154 research outputs found
MRI channel flows and their parasites
Local simulations of the magnetorotational instability (MRI) in accretion
disks can exhibit recurrent coherent structures called channel flows. The
formation and destruction of these structures may play a role in the
development and saturation of MRI-induced turbulence, and consequently help us
understand the time-dependent accretion behaviour of certain astrophysical
objects. Previous investigations have revealed that channel solutions are
attacked by various parasitic modes, foremost of which is an analogue of the
Kelvin-Helmholtz instability. We revisit these instabilities and show how they
relate to the classical instabilities of plasma physics, the kink and pinch
modes. However, we argue that in most cases channels emerge from developed
turbulence and are eventually destroyed by turbulent mixing, not by the
parasites. The exceptions are the clean isolated channels which appear in
systems near criticality or which emerge from low amplitude initial conditions.
These structures inevitably achieve large amplitudes and are only then
destroyed, giving rise to eruptive behaviour.Comment: 17 pages, 17 figures, accepted by MNRA
Summarising salient information on historical controls: A structured assessment of validity and comparability across studies
BACKGROUND: While placebo-controlled randomised controlled trials remain the standard way to evaluate drugs for efficacy, historical data are used extensively across the development cycle. This ranges from supplementing contemporary data to increase the power of trials to cross-trial comparisons in estimating comparative efficacy. In many cases, these approaches are performed without in-depth review of the context of data, which may lead to bias and incorrect conclusions. METHODS: We discuss the original 'Pocock' criteria for the use of historical data and how the use of historical data has evolved over time. Based on these factors and personal experience, we created a series of questions that may be asked of historical data, prior to their use. Based on the answers to these questions, various statistical approaches are recommended. The strategy is illustrated with a case study in colorectal cancer. RESULTS: A number of areas need to be considered with historical data, which we split into three categories: outcome measurement, study/patient characteristics (including setting and inclusion/exclusion criteria), and disease process/intervention effects. Each of these areas may introduce issues if not appropriately handled, while some may preclude the use of historical data entirely. We present a tool (in the form of a table) for highlighting any such issues. Application of the tool to a colorectal cancer data set demonstrates under what conditions historical data could be used and what the limitations of such an analysis would be. CONCLUSION: Historical data can be a powerful tool to augment or compare with contemporary trial data, though caution is required. We present some of the issues that may be considered when involving historical data and what (if any) statistical approaches may account for differences between studies. We recommend that, where historical data are to be used in analyses, potential differences between studies are addressed explicitly
Model and Stability Analysis of a Flexible Bladed Rotor
This paper presents a fully bladed flexible rotor and outlines the
associated stability analysis. From an energetic approach based on
the complete energies and potentials for Euler-Bernoulli beams, a
system of equations is derived, in the rotational frame, for the
rotor. This later one is made of a hollow shaft modelled by an
Euler-Bernoulli beam supported by a set of bearings. It is
connected to a rigid disk having a rotational inertia. A full set
of flexible blades is also modelled by Euler-Bernoulli beams
clamped in the disk. The flexural vibrations of the blades as well
as those of the shaft are considered. The evolution of the
eigenvalues of this rotor, in the corotational frame,
is studied. A stability detection method, bringing coalescence and
loci separation phenomena to the fore, in case of an asymmetric
rotor, is undertaken in order to determine a parametric domain
where turbomachinery cannot encounter damage. Finally, extensive
parametric studies including the length and the stagger angle of
the blades as well as their flexibility are presented in order to
obtain robust criteria for stable and unstable areas prediction
Stability Analysis of Beams Rotating on an Elastic Ring Application to Turbo machinery Rotor-Stator Contacts
Summary This paper presents a model of flexible beams rotating on the inner surface of an elastic stationary ring. The beams possesses two degrees of freedom, traction/compression and flexure. The in-plane deformations of the ring are considered and a single mode approximation is used. The model has been developed within the rotating frame by use of an energetic method. To better understand the phenomena occurring, the degrees of freedom of the beams can first be treated separately then together. Stability analysis show that even without rubbing, the radial degree of freedom of a beam rotating on an elastic ring can create divergence instabilities as well as mode couplings of the circular structure. When rubbing is considered, the system is unstable as soon as the rotational speed is non null. Moreover rubbing can couple the beams and the ring giving rise to mode coupling instabilities and locus veering phenomena. Finally, a comparison to a more complicated model of a flexible bladed-rotor in contact with an elastic casing shows a very good accordance with the phenomena occurring
3-D image-based numerical computations of snow permeability: links to specific surface area, density, and microstructural anisotropy
We used three-dimensional (3-D) images of snow microstructure to carry out numerical estimations of the full tensor of the intrinsic permeability of snow (<b>K</b>). This study was performed on 35 snow samples, spanning a wide range of seasonal snow types. For several snow samples, a significant anisotropy of permeability was detected and is consistent with that observed for the effective thermal conductivity obtained from the same samples. The anisotropy coefficient, defined as the ratio of the vertical over the horizontal components of <b>K</b>, ranges from 0.74 for a sample of decomposing precipitation particles collected in the field to 1.66 for a depth hoar specimen. Because the permeability is related to a characteristic length, we introduced a dimensionless tensor <b>K</b>*=<b>K</b>/<i>r</i><sub>es</sub><sup>2</sup>, where the equivalent sphere radius of ice grains (<i>r</i><sub>es</sub>) is computed from the specific surface area of snow (SSA) and the ice density (&rho;<sub>i</sub>) as follows: <i>r</i><sub>es</sub>=3/(SSA&times;&rho;<sub>i</sub>. We define <i>K</i> and <i>K</i>* as the average of the diagonal components of <b>K</b> and <b>K</b>*, respectively. The 35 values of <i>K</i>* were fitted to snow density (&rho;<sub>s</sub>) and provide the following regression: <i>K</i> = (3.0 &pm; 0.3) <i>r</i><sub>es</sub><sup>2</sup> exp((&minus;0.0130 &pm; 0.0003)&rho;<sub>s</sub>). We noted that the anisotropy of permeability does not affect significantly the proposed equation. This regression curve was applied to several independent datasets from the literature and compared to other existing regression curves or analytical models. The results show that it is probably the best currently available simple relationship linking the average value of permeability, <i>K</i>, to snow density and specific surface area
Stability analysis of rotating beams rubbing on an elastic circular structure
This paper presents the stability analysis of a system composed of rotating
beams on a flexible, circular fixed ring, using the Routh-Hurwitz criterion.
The model displayed has been fully developed within the rotating frame by use
of an energy approach. The beams considered possess two degrees of freedom
(dofs), a flexural motion as well as a traction/compression motion. In-plane
deformations of the ring will be considered. Divergences and mode couplings
have thus been underscored within the rotating frame and in order to simplify
understanding of all these phenomena, the dofs of the beams will first be
treated separately and then together. The dynamics of radial rotating loads on
an elastic ring can create divergence instabilities as well as post-critical
mode couplings. Moreover, the flexural motion of beam rubbing on the ring can
also lead to mode couplings and to the locus-veering phenomenon. The presence
of rubbing seems to make the system unstable as soon as the rotational speed of
the beams is greater than zero. Lastly, the influence of an angle between the
beams and the normal to the ring's inner surface will be studied with respect
to system stability, thus highlighting a shift frequency phenomenon
Delayed Decision-making in Real-time Beatbox Percussion Classification
This is an electronic version of an article published in Journal of New Music Research, 39(3), 203-213, 2010. doi:10.1080/09298215.2010.512979. Journal of New Music Research is available online at: www.tandfonline.com/openurl?genre=article&issn=1744-5027&volume=39&issue=3&spage=20
Summarising salient information on historical controls: A structured assessment of validity and comparability across studies
Background
While placebo-controlled randomised controlled trials remain the standard way to evaluate drugs for efficacy, historical data are used extensively across the development cycle. This ranges from supplementing contemporary data to increase the power of trials to cross-trial comparisons in estimating comparative efficacy. In many cases, these approaches are performed without in-depth review of the context of data, which may lead to bias and incorrect conclusions.
Methods
We discuss the original ‘Pocock’ criteria for the use of historical data and how the use of historical data has evolved over time. Based on these factors and personal experience, we created a series of questions that may be asked of historical data, prior to their use. Based on the answers to these questions, various statistical approaches are recommended. The strategy is illustrated with a case study in colorectal cancer.
Results
A number of areas need to be considered with historical data, which we split into three categories: outcome measurement, study/patient characteristics (including setting and inclusion/exclusion criteria), and disease process/intervention effects. Each of these areas may introduce issues if not appropriately handled, while some may preclude the use of historical data entirely. We present a tool (in the form of a table) for highlighting any such issues. Application of the tool to a colorectal cancer data set demonstrates under what conditions historical data could be used and what the limitations of such an analysis would be.
Conclusion
Historical data can be a powerful tool to augment or compare with contemporary trial data, though caution is required. We present some of the issues that may be considered when involving historical data and what (if any) statistical approaches may account for differences between studies. We recommend that, where historical data are to be used in analyses, potential di
- …