20,457 research outputs found
A coupled approximate deconvolution and dynamic mixed scale model for large-eddy simulation
Large-eddy simulations of incompressible Newtonian fluid flows with
approximate deconvolution models based on the van Cittert method are reported.
The Legendre spectral element method is used for the spatial discretization to
solve the filtered Navier--Stokes equations. A novel variant of approximate
deconvolution models blended with a mixed scale model using a dynamic
evaluation of the subgrid-viscosity constant is proposed. This model is
validated by comparing the large-eddy simulation with the direct numerical
simulation of the flow in a lid-driven cubical cavity, performed at a Reynolds
number of 12'000. Subgrid modeling in the case of a flow with coexisting
laminar, transitional and turbulent zones such as the lid-driven cubical cavity
flow represents a challenging problem. Moreover, the coupling with the spectral
element method having very low numerical dissipation and dispersion builds a
well suited framework to analyze the efficiency of a subgrid model. First- and
second-order statistics obtained using this new model are showing very good
agreement with the direct numerical simulation. Filtering operations rely on an
invertible filter applied in a modal basis and preserving the C0-continuity
across elements. No clipping on dynamic parameters was needed to preserve
numerical stability
Kalman-filter control schemes for fringe tracking. Development and application to VLTI/GRAVITY
The implementation of fringe tracking for optical interferometers is
inevitable when optimal exploitation of the instrumental capacities is desired.
Fringe tracking allows continuous fringe observation, considerably increasing
the sensitivity of the interferometric system. In addition to the correction of
atmospheric path-length differences, a decent control algorithm should correct
for disturbances introduced by instrumental vibrations, and deal with other
errors propagating in the optical trains. We attempt to construct control
schemes based on Kalman filters. Kalman filtering is an optimal data processing
algorithm for tracking and correcting a system on which observations are
performed. As a direct application, control schemes are designed for GRAVITY, a
future four-telescope near-infrared beam combiner for the Very Large Telescope
Interferometer (VLTI). We base our study on recent work in adaptive-optics
control. The technique is to describe perturbations of fringe phases in terms
of an a priori model. The model allows us to optimize the tracking of fringes,
in that it is adapted to the prevailing perturbations. Since the model is of a
parametric nature, a parameter identification needs to be included. Different
possibilities exist to generalize to the four-telescope fringe tracking that is
useful for GRAVITY. On the basis of a two-telescope Kalman-filtering control
algorithm, a set of two properly working control algorithms for four-telescope
fringe tracking is constructed. The control schemes are designed to take into
account flux problems and low-signal baselines. First simulations of the
fringe-tracking process indicate that the defined schemes meet the requirements
for GRAVITY and allow us to distinguish in performance. In a future paper, we
will compare the performances of classical fringe tracking to our Kalman-filter
control.Comment: 17 pages, 8 figures, accepted for publication in A&
Large-eddy simulation of the flow in a lid-driven cubical cavity
Large-eddy simulations of the turbulent flow in a lid-driven cubical cavity
have been carried out at a Reynolds number of 12000 using spectral element
methods. Two distinct subgrid-scales models, namely a dynamic Smagorinsky model
and a dynamic mixed model, have been both implemented and used to perform
long-lasting simulations required by the relevant time scales of the flow. All
filtering levels make use of explicit filters applied in the physical space (on
an element-by-element approach) and spectral (modal) spaces. The two
subgrid-scales models are validated and compared to available experimental and
numerical reference results, showing very good agreement. Specific features of
lid-driven cavity flow in the turbulent regime, such as inhomogeneity of
turbulence, turbulence production near the downstream corner eddy, small-scales
localization and helical properties are investigated and discussed in the
large-eddy simulation framework. Time histories of quantities such as the total
energy, total turbulent kinetic energy or helicity exhibit different evolutions
but only after a relatively long transient period. However, the average values
remain extremely close
High-resolution modal analysis
Usual modal analysis techniques are based on the Fourier transform. Due to
the Delta T . Delta f limitation, they perform poorly when the modal overlap mu
exceeds 30%. A technique based on a high-resolution analysis algorithm and an
order-detection method is presented here, with the aim of filling the gap
between the low- and the high-frequency domains (30%<mu<100%). A pseudo-impulse
force is applied at points of interests of a structure and the response is
measured at a given point. For each pair of measurements, the impulse response
of the structure is retrieved by deconvolving the pseudo-impulse force and
filtering the response with the result. Following conditioning treatments, the
reconstructed impulse response is analysed in different frequency-bands. In
each frequency-band, the number of modes is evaluated, the frequencies and
damping factors are estimated, and the complex amplitudes are finally
extracted. As examples of application, the separation of the twin modes of a
square plate and the partial modal analyses of aluminium plates up to a modal
overlap of 70% are presented. Results measured with this new method and those
calculated with an improved Rayleigh method match closely
Mid-infrared laser light nulling experiment using single-mode conductive waveguides
Aims: In the context of space interferometry missions devoted to the search
of exo-Earths, this paper investigates the capabilities of new single mode
conductive waveguides at providing modal filtering in an infrared and
monochromatic nulling experiment; Methods: A Michelson laser interferometer
with a co-axial beam combination scheme at 10.6 microns is used. After
introducing a Pi phase shift using a translating mirror, dynamic and static
measurements of the nulling ratio are performed in the two cases where modal
filtering is implemented and suppressed. No additional active control of the
wavefront errors is involved. Results: We achieve on average a statistical
nulling ratio of 2.5e-4 with a 1-sigma upper limit of 6e-4, while a best null
of 5.6e-5 is obtained in static mode. At the moment, the impact of external
vibrations limits our ability to maintain the null to 10 to 20 seconds.;
Conclusions: A positive effect of SM conductive waveguide on modal filtering
has been observed in this study. Further improvement of the null should be
possible with proper mechanical isolation of the setup.Comment: Accepted in A&A, 7 pages, 5 figure
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