Wind speed data of multi-heights have been examined to investigate the spatial and temporal characteristics of coherent structures in the near neutral marine atmospheric surface layer. With Taylor's hypothesis, the temporal velocity signals have been transformed to spatial fluctuations and then visualize these spatial velocity fluctuations to identify the coherent structures. It has been confirmed that there exist similar coherent structures in the marine atmospheric surface layer to those in laboratory turbulent boundary layer. These similar coherent structures include ejections, sweeps, shear layers, transverse vortices, and combined events of the shear layers and transverse vortices. Besides these similar coherent structures, there exist the plume and downdraft motions in the unstable marine atmospheric surface layer. It has been observed that the streamwise spatial length of the ejections and sweeps is 20-250 m and their mean frequency is of order of 0.01-0.001 /s at mean wind speed of 5-12.6 m/s. Between the region of the upstream ejection and downstream sweep motions an inclined shear layer is often seen. The inclined angle of the shear layer has been observed to vary from 30 to 70 degree with the height and length of the the shear layer. The transverse vortices are seen to exist in every region from the wall up to a height of 45 m and their diameter is up to 40 m. The mean frequency of the shear layers and the transverse vortices is of order of 0.001 /s. In the fully developed stage of the combined event of the shear layer and transverse vortex, the shear layer is generally longer and the diameter of the transverse vortex is larger. The mean frequency of the combined event of the shear layers and the transverse vortices is of order of 0.001 /s. The streamwise spatial length of the plume and downdraft motions is generally from 20 m to 50 m. Analysis indicates that the mean wind speed is a dominant factor in affecting the spatial and temporal characteristics of the coherent structures in the near neutral marine atmospheric surface layer. As the mean wind speed increases, the frequency of the shear related coherent events will increase, while the frequency of the buoyancy related coherent events (plumes and downdrafts) will decrease. The temperature difference between higher level of the surface layer and sea surface is the second main factor in affecting the spatial and temporal characteristics of the coherent structures. As the marine atmospheric surface layer becomes more stable the coherent motions will be suppressed. The effect of the temperature difference on the buoyancy related plume and downdraft motions is more evident than on the other shear related coherent motions. / Master of Science

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Avec le développement de la technologie, les mesures des champs de vitesse instationnaire sont disponibles maintenant. Il s'en suit une augmentation de l'intérêt de l'analyse lagrangienne des données. Un outil central pour analyser les écoulements est l'exposant de Lyapunov à temps fini (FTLE). Il permet d’identifier les structures cohérentes lagrangiennes LCS qui apparaissent comme des crêtes du champ de FTLE. Les LCS sont des quasi barrières de transport et séparent le domaine fluide en régions aux propriétés dynamiques différentes. Cependant, la méthodologie de calcul actuelle des FTLE exige l'évaluation numérique d'un grand nombre de trajectoires de particules fluides sur un maillage cartésien ou adaptatif qui est superposé aux champs de vitesses simulées ou mesurées.Dans ce travail de thèse, nous proposons une nouvelle méthode de calcul du champ de l'exposant de Lyapunov à temps fini FTLE. Pour cela, nous utilisons la méthode des moments d'ordre 2 qui permet d'évaluer au cours du temps la dispersion des particules distribuées uniformément dans un domaine circulaire ou elliptique. Nous appelons ce nouveau champ scalaire, champ de M-FTLE. Nous validons cette approche, théoriquement en tout point du domaine fluide en comparant M-FTLE et FTLE et aussi en faisant la comparaison sur des exemples classiques (champ de vitesse linéaire, circulaire ou hyperbolique) et sur un exemple numérique (champ de vitesse du double gyre). Cette méthode est alors appliquée sur des données expérimentales du champ de vitesse du mascaret, obtenues au sein l'institut 'Pprime' par vélocimétrie par image de particules PIV. / With the development of technology, instantaneous flow fields coming from experiments or numerical simulation are available now. It has been followed by a rise of interest for the Lagrangian analysis of such data. One central tool to analyze the flow fields is the Finite Time Lyapunov Exponent (FTLE). It allows to the identify of the Lagrangian Coherent Structures (LCS) which appear as ridges in the FTLE fields. The LCS are quasi transport bareers and separatte the fluid domain into regions which have different dynamic properties. However, the computation methodology currently used in order to obtain the FTLE requires numerical evalution of a large number of fluid particle trajectories on cartesian or adaptive meshes that are superimposed on the original data grid.In this thesis, we propose a new method for calculating the Finite Time Lyapunov Exponent FTLE fields. For this, we use the method of second-order moments which allows to evaluate over time the dispersion of particles uniformly distributed in a circular or elliptical domain. We call this new scalar field, the M-FTLE field. We validate this approach theoretically, at every point of the fluid domain by comparing FTLE and M-FTLE and also by the comparison of the classic examples (linear velocity field, circular and hyperbolic) and a numerical example (velocity field of double gyre). This method is then applied on experimental measurements of tidal bore velocity fields, obtained within the institute 'Pprime' by using a measurement technique called particle image velocimetry (PIV).

The turbulent boundary layer is an aspect of fluid flow which dominates the performance of many engineering systems - yet the analytic solution of such flows is intractable for most applications. Our understanding of boundary layers is therefore limited by our ability to simulate and measure them. Tomographic Particle Image Velocimetry (TPIV) is a recently developed technique for direct measurement of fluid velocity within a 3D region. This allows new insight into the topological structure of turbulent boundary layers. Increasing Reynolds Number increases the range of scales at which turbulence exists; a measurement technique must have a larger 'dynamic range' to fully resolve the flow. Tomographic PIV is currently limited in spatial dynamic range (which is also linked to the spatial and temporal resolution) due to a high degree of noise. Results also contain significant bias error. This work proposes a modification of the technique to use more than two exposures in the PIV process, which (for four exposures) is shown to improve random error by a factor of 2 to 7 depending on experimental setup parameters. The dynamic range increases correspondingly and can be doubled again in highly turbulent flows. Bias error is reduced by up to 40%. An alternative reconstruction approach is also presented, based on application of a reduction strategy (elimination of coefficients based on a first guess) to the tomographic weightings matrix Wij. This facilitates a potentially significant increase in computational efficiency. Despite the achieved reduction in error, measurements contain non-zero divergence due to noise and sampling errors. The same problem affects visualisation of topology and coherent fluid structures. Using Projection Onto Convex Sets, a framework for post-processing operators is implemented which includes a divergence minimisation procedure and a scale-limited denoising strategy which is resilient to 'false' vectors contained in the data. Finally, developed techniques are showcased by visualisation of topological information in the inner region of a high Reynolds Number boundary layer (δ+ = 1890, Reθ = 3650). Comments are made on the visible flow structures and tentative conclusions are drawn.