Computing return times or return periods with rare event algorithms

The average time between two occurrences of the same event, referred to as its return time (or return period), is a useful statistical concept for practical applications. For instance insurances or public agencies may be interested by the return time of a 10 m flood of the Seine river in Paris. However, due to their scarcity, reliably estimating return times for rare events is very difficult using either observational data or direct numerical simulations. For rare events, an estimator for return times can be built from the extrema of the observable on trajectory blocks. Here, we show that this estimator can be improved to remain accurate for return times of the order of the block size. More importantly, we show that this approach can be generalised to estimate return times from numerical algorithms specifically designed to sample rare events. So far those algorithms often compute probabilities, rather than return times. The approach we propose provides a computationally extremely efficient way to estimate numerically the return times of rare events for a dynamical system, gaining several orders of magnitude of computational costs. We illustrate the method on two kinds of observables, instantaneous and time-averaged, using two different rare event algorithms, for a simple stochastic process, the Ornstein-Uhlenbeck process. As an example of realistic applications to complex systems, we finally discuss extreme values of the drag on an object in a turbulent flow. © 2018 IOP Publishing Ltd and SISSA Medialab srl

Maritime ports and inland interconnections: a transactional analysis of container barge transport in France

International audienceRecent research on maritime ports hinterlands points out the relevance of mass ground transport modes such as barge transport for enormous flows of containers to and from harbours, especially when a maritime port is located at the mouth of a river. Though, the modal share of container barge transport in French maritime ports (9% of TEU in Le Havre and 5% in Marseille in 2007) is significantly lower than elsewhere (32% in Rotterdam and 33% in Antwerp). Some reports and studies explain the viscosity of container barge transport flows as a result of several factors, generally concentrated around the seaport community. In continuation of previous seminal works, this paper adopts a neo-institutional approach (Williamson, 1985; 1996) of container barge transport to understand how the factors generating this viscosity are managed. Section 2 describes the characteristics of the transaction of container barge transport. Section 3 is devoted to its attributes (asset specificity, frequency, uncertainty). According to Williamson’s (1996) remediableness criterion, the observed governance structure of a given transaction is presumed efficient and aligned to its attributes. Thus, Section 4 deals with observed governance structures of container barge transport chains with a focus on Le Havre, main French container seaport and shows how agents try to limit opportunism in ex-post haggling over quasi-rents or under-investments. Implementation of a new institutional environment to modify governance structures is analysed, and a comparison with currently implemented governance structures observed in Rhine is made. Finally, Section 5 suggests ways of dealing with the remaining coordination problems impeding the development of container barge transport in France

Room Temperature Optically Tunability and Inhomogeneous Broadening in 2D-Layered Organic-Inorganic Perovskite Pseudobinary Alloys.

International audienceWe focus here our attention on a particular family of 2D-layered and 3D hybrid perovskite molecular crystals, the mixed perovskites (C6H5–C2H4–NH3)2PbZ4(1–x)Y4x and (CH3–NH3)PbZ3(1–x)Y3x, where Z and Y are halogen ions such as I, Br, and Cl. Studying experimentally the disorder-induced effects on the optical properties of the 2D mixed layered materials, we demonstrate that they can be considered as pseudobinary alloys, exactly like Ga1–xAlxAs, Cd1–xHgxTe inorganic semiconductors, or previously reported 3D mixed hybrid perovskite compounds. 2D-layered and 3D hybrid perovskites afford similar continuous optical tunability at room temperature. Our theoretical analysis allows one to describe the influence of alloying on the excitonic properties of 2D-layered perovskite molecular crystals. This model is further refined by considering different Bohr radii for pure compounds. This study confirms that despite a large binding energy of several 100 meV, the 2D excitons present a Wannier character rather than a Frenkel character. The small inhomogeneous broadening previously reported in 3D hybrid compounds at low temperature is similarly consistent with the Wannier character of free excitons