39 research outputs found
Marangoni driven turbulence in high energy surface melting processes
Experimental observations of high-energy surface melting processes, such as
laser welding, have revealed unsteady, often violent, motion of the free
surface of the melt pool. Surprisingly, no similar observations have been
reported in numerical simulation studies of such flows. Moreover, the published
simulation results fail to predict the post-solidification pool shape without
adapting non-physical values for input parameters, suggesting the neglect of
significant physics in the models employed. The experimentally observed violent
flow surface instabilities, scaling analyses for the occurrence of turbulence
in Marangoni driven flows, and the fact that in simulations transport
coefficients generally have to be increased by an order of magnitude to match
experimentally observed pool shapes, suggest the common assumption of laminar
flow in the pool may not hold, and that the flow is actually turbulent. Here,
we use direct numerical simulations (DNS) to investigate the role of turbulence
in laser melting of a steel alloy with surface active elements. Our results
reveal the presence of two competing vortices driven by thermocapillary forces
towards a local surface tension maximum. The jet away from this location at the
free surface, separating the two vortices, is found to be unstable and highly
oscillatory, indeed leading to turbulence-like flow in the pool. The resulting
additional heat transport, however, is insufficient to account for the observed
differences in pool shapes between experiment and simulations
Droplets on Inclined Plates: Local and Global Hysteresis of Pinned Capillary Surfaces
Local contact line pinning prevents droplets from rearranging to minimal
global energy, and models for droplets without pinning cannot predict their
shape. We show that experiments are much better described by a theory,
developed herein, that does account for the constrained contact line motion,
using as example droplets on tilted plates. We map out their shapes in suitable
phase spaces. For 2D droplets, the critical point of maximum tilt depends on
the hysteresis range and Bond number. In 3D, it also depends on the initial
width, highlighting the importance of the deposition history.Comment: 5 pages, 5 figures, accepted for publication in Phys. Rev. Let
CHANGING NURSING CARE TIME AS AN EFFECT OF CHANGED CHARACTERISTICS OF THE DIALYSIS POPULATION
Background: The population of dialysis patients is ageing. Dialysis nurses are confronted with geriatric patients with multiple comorbidities. Nurses are confronted with an increasing burden of care. Objectives: The present study focused on the question of whether, over time, the increasing age and comorbidities of the haemodialysis population increased nursing care time. Furthermore, we studied potential changes in the predictors of the required nursing time. Design: Observational study. Participants: A total of 980 dialysis patients from 12 dialysis centres were included. Measurements: Nurses filled out the classification tool for each patient and completed a form for reporting patient characteristics for groups of relevant haemodialysis patients at baseline and after 1 and four years. Changes in patient and dialysis characteristics were analysed, as well as the estimated nursing care time needed. Results: An increase in the nursing time needed for dialysis was largely due to decreased mobility, closing of the vascular access and a greater need for psychosocial attention and was most strongly present in incident dialysis patients. The time needed for dialysis decreased as patient participation increased and vascular access changed from catheters to fistulae. Over the four-year period, the average overall needed nursing care time per haemodialysis session did not change. Conclusions: Our study shows that the average nursing time needed per patient did not change in the four-year observation period. However, more time is required for incident patients; thus, if a centre has high patient turnover, more nursing care time is needed
Revealing internal flow behaviour in arc welding and additive manufacturing of metals
Internal flow behaviour during melt-pool-based metal manufacturing remains unclear and hinders progression to process optimisation. In this contribution, we present direct time-resolved imaging of melt pool flow dynamics from a high-energy synchrotron radiation experiment. We track internal flow streams during arc welding of steel and measure instantaneous flow velocities ranging from 0.1 m s−1 to 0.5 m s−1. When the temperature-dependent surface tension coefficient is negative, bulk turbulence is the main flow mechanism and the critical velocity for surface turbulence is below the limits identified in previous theoretical studies. When the alloy exhibits a positive temperature-dependent surface tension coefficient, surface turbulence occurs and derisory oxides can be entrapped within the subsequent solid as result of higher flow velocities. The widely used arc welding and the emerging arc additive manufacturing routes can be optimised by controlling internal melt flow through adjusting surface active elements
KiDS-1000 Cosmology:Multi-probe weak gravitational lensing and spectroscopic galaxy clustering constraints
We present a joint cosmological analysis of weak gravitational lensing
observations from the Kilo-Degree Survey (KiDS-1000), with redshift-space
galaxy clustering observations from the Baryon Oscillation Spectroscopic Survey
(BOSS), and galaxy-galaxy lensing observations from the overlap between
KiDS-1000, BOSS and the spectroscopic 2-degree Field Lensing Survey (2dFLenS).
This combination of large-scale structure probes breaks the degeneracies
between cosmological parameters for individual observables, resulting in a
constraint on the structure growth parameter , that has the same overall precision as that
reported by the full-sky cosmic microwave background observations from Planck.
The recovered amplitude is low, however, by % relative to
Planck. This result builds from a series of KiDS-1000 analyses where we
validate our methodology with variable depth mock galaxy surveys, our lensing
calibration with image simulations and null-tests, and our
optical-to-near-infrared redshift calibration with multi-band mock catalogues
and a spectroscopic-photometric clustering analysis. The systematic
uncertainties identified by these analyses are folded through as nuisance
parameters in our cosmological analysis. Inspecting the offset between the
marginalised posterior distributions, we find that the -difference with
Planck is driven by a tension in the matter fluctuation amplitude parameter,
. We quantify the level of agreement between the CMB and our
large-scale structure constraints using a series of different metrics, finding
differences with a significance ranging between , when
considering the offset in , and , when considering the
full multi-dimensional parameter space.Comment: 25 pages, 11 figures, 5 tables, A&A accepted, including a new
appendix on Intrinsic Alignments. The KiDS-1000 data products are available
for download at http://kids.strw.leidenuniv.nl/DR4/lensing.php. This data
release includes open source software, the shear-photo-z catalogue, the
cosmic shear and 3x2pt data vectors and covariances, and posteriors in the
form of Multinest chain