19,646 research outputs found
Parametric instability and wave turbulence driven by tidal excitation of internal waves
We investigate the stability of stratified fluid layers undergoing
homogeneous and periodic tidal deformation. We first introduce a local model
which allows to study velocity and buoyancy fluctuations in a Lagrangian domain
periodically stretched and sheared by the tidal base flow. While keeping the
key physical ingredients only, such a model is efficient to simulate planetary
regimes where tidal amplitudes and dissipation are small. With this model, we
prove that tidal flows are able to drive parametric subharmonic resonances of
internal waves, in a way reminiscent of the elliptical instability in rotating
fluids. The growth rates computed via Direct Numerical Simulations (DNS) are in
very good agreement with WKB analysis and Floquet theory. We also investigate
the turbulence driven by this instability mechanism. With spatio-temporal
analysis, we show that it is a weak internal wave turbulence occurring at small
Froude and buoyancy Reynolds numbers. When the gap between the excitation and
the Brunt-V\"ais\"al\"a frequencies is increased, the frequency spectrum of
this wave turbulence displays a -2 power law reminiscent of the high-frequency
branch of the Garett and Munk spectrum (Garrett & Munk 1979) which has been
measured in the oceans. In addition, we find that the mixing efficiency is
altered compared to what is computed in the context of DNS of stratified
turbulence excited at small Froude and large buoyancy Reynolds numbers and is
consistent with a superposition of waves.Comment: Accepted for publication in Journal of Fluid Mechanics, 27 pages, 21
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The Universal Aspect Ratio of Vortices in Rotating Stratifi?ed Flows: Experiments and Observations
We validate a new law for the aspect ratio of vortices in a
rotating, stratified flow, where and are the vertical half-height and
horizontal length scale of the vortices. The aspect ratio depends not only on
the Coriolis parameter f and buoyancy (or Brunt-Vaisala) frequency of
the background flow, but also on the buoyancy frequency within the vortex
and on the Rossby number of the vortex such that . This law for is obeyed precisely by the
exact equilibrium solution of the inviscid Boussinesq equations that we show to
be a useful model of our laboratory vortices. The law is valid for both
cyclones and anticyclones. Our anticyclones are generated by injecting fluid
into a rotating tank filled with linearly-stratified salt water. The vortices
are far from the top and bottom boundaries of the tank, so there is no Ekman
circulation. In one set of experiments, the vortices viscously decay, but as
they do, they continue to obey our law for , which decreases over time.
In a second set of experiments, the vortices are sustained by a slow continuous
injection after they form, so they evolve more slowly and have larger |Ro|, but
they also obey our law for . The law for is not only validated
by our experiments, but is also shown to be consistent with observations of the
aspect ratios of Atlantic meddies and Jupiter's Great Red Spot and Oval BA. The
relationship for is derived and examined numerically in a companion
paper by Hassanzadeh et al. (2012).Comment: Submitted to the Journal of Fluid Mechanics. Also see the companion
paper by Hassanzadeh et al. "The Universal Aspect Ratio of Vortices in
Rotating Stratifi?ed Flows: Theory and Simulation" 201
The linear instability of the stratified plane Couette flow
We present the stability analysis of a plane Couette flow which is stably
stratified in the vertical direction orthogonally to the horizontal shear.
Interest in such a flow comes from geophysical and astrophysical applications
where background shear and vertical stable stratification commonly coexist. We
perform the linear stability analysis of the flow in a domain which is periodic
in the stream-wise and vertical directions and confined in the cross-stream
direction. The stability diagram is constructed as a function of the Reynolds
number Re and the Froude number Fr, which compares the importance of shear and
stratification. We find that the flow becomes unstable when shear and
stratification are of the same order (i.e. Fr 1) and above a moderate
value of the Reynolds number Re700. The instability results from a
resonance mechanism already known in the context of channel flows, for instance
the unstratified plane Couette flow in the shallow water approximation. The
result is confirmed by fully non linear direct numerical simulations and to the
best of our knowledge, constitutes the first evidence of linear instability in
a vertically stratified plane Couette flow. We also report the study of a
laboratory flow generated by a transparent belt entrained by two vertical
cylinders and immersed in a tank filled with salty water linearly stratified in
density. We observe the emergence of a robust spatio-temporal pattern close to
the threshold values of F r and Re indicated by linear analysis, and explore
the accessible part of the stability diagram. With the support of numerical
simulations we conclude that the observed pattern is a signature of the same
instability predicted by the linear theory, although slightly modified due to
streamwise confinement
Spontaneous generation of inertial waves from boundary turbulence in a librating sphere
In this work, we report the excitation of inertial waves in a librating
sphere even for libration frequencies where these waves are not directly
forced. This spontaneous generation comes from the localized turbulence induced
by the centrifugal instabilities in the Ekman boundary layer near the equator
and does not depend on the libration frequency. We characterize the key
features of these inertial waves in analogy with previous studies of the
generation of internal waves in stratified flows from localized turbulent
patterns. In particular, the temporal spectrum exhibits preferred values of
excited frequency. This first-order phenomenon is generic to any rotating flow
in the presence of localized turbulence and is fully relevant for planetary
applications
Image metadata estimation using independent component analysis and regression
In this paper, we describe an approach to camera metadata estimation using regression based on Independent Component Analysis (ICA). Semantic scene classification of images using camera metadata related to capture conditions has had some success in the past. However, different makes and models of camera capture different types of metadata and this severely hampers the application of this kind of approach in real systems that consist of photos captured by many different users. We propose to address this issue by using regression to predict the missing metadata from observed data, thereby providing more complete (and hence more useful) metadata for the entire image corpus. The proposed approach uses an ICA based approach to regression
Experience with Ada on the F-18 High Alpha Research Vehicle Flight Test Program
Considerable experience was acquired with Ada at the NASA Dryden Flight Research Facility during the on-going High Alpha Technology Program. In this program, an F-18 aircraft was highly modified by the addition of thrust-vectoring vanes to the airframe. In addition, substantial alteration was made in the original quadruplex flight control system. The result is the High Alpha Research Vehicle. An additional research flight control computer was incorporated in each of the four channels. Software for the research flight control computer was written in Ada. To date, six releases of this software have been flown. This paper provides a detailed description of the modifications to the research flight control system. Efficient ground-testing of the software was accomplished by using simulations that used the Ada for portions of their software. These simulations are also described. Modifying and transferring the Ada for flight software to the software simulation configuration has allowed evaluation of this language. This paper also discusses such significant issues in using Ada as portability, modifiability, and testability as well as documentation requirements
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