1,422 research outputs found
Gravity from the entropy of light
The holographic principle, considered in a semiclassical setting, is shown to
have direct consequences on physics at a fundamental level. In particular, a
certain relation is pointed out to be the expression of holography in basic
thermodynamics. It is argued moreover that through this relation holography can
be recognized to induce gravity, and an expression for the gravitational
lensing is obtained in terms of entropy over wavelength of black-body
radiation, or, at a deeper level, in terms of maximum entropy over associated
space to the elementary bit of information.Comment: 7 pages; v2: completion of the list of references; v3: the discussion
is divided in Sections and the argument is described in more detail; v4: a
statement is added (below eq.13) on what is the supposed difference between
Jacobson's work in ref.21 and this attempt; addition of a paragraph in last
Sectio
Growth and instability of a laminar plume in a strongly stratified environment
Experimental studies of laminar plumes descending under gravity into stably stratified environments have shown the existence of a critical injection velocity beyond which the plume exhibits a bifurcation to a coiling instability in three dimensions or a sinuous instability in a Hele-Shaw flow. In addition, flow visualization has shown that, prior to the onset of the instability, a stable base flow is established in which the plume penetrates to a depth significantly smaller than the neutral buoyancy depth. Moreover, the fresh water that is viscously entrained by the plume recirculates within a āconduitā whose boundary with the background stratification appears sharp. Beyond the bifurcation, the buckling plume takes the form of a travelling wave of varying amplitude, confined within the conduit, which disappears at the penetration depth. To determine the mechanisms underlying these complex phenomena, which take place at a strikingly low Reynolds number but a high Schmidt number, we study here a two-dimensional arrangement, as it is perhaps the simplest system which possesses all the key experimental features. Through a combination of numerical and analytical approaches, a scaling law is found for the plumeās penetration depth within the base flow (i.e. the flow where the instability is either absent or artificially suppressed), and the horizontal cross-stream velocity and concentration profile outside the plume are determined from an asymptotic analysis of a simplified model. Direct numerical simulations show that, with increasing flow rate, a sinuous global mode is destabilized giving rise to the self-sustained oscillations as in the experiment. The sinuous instability is shown to be a consequence of the baroclinic generation of vorticity, due to the strong horizontal gradients at the edge of the conduit, a mechanism that is relevant even at very low Reynolds numbers. Despite the strength of this instability, the penetration depth is not significantly affected by it, instead being determined by the properties of the plume in the vicinity of the source. This scenario is confirmed by a local stability analysis. A finite region of local absolute instability is found near the source for sinuous modes prior to the onset of the global instability. Sufficiently far from the source the flow is locally stable. Near the onset of the global instability, varicose modes are also found to be locally, but only convectively, unstable
Laser scanning the Garisenda and Asinelli towers in Bologna (Italy): detailed deformation patterns of two ancient leaning buildings
The Asinelli and Garisenda towers are the main symbol of the city of Bologna (Italy). These
leaning towers, whose heights are about 97 m and 48 m respectively, were built during the
early 12th century and are two of the few surviving ones from about a hundred tall medieval
buildings that once characterized the city. Therefore, they are part of the Italian cultural
heritage and their safeguard is extremely important. In order to evaluate in detail the
deformations of these towers, in particular the deviations from a regular inclination of their
walls, the terrestrial laser scanning (TLS) has been used and an efficient direct analysis
method has been developed. The towers have been scanned from 6 viewpoints, providing 19
point clouds with a complete coverage of the visible surfaces with large overlap areas. For
each tower, after the registration of the partial point clouds into a common reference frame, an
accurate morphological analysis of the acquired surfaces has been carried out. The results
show several zones affected by significant deformations and inclination changes. In the case of the Asinelli tower, for which a finite element model is available, the results have also been
interpreted on the basis of the static load and normal modes. The correspondence between the measured deformation and the theoretically expected deformation, caused by a seismic sequence, is clear. This fact suggests a high sensibility of the tower to dynamic loads.
Although a direct evaluation of the risk cannot be carried out with the obtained results, they lead to the general indication that the structural health of these buildings must be frequently
checked and that man-made loads (e.g. vibration due to vehicular traffic) should be avoided or at least reduced
Mixing by Swimming Algae
In this fluid dynamics video, we demonstrate the microscale mixing
enhancement of passive tracer particles in suspensions of swimming microalgae,
Chlamydomonas reinhardtii. These biflagellated, single-celled eukaryotes (10
micron diameter) swim with a "breaststroke" pulling motion of their flagella at
speeds of about 100 microns/s and exhibit heterogeneous trajectory shapes.
Fluorescent tracer particles (2 micron diameter) allowed us to quantify the
enhanced mixing caused by the swimmers, which is relevant to suspension feeding
and biogenic mixing. Without swimmers present, tracer particles diffuse slowly
due solely to Brownian motion. As the swimmer concentration is increased, the
probability density functions (PDFs) of tracer displacements develop strong
exponential tails, and the Gaussian core broadens. High-speed imaging (500 Hz)
of tracer-swimmer interactions demonstrates the importance of flagellar beating
in creating oscillatory flows that exceed Brownian motion out to about 5 cell
radii from the swimmers. Finally, we also show evidence of possible cooperative
motion and synchronization between swimming algal cells.Comment: 1 page, APS-DFD 2009 Gallery of Fluid Motio
Antiphase Synchronization in a Flagellar-Dominance Mutant of Chlamydomonas
Groups of beating flagella or cilia often synchronize so that neighboring
filaments have identical frequencies and phases. A prime example is provided by
the unicellular biflagellate Chlamydomonas reinhardtii, which typically
displays synchronous in-phase beating in a low-Reynolds number version of
breaststroke swimming. We report here the discovery that ptx1, a flagellar
dominance mutant of C. reinhardtii, can exhibit synchronization in precise
antiphase, as in the freestyle swimming stroke. Long-duration high-speed
imaging shows that ptx1 flagella switch stochastically between in-phase and
antiphase states, and that the latter has a distinct waveform and significantly
higher frequency, both of which are strikingly similar to those found during
phase slips that stochastically interrupt in-phase beating of the wildtype.
Possible mechanisms underlying these observations are discussed.Comment: 5 pages, 4 figure
Elastohydrodynamic synchronization of adjacent beating flagella
It is now well established that nearby beating pairs of eukaryotic flagella or cilia typically synchronize in phase. A substantial body of evidence supports the hypothesis that hydrodynamic coupling between the active filaments, combined with waveform compliance, provides a robust mechanism for synchrony. This elastohydrodynamic mechanism has been incorporated into bead-spring models in which the beating flagella are represented by microspheres tethered by radial springs as they are driven about orbits by internal forces. While these low-dimensional models reproduce the phenomenon of synchrony, their parameters are not readily relatable to those of the filaments they represent. More realistic models, which reflect the underlying elasticity of the axonemes and the active force generation, take the form of fourth-order nonlinear partial differential equations (PDEs). While computational studies have shown the occurrence of synchrony, the effects of hydrodynamic coupling between nearby filaments governed by such continuum models have been examined theoretically only in the regime of interflagellar distances d large compared to flagellar length . Yet in many biological situations āŖ1. Here we present an asymptotic analysis of the hydrodynamic coupling between two extended filaments in the regime āŖ1 and find that the form of the coupling is independent of the microscopic details of the internal forces that govern the motion of the individual filaments. The analysis is analogous to that yielding the localized induction approximation for vortex filament motion, extended to the case of mutual induction. In order to understand how the elastohydrodynamic coupling mechanism leads to synchrony of extended objects, we introduce a heuristic model of flagellar beating. The model takes the form of a single fourth-order nonlinear PDE whose form is derived from symmetry considerations, the physics of elasticity, and the overdamped nature of the dynamics. Analytical and numerical studies of this model illustrate how synchrony between a pair of filaments is achieved through the asymptotic coupling.This work was supported by Wellcome Trust Senior Investigator Award 097855MA (R.E.G. and A.I.P.) and by a Marie Curie Career Integration Grant (E.L.)
UN METODO PER LO STUDIO DI EDIFICI STORICI MEDIANTE MISURE LASER A SCANSIONE TERRESTRE: LE DUE TORRI DI BOLOGNA
La presente relazione descrive i lavori effettuati a Bologna nel Settembre 2010 con lo scopo di
contribuire allo studio sullo stato di salute delle Due Torri, verificandone le condizioni, mediante lāutilizzo
del sistema laser a scansione terrestre Optech ILRIS 3D in dotazione allāINGV.
Tale sistema di misura non invasivo ha permesso di osservare nel dettagli
- ā¦