4,718 research outputs found
Three-dimensional structure of a low-Reynolds-number turbulent boundary layer
A low-Reynolds-number zero-pressure-gradient incompressible turbulent boundary layer was investigated using a volumetric imaging technique. The Reynolds number based on momentum thickness was 700. The flow was tagged with a passive scalar from two spanwise dye slots to distinguish between fluid motions originating in the inner and outer portions of the boundary layer. The resulting volumetric scalar field was interrogated using a laser sheet scanner developed for this study. Two- and three-dimensional time-dependent visualizations of a 50 volume time series are presented (equivalent to 17δ in length). In the outer portion of the boundary layer, scalar structures were observed to lie along lines in the (x, z)-plane, inclined to the streamwise (x-)direction in the range ±50°. The ejection of brightly dyed fluid packets from the near-wall region was observed to be spatially organized, and related to the passage of the large-scale scalar structures.Carl J Delo, Richard M Kelson and Alexander J Smit
Longitudinal spin-relaxation in nitrogen-vacancy centers in electron irradiated diamond
We present systematic measurements of longitudinal relaxation rates ()
of spin polarization in the ground state of the nitrogen-vacancy (NV) color
center in synthetic diamond as a function of NV concentration and magnetic
field . NV centers were created by irradiating a Type 1b single-crystal
diamond along the [100] axis with 200 keV electrons from a transmission
electron microscope with varying doses to achieve spots of different NV
center concentrations. Values of () were measured for each spot as a
function of .Comment: 4 pages, 8 figure
Consonant and vowel confusion patterns by American English listeners
This study investigated the perception of American English phonemes by native listeners. Listeners identified either the consonant or the vowel in all possible English CV and VC syllables. The syllables were embedded in multispeaker babble at three signal-to-noise ratios (0 dB, 8 dB, and 16 dB). Effects of syllable position, signal-to-noise ratio, and articulatory features on vowel and consonant identification are discussed. The results constitute the largest source of data that is currently available on phoneme confusion patterns of American English phonemes by native listeners
Improving Predictions for Helium Emission Lines
We have combined the detailed He I recombination model of Smits with the
collisional transitions of Sawey & Berrington in order to produce new accurate
helium emissivities that include the effects of collisional excitation from
both the 2 (3)S and 2 (1) S levels. We present a grid of emissivities for a
range of temperature and densities along with analytical fits and error
estimates.
Fits accurate to within 1% are given for the emissivities of the brightest
lines over a restricted range for estimates of primordial helium abundance. We
characterize the analysis uncertainties associated with uncertainties in
temperature, density, fitting functions, and input atomic data. We estimate
that atomic data uncertainties alone may limit abundance estimates to an
accuracy of 1.5%; systematic errors may be greater than this. This analysis
uncertainty must be incorporated when attempting to make high accuracy
estimates of the helium abundance. For example, in recent determinations of the
primordial helium abundance, uncertainties in the input atomic data have been
neglected.Comment: ApJ, accepte
Wall-bounded turbulent flows at high Reynolds numbers: Recent advances and key issues
Wall-bounded turbulent flows at high Reynolds numbers have become an increasingly active area of
research in recent years. Many challenges remain in theory, scaling, physical understanding,
experimental techniques, and numerical simulations. In this paper we distill the salient advances of
recent origin, particularly those that challenge textbook orthodoxy. Some of the outstanding
questions, such as the extent of the logarithmic overlap layer, the universality or otherwise of the
principal model parameters such as the von Kármán “constant,” the parametrization of roughness
effects, and the scaling of mean flow and Reynolds stresses, are highlighted. Research avenues that
may provide answers to these questions, notably the improvement of measuring techniques and the
construction of new facilities, are identified. We also highlight aspects where differences of opinion
persist, with the expectation that this discussion might mark the beginning of their resolution
Friction factors for smooth pipe flow
Friction factor data from two recent pipe flow experiments are combined to provide a comprehensive picture of the friction factor variation for Reynolds numbers from 10 to 36,000,000
Categorization of sounds
The authors conducted 4 experiments to test the decision-bound, prototype, and distribution theories for the categorization of sounds. They used as stimuli sounds varying in either resonance frequency or duration. They created different experimental conditions by varying the variance and overlap of 2 stimulus distributions used in a training phase and varying the size of the stimulus continuum used in the subsequent test phase. When resonance frequency was the stimulus dimension, the pattern of categorization-function slopes was in accordance with the decision-bound theory. When duration was the stimulus dimension, however, the slope pattern gave partial support for the decision-bound and distribution theories. The authors introduce a new categorization model combining aspects of decision-bound and distribution theories that gives a superior account of the slope patterns across the 2 stimulus dimensions
Paradoxical popups: Why are they hard to catch?
Even professional baseball players occasionally find it difficult to
gracefully approach seemingly routine pop-ups. This paper describes a set of
towering pop-ups with trajectories that exhibit cusps and loops near the apex.
For a normal fly ball, the horizontal velocity is continuously decreasing due
to drag caused by air resistance. But for pop-ups, the Magnus force (the force
due to the ball spinning in a moving airflow) is larger than the drag force. In
these cases the horizontal velocity decreases in the beginning, like a normal
fly ball, but after the apex, the Magnus force accelerates the horizontal
motion. We refer to this class of pop-ups as paradoxical because they appear to
misinform the typically robust optical control strategies used by fielders and
lead to systematic vacillation in running paths, especially when a trajectory
terminates near the fielder. In short, some of the dancing around when
infielders pursue pop-ups can be well explained as a combination of bizarre
trajectories and misguidance by the normally reliable optical control strategy,
rather than apparent fielder error. Former major league infielders confirm that
our model agrees with their experiences.Comment: 28 pages, 10 figures, sumitted to American Journal of Physic
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