Laboratoire des Écoulements Géophysiques et Industriels

Low-cost aerosol sensors give the opportunity to build air quality monitoring network, allowing for increased spatial resolution of particulate matter (PM) measurements [1]. This type of network has already been used in urban environments to capture variation of PM at the city scale [2]. Another environment where PM can change quickly with space is alpine valleys. Indeed, the vertical stratification of air caused by nighttime ground cooling induces important PM concentration variation along a slope. While it is well known that aerosol concentration is greater at the valley floor, the aim of this study is to describe how PM concentration evolves with altitude at the ground and what meteorological phenomenon can change the vertical distribution of PM. Indeed, complex topography influences vertical transport of aerosols through upward and downward motions as well as through local to regional sources of PM.In the development of a monitoring station for mountainous regions, it is necessary to incorporate specific constraints, such as challenging access to measurement sites, a lack of phone network, and limited access to power. Optical particle counters are low-cost PM sensors that are both small and energy-efficient, enabling the development of portable and autonomous stations. These are key features for building a monitoring network in an alpine valley. We designed monitoring stations based on these constraints. The stations have several sensors that measure the temperature, humidity, and PM10, PM2.5, and PM1 levels. Every fifteen minutes, a microcontroller gathers data and stores them on an SD card. The energy is provided by a battery recharged by a solar panel recessed at the top of the 3D-printed framework. The overall size of the stations is a cube with a 30 cm side length.These laboratory-made stations have been used to study PM concentration in the Arve River valley, located in the northern French Alps and encompassing cities like Sallanches, Passy, and Chamonix. The network is composed of 12 monitoring stations and 6 monitored sites positioned on the slopes around the Passy basin. One monitoring site is located on the valley floor, on an air quality agency monitoring station containing a TEOM-FDMS. The discussion will focus on the comparison of low-cost stations with the reference measurements, the benefits of using a low-cost monitoring network to study PM concentrations in mountainous terrain, and the limitations inherent to low-cost sensors and autonomous stations.[1] Bagkis, E., Hassani, A., Schneider, P., DeSouza, P., Shetty, S., Kassandros, T., Salamalikis, V., Castell, N., Karatzas, K., Ahlawat, A., Khan, J. Evolving trends in application of low-cost air quality sensor networks: challenges and future directions. npj Clim Atmos Sci 8, 335 (2025). https://doi.org/10.1038/s41612-025-01216-4[2] Feinberg, S. N., Williams, R., Hagler, G., Low, J., Smith, L., Brown, R., Garver, D., Davis, M., Morton, M., Schaefer, J. & Campbell, J. Examining spatiotemporal variability of urban particulate matter and application of high-time resolution data from a network of low-cost air pollution sensors. Atmospheric Environment 213, 579–584 (2019). https://doi.org/10.1016/j.atmosenv.2019.06.026

Abstract Relaxation of the turbulent and fluctuating passive scalar fields generated by a specific staggered roughness distribution in the entrance region of an initially laminar channel flow is studied through direct numerical simulations. Large staggered roughness elements of height k = 0.27 H partly recover the entry zone, prior to a smooth channel (control channel) wherein both the turbulent flow and scalar fields decay, because the Karman (friction) number <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>R</mml:mi> <mml:msub> <mml:mi>e</mml:mi> <mml:mi>τ</mml:mi> </mml:msub> <mml:mo>=</mml:mo> <mml:mi>H</mml:mi> <mml:mover> <mml:msub> <mml:mi>u</mml:mi> <mml:mi>τ</mml:mi> </mml:msub> <mml:mo stretchy="true">_</mml:mo> </mml:mover> <mml:mo>/</mml:mo> <mml:mi>ν</mml:mi> <mml:mo>≈</mml:mo> <mml:mn>70</mml:mn> </mml:math> is significantly below the subcritical limit. Here u τ stands for the friction velocity, and H for the channel half width. Despite the low Reynolds numbers, the roughness elements are able to trigger the by-pass transition mechanism leading to massive clusters of quasi-streamwise vortices entering into the smooth channel. The characteristics of the turbulence decay at the centerline of the control channel collapse with those of a homogeneous isotropic turbulence. Despite the fact that the turbulent activity decreases and the flow approaches relaminarization, the Nusselt number averaged over large streamwise distances in the smooth channel remains close to a fully developed turbulent flow.

Abstract This study investigates the phenomenon of fog clearing in the lee of Sable Island, Nova Scotia, termed a “fog shadow”, using observations from the 2022 Fog And Turbulence Interactions in the Marine Atmosphere field experiment and Coupled Ocean–Atmosphere Mesoscale Prediction System numerical weather prediction model. The fog shadow occurs when a shallow layer of marine fog dissipates downstream of this low‐elevation island due to surface heating and turbulent mixing. Analysis focuses on two cases: July 24, 2022, when a fog shadow was both predicted and observed through multiple platforms, including satellite imagery, and July 26, 2022, when the model erroneously forecast a fog shadow. Though the model successfully predicted some fog shadow events, it consistently overestimated surface heat fluxes over the island, leading to excessive fog dissipation in forecasts. The study reveals that fog shadows can form when sufficient surface heating combines with turbulent mixing to erode shallow fog layers, though the precise mechanisms and conditions required remain to be fully understood. These findings highlight the challenges in accurately modeling air–sea interactions and fog evolution around small islands, suggesting several areas for model improvement, including enhanced resolution, better surface flux parameterizations, and more sophisticated turbulence schemes.

The OPTIDE project aims to improve the efficiency and durability of hydrokinetic cross-flow tidal turbines (CFTT). CFTT are attractive for the exploitation of tidal energy, because of high area-based power densities of such turbine arrays, a simple design and the ability to operate under varying flow conditions.Nevertheless, the efficiency of single CFTT is lower relative to the most commonly used axial turbine type. Furthermore the life time is often limited due to alternating and pulsating stresses.A promising approach to overcome these drawbacks is intracycle blade pitching. In this case the angle of attack is continuously adjusted individually for each blade during the rotation. The project aims to explore the influence of active blade pitching on CFTTs and to optimize it by numerical and experimental means.Therefore, a lab-scaled three-bladed experimental turbine with embedded pitch actuators is developed.The turbine is equipped with multiple full-bridges of strain gauges for the detection of tangential and radial components of the blade loads, from which the structural stress is calculated subsequently.To ensure the turbines mechanical durability, weakly coupled fluid-solid-interaction (FSI) simulations have been performed. Additionally, the FSI simulations show that a setup with strain gauges on a single blade support only allows for the measurement of the radial blade load component, as the measurements are distorted by pitching moments and secondary force paths.A revised instrumentation setup is developed and presented. The FSI simulations are adapted, to prove the suitability of the revised setup.

Le bâchage, méthode mécanique consistant à opposer une barrière physique à la croissance des plantes en plaquant un matériau sur le sol, est régulièrement cité comme une méthode de lutte contre les renouées asiatiques, mais reste très peu documentée. Nous avons réalisé une revue de littérature et une collecte de retours d’expériences afin d’améliorer les connaissances sur la méthode et tenter d'identifier les facteurs clés de succès. Le bâchage semble de plus en plus fréquemment utilisé en France comme à l'étranger, mais les pratiques et les itinéraires techniques très variés observés amènent à des résultats contrastés. Cependant, en analysant les données collectées, nous montrons que le bâchage peut être efficace sous certaines conditions et même mener à une éradication complète. Pour cela, la prise en compte des caractéristiques biologiques des renouées est indispensable. Ainsi, nous recommandons (si l’objectif visé est l’éradication) de bâcher l’intégralité du massif avec une distance de pose de quatre mètres au-delà des limites du massif (compte tenu des capacités d’expansion des rhizomes souterrains) pour une durée minimale de six ans, avec l’utilisation d’un matériau résistant d’une durée de vie équivalente. Un suivi régulier pendant toute la durée de l’opération est nécessaire afin d’assurer l’étanchéité du dispositif.

A boundary integral representation is derived for the translational oscillations of a triaxial ellipsoid in a uniformly stratified fluid. The representation is of single-layer type, a distribution of sources and sinks over the surface of the ellipsoid. The added mass tensor of the ellipsoid is deduced from it and, from this tensor, the impulse response function together with the energy radiated away as internal waves. Horizontal oscillations correspond to the generation of an internal or baroclinic tide by the oscillation of the barotropic tide over ellipsoidal topography at the bottom of the stratified ocean. Such topography is unconditionally supercritical, namely of slope larger than the slope of the wave rays, irrespective of the frequency of oscillation. So far, analytical work on supercritical topographies has been limited, for the most part, to two-dimensional set-ups. Here, for the ellipsoidal seamount, the orientation of the barotropic tide and the anisotropy of the topography have their effects analysed in detail. As the height of the seamount increases, the rate of conversion of barotropic energy into baroclinic form is seen to first increase according to the square law expected for a topography of small slope, then saturate and eventually decrease.