Directional Dynamics along Arbitrary Curves in Cellular Automata

This paper studies directional dynamics in cellular automata, a formalism previously introduced by the third author. The central idea is to study the dynamical behaviour of a cellular automaton through the conjoint action of its global rule (temporal action) and the shift map (spacial action): qualitative behaviours inherited from topological dynamics (equicontinuity, sensitivity, expansivity) are thus considered along arbitrary curves in space-time. The main contributions of the paper concern equicontinuous dynamics which can be connected to the notion of consequences of a word. We show that there is a cellular automaton with an equicontinuous dynamics along a parabola, but which is sensitive along any linear direction. We also show that real numbers that occur as the slope of a limit linear direction with equicontinuous dynamics in some cellular automaton are exactly the computably enumerable numbers

μ\mu-Limit Sets of Cellular Automata from a Computational Complexity Perspective

This paper concerns $\mu$-limit sets of cellular automata: sets of configurations made of words whose probability to appear does not vanish with time, starting from an initial $\mu$-random configuration. More precisely, we investigate the computational complexity of these sets and of related decision problems. Main results: first, $\mu$-limit sets can have a $\Sigma\_3^0$-hard language, second, they can contain only $\alpha$-complex configurations, third, any non-trivial property concerning them is at least $\Pi\_3^0$-hard. We prove complexity upper bounds, study restrictions of these questions to particular classes of CA, and different types of (non-)convergence of the measure of a word during the evolution.Comment: 41 page

Approche numérique et expérimentale du transfert de chaleur en écoulement de Gortler

Nous avons observé un comportement inattendu du transfert thermique dans une couche limite laminaire sur paroi concave à basse vitesse nominale, une configuration particulièrement ignorée dans la littérature. Cette couche limite contient des tourbillons de Görtler intenses. Dans cette situation, les mesures précises du flux de chaleur pariétal démontrent que l'amélioration du transfert thermique est extrêmement grande, au-dessus de celui correspondant au cas d'une couche limite turbulente sur plaque plane. L'analyse démontre que pour une gamme de vitesse inférieure à une certaine vitesse critique, la couche limite transitionnelle est dominée par l'instabilité centrifuge. D'autres résultats renforcent cette discussion dont l'analyse de l'influence du forçage des perturbations amonts ainsi qu'un ensemble de visualisations d'écoulement. Une méthode numérique, résolvant les équations de Navier-Stokes pour une couche limite temporelle, permet une analogie avec le développement linéaire et non-linéaire des tourbillons réguliers de Görtler et leur instabilité primaire. Ces résultats sont comparés aux mesures expérimentales

Mise en œuvre d’un calcul couplé fluide/structure avec un code CFD généraliste

Le présent article est relatif à la simulation numérique d’un problème couplé fluide/structure. On s’intéresse ici au cas de deux cylindres concentriques couplés par un fluide visqueux incompressible. L’étude s’intéresse d’une part à la prédiction des efforts fluide (efforts de pression et de viscosité) sur la structure, d’autre part à la mise en œuvre d’un algorithme de couplage pour la résolution du problème couplé au moyen d’un code CFD généraliste

Approche numérique et expérimentale du transfert de chaleur en écoulement de Gortler

Colloque avec actes et comité de lecture. Internationale.International audienceNous avons observé un comportement inattendu du transfert thermique dans une couche limite laminaire sur paroi concave à basse vitesse nominale, une configuration particulièrement ignorée dans la littérature. Cette couche limite contient des tourbillons de Görtler intenses. Dans cette situation, les mesures précises du flux de chaleur pariétal démontrent que l'amélioration du transfert thermique est extrêmement grande, au-dessus de celui correspondant au cas d'une couche limite turbulente sur plaque plane. L'analyse démontre que pour une gamme de vitesse inférieure à une certaine vitesse critique, la couche limite transitionnelle est dominée par l'instabilité centrifuge. D'autres résultats renforcent cette discussion dont l'analyse de l'influence du forçage des perturbations amonts ainsi qu'un ensemble de visualisations d'écoulement. Une méthode numérique, résolvant les équations de Navier-Stokes pour une couche limite temporelle, permet une analogie avec le développement linéaire et non-linéaire des tourbillons réguliers de Görtler et leur instabilité primaire. Ces résultats sont comparés aux mesures expérimentales

STABILITY OF CONCAVE BOUNDARY LAYERS-OVERVIEW OF STABILITY MECHANISM AND RECENT FINDINGS

A series of experimental measurements of flow and heat transfer under streamwise Gortler vortices shows conclusively that the local surface beat transfer rates can exceed that of the turbulent boundary layer even in the absence of turbulence. We have observed unexpected behavior of heat transfer in a laminar boundary layer on a concave wall at low nominal velocity, a configuration ignored in the literature. In this situation. precise measurements of the wall heat flux show that the heat transfer enhancement is extremely elevated, above that corresponding to the case of a turbulent boundary layer on a flat plate. The nonlinearly developing steady streamwise vortex (primary instability) heat transfer can already bridge the local laminar to turbulent beat transfer values in the absence of turbulence. The analysis shows that for a range of velocities less than a certain critical velocity, the transitional boundary layer is dominated by centrifugal instability. However, the steady streamwise vortices, like steady Taylor vortices between coaxial rotating cylinders, arc susceptible to secondary instabilities of the varicose and sinuous nodes. In experiments both modes appear to coexist and cause waviness of the primary streamwise vortices. Other results confirm this discussion based on analysis of the influence of a forcing upstream disturbance

A French research infrastructure for Coastal Ocean and Seashore Observations

ILICO, the French Research Infrastructure for Coastal Ocean and Nearshore Observations, is a notable example of national and pan-institutional efforts to expand knowledge of the complex processes at work within the critical coastal zone in line with the European Ocean Observing System perspective. At the interface between land and sea, ILICO is necessarily multiscale and pluri-disciplinary. It federates complementary distributed observation services (networks) monitoring coastline dynamics, sea level evolution, physical and biogeochemical water properties, coastal water dynamics, phytoplankton, benthos composition and coral reef health in order to monitor and detect episodic events (e.g. extreme events), understand physics-biology multi-scale coupling and answer operational needs and societal demands. Each network is accredited and receives funding from the French Ministry for Higher Education, Research and Innovation and national public research institutions. In addition to the sustained and long-term nature of its time-series data, ILICO's observation sites have unique geographical coverage spanning both metropolitan coastlines and those of overseas national territories. Significantly, although its scope is not strictly limited to coastal marine systems, ILICO is the French-node of the Joint European Research Infrastructure for Coastal Marine systems (JERICO-RI) led by France. ILICO's latest advances include the implementation of an open data strategy, aggregating multisource data to ensure optimal access and re-use by the scientific community, for operational ocean observing and forecasting, and by public authorities and citizens

Suspended sediment concentration field quantified from a calibrated MultiBeam EchoSounder

Acoustic scattering can be used to estimate Suspended Sediment Concentration (SSC) through acoustic inversion methods. Current SSC quantification methods are mostly unable to observe both spatial and temporal variations. Here, we assess the possibility to measure both using a Multibeam Echosounder (MBES). MBES combine a large spatial covering in the water column and the capability to measure ‘on route’, allowing a better representativity of the measurements. Time-series of raw EM3002-MBES data at 300 kHz were acquired during a 5-hours field experiment at a fixed location in the Aulne macrotidal estuary (France) during ebb, ensuring sufficient SSC variations. Concurrently, 4-frequencies Acoustic Backscattering System (ABS) profiles were acquired in the water column, as well as turbidity profiles, further converted into SSC using collected water samples. An original in-situ calibration was performed on the MBES, using a tungsten sphere of known properties, which allowed corrections to be made to the volume backscattered levels over the echosounder fan. Using ABS-derived equivalent radii, the MBES backscattered signal was inverted to retrieve an SSC estimate. Good consistency between MBES time-series observations and turbidity-derived SSC is observed. This experiment demonstrates the potential use of MBES for 3-dimensional turbidity observations in coastal areas, which is of great interest for sediment flux quantification

Short-term morphodynamics of a submarine dune bank in a macrotidal environment: observations from the Creïzic bank (South Brittany, France)

International audienceMarine sand dunes and sand banks can exhibit rapid morphological changes in coastal areas, which may posea hazard for navigation in shallow water or generally in areas near marine infrastructures. Understanding theirdynamics is still a scientific challenge, especially their migration processes and its consequences on the seabedtopography and on sediment transport. In this study, the morphodynamics of a submarine sandbank in a macrotidalenvironment is investigated at short time scale. The aim is to quantify the morphological changes and migrationof the bank and overlying dunes (primary and secondary sedimentary structures) in order to estimate sedimenttransport corresponding to bedform migration. The study site is the Creïzic submarine dune bank, located in theGulf of Morbihan (South Brittany, France). The Creïzic bank is about 1200 m long, 600 m wide with dune heightsfrom 0.1 m to 3.5 m. It is situated in water depths between 5 m and 20 m and is mainly composed of shellyand coarse sand. The main hydrodynamic forcing factor at play on this sand bank is a strong and unsteady tidalcurrent. During storm events, waves, mostly from fetch, also have an impact as the bank is shallow. The bankfeatures three main sedimentary structures: a central dune field at the middle of the bank and two dune fields oneither side of the bank. Former studies at yearly time scales have shown that the central area is highly mobilewith significant net migration of the dunes. To investigate the morphodynamics of the Creïzic bank, a 37-day fieldcampaign has been carried out in the fall of 2017. A high-resolution (50 cm grid size) bathymetric dataset has beenacquired using a mulitibeam echosounder (MBES), with repeated surveys at the time scales of a month (entirebank), a tidal cycle and hour scales (portions and patches on the bank). From this data, velocities and directionsof migration are estimated by computing differential Digital Terrain Models (DTM), and quantified with a spatialcross-correlation algorithm. The preliminary analysis of the data set confirms the high mobility of the primaryand secondary sedimentary structures at short time scales, with migration distances up to 1 m over one hour forbedforms with metric heights and wavelengths of the order of 30 m in the central zone, and reaching 50 cm overtwo hours for dunes with heights between 0.2 m and 0. 5 m and wavelengths between 2 m and 4 m on the south-eastside of the bank

Short-term morphodynamics of a submarine dune bank in a macrotidal environment: observations from the Creïzic bank (South Brittany, France)

International audienceMarine sand dunes and sand banks can exhibit rapid morphological changes in coastal areas, which may posea hazard for navigation in shallow water or generally in areas near marine infrastructures. Understanding theirdynamics is still a scientific challenge, especially their migration processes and its consequences on the seabedtopography and on sediment transport. In this study, the morphodynamics of a submarine sandbank in a macrotidalenvironment is investigated at short time scale. The aim is to quantify the morphological changes and migrationof the bank and overlying dunes (primary and secondary sedimentary structures) in order to estimate sedimenttransport corresponding to bedform migration. The study site is the Creïzic submarine dune bank, located in theGulf of Morbihan (South Brittany, France). The Creïzic bank is about 1200 m long, 600 m wide with dune heightsfrom 0.1 m to 3.5 m. It is situated in water depths between 5 m and 20 m and is mainly composed of shellyand coarse sand. The main hydrodynamic forcing factor at play on this sand bank is a strong and unsteady tidalcurrent. During storm events, waves, mostly from fetch, also have an impact as the bank is shallow. The bankfeatures three main sedimentary structures: a central dune field at the middle of the bank and two dune fields oneither side of the bank. Former studies at yearly time scales have shown that the central area is highly mobilewith significant net migration of the dunes. To investigate the morphodynamics of the Creïzic bank, a 37-day fieldcampaign has been carried out in the fall of 2017. A high-resolution (50 cm grid size) bathymetric dataset has beenacquired using a mulitibeam echosounder (MBES), with repeated surveys at the time scales of a month (entirebank), a tidal cycle and hour scales (portions and patches on the bank). From this data, velocities and directionsof migration are estimated by computing differential Digital Terrain Models (DTM), and quantified with a spatialcross-correlation algorithm. The preliminary analysis of the data set confirms the high mobility of the primaryand secondary sedimentary structures at short time scales, with migration distances up to 1 m over one hour forbedforms with metric heights and wavelengths of the order of 30 m in the central zone, and reaching 50 cm overtwo hours for dunes with heights between 0.2 m and 0. 5 m and wavelengths between 2 m and 4 m on the south-eastside of the bank