1] = 5.285, p = 0.022), reported feeling more alert (Karolinska Sleepiness Scale: F1,77 = 4.955, p = 0.029; Visual Analogue Scalealert: F1,77 = 8.226, p = 0.005), and reported improved mood (Visual Analogue Scalecheerful: F1,77 = 8.615, p = 0.004). There was no significant difference in sleep-onset latency between conditions following the testing period (t10 = 1.024, p = 0.330). Our results suggest that exposure to polychromatic short-wavelength-enriched light immediately after waking from slow-wave sleep at night may help improve vigilant attention, subjective alertness, and mood. Future studies should explore the potential mechanisms of this countermeasure and its efficacy in real-world environments
Modeling sediment mobilization using a distributed hydrological model coupled with a bank stability model
In addition to surface erosion, stream bank erosion and failure contributes significant sediment and sediment-bound nutrients to receiving waters during high flow events. However, distributed and mechanistic simulation of stream bank sediment contribution to sediment loads in a watershed has not been achieved. Here we present a full coupling of existing distributed watershed and bank stability models and apply the resulting model to the Mad River in central Vermont. We fully coupled the Bank Stability and Toe Erosion Model (BSTEM) with the Distributed Hydrology Soil Vegetation Model (DHSVM) to allow the simulation of stream bank erosion and potential failure in a spatially explicit environment. We demonstrate the model\u27s ability to simulate the impacts of unstable streams on sediment mobilization and transport within a watershed and discuss the model\u27s capability to simulate watershed sediment loading under climate change. The calibrated model simulates total suspended sediment loads and reproduces variability in suspended sediment concentrations at watershed and subbasin outlets. In addition, characteristics such as land use and road-to-stream ratio of subbasins are shown to impact the relative proportions of sediment mobilized by overland erosion, erosion of roads, and stream bank erosion and failure in the subbasins and watershed. This coupled model will advance mechanistic simulation of suspended sediment mobilization and transport from watersheds, which will be particularly valuable for investigating the potential impacts of climate and land use changes, as well as extreme events
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