- Maciel, Yvan

## Personne : Maciel, Yvan

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Maciel

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Yvan

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Université Laval. Département de génie mécanique

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ncf11860329

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- PublicationAccès libreOuter scales and parameters of adverse-pressure-gradient turbulent boundary layers(Cambridge University Press, 2018-02-22) Maciel, Yvan; Wei, Tie; Güngör, Ayşe Gül; Simens, Mark P.
Voir plus A clear and consistent framework for the analysis of the outer region of adverse-pressure-gradient turbulent boundary layers is established in this paper based on basic principles and theory, and the help of six adverse-pressure-gradient turbulent boundary layer databases and a zero-pressure-gradient one. Outer velocity and length scales for the mean velocity defect and the Reynolds stresses are discussed first. The conditions of validity of four velocity scales are determined in terms of the shape factor, since one scale is restricted to small velocity-defect boundary layers (the friction velocity 𝜏 ), one to large-defect ones (the pressure-gradient velocity ), while the two others are proper scales for all velocity-defect conditions (the Zagarola–Smits velocity and the mixing-layer-type velocity ). The turbulent boundary layer equations are then used to bring out, in a consistent manner and without assuming any self-similar behaviour, a set of non-dimensional parameters characterizing the outer region of turbulent boundary layers with arbitrary pressure gradients. In terms of a generic outer length scale and velocity scale , these non-dimensional parameters are the pressure-gradient parameter 𝛽𝜌 , the Reynolds number 𝜈 and the inertial parameter 𝛼 , where and are respectively the streamwise and wall-normal components of mean velocity at the boundary layer edge. These parameters have a clear physical meaning: they are ratios of the order of magnitude of forces, with the Reynolds shear stress gradient (apparent turbulent force) as the reference force – inertial to apparent turbulent forces for 𝛼 , pressure to apparent turbulent forces for 𝛽 and apparent turbulent to viscous forces for . We discuss at length their significance and determine under what conditions they retain their meaning depending on the outer velocity scale that is considered. The seven boundary layer databases are analysed and compared using the established framework. An astonishing and impressive result is obtained: by choosing , the streamwise evolution of the three ratios of forces in the outer region can be accurately followed with 𝛽 , 𝛼 and in all these flows – not just the order of magnitude of these ratios. This cannot be achieved with 𝜏 and , and only imperfectly with . Consequently, 𝛽 , 𝛼 and can be used to follow – in a global sense – the streamwise evolution of the streamwise mean momentum balance in the outer region.Voir plus - PublicationRestreintSecondary flow and roll cells interaction in high-aspect-ratio rotating turbulent duct flows(Gordon and Breach Publishing Group, 2008-04-02) Julien, Steve; Dumas, Guy; Torriano, Federico; Maciel, Yvan
Voir plus End-wall effects for high aspect ratio (AR) turbulent duct flows under moderate spanwise rotation are investigated using Reynolds-Averaged Navier–Stokes (RANS) calculations with a Reynolds stress turbulence closure model. It is shown that despite an important uniformisation of the mean streamwise flow compared to the non-rotating case, the channel flow solution (AR ¼ 1) is not recovered in practical high AR ducts used in experiments. The unavoidable end-wall generated secondary flow causes transverse advection which is capable of altering the mean velocity profile, even for AR as high as 22. In addition, for Re ¼ 40,000 and Ro ¼ 0.22, persistent longitudinal roll cells are found in the RANS solutions. The results suggest that their interaction with the secondary flow may challenge the prospect of formally reaching a steady, streamwise invariant regime in actual rotating duct experiments.Voir plus - PublicationAccès libreCoherent structures in a non-equilibrium large-velocity-defect turbulent boundary layer(Springer, 2016-04-25) Maciel, Yvan; Simens, Mark P.; Gungor, Ayse G.
Voir plus The characteristics of the coherent structures in a strongly decelerated large velocity-defect boundary layer are analysed by direct numerical simulation. The simulated boundary layer starts as a zero-pressure-gradient boundary layer, decelerates under a strong adverse pressure gradient, and separates near the end of the domain, in the form of a very thin separation bubble. The Reynolds number at separation is Re 𝜃 = 3912 and the shape factor H = 3.43. The three-dimensional spatial correlations of (u, u) and (u, v) are investigated and compared to those of a zero-pressure-gradient boundary layer and another strongly decelerated boundary layer. These velocity pairs lose coherence in the streamwise and spanwise directions as the velocity defect increases. In the outer region, the shape of the correlations suggest that large-scale u structures are less streamwise elongated and more inclined with respect to the wall in large-defect boundary layers. The three-dimensional properties of sweeps and ejections are characterized for the first time in both the zeropressure-gradient and adverse-pressure-gradient boundary layers, following the method of Lozano-Duran et al. (J. Fluid Mech. ´ 694, 100–130, 2012). Although longer sweeps and ejections are found in the zero-pressure-gradient boundary layer, with ejections reaching streamwise lengths of 5 boundary layer thicknesses, the sweeps and ejections tend to be bigger in the adverse-pressure-gradient boundary layer. Moreover, small near-wall sweeps and ejections are much less numerous in the large-defect boundary layer. Large sweeps and ejections that reach the wall region (wall-attached) are also less numerous, less streamwise elongated and they occupy less space than in the zero-pressure-gradient boundary layer.Voir plus - PublicationAccès libreInvestigation of flow separation in a diffuser of a bulb turbine(American Society of Mechanical Engineers, 2015-09-03) Maciel, Yvan; Duquesne, Pierre; Deschênes, Claire.
Voir plus A three-dimensional unsteady flow separation in the straight diffuser of a model bulb turbine is investigated using tuft visualizations, unsteady wall pressure sensors, and particle image velocimetry (PIV). Experimental results reveal a link between the flow separation zone extension and the sudden drop in turbine performances. The flow separation zone grows as the flow rate increases past the best efficiency operating point (OP). It starts on the bottom wall and expands azimuthally and upstream. It deviates and perturbs the flow far upstream. Despite high unsteadiness, a global separation streamline pattern composed of a saddle point and a convergence line emerges.Voir plus