5,323 research outputs found
Investigations on the Turbulent Wake of a Generic Space Launcher Geometry in the Hypersonic Flow Regime
The turbulent wake flow of generic rocket configurations is investigated experimentally and numerically at a freestream Mach number of 6.0 and a unit Reynolds number of 10 x 10^6. The flow condition is based on the trajectory of Ariane V at an altitude of 50 km, which is used as baseline to address the overarching tasks of wake flows in the hypersonic regime like fluid-structural coupling, reverse hot jets and base heating. Experiments using pressure transducers and high-speed schlieren measurement technique were conducted to gain insight into the local pressure fluctuations on the base and the oscillations of the recompression shock. This experimental configuration features a wedge-profiled strut orthogonally mounted to the main body. Additionally, the influence of cylindrical nozzle extensions attached to the base of the rocket is investigated, which is the link to the numerical investigations. Here, the axisymmetric model possesses a cylindrical sting support of the same diameter as the nozzle extensions. The sting support allows investigations of a undisturbed wake flow. A time-accurate zonal RANS/LES approach was applied to identify shocks, expansion waves, and the highly unsteady recompression region numerically. Subsequently, experimental and numerical results in the strut-averted region are opposed with regard to the wall pressure and recompression shock frequency spectra. For the compared configurations, experimental pressure spectra exhibit dominant Strouhal numbers at about S rD = 0.03 and 0.27 and the recompression
shock oscillates at 0.2. In general, the numerical pressure and recompression shock fluctuations agree satisfactorily to the experimental results. The experiments with a blunt base reveal base-pressure spectra with dominant Strouhal numbers at 0.08 at the center position and 0.145, 0.21 − 0.22 and 0.31 − 0.33 at the outskirts of the base
Relational Quantum Cosmology
The application of quantum theory to cosmology raises a number of conceptual
questions, such as the role of the quantum-mechanical notion of "observer" or
the absence of a time variable in the Wheeler-DeWitt equation. I point out that
a relational formulation of quantum mechanics, and more in general the
observation that evolution is always relational, provides a coherent solution
to this tangle of problems.Comment: 20 pages, 4 figures. Contribution to the forthcoming book on
Philosophy of Cosmology edited by K. Chamcham, J. Barrow, J. Silk and S.
Saunders for Cambridge University Pres
Nonlinear diffusion & thermo-electric coupling in a two-variable model of cardiac action potential
This work reports the results of the theoretical investigation of nonlinear
dynamics and spiral wave breakup in a generalized two-variable model of cardiac
action potential accounting for thermo-electric coupling and diffusion
nonlinearities. As customary in excitable media, the common Q10 and Moore
factors are used to describe thermo-electric feedback in a 10-degrees range.
Motivated by the porous nature of the cardiac tissue, in this study we also
propose a nonlinear Fickian flux formulated by Taylor expanding the voltage
dependent diffusion coefficient up to quadratic terms. A fine tuning of the
diffusive parameters is performed a priori to match the conduction velocity of
the equivalent cable model. The resulting combined effects are then studied by
numerically simulating different stimulation protocols on a one-dimensional
cable. Model features are compared in terms of action potential morphology,
restitution curves, frequency spectra and spatio-temporal phase differences.
Two-dimensional long-run simulations are finally performed to characterize
spiral breakup during sustained fibrillation at different thermal states.
Temperature and nonlinear diffusion effects are found to impact the
repolarization phase of the action potential wave with non-monotone patterns
and to increase the propensity of arrhythmogenesis
Spacetime states and covariant quantum theory
In it's usual presentation, classical mechanics appears to give time a very
special role. But it is well known that mechanics can be formulated so as to
treat the time variable on the same footing as the other variables in the
extended configuration space. Such covariant formulations are natural for
relativistic gravitational systems, where general covariance conflicts with the
notion of a preferred physical-time variable. The standard presentation of
quantum mechanics, in turns, gives again time a very special role, raising well
known difficulties for quantum gravity. Is there a covariant form of
(canonical) quantum mechanics? We observe that the preferred role of time in
quantum theory is the consequence of an idealization: that measurements are
instantaneous. Canonical quantum theory can be given a covariant form by
dropping this idealization. States prepared by non-instantaneous measurements
are described by "spacetime smeared states". The theory can be formulated in
terms of these states, without making any reference to a special time variable.
The quantum dynamics is expressed in terms of the propagator, an object
covariantly defined on the extended configuration space.Comment: 20 pages, no figures. Revision: minor corrections and references
adde
Kant, Heidegger and spacing
Kant's metaphysics of space
periods is explored via his
in both the pre-critical and critical
relation to Leibniz, the incongruent
counterparts argument, and the distinctive arguments of the
Transcendental Aesthetic. Sequentially, Heidegger's phenomenology of
space from the period of Being and Time is explicated in terms of
concepts like deseverance, directionality, region and equipmentality.
The two analyses are found to overlap on several key points. These
include: the priority of the whole over the parts, openness, and
exteriority and thus non-discursivity.
The points of overlap we call
'spacing' . Through further analyses,
it is discovered that the
concepts of spacing are precisely the concepts required by these two
philosophers even when they treat of subjects not normally considered
essentially spacial. These subjects include the nature of temporal
relations, of selfhood and self-constancy, and of the experience and
significance of art. The importance of spacing for these subjects is
individually discussed, as are possible reasons why the language of
space should be required
Robustness Analysis of Video-Language Models Against Visual and Language Perturbations
Joint visual and language modeling on large-scale datasets has recently shown
good progress in multi-modal tasks when compared to single modal learning.
However, robustness of these approaches against real-world perturbations has
not been studied. In this work, we perform the first extensive robustness study
of video-language models against various real-world perturbations. We focus on
text-to-video retrieval and propose two large-scale benchmark datasets,
MSRVTT-P and YouCook2-P, which utilize 90 different visual and 35 different
text perturbations. The study reveals some interesting initial findings from
the studied models: 1) models are generally more susceptible when only video is
perturbed as opposed to when only text is perturbed, 2) models that are
pre-trained are more robust than those trained from scratch, 3) models attend
more to scene and objects rather than motion and action. We hope this study
will serve as a benchmark and guide future research in robust video-language
learning. The benchmark introduced in this study along with the code and
datasets is available at https://bit.ly/3CNOly4.Comment: NeurIPS 2022 Datasets and Benchmarks Track. This projects webpage is
located at https://bit.ly/3CNOly
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