Icephobicity of Superhydrophobic Surfaces Under Atmospheric Icing, the Role of Surface Wettability on Impact Dynamics and Ice Growth Kinetics

Le givrage atmosphérique est un problème difficile pour l'exploitation et la sécurité des aéronefs. Il se produit en vol lorsque des nuages de micrométriques gouttelettes d'eau en surfusion percutent et gèlent sur les surfaces exposées, entrainant diverses défaillances et pertes d'efficacité. Ces gouttelettes, dans une phase liquide instable, possèdent des diamètres de l'ordre de 10 à 50 micromètres, peuvent être refroidies à des températures inférieures à moins 20 C et percuter à des vitesses de dizaines de mètres par seconde. Les systèmes actuels de protection contre la glace reposent sur des technologies telles que des éléments chauffants ou des fluides de dégivrages. Ils sont énergivores, toxiques et peu fiables dans des conditions sévères. Récemment, le développement de surfaces dites glaciophobes a attiré l’attention pour exploiter leurs potentielles propriétés afin d'empêcher l'accumulation de glace, de faciliter le déglaçage ou d'améliorer le dégivrage thermique. Parmi ces surfaces, les revêtements superhydrophobes, qui combinent des matériaux à faible énergie de surface et une rugosité de surface élevée, ont été proposés en raison de leur résistance à l’eau exceptionnelle. Il a été démontré que ces surfaces hydrophobes conservent leurs propriétés dans certaines conditions de givrage, affichant une faible adhérence à la glace, une répulsion des gouttes dans des environnements glacials ou facilitant la fonte et l'élimination de la glace. Cependant, l'adoption de surfaces glaciophobes pour des applications aérospatiales est entravée par un manque de connaissance de leur performance dans des conditions environnementales réalistes. Les conditions de givrage des aéronefs nécessitent des installations dédiées d'essai en soufflerie givrante pour reproduire un environnement atmosphérique réaliste. Le comportement de mouillage et de glaciophobicité des revêtements superhydrophobes sous l’effet combiné de microgouttelettes, de températures glaciales et d'une vitesse d'impact élevée est encore largement inexploré. Par conséquent, l'objectif de cette thèse est d'étudier le rôle de la mouillabilité et de la superhydrophobicité sur la glaciophobicité de surfaces spécifiquement modifiées dans des conditions de givrage de microgouttelettes en surfusion telles que subies en vol. L’essentiel du travail expérimental a été réalisé sur une soufflerie de givre à petite échelle capable de reproduire des conditions de givrage quasi réalistes nécessaires pour tester la glaciophobicité des surfaces fabriquées. Nous avons étudié l'influence de la mouillabilité et de la rugosité de surface en examinant successivement la dynamique d'impact des gouttelettes, la cinétique de croissance de la glace et le comportement des surfaces nanotexturées.Abstract----------Atmospheric icing is a challenging problem for aircraft operation and safety. It occurs in flight when clouds of micrometric supercooled water droplets impact and freeze on exposed surfaces, potentially leading to diverse system malfunctions and efficiency losses. These droplets, in an unstable liquid phase, feature diameters in the range of 10 to 50 micrometers, can be cooled to temperatures below -20oC and impact at speeds of tens of meters per second. Current ice protection methods rely on technologies such as heaters or freezing-point depressant chemicals. These are energy-consumptive, environmentally damaging and prone to failure in severe weather. Recently, the development of so-called icephobic surfaces has attracted a lot of research to harness potential properties to prevent ice accretion, facilitate ice shedding or enhanced thermal de-icing. Among these surfaces, superhydrophobic coatings, which combine low surface energy materials and high surface roughness, have been proposed due to their exceptional water repellency. These water-repellent surfaces have been shown to maintain their properties under certain icing conditions, displaying low ice adhesion, droplet repellency in cold environment, or facilitating ice melting and removal. However, the adoption of icephobic surfaces in aerospace applications has been hindered by a lack of research related to their performance under realistic environmental conditions. Aircraft icing conditions require dedicated icing wind-tunnel testing facilities to reproduce realistic atmospheric environment. The wetting and icephobic behavior of superhydrophobic coatings under the combination of micrometric droplets, sub-zero temperatures, and high impact velocity is still largely unexplored. Therefore, the objective of this thesis is to investigate the role of wettability and superhydrophobicity on the icephobicity of engineered surfaces under supercooled water microdroplet icing conditions as experienced by aircraft. The core of the experimental work was performed on a small-scale icing wind-tunnel able to reproduce near-realistic icing conditions needed to test icephobicity of the desired surfaces. We investigated the influence of surface wettability and roughness by looking successively at the droplet impact dynamics, the ice growth kinetics and the behavior of nanotextured surfaces. In the first part of the study, we used ultra-high-speed imaging in the wind-tunnel to record in detail the impact dynamics of microdroplets at different levels of supercooling on four reference surfaces with wettability ranging from smooth near-superhydrophilic to micro/nanotextured superhydrophobic

The role of the basic state in the ENSO-monsoon relationship and implications for predictability

The impact of systematic model errors on a coupled simulation of the Asian Summer monsoon and its interannual variability is studied. Although the mean monsoon climate is reasonably well captured, systematic errors in the equatorial Pacific mean that the monsoon-ENSO teleconnection is rather poorly represented in the GCM. A system of ocean-surface heat flux adjustments is implemented in the tropical Pacific and Indian Oceans in order to reduce the systematic biases. In this version of the GCM, the monsoon-ENSO teleconnection is better simulated, particularly the lag-lead relationships in which weak monsoons precede the peak of El Nino. In part this is related to changes in the characteristics of El Nino, which has a more realistic evolution in its developing phase. A stronger ENSO amplitude in the new model version also feeds back to further strengthen the teleconnection. These results have important implications for the use of coupled models for seasonal prediction of systems such as the monsoon, and suggest that some form of flux correction may have significant benefits where model systematic error compromises important teleconnections and modes of interannual variability

Piezoelectric and optical setup to measure an electrical field: Application to the longitudinal near-field generated by a tapered coax

We propose a new setup to measure an electrical field in one direction. This setup is made of a piezoelectric sintered lead zinconate titanate film and an optical interferometric probe. We used this setup to investigate how the shape of the extremity of a coaxial cable influences the longitudinal electrical near-field generated by it. For this application, we designed our setup to have a spatial resolution of 100 um in the direction of the electrical field. Simulations and experiments are presented

Salinity Measurements Collected by Fishermen Reveal a “River in the Sea” Flowing Along the Eastern Coast of India

Being the only tropical ocean bounded by a continent to the north, the Indian Ocean is home to the most powerful monsoon system on Earth. Monsoonal rains and winds induce huge river discharges and strong coastal currents in the northern Bay of Bengal. To date, the paucity of salinity data has prevented a thorough description of the spreading of this freshwater into the bay. The potential impact of the salinity on cyclones and regional climate in the Bay of Bengal is, however, a strong incentive for a better description of the water cycle in this region. Since May 2005, the National Institute of Oceanography conducts a program in which fishermen collect seawater samples in knee-deep water at eight stations along the Indian coastline every 5 days. Comparison with open-ocean samples shows that this cost-effective sampling strategy is representative of offshore salinity evolution. This new dataset reveals a salinity drop exceeding 10 g kg−1 in the northern part of the bay at the end of the summer monsoon. This freshening signal propagates southward in a narrow (~100 km wide) strip along the eastern coast of India, and reaches its southern tip after 2.5 months. Satellite-derived alongshore-current data shows that the southward propagation of this “river in the sea” is consistent with transport by seasonal coastal currents, while other processes are responsible for the ensuing erosion of this coastal freshening. This simple procedure of coastal seawater samples collection could further be used to monitor phytoplankton concentration, bacterial content, and isotopic composition of seawater along the Indian coastlin

Western Pacific oceanic heat content: a better predictor of La Niña than of El Niño

The western equatorial Pacific oceanic heat content (Warm Water Volume in the west or WWVW) is the best El Niño–Southern Oscillation (ENSO) predictorbeyond1‐year lead. Using observations and selected Coupled Model Intercomparison Project Phase 5 (CMIP5) simulations, we show that a discharged WWVW in boreal fall is a better predictor of La Niña than a recharged WWVW for El Niño13 months later, both in terms of occurrence and amplitude. These results are robust when considering the heat content across the entire equatorial Pacific (WWV) at shorter lead‐times, including all CMIP5 models or excluding Niño‐Niña and Niña‐Niño phase transitions. Suggested mechanisms for this asymmetry include 1) the negatively skewed WWVW distribution with stronger discharges related to stronger wind stress anomalies during El Niño and 2) the stronger positive Bjerknes feedback loop during El Niño. The possible role of stronger subseasonal wind variations during El Niño is also discussed

The critical role of the routing scheme in simulating peak river discharge in global hydrological models

Global hydrological models (GHMs) have been applied to assess global flood hazards, but their capacity to capture the timing and amplitude of peak river discharge—which is crucial in flood simulations—has traditionally not been the focus of examination. Here we evaluate to what degree the choice of river routing scheme affects simulations of peak discharge and may help to provide better agreement with observations. To this end we use runoff and discharge simulations of nine GHMs forced by observational climate data (1971–2010) within the ISIMIP2a project. The runoff simulations were used as input for the global river routing model CaMa-Flood. The simulated daily discharge was compared to the discharge generated by each GHM using its native river routing scheme. For each GHM both versions of simulated discharge were compared to monthly and daily discharge observations from 1701 GRDC stations as a benchmark. CaMa-Flood routing shows a general reduction of peak river discharge and a delay of about two to three weeks in its occurrence, likely induced by the buffering capacity of floodplain reservoirs. For a majority of river basins, discharge produced by CaMa-Flood resulted in a better agreement with observations. In particular, maximum daily discharge was adjusted, with a multi-model averaged reduction in bias over about 2/3 of the analysed basin area. The increase in agreement was obtained in both managed and near-natural basins. Overall, this study demonstrates the importance of routing scheme choice in peak discharge simulation, where CaMa-Flood routing accounts for floodplain storage and backwater effects that are not represented in most GHMs. Our study provides important hints that an explicit parameterisation of these processes may be essential in future impact studies

Tentative reconstruction of the 1998–2012 hiatus in global temperature warming using the IPSL–CM5A–LR climate model

The period running from 1998 to 2012 has experienced a slower increase in global temperature at the surface of the Earth than the decades before. Several explanations have been proposed, ranging from internal variability of the climate system to a contribution of the natural external forcing. In this study, we use the IPSL-CM5A-LR climate model to test these different hypotheses. We consider historical simulations, including observed external forcing, in which nudging towards observed sea surface temperature has been applied to different regions of the ocean to phase the decadal variability of large-scale modes in the Atlantic and the Pacific to observations. We find that phasing the tropical Pacific is reducing the warming trend detected in historical simulations by a factor of two, but the remaining trend is still twice as large as the observed one. Combining the tropical Pacific phasing and the potential effect of recent eruptions allows us to fully reproduce the observed hiatus. Conversely, nudging the Atlantic does not drive any hiatus in this model