10,660 research outputs found
The optical calcium frequency standards of PTB and NIST
We describe the current status of the Ca optical frequency standards with
laser-cooled neutral atoms realized in two different laboratories for the
purpose of developing a possible future optical atomic clock.
Frequency measurements performed at the Physikalisch-Technische Bundesanstalt
(PTB) and the National Institute of Standards and Technology (NIST) make the
frequency of the clock transition of 40Ca one of the best known optical
frequencies (relative uncertainty 1.2e-14) and the measurements of this
frequency in both laboratories agree to well within their respective
uncertainties.
Prospects for improvement by orders of magnitude in the relative uncertainty
of the standard look feasible.Comment: 13 pages, 11 figures, to appear in Comptes Rendus Physiqu
Reduced order modeling of fluid flows: Machine learning, Kolmogorov barrier, closure modeling, and partitioning
In this paper, we put forth a long short-term memory (LSTM) nudging framework
for the enhancement of reduced order models (ROMs) of fluid flows utilizing
noisy measurements. We build on the fact that in a realistic application, there
are uncertainties in initial conditions, boundary conditions, model parameters,
and/or field measurements. Moreover, conventional nonlinear ROMs based on
Galerkin projection (GROMs) suffer from imperfection and solution instabilities
due to the modal truncation, especially for advection-dominated flows with slow
decay in the Kolmogorov width. In the presented LSTM-Nudge approach, we fuse
forecasts from a combination of imperfect GROM and uncertain state estimates,
with sparse Eulerian sensor measurements to provide more reliable predictions
in a dynamical data assimilation framework. We illustrate the idea with the
viscous Burgers problem, as a benchmark test bed with quadratic nonlinearity
and Laplacian dissipation. We investigate the effects of measurements noise and
state estimate uncertainty on the performance of the LSTM-Nudge behavior. We
also demonstrate that it can sufficiently handle different levels of temporal
and spatial measurement sparsity. This first step in our assessment of the
proposed model shows that the LSTM nudging could represent a viable realtime
predictive tool in emerging digital twin systems
Comments on actuator fault accommodation
International audienceThe present work concerns the problem of progressive accommodation to actuator failure. An optimal nonlinear controller synthesis approach is formulated on the basis of the closed loop stability objective. The authors show the interest of the proposed method even for a local analysis when a linear approximation is used. This work focuses on a solution to ensure stability while accommodating to actuator failure. The approach is illustrated in an academic example
Nonlinear Feedback Control of Axisymmetric Aerial Vehicles
We investigate the use of simple aerodynamic models for the feedback control
of aerial vehicles with large flight envelopes. Thrust-propelled vehicles with
a body shape symmetric with respect to the thrust axis are considered. Upon a
condition on the aerodynamic characteristics of the vehicle, we show that the
equilibrium orientation can be explicitly determined as a function of the
desired flight velocity. This allows for the adaptation of previously proposed
control design approaches based on the thrust direction control paradigm.
Simulation results conducted by using measured aerodynamic characteristics of
quasi-axisymmetric bodies illustrate the soundness of the proposed approach
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