6,360 research outputs found
Porous squeeze-film flow
The squeeze-film flow of a thin layer of Newtonian fluid filling the gap between a flat impermeable surface moving under a prescribed constant load and a flat thin porous bed coating a stationary flat impermeable surface is considered. Unlike in the classical case of an impermeable bed, in which an infinite time is required for the two surfaces to touch, for a porous bed contact occurs in a finite contact time. Using a lubrication approximation an implicit expression for the fluid layer thickness and an explicit expression for the contact time are obtained and analysed. In addition, the fluid particle paths are calculated, and the penetration depths of fluid particles into the porous bed are determined. In particular, the behaviour in the asymptotic limit of small permeability, in which the contact time is large but finite, is investigated. Finally, the results are interpreted in the context of lubrication in the human knee joint, and some conclusions are drawn about the contact time of the cartilage-coated femoral condyles and tibial plateau and the penetration of nutrients into the cartilage
Squeeze-Film Flow in the Presence of a Thin Porous Bed, with Application to the Human Knee Joint
Motivated by the desire for a better understanding of the lubrication of the human knee joint, the squeeze-film flow of a thin layer of Newtonian fluid (representing the synovial fluid) filling the gap between a flat impermeable surface (representing the femoral condyles) and a flat thin porous bed (representing the articular cartilage) coating a stationary flat impermeable surface (representing the tibial plateau) is considered. As the impermeable surface approaches the porous bed under a prescribed constant load all of the fluid is squeezed out of the gap in a finite contact time. In the context of the knee, the size of this contact time suggests that when a person stands still for a short period of time their knees may be fluid lubricated, but that when they stand still for a longer period of time contact between the cartilage-coated surfaces may occur. The fluid particle paths are calculated, and the penetration depths of fluid particles into the porous bed are determined. In the context of the knee, these penetration depths provide a measure of how far into the cartilage nutrients are carried by the synovial fluid, and suggest that when a person stands still nutrients initially in the fluid layer penetrate only a relatively small distance into the cartilage. However, the model also suggests that the cumulative effect of repeated loading and unloading of the knees during physical activity such as walking or running may be sufficient to carry nutrients deep into the cartilage
The Kinematics of Molecular Cloud Cores in the Presence of Driven and Decaying Turbulence: Comparisons with Observations
In this study we investigate the formation and properties of prestellar and
protostellar cores using hydrodynamic, self-gravitating Adaptive Mesh
Refinement simulations, comparing the cases where turbulence is continually
driven and where it is allowed to decay. We model observations of these cores
in the CO, NH, and NH lines, and from
the simulated observations we measure the linewidths of individual cores, the
linewidths of the surrounding gas, and the motions of the cores relative to one
another. Some of these distributions are significantly different in the driven
and decaying runs, making them potential diagnostics for determining whether
the turbulence in observed star-forming clouds is driven or decaying. Comparing
our simulations with observed cores in the Perseus and Ophiuchus clouds
shows reasonably good agreement between the observed and simulated core-to-core
velocity dispersions for both the driven and decaying cases. However, we find
that the linewidths through protostellar cores in both simulations are too
large compared to the observations. The disagreement is noticably worse for the
decaying simulation, in which cores show highly supersonic infall signatures in
their centers that decrease toward their edges, a pattern not seen in the
observed regions. This result gives some support to the use of driven
turbulence for modeling regions of star formation, but reaching a firm
conclusion on the relative merits of driven or decaying turbulence will require
more complete data on a larger sample of clouds as well as simulations that
include magnetic fields, outflows, and thermal feedback from the protostars.Comment: 18 pages, 12 figures, accepted to A
The Protostellar Luminosity Function
The protostellar luminosity function (PLF) is the present-day luminosity
function of the protostars in a region of star formation. It is determined
using the protostellar mass function (PMF) in combination with a stellar
evolutionary model that provides the luminosity as a function of instantaneous
and final stellar mass. As in McKee & Offner (2010), we consider three main
accretion models: the Isothermal Sphere model, the Turbulent Core model, and an
approximation of the Competitive Accretion model. We also consider the effect
of an accretion rate that tapers off linearly in time and an accelerating star
formation rate. For each model, we characterize the luminosity distribution
using the mean, median, maximum, ratio of the median to the mean, standard
deviation of the logarithm of the luminosity, and the fraction of very low
luminosity objects. We compare the models with bolometric luminosities observed
in local star forming regions and find that models with an approximately
constant accretion time, such as the Turbulent Core and Competitive Accretion
models, appear to agree better with observation than those with a constant
accretion rate, such as the Isothermal Sphere model. We show that observations
of the mean protostellar luminosity in these nearby regions of low-mass star
formation suggest a mean star formation time of 0.30.1 Myr. Such a
timescale, together with some accretion that occurs non-radiatively and some
that occurs in high-accretion, episodic bursts, resolves the classical
"luminosity problem" in low-mass star formation, in which observed protostellar
luminosities are significantly less than predicted. An accelerating star
formation rate is one possible way of reconciling the observed star formation
time and mean luminosity.Comment: 22 pages, 9 figures, accepted to Ap
The HEAO A-2 survey of Abell clusters and the X-ray luminosity function
The HEAO A-2 all sky data base was surveyed for 2-10KeV X-rau emission from the 225 Abell clusters og galaxies listed in Abell's (1958) catalog which are of distance class four or less, and are within the fraction of the sky surveyed completely by Abell. Thirty-two identifications of clusters with X-ray sources were made, for which 2-10 KeV fluxes and 90% error boxes are presented. Twelve of these identification are new. The X-ray luminosity function was derived for this statistically complete sample and the best exponential fit was found to be f(L) = 20.2 x 10 to the minus 8 power exp (-l(44)/1.9) per cu Mpc 2-10KeV. The relationship between X-ray luminosity and richness was examined and a correlation was found for richness classes 0, 1, and 2. The relationship of X-rau luminosity, Bautz-Morgan type, and Rood-Sastry type was examined. It was found that BM type I's and RS type cD and B have the greatest average luminosity. The contribution of clusters to the X-ray background was calculated from the luminosity function and was found to be 5%, and with 90% certainty, less than 8% in the 2-10 KeV band pass
The Direct Sensing of Damage to Ion Implanted Materials
Material damage caused by the implantation of a high concentration of hydrogenic ions requires regular remote monitoring in order to study the atomic and nuclear reaction processes taking place within each sample. Real time continuous measurements of acoustic emission, X-ray production and emitted particle flux enable processes such as bubble or crack formation, changes in crystalline order, and nuclear fusion reactions can be studied in detail through examination of secondary or associated emission products. Fracturing of a material may generate a unique signature which, when taken in conjunction with time-averaged quantities such as changes in resistivity, surface strain, and induced radioactivity, enable an overall picture of the onset and nature of crack formation to be acquired. The overall usefulness of the remote sensing of damage processes and nuclear reactions is discussed. Surface studies involving inelastic Raman scattering and atomic force spectroscopy can contribute substantially to the overall picture, and identify clustering and cluster processes
Squeeze-film flow between a curved impermeable bearing and a flat porous bed
Axisymmetric squeeze-film flow in the thin gap between a stationary flat thin porous bed and a curved impermeable bearing moving under a prescribed constant load is analysed. The unsteady Reynolds equation is formulated and solved for the fluid pressure. This solution is used to obtain the time for the minimum fluid layer thickness to reduce to a given value, and, in particular, the finite time for the bearing and the bed to come into contact. The effect of varying the shape of the bearing and the permeability of the layer is investigated, and, in particular, it is found that both the contact time and the fluid pressure behave qualitatively differently for beds with small and large permeabilities. In addition, the paths of fluid particles initially situated in both the fluid layer and the porous bed are calculated. In particular, it is shown that, unlike in the case of a flat bearing, for a curved bearing there are fluid particles, initially situated in the fluid layer, that flow from the fluid layer into the porous bed and then re-emerge into the fluid layer, and the region in which these fluid particles are initially situated is determined
The Protostellar Mass Function
The protostellar mass function (PMF) is the Present-Day Mass Function of the
protostars in a region of star formation. It is determined by the initial mass
function weighted by the accretion time. The PMF thus depends on the accretion
history of protostars and in principle provides a powerful tool for
observationally distinguishing different protostellar accretion models. We
consider three basic models here: the Isothermal Sphere model (Shu 1977), the
Turbulent Core model (McKee & Tan 2003), and an approximate representation of
the Competitive Accretion model (Bonnell et al. 1997, 2001a). We also consider
modified versions of these accretion models, in which the accretion rate tapers
off linearly in time. Finally, we allow for an overall acceleration in the rate
of star formation. At present, it is not possible to directly determine the PMF
since protostellar masses are not currently measurable. We carry out an
approximate comparison of predicted PMFs with observation by using the theory
to infer the conditions in the ambient medium in several star-forming regions.
Tapered and accelerating models generally agree better with observed
star-formation times than models without tapering or acceleration, but
uncertainties in the accretion models and in the observations do not allow one
to rule out any of the proposed models at present. The PMF is essential for the
calculation of the Protostellar Luminosity Function, however, and this enables
stronger conclusions to be drawn (Offner & McKee 2010).Comment: 16 pages, 8 figures, published in Ap
- …