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Publié le 19 novembre 2020 | Mis à jour le 19 novembre 2020

14h55 - 15h20 : Lagrangian diffusion properties of a free-shearing turbulent jet

Thomas Basset (LPENSL, ENS de Lyon)

A Lagrangian study of an axisymmetric turbulent water jet is performed to investigate the highly anisotropic and inhomogeneous flow field. The measurements were conducted at the École Normale Supérieure de Lyon, within a Lagrangian Exploration Module, an icosahedron apparatus, to facilitate optical access of three cameras. The stereoscopic particle tracking velocimetry results in three component tracks of position, velocity and acceleration of the tracer particles within the vertically-oriented jet with a Taylor-based Reynolds number Reλ = 260. Analysis is performed at seven locations from near the jet exit at 15 diameters downstream up to 45 diameters downstream. Eulerian analysis is first carried out to obtain critical parameters of the jet and relevant scales, namely the Kolmogorov and large turnover (integral) scales as well as the energy dissipation rate. Lagrangian statistical analysis is then executed on velocity components stationarised following methods inspired by Batchelor (Batchelor 1957 J. Fluid Mech.) which aim to extending Taylor's stationary Lagrangian theory of diffusion to the case of self-similar flows. The evolution of typical Lagrangian scaling parameters as a function of the developing jet is explored and results show validation of the proposed stationarisation. The universal scaling constant C0 (for the Lagrangian second-order structure function), as well as Eulerian and Lagrangian integral time scales are discussed in this context. C0 is found to converge to a constant value (of the order of C0 = 3) within 30 diameters downstream of the nozzle. Finally, the existence of finite particle size effects are investigated through consideration of acceleration dependent quantities.