15,123 research outputs found
Flow-Induced Draping
Crumpled paper or drapery patterns are everyday examples of how elastic
sheets can respond to external forcing. In this Letter, we study experimentally
a novel sort of forcing. We consider a circular flexible plate clamped at its
center and subject to a uniform flow normal to its initial surface. As the flow
velocity is gradually increased, the plate exhibits a rich variety of bending
deformations: from a cylindrical taco-like shape, to isometric developable
cones with azimuthal periodicity two or three, to eventually a rolled-up
period-three cone. We show that this sequence of flow-induced deformations can
be qualitatively predicted by a linear analysis based on the balance between
elastic energy and pressure force work
Steady-State Magnetohydrodynamic Flow Around an Unmagnetized Conducting Sphere
The non-collisional interaction between conducting obstacles and magnetized
plasma winds can be found in different scenarios, from the interaction
occurring between regions inside galaxy clusters to the interaction between the
solar wind and Mars, Venus, active comets or even the interaction between Titan
and the Saturnian's magnetospheric flow. These objects generate, through
several current systems, perturbations in the streaming magnetic field leading
to its draping around the obstacle's effective conducting surface. Recent
observational results suggest that several properties associated with the
magnetic field draping, such as the location of the polarity reversal layer of
the induced magnetotail, are affected by variations in the conditions of the
streaming magnetic field. To improve our understanding of these phenomena, we
perform a characterization of several magnetic field draping signatures by
analytically solving an ideal problem in which a perfectly conducting
magnetized plasma (with frozen-in magnetic field conditions) flows around a
spherical body for various orientations of the streaming magnetic field. In
particular, we compute the shift of the inverse polarity reversal layer as the
orientation of the background magnetic field is changed.Comment: Preprint submitted to Astrophysical Journa
The impact of draping effects on the stiffness and failure behavior of unidirectional non-crimp fabric fiber reinforced composites
Unidirectional non-crimp fabrics (UD-NCF) are often used to exploit the lightweight potential of continuous fiber reinforced plastics (CoFRP). During the draping process, the UD-NCF fabric can undergo large deformations that alter the local fiber orientation, the local fiber volume content (FVC) and create local fiber waviness. Especially the FVC is affected and has a large impact on the mechanical properties. This impact, resulting from different deformation modes during draping, is in general not considered in composite design processes. To analyze the impact of different draping effects on the mechanical properties and the failure behavior of UD-NCF composites, experimental results of reference laminates are compared to the results of laminates with specifically induced draping effects, such as non-constant FVC and fiber waviness. Furthermore, an analytical model to predict the failure strengths of UD laminates with in-plane waviness is introduced. The resulting stiffness and strength values for different FVC or amplitude to wavelength configurations are presented and discussed. In addition, failure envelopes based on the PUCK failure criterion for each draping effect are derived, which show a clear specific impact on the mechanical properties. The findings suggest that each draping effect leads to a “new fabric” type. Additionally, analytical models are introduced and the experimental results are compared to the predictions. Results indicate that the models provide reliable predictions for each draping effect. Recommendations regarding necessary tests to consider each draping effect are presented. As a further prospect the resulting stiffness and strength values for each draping effect can be used for a more accurate prediction of the structural performance of CoFRP parts
Detecting the orientation of magnetic fields in galaxy clusters
Clusters of galaxies, filled with hot magnetized plasma, are the largest
bound objects in existence and an important touchstone in understanding the
formation of structures in our Universe. In such clusters, thermal conduction
follows field lines, so magnetic fields strongly shape the cluster's thermal
history; that some have not since cooled and collapsed is a mystery. In a
seemingly unrelated puzzle, recent observations of Virgo cluster spiral
galaxies imply ridges of strong, coherent magnetic fields offset from their
centre. Here we demonstrate, using three-dimensional magnetohydrodynamical
simulations, that such ridges are easily explained by galaxies sweeping up
field lines as they orbit inside the cluster. This magnetic drape is then lit
up with cosmic rays from the galaxies' stars, generating coherent polarized
emission at the galaxies' leading edges. This immediately presents a technique
for probing local orientations and characteristic length scales of cluster
magnetic fields. The first application of this technique, mapping the field of
the Virgo cluster, gives a startling result: outside a central region, the
magnetic field is preferentially oriented radially as predicted by the
magnetothermal instability. Our results strongly suggest a mechanism for
maintaining some clusters in a 'non-cooling-core' state.Comment: 48 pages, 21 figures, revised version to match published article in
Nature Physics, high-resolution version available at
http://www.cita.utoronto.ca/~pfrommer/Publications/pfrommer-dursi.pd
Impact of tangled magnetic fields on AGN-blown bubbles
There is growing consensus that feedback from AGN is the main mechanism
responsible for stopping cooling flows in clusters of galaxies. AGN are known
to inflate buoyant bubbles that supply mechanical power to the intracluster gas
(ICM). High Reynolds number hydrodynamical simulations show that such bubbles
get entirely disrupted within 100 Myr, as they rise in cluster atmospheres,
which is contrary to observations. This artificial mixing has consequences for
models trying to quantify the amount of heating and star formation in cool core
clusters of galaxies. It has been suggested that magnetic fields can stabilize
bubbles against disruption. We perform MHD simulations of fossil bubbles in the
presence of tangled magnetic fields using the high order PENCIL code. We focus
on the physically-motivated case where thermal pressure dominates over magnetic
pressure and consider randomly oriented fields with and without maximum
helicity and a case where large scale external fields drape the bubble.We find
that helicity has some stabilizing effect. However, unless the coherence length
of magnetic fields exceeds the bubble size, the bubbles are quickly shredded.
As observations of Hydra A suggest that lengthscale of magnetic fields may be
smaller then typical bubble size, this may suggest that other mechanisms, such
as viscosity, may be responsible for stabilizing the bubbles. However, since
Faraday rotation observations of radio lobes do not constrain large scale ICM
fields well if they are aligned with the bubble surface, the draping case may
be a viable alternative solution to the problem. A generic feature found in our
simulations is the formation of magnetic wakes where fields are ordered and
amplified. We suggest that this effect could prevent evaporation by thermal
conduction of cold Halpha filaments observed in the Perseus cluster.Comment: accepted for publication in MNRAS, (downgraded resolution figures,
color printing recommended
Development of a Computationally Efficient Fabric Model for Optimization of Gripper Trajectories in Automated Composite Draping
An automated prepreg fabric draping system is being developed which consists
of an array of actuated grippers. It has the ability to pick up a fabric ply
and place it onto a double-curved mold surface. A previous research effort
based on a nonlinear Finite Element model showed that the movements of the
grippers should be chosen carefully to avoid misplacement and induce of
wrinkles in the draped configuration. Thus, the present study seeks to develop
a computationally efficient model of the mechanical behavior of a fabric based
on 2D catenaries which can be used for optimization of the gripper
trajectories. The model includes bending stiffness, large deflections, large
ply shear and a simple contact formulation. The model is found to be quick to
evaluate and gives very reasonable predictions of the displacement field
Draping of Cluster Magnetic Fields over Bullets and Bubbles -- Morphology and Dynamic Effects
High-resolution X-ray observations have revealed cavities and `cold fronts'
with sharp edges in temperature, density, and metallicity within galaxy
clusters. Their presence poses a puzzle since these features are not expected
to be hydrodynamically stable, or to remain sharp in the presence of diffusion.
However, a moving core or bubble in even a very weakly magnetized plasma
necessarily sweeps up enough magnetic field to build up a dynamically important
sheath around the object; the layer's strength is set by a competition between
`plowing up' of field and field lines slipping around the core. We show that a
two-dimensional approach to the problem is quite generally not possible. In
three dimensions, we show with analytic arguments and in numerical experiments,
that this magnetic layer modifies the dynamics of a plunging core, greatly
modifies the effects of hydrodynamic instabilities on the core, modifies the
geometry of stripped material, and even slows the fall of the core through
magnetic tension. We derive an expression for the maximum magnetic field
strength, the thickness of the layer, and the opening angle of the magnetic
wake. The morphology of the magnetic draping layer implies the suppression of
thermal conduction across the layer, thus conserving strong temperature
gradients over the contact surface. The intermittent amplification of the
magnetic field as well as the injection of MHD turbulence in the wake of the
core is identified to be due to vorticity generation within the magnetic
draping layer. These results have important consequences for understanding the
physical properties and the complex gasdynamical processes of the intra-cluster
medium, and apply quite generally to motions through other magnetized
environments, e.g., the ISM.Comment: For version of this paper with interactive 3D graphics and
full-resolution figures, see http://www.cita.utoronto.ca/~ljdursi/draping/ .
19p, 26 figures, emulateapj format. Version accepted by ApJ - new references,
improved figure
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