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 figure

An Anatomy of the French Labour Market

[Excerpt] Over the last decades, many European countries have experienced high and persistent unemployment rates. The bulk of labour market research has tackled this issue by emphasizing the effect of employment protection legislation, hereafter EPL, on labour market performance. As a result, the importance of labour market flexibility has been widely acknowledged. This view can be summarized by the expressed desire of the E.U. council to give member States incentives to “review and, where appropriate, reform overly restrictive elements in employment legislation that affect labour market dynamics [...] and to undertake other appropriate measures to promote a better balance between work and private life and between flexibility and security”. It is however striking that most of the reforms undertaken have contrasted sharply with this latter recommendation by favouring reforms at the margin. Those reforms have fostered two-tier systems, as the increase in labour market flexibility has taken place mainly through a series of marginal reforms that liberalized the use of fixed-term and/or non-standard employment contracts. Two-tier systems have promoted the emergence of dual employment protection which can be broadly defined as the coexistence of both long-term contracts, which benefit from stringent protection, and short-term contracts with little or no protection. It is often argued that this combination creates labour market segmentation, traps workers in a recurring sequence of frequent unemployment spells, favours unequal repartition of risk between workers and enhances inequalities. In particular, two-tier systems create excess labour turnover as they increase the incentives to create temporary rather than permanent jobs, reduce job destruction for stable jobs, but increase churning for temporary jobs. For instance in countries with stringent legal constraints on the termination of permanent jobs, such as France or Spain, it turns out that about 70 per cent to 90 per cent of entries into employment are in temporary jobs with very short duration (on average less than one month and a half in France). If excess labour turnover and its consequences are a concern for the economy as a whole, the dramatic spread of temporary jobs is even more a concern for young/less experienced workers as they are more likely to be negatively affected by the adverse effects of dual employment protection. The French labour market is no exception and has faced similar trends during the 1990s. Given the pervasiveness of temporary jobs on the labour market and their consequences on the society and economic outcomes, it is urgent to understand how two-tier systems shape the functioning of the labour market. This is the very purpose of the present report. After having described in details the salient features of the French dual labour market and having discussed the legislation at the root of French dualism, we review the different mechanisms through which dualism affect labour markets: labour market dynamics, wage inequality, human capital accumulation, job satisfaction, social integration and health. We consider whenever possible both theoretical insights and empirical evaluations. We finally conclude this report by providing possible directions to reform the labour market

Torque of guided light on an atom near an optical nanofiber

We calculate analytically and numerically the axial orbital and spin torques of guided light on a two-level atom near an optical nanofiber. We show that the generation of these torques is governed by the angular momentum conservation law in the Minkowski formulation. The orbital torque on the atom near the fiber has a contribution from the average recoil of spontaneously emitted photons. Photon angular momentum and atomic spin angular momentum can be converted into atomic orbital angular momentum. The orbital and spin angular momenta of the guided field are not transferred separately to the orbital and spin angular momenta of the atom

Directed polymer near a hard wall and KPZ equation in the half-space

We study the directed polymer with fixed endpoints near an absorbing wall, in the continuum and in presence of disorder, equivalent to the KPZ equation on the half space with droplet initial conditions. From a Bethe Ansatz solution of the equivalent attractive boson model we obtain the exact expression for the free energy distribution at all times. It converges at large time to the Tracy Widom distribution $F_4$ of the Gaussian Symplectic Ensemble (GSE). We compare our results with numerical simulations of the lattice directed polymer, both at zero and high temperature.Comment: 7 pages 4 figures one paragraph and one reference adde

Reconstructing intelligible audio speech from visual speech features

This work describes an investigation into the feasibility of producing intelligible audio speech from only visual speech fea- tures. The proposed method aims to estimate a spectral enve- lope from visual features which is then combined with an arti- ficial excitation signal and used within a model of speech pro- duction to reconstruct an audio signal. Different combinations of audio and visual features are considered, along with both a statistical method of estimation and a deep neural network. The intelligibility of the reconstructed audio speech is measured by human listeners, and then compared to the intelligibility of the video signal only and when combined with the reconstructed audio

Voicing classification of visual speech using convolutional neural networks

The application of neural network and convolutional neural net- work (CNN) architectures is explored for the tasks of voicing classification (classifying frames as being either non-speech, unvoiced, or voiced) and voice activity detection (VAD) of vi- sual speech. Experiments are conducted for both speaker de- pendent and speaker independent scenarios. A Gaussian mixture model (GMM) baseline system is de- veloped using standard image-based two-dimensional discrete cosine transform (2D-DCT) visual speech features, achieving speaker dependent accuracies of 79% and 94%, for voicing classification and VAD respectively. Additionally, a single- layer neural network system trained using the same visual fea- tures achieves accuracies of 86 % and 97 %. A novel technique using convolutional neural networks for visual speech feature extraction and classification is presented. The voicing classifi- cation and VAD results using the system are further improved to 88 % and 98 % respectively. The speaker independent results show the neural network system to outperform both the GMM and CNN systems, achiev- ing accuracies of 63 % for voicing classification, and 79 % for voice activity detection

On Time Synchronization Issues in Time-Sensitive Networks with Regulators and Nonideal Clocks

Flow reshaping is used in time-sensitive networks (as in the context of IEEE TSN and IETF Detnet) in order to reduce burstiness inside the network and to support the computation of guaranteed latency bounds. This is performed using per-flow regulators (such as the Token Bucket Filter) or interleaved regulators (as with IEEE TSN Asynchronous Traffic Shaping). Both types of regulators are beneficial as they cancel the increase of burstiness due to multiplexing inside the network. It was demonstrated, by using network calculus, that they do not increase the worst-case latency. However, the properties of regulators were established assuming that time is perfect in all network nodes. In reality, nodes use local, imperfect clocks. Time-sensitive networks exist in two flavours: (1) in non-synchronized networks, local clocks run independently at every node and their deviations are not controlled and (2) in synchronized networks, the deviations of local clocks are kept within very small bounds using for example a synchronization protocol (such as PTP) or a satellite based geo-positioning system (such as GPS). We revisit the properties of regulators in both cases. In non-synchronized networks, we show that ignoring the timing inaccuracies can lead to network instability due to unbounded delay in per-flow or interleaved regulators. We propose and analyze two methods (rate and burst cascade, and asynchronous dual arrival-curve method) for avoiding this problem. In synchronized networks, we show that there is no instability with per-flow regulators but, surprisingly, interleaved regulators can lead to instability. To establish these results, we develop a new framework that captures industrial requirements on clocks in both non-synchronized and synchronized networks, and we develop a toolbox that extends network calculus to account for clock imperfections.Comment: ACM SIGMETRICS 2020 Boston, Massachusetts, USA June 8-12, 202