Accélération par effet de marée

Résumé

vignette|droite|redresse=1|La Terre et la Lune vues de Mars : la présence de la Lune, dont la masse est 1/81 de celle de la Terre, ralentit la rotation de cette dernière d’environ par siècle. L’accélération par effet de marée est un effet de la force de marée entre un satellite naturel (par exemple la Lune) et la planète autour de laquelle il orbite (par exemple la Terre). Cet effet provoque un éloignement progressif du satellite s'il a une orbite prograde et une décélération de la vitesse de rotation de la planète. Ce double effet mène à une rotation synchrone du plus petit corps d’abord, puis du corps le plus important. Le système Terre-Lune est le cas le mieux étudié : la Lune s'éloigne ainsi de la Terre d'environ 3,8 mètres par siècle et la période de rotation de la Terre (la journée) s'allonge dans le même temps d'environ 2 millisecondes. Le processus est appelé décélération par effet de marée lorsque le satellite a une période orbitale plus courte que la période de rotation d

Source officielle

https://fr.wikipedia.org/wiki/Acc%C3%A9l%C3%A9ration_par_effet_de_mar%C3%A9e

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At sea, wind forcing is responsible for the formation and development of surface waves and represents an important source of near-surface turbulence. Therefore, processes related to near-surface turbulence and wave breaking, such as sea spray emission and air–sea gas exchange, are often parameterised with wind speed. Thus, shipborne wind speed measurements provide highly relevant observations. They can, however, be compromised by flow distortion due to the ship's structure and objects near the anemometer that modify the airflow, leading to a deflection of the apparent wind direction and positive or negative acceleration of the apparent wind speed. The resulting errors in the estimated true wind speed can be greatly magnified at low wind speeds. For some research ships, correction factors have been derived from computational fluid dynamic models or through direct comparison with wind speed measurements from buoys. These correction factors can, however, lose their validity due to changes in the structures near the anemometer and, thus, require frequent re-evaluation, which is costly in either computational power or ship time. Here, we evaluate if global atmospheric reanalysis data can be used to quantify the flow distortion bias in shipborne wind speed measurements. The method is tested on data from the Antarctic Circumnavigation Expedition onboard the R/V Akademik Tryoshnikov, which are compared to ERA-5 reanalysis wind speeds. We find that, depending on the relative wind direction, the relative wind speed and direction measurements are biased by −37 % to +22 % and -17∘ to +11∘ respectively. The resulting error in the true wind speed is +11.5 % on average but ranges from −4 % to +41 % (5th and 95th percentile). After applying the bias correction, the uncertainty in the true wind speed is reduced to ±5 % and depends mainly on the average accuracy of the ERA-5 data over the period of the experiment. The obvious drawback of this approach is the potential intrusion of model biases in the correction factors. We show that this problem can be somewhat mitigated when the error propagation in the true wind correction is accounted for and used to weight the observations. We discuss the potential caveats and limitations of this approach and conclude that it can be used to quantify flow distortion bias for ships that operate on a global scale. The method can also be valuable to verify computational fluid dynamic studies of airflow distortion on research vessels.

2020

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Third-Order Optical Nonlinearity in Two-Dimensional Transition Metal Dichalcogenides

Sina Khorasani

We present a detailed calculation of the linear and nonlinear optical response of four types of monolayer twodimensional (2D) transition-metal dichalcogenides (TMDCs), having the formula MX2 with M = Mo, W and X = S, Se. The calculations are based on 6-band tight-binding model of TMDCs, and then performing a semi-classical perturbation analysis of response functions. We numerically calculate the linear chi((1))(mu nu) (-omega;omega) and nonlinear surface susceptibility tensors chi mu nu zeta eta((3)) (-omega(Sigma);omega(r),omega(s),omega(t)) with omega(Sigma) = omega(r)+ omega(s) +omega(t) . Both non-degenerate and degenerate cases are studied for thirdharmonic generation and nonlinear refractive index, respectively. Computational results obtained with no external fitting parameters are discussed regarding two recent reported experiments on MoS2 , and thus we can confirm the extraordinarily strong optical nonlinearity of TMDCs. As a possible application, we demonstrate generation of a pi/4-rotated squeezed state by means of nonlinear response of TMDCs, in a silica micro-disk resonator covered with the 2D material. Our proposed method will enable accurate calculations of nonlinear optical response, such as four-wave mixing and highharmonic generation in 2D materials and their heterostructures, thus enabling study of novel functionalities of 2D photonic integrated circuits.

2018

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Energy expenditure estimation using accelerometry and heart rate for multiple sclerosis and healthy older adults

Kamiar Aminian, Arash Arami, Alan Kevin Bourke, Fabien Massé

Accurate estimation of Energy Expenditure (EE) in ambulatory settings provides greater insight into the underlying relation between different human physical activity and health. This paper describes the development and validation of energy expenditure estimation algorithms. A total of 4 healthy subjects and 3 suffering from multiple sclerosis were monitored using a gold-standard energy expenditure measurement system, a heart rate monitor and accelerometry. We demonstrated that greater improvements can be achieved by estimating energy expenditure during normal activities of daily living by combining both whole body acceleration estimates, vertical body acceleration estimates, body posture and heart rate data as part of a flex heart rate algorithm in subject specific models when compared to using accelerometry or heart rate data alone. This will allow more accurate EE estimation during normal activities of daily living.

2014

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2020