6,936 research outputs found
Turbulent Fluid Flow Over Aerodynamically Rough Surfaces Using Direct Numerical Simulations
Incompressible turbulent fluid flow in aerodynamically rough channels is investigated
using direct numerical simulations. A comprehensive database of simulation
data for rough surfaces with different topographical properties has been developed
for 17 industrially relevant rough surface samples. It includes numerous commonlyseen
industrial rough surfaces such as concrete, graphite, carbon-carbon composite
and ground, shotblasted and spark-eroded steel. Other surfaces such as cast, filed
and gritblasted steel are also studied, along with replicas of ship propeller surfaces
eroded by periods of service. The Reynolds number considered is Reτ = 180, for
which the flow is in the transitionally rough regime. A study with variable δ/Sq ratio
while keeping S
+
q
constant, where Sq is the root mean squared roughness height,
is conducted for one of the samples with the mean profiles showing convergence
for δ/Sq >≈ 25. A Reynolds number dependence study is conducted for two of the
samples with Reτ up to 720 showing a more complete range up to the fully rough
flow regime, allowing the equivalent sandgrain roughness height, ks
to be computed.
A correlation based on the frontal and wetted roughness area is found to be superior
to the surface skewness in predicting ∆U
+ based on the topographic surface
parameters
Fluctuation dynamo amplified by intermittent shear bursts in convectively driven magnetohydrodynamic turbulence
Intermittent large-scale high-shear flows are found to occur frequently and
spontaneously in direct numerical simulations of statistically stationary
turbulent Boussinesq magnetohydrodynamic (MHD) convection. The energetic
steady-state of the system is sustained by convective driving of the velocity
field and small-scale dynamo action. The intermittent emergence of flow
structures with strong velocity and magnetic shearing generates magnetic energy
at an elevated rate over time-scales longer than the characteristic time of the
large-scale convective motion. The resilience of magnetic energy amplification
suggests that intermittent shear-bursts are a significant driver of dynamo
action in turbulent magnetoconvection
Cost-sharing in the German health care system
In Germany, cost-sharing for health care has been used as a financing mechanism since 1923. In this article, the historical development of user charges in Germany since the 1980s is presented in more detail by type of private expenditure, including direct payments, cost-sharing measures, and voluntary health insurance. This is followed by a mapping of current cost-sharing measures including a discussion of protection mechanisms and responsibility for decision-making on cost-sharing measures and a summary of national policy debates. In the final section, the results of a systematic review of the literature on the impact of cost-sharing on equity, efficiency and health outcomes in Germany are presented. -- Die Selbstbeteiligung des Patienten an den Gesundheitsversorgungskosten hat in Deutschland eine lange Tradition und geht auf das Jahr 1923 zurück. In dieser Arbeit wird die historische Entwicklung und Bedeutung von Kostenselbstbeteiligung im Gesundheitswesen seit 1980 detailliert nach Art der Gesundheitsausgaben dargestellt. Dies beinhaltet direkte Zahlungen, Kostenbeteiligung, und private Krankenversicherung. Darauf folgt eine Darstellung der derzeitigen Regelungen zur Selbstbeteiligung mit Berücksichtigung der verschiedenen Mechanismen zum Schutz vor katastrophalen Gesundheitsausgaben und der Zuständigkeit für politische und administrative Entscheidungsfindungen zur Selbstbeteiligung. Im letzten Abschnitt werden die Ergebnisse einer systematischen Literatursuche zu den Auswirkungen von Kostenbeteiligungen auf Effizienz, Gerechtigkeit und Gesundheitsstatus in Deutschland dargestellt.
Lagrangian Statistics of Navier-Stokes- and MHD-Turbulence
We report on a comparison of high-resolution numerical simulations of
Lagrangian particles advected by incompressible turbulent hydro- and
magnetohydrodynamic (MHD) flows. Numerical simulations were performed with up
to collocation points and 10 million particles in the Navier-Stokes
case and collocation points and 1 million particles in the MHD case. In
the hydrodynamics case our findings compare with recent experiments from
Mordant et al. [1] and Xu et al. [2]. They differ from the simulations of
Biferale et al. [3] due to differences of the ranges choosen for evaluating the
structure functions. In Navier-Stokes turbulence intermittency is stronger than
predicted by a multifractal approach of [3] whereas in MHD turbulence the
predictions from the multifractal approach are more intermittent than observed
in our simulations. In addition, our simulations reveal that Lagrangian
Navier-Stokes turbulence is more intermittent than MHD turbulence, whereas the
situation is reversed in the Eulerian case. Those findings can not consistently
be described by the multifractal modeling. The crucial point is that the
geometry of the dissipative structures have different implications for
Lagrangian and Eulerian intermittency. Application of the multifractal approach
for the modeling of the acceleration PDFs works well for the Navier-Stokes case
but in the MHD case just the tails are well described.Comment: to appear in J. Plasma Phy
Parametric forcing approach to rough-wall turbulent channel flow
The effects of rough surfaces on turbulent channel flow are modelled by an extra force term in the Navier–Stokes equations. This force term contains two parameters, related to the density and the height of the roughness elements, and a shape function, which regulates the influence of the force term with respect to the distance from the channel wall. This permits a more flexible specification of a rough surface than a single parameter such as the equivalent sand grain roughness. The effects of the roughness force term on turbulent channel flow have been investigated for a large number of parameter combinations and several shape functions by direct numerical simulations. It is possible to cover the full spectrum of rough flows ranging from hydraulically smooth through transitionally rough to fully rough cases. By using different parameter combinations and shape functions, it is possible to match the effects of different types of rough surfaces. Mean flow and standard turbulence statistics have been used to compare the results to recent experimental and numerical studies and a good qualitative agreement has been found. Outer scaling is preserved for the streamwise velocity for both the mean profile as well as its mean square fluctuations in all but extremely rough cases. The structure of the turbulent flow shows a trend towards more isotropic turbulent states within the roughness layer. In extremely rough cases, spanwise structures emerge near the wall and the turbulent state resembles a mixing layer. A direct comparison with the study of Ashrafian, Andersson & Manhart (Intl J. Heat Fluid Flow, vol. 25, 2004, pp. 373–383) shows a good quantitative agreement of the mean flow and Reynolds stresses everywhere except in the immediate vicinity of the rough wall. The proposed roughness force term may be of benefit as a wall model for direct and large-eddy numerical simulations in cases where the exact details of the flow over a rough wall can be neglecte
Extreme-value statistics from Lagrangian convex hull analysis for homogeneous turbulent Boussinesq convection and MHD convection
We investigate the utility of the convex hull of many Lagrangian tracers to
analyze transport properties of turbulent flows with different anisotropy. In
direct numerical simulations of statistically homogeneous and stationary
Navier-Stokes turbulence, neutral fluid Boussinesq convection, and MHD
Boussinesq convection a comparison with Lagrangian pair dispersion shows that
convex hull statistics capture the asymptotic dispersive behavior of a large
group of passive tracer particles. Moreover, convex hull analysis provides
additional information on the sub-ensemble of tracers that on average disperse
most efficiently in the form of extreme value statistics and flow anisotropy
via the geometric properties of the convex hulls. We use the convex hull
surface geometry to examine the anisotropy that occurs in turbulent convection.
Applying extreme value theory, we show that the maximal square extensions of
convex hull vertices are well described by a classic extreme value
distribution, the Gumbel distribution. During turbulent convection,
intermittent convective plumes grow and accelerate the dispersion of Lagrangian
tracers. Convex hull analysis yields information that supplements standard
Lagrangian analysis of coherent turbulent structures and their influence on the
global statistics of the flow.Comment: 18 pages, 10 figures, preprin
3D Micron-scale Imaging of the Cortical Bone Canal Network in Human Osteogenesis Imperfecta (OI)
Osteogenesis imperfecta (OI) is a genetic disorder leading to increased bone fragility. Recent work has shown that the hierarchical structure of bone plays an important role in determining its mechanical properties and resistance to fracture. The current study represents one of the first attempts to characterize the 3D structure and composition of cortical bone in OI at the micron-scale. A total of 26 pediatric bone fragments from 18 individuals were collected during autopsy (Nc=5) or routing orthopaedic procedures (NOI=13) and imaged by microtomography with a synchrotron light source (SRµCT) for several microstructural parameters including cortical porosity (Ca.V/TV), canal surface to tissue volume (Ca.S/TV), canal diameter (Ca.Dm), canal separation (Ca.Sp), canal connectivity density (Ca.ConnD), and volumetric tissue mineral density (TMD). Results indicated significant differences in all imaging parameters between pediatric controls and OI tissue, with OI bone showing drastically increased cortical porosity, canal diameter, and connectivity. Preliminary mechanical testing revealed a possible link between cortical porosity and strength. Together these results suggest that the pore network in OI contributes greatly to its reduced mechanical properties
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