5,218 research outputs found
Experimental investigation of transitional flow in a toroidal pipe
The flow instability and further transition to turbulence in a toroidal pipe
(torus) with curvature (tube-to-coiling diameter) 0.049 is investigated
experimentally. The flow inside the toroidal pipe is driven by a steel sphere
fitted to the inner pipe diameter. The sphere is moved with constant azimuthal
velocity from outside the torus by a moving magnet. The experiment is designed
to investigate curved pipe flow by optical measurement techniques. Using
stereoscopic particle image velocimetry, laser Doppler velocimetry and pressure
drop measurements, the flow is measured for Reynolds numbers ranging from 1000
to 15000. Time- and space-resolved velocity fields are obtained and analysed.
The steady axisymmetric basic flow is strongly influenced by centrifugal
effects. On an increase of the Reynolds number we find a sequence of
bifurcations. For Re=4075 a supercritical bifurcation to an oscillatory flow is
found in which waves travel in the streamwise direction with a phase velocity
slightly faster than the mean flow. The oscillatory flow is superseded by a
presumably quasi-periodic flow at a further increase of the Reynolds number
before turbulence sets in. The results are found to be compatible, in general,
with earlier experimental and numerical investigations on transition to
turbulence in helical and curved pipes. However, important aspects of the
bifurcation scenario differ considerably
Design and evaluation of dynamic policy-based flow redirection for multihomed mobile netwotks
This paper presents the design, implementation and evaluation of a solution for dynamic redirection of traffic flows for multihomed mobile networks. The solution was developed for a mobile user that disposes of a Personal Area Network (PAN) with a Personal Mobile Router (PMR), in order to achieve Always Best Connected(ABC) service by distributing flows belonging to different applications among the most appropriate access networks. Designed in a modular way for a NEMO based mobility and multihoming support, the proposed flow redirection solution can be easily coupled with and controlled by dynamic traffic policies that come from advanced network intelligence, according to the currently available network resources and user and application requirements. A prototype implementation was validated and assessed on a testbed as proof-of-concept
How to Mix Molecules with Mathematics
In this paper we develop two methods to calculate thermodynamic properties of mixtures. Starting point are the basic assumptions that also form the basis for the COSMO-RS model. In this approach, the individual molecules are represented by their geometrical shape with an electrical charge density on their surfaces. Next, the surface is split up into surface segments each with its own charge. In COSMO-RS a strong reduction is introduced by treating the segments as if they are completely independent. In the present study we take into account that the coupling between two patches is essentially dependent on the charge distribution on neighboring segments and on the local geometrical structure of the surface. Two approaches are followed. The first one points out how the model
equations, which comprise the optimization of the entropy and conservation of internal energy, can efficiently be solved in general, thus also if the dependency between segments and the local geometry is included in the expression for the coupling energy between segments. In the second method the configuration with maximal entropy and prescribed energy is sought via simulation. Successive molecular configurations of the mixture are simulated and updated via a genetic algorithm to optimize the entropy. The second method is more time consuming but very general
Data-driven and Model-based Verification: a Bayesian Identification Approach
This work develops a measurement-driven and model-based formal verification
approach, applicable to systems with partly unknown dynamics. We provide a
principled method, grounded on reachability analysis and on Bayesian inference,
to compute the confidence that a physical system driven by external inputs and
accessed under noisy measurements, verifies a temporal logic property. A case
study is discussed, where we investigate the bounded- and unbounded-time safety
of a partly unknown linear time invariant system
Observer-based correct-by-design controller synthesis
Current state-of-the-art correct-by-design controllers are designed for
full-state measurable systems. This work first extends the applicability of
correct-by-design controllers to partially observable LTI systems. Leveraging
2nd order bounds we give a design method that has a quantifiable robustness to
probabilistic disturbances on state transitions and on output measurements. In
a case study from smart buildings we evaluate the new output-based
correct-by-design controller on a physical system with limited sensor
information
Subcritical versus supercritical transition to turbulence in curved pipes
Transition to turbulence in straight pipes occurs in spite of the linear
stability of the laminar Hagen--Poiseuille flow if the amplitude of flow
perturbations as well as the Reynolds number exceed a minimum threshold
(subcritical transition). As the pipe curvature increases centrifugal effects
become important, modifying the basic flow as well as the most unstable linear
modes. If the curvature (tube-to-coiling diameter ) is sufficiently large
a Hopf bifurcation (supercritical instability) is encountered before turbulence
can be excited (subcritical instability). We trace the instability thresholds
in the parameter space in the range by means
of laser-Doppler velocimetry and determine the point where the subcritical and
supercritical instabilities meet. Two different experimental setups were used:
a closed system where the pipe forms an axisymmetric torus and an open system
employing a helical pipe. Implications for the measurement of friction factors
in curved pipes are discussed
Human cells in culture: Revislted
Human cells in vitro offer considerable advantages in the study and detection of defects in human chromosomes and gene products. Of all the euploid cells of the mammalian organism the fibroblasts are capable of reflecting many different metabolic disorders. These methods, combined with recently developed techniques for the early detection of genetic disorders allow the assessment of a 'high risk' pregnancy. Cell hybridization has become of paramount importance to genetic analysis and has proved very useful in assigning gene information to certain chromosome
Gate Oxide Reliability and Deuterated CMOS Processing
In recent literature, a controversy has arisen over the question whether deuterium improves the stability of the MOS gate dielectric. It appears as if this controversy finds its origin in the different stages (e.g. oxidation or post metal anneal) deuterium is introduced in the CMOS process. This paper investigates this in detail. The obtained results show that the hot carrier degradation only benefits from an isotope effect when deuterium is introduced in the post metal anneal. At the same time, charge to breakdown for high quality oxides does not benefit from an isotope effect, regardless of the processing stage deuterium is introduced, or the gate oxide thickness used. This is verified on two different sets of wafers fabricated in two different laboratories
Genetic mental services for retardation patients with severe - The South African situation
In South Africa mental retardation is still ill-defined as regards the aetiology and general epidemiology. A systematic diagnostic/genetics programme implemented at various institutions for the mentally retarded within the framework of a comprehensive genetic service is described. The progress made is reported and the contribution of genetic services to the prevention and the management of mental retardation highlighted
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