Global and Koopman modes analysis of sound generation in mixing layers

It is now well established that linear and nonlinear instability waves play a significant role in the noise generation process for a wide variety of shear flows such as jets or mixing layers. In that context, the problem of acoustic radiation generated by spatially growing instability waves of two-dimensional subsonic and supersonic mixing layers are revisited in a global point of view, i.e., without any assumption about the base flow, in both a linear and a nonlinear framework by using global and Koopman mode decompositions. In that respect, a timestepping technique based on disturbance equations is employed to extract the most dynamically relevant coherent structures for both linear and nonlinear regimes. The present analysis proposes thus a general strategy for analysing the near-field coherent structures which are responsible for the acoustic noise in these configurations. In particular, we illustrate the failure of linear global modes to describe the noise generation mechanism associated with the vortex pairing for the subsonic regime whereas they appropriately explain the Mach wave radiation of instability waves in the supersonic regime. By contrast, the Dynamic Mode Decomposition (DMD) analysis captures both the near-field dynamics and the far-field acoustics with a few number of modes for both configurations. In addition, the combination of DMD and linear global modes analyses provides new insight about the influence on the radiated noise of nonlinear interactions and saturation of instability waves as well as their interaction with the mean flow

Commitment and Matching Contributions to Public Goods

This paper studies multi-stage processes of non-cooperative voluntary provision of public goods. In the first stage, one or more players announce contributions that may be conditional on the subsequent contributions of others. In later stages, players choose their own contributions and fulfill any commitments made in the first stage. Equilibrium contributions are characterized under different assumptions about the commitment ability of players, the number of public goods and whether players commit to matching rates or to discrete quantities. We focus on contribution mechanisms that can emerge and be sustainable without a central authority, and that therefore may be particularly relevant for the provision of international public goods. Efficient levels of public goods can be achieved under some circumstances.voluntary provision, matching contributions, commitment, multiple public goods

The Efficiency of Voluntary Pollution Abatement when Countries can Commit

We characterize a mechanism for reducing pollution emissions in which countries,　acting non-cooperatively, commit to match each others’ abatement levels and may　subsequently engage in emissions quota trading. The mechanism leads to an efficient level of emissions, and if the matching abatements process includes a quota trading stage, the marginal benefits of emissions are also equalized across countries. Given equilibrium matching rates, the initial allocation of emission quotas (before trading) reflects each country’s marginal valuation for lower pollution relative to its marginal benefit from emissions. These results hold for any number of countries, in an environment where countries have different abatement technologies and different benefits from emissions, and even if the emissions of countries are imperfect substitutes in each country’s damage function. In a dynamic twoperiod setting, the mechanism achieves both intra-temporal and inter-temporal efficiency. We extend the model by assuming that countries are voluntarily contributing to an international public good, in addition to undertaking pollution abatements, and find that the level of emissions may be efficient even without any matching abatement commitments, and the marginal benefits of emissions may be equalized across countries even without quota trading.Voluntary pollution abatement, matching commitments, emissions quota trading

The Efficiency of Voluntary Pollution Abatement when Countries can Commit

In this paper, we characterize a mechanism for reducing pollution emissions in which countries, acting non-cooperatively, commit to match each others' abatement levels and may subsequently engage in emissions quota trading. The analysis shows that the mechanism leads to efficient outcomes. The level of emissions is efficient, and if the matching abatements process includes a quota trading stage, the marginal benefits of emissions are also equalized across countries. Given the equilibrium matching rates, the initial allocation of emission quotas (before trading) reflects each country's marginal valuation for lower pollution relative to its marginal benefit from emissions. These results hold for any number of countries, in an environment where countries have different abatement technologies and different benefits from emissions, and even if the emissions of countries are imperfect substitutes in each country's damage function. In a dynamic two-period setting, the mechanism achieves both intra-temporal and inter-temporal efficiency. We extend the model by assuming that countries are voluntarily contributing to an international public good, in addition to undertaking pollution abatements, and find that the level of emissions may be efficient even without any matching abatement commitments, and the marginal benefits of emissions may be equalized across countries even without quota trading.voluntary pollution abatement, matching commitments, emissions quota trading

EBW technology applied on the ICRF antenna component

Central conductor is one of the key components of ion cyclotron ranges of heating antenna, which is usually formed by welding due to the complex structures. High level of welding seam quality and small deformation are very important to central conductor. Electron beam welding (EBW) is suggested as the central conductor welding. To meet EBW requirements and reduce the risk, complex and high level of the accuracy welding fixture have been designed for central conductor EBW. Some samples were manufactured to do test and examination for EBW qualification before central conductor welding. Based on the welding parameters, thermal analysis using finite element method for the welding seam have been carried out. One mockup of central conductor for EBW has been made for proving welding parameters. In addition, some postwelding process were employed after one central conductor EBW. Results of examination and inspection of one central conductor using EBW are presented in this paper

Fast optimal transition between two equilibrium states

We demonstrate a technique based on invariants of motion for a time-dependent Hamiltonian, allowing a fast transition to a final state identical in theory to that obtained through a perfectly adiabatic transformation. This method is experimentally applied to the fast decompression of an ultracold cloud of Rubidium 87 atoms held in a harmonic magnetic trap, in the presence of gravity. We are able to decompress the trap by a factor of 15 within 35 ms with a strong suppression of the sloshing and breathing modes induced by the large vertical displacement and curvature reduction of the trap. When compared to a standard linear decompression, we achieve a gain of a factor of 37 on the transition time.Comment: 5 pages, 4 figures, an error in Eq. (2) has been correcte

Self-management of context-aware overlay ambient networks

Ambient Networks (ANs) are dynamically changing and heterogeneous as they consist of potentially large numbers of independent, heterogeneous mobile nodes, with spontaneous topologies that can logically interact with each other to share a common control space, known as the Ambient Control Space. ANs are also flexible i.e. they can compose and decompose dynamically and automatically, for supporting the deployment of cross-domain (new) services. Thus, the AN architecture must be sophisticatedly designed to support such high level of dynamicity, heterogeneity and flexibility. We advocate the use of service specific overlay networks in ANs, that are created on-demand according to specific service requirements, to deliver, and to automatically adapt services to the dynamically changing user and network context. This paper presents a self-management approach to create, configure, adapt, contextualise, and finally teardown service specific overlay networks

Personalized Federated X -armed Bandit

In this work, we study the personalized federated $\mathcal{X}$-armed bandit problem, where the heterogeneous local objectives of the clients are optimized simultaneously in the federated learning paradigm. We propose the \texttt{PF-PNE} algorithm with a unique double elimination strategy, which safely eliminates the non-optimal regions while encouraging federated collaboration through biased but effective evaluations of the local objectives. The proposed \texttt{PF-PNE} algorithm is able to optimize local objectives with arbitrary levels of heterogeneity, and its limited communications protects the confidentiality of the client-wise reward data. Our theoretical analysis shows the benefit of the proposed algorithm over single-client algorithms. Experimentally, \texttt{PF-PNE} outperforms multiple baselines on both synthetic and real life datasets