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Ruban V.,Moscow State Textile University | Kodama Y.,Ohio State University | Ruderman M.,University of Sheffield | Dudley J.,University of Franche Comte | And 15 more authors.
European Physical Journal: Special Topics | Year: 2010

This paper contains the discussion inputs by the contributors of the special issue on the subject of rogue waves. © 2010 EDP Sciences and Springer.

Carballido-Landeira J.,Free University of Colombia | Trevelyan P.M.J.,University of Santiago de Compostela | Almarcha C.,University of South Wales | De Wit A.,IRPHE
Physics of Fluids | Year: 2013

In a gravitational field, a horizontal interface between two miscible fluids can be buoyantly unstable because of double diffusive effects or because of a Rayleigh-Taylor instability arising when a denser fluid lies on top of a less dense one. We show here both experimentally and theoretically that, besides such classical buoyancy-driven instabilities, a new mixed mode dynamics exists when these two instabilities act cooperatively. This is the case when the upper denser solution contains a solute A, which diffuses sufficiently faster than a solute B initially in the lower layer to yield non-monotonic density profiles after contact of the two solutions. We derive parameter plane, where R is the buoyancy ratio between the two solutions and δ is the ratio of diffusion coefficient of the solutes. We find an excellent agreement of these theoretical predictions with experiments performed in Hele-Shaw cells and with numerical simulations. © 2013 American Institute of Physics.

Mercier F.,IRSTEA | Golay F.,University of Toulon | Bonelli S.,IRSTEA | Anselmet F.,IRPHE | And 2 more authors.
European Journal of Mechanics, B/Fluids | Year: 2014

This study focuses on 2D Computational Fluid Dynamics (CFD) numerical modelling of the erosion of a cohesive soil by a circular impinging turbulent jet. Initially, the model is validated in the case of a non erodible flat plate. Several turbulence models are compared to experimental results and to simplified formulas available in the literature. The results obtained show that the Reynolds Stress Model (RSM) is in good agreement with the semi-empirical results in the literature. Nonetheless, the RSM cannot be used with successive remeshings, due to its convergence issues. The shear stress at the wall is well-described by the k-ε model while the pressure is better-described by the k-ω model. The numerical model of erosion is based on adaptive remeshing of the water/soil interface to ensure the good precision of the mechanical values at the wall. The two erosion parameters are the critical shear stress and the erosion coefficient. The results obtained are compared with the semi-empirical model interpreting the Jet Erosion Test. The k-ε model underestimates the shear stress and does not allow simulation of the entire erosion process, whereas the results obtained with the k-ω model agree well with the semi-empirical model and experimental data. A study of the influence of erosion parameters on erosion kinetics and scouring depth shows that the shape and depth of scouring are influenced solely by the critical shear stress while the duration of scouring depends on both erosion parameters. Further research is nonetheless required to better understand the erosion mechanisms in the stagnation zone. © 2013 Elsevier Masson SAS. All rights reserved.

Kharif C.,IRPHE | Touboul J.,University of Toulon
European Physical Journal: Special Topics | Year: 2010

Within the framework of the fully nonlinear water waves equations, we consider a Stokes wavetrain modulated by the Benjamin-Feir instability in the presence of both viscous dissipation and forcing due to wind. The wind model corresponds to the Miles' theory. By introducing wind effect on the waves, the present paper extends the previous works of [6] and [7] who neglected wind input. It is also a continuation of the study developed by [9] who considered a similar problem within the framework of the NLS equation. The marginal stability curve derived from the fully nonlinear numerical simulations coincides with the curve obtained by [9] from a linear stability analysis. Furthermore, it is found that wind input goes in the subharmonic mode of the modulation whereas dissipation damps the fundamental mode of the initial Stokes wavetrain. © 2010 EDP Sciences and Springer.

Chaouat B.,ONERA | Schiestel R.,IRPHE | Schiestel R.,French National Center for Scientific Research
Computers and Fluids | Year: 2013

We apply the partially integrated transport modeling (PITM) method with a stress transport subfilter model [Chaouat B, Schiestel R. A new partially integrated transport model for subgrid-scale stresses and dissipation rate for turbulent developing flows. Phys Fluids 2005:17] to perform continuous hybrid non-zonal RANS/LES numerical simulations of turbulent flows over two-dimensional periodic hills at high Reynolds number Re=37,000 on coarse and medium meshes. The fine scale turbulence is described using a subfilter scale stress transport model deduced from PITM. This work extends the previous simulations of the turbulent flow over periodic hills performed at the lower Reynolds number Re=10,595 [Chaouat B. Subfilter-scale transport model for hybrid RANS/LES simulations applied to a complex bounded flow. J Turbul 2010:11] to the higher value 37,000 considering that studying the effects of the Reynolds number on the turbulence field constitutes a new material that deserves interest in CFD. So that, the aim of this paper is to explore the extension of a PITM subfilter model to high Reynolds numbers where conventional LES is not any more accessible because of the highly consuming cost. The effects of the grid refinement at Re=37,000 are investigated in detail through the use of different mesh sizes with a very coarse grid and with a several medium grids. For comparison purposes, the channel flow over 2D hills is also computed using a full statistical Reynolds stress transport model developed in RANS methodology. As a result of the simulations, it appears that the PITM simulations, although performed on coarse meshes, reproduce this complex flow governed by interacting turbulence mechanisms associated with separation, recirculation, reattachment, acceleration and wall effects with a relatively good agreement. The mean velocity and turbulent stresses are compared with reference data of this experiment at the flow Reynolds number Re=37,000 [Rapp Ch, Manhart M. Flow over periodic hills - an experimental study. Exp Fluids 2011:51]. Some discrepancies are observed in the immediate vicinity of the lower wall for the coarse simulations but as it could be expected, the simulation performed on the medium mesh provides better results than those performed on the coarse meshes thanks to the higher resolution due to the grid refinement in the streamwise and spanwise directions that allows a better account of the three-dimensional character of the flow. As usual in LES calculations, the instantaneous large flow structures are investigated in detail providing some interesting insights into the structures of the present turbulent flow. Comparatively to the PITM simulation results, it is found that the RANS Reynolds stress model based on second moment closures fails to predict correctly this flow in several respects, although being one of the most advanced model in RANS methodology. Important discrepancies with the experimental data are noticed. This work suggests that the present subfilter-scale stress model derived form the PITM method is well suited for simulating complex flows at high Reynolds numbers, with a sufficient accuracy from an engineering point of view, even if the grids are not as so fine as those used in conventional LES, while at the same time allowing a drastic reduction of the computational cost. Beside, these calculations give some ideas on the influence of the Reynolds number on the flow. © 2013 Elsevier Ltd.

Duclaux V.,IRPHE | Gallaire F.,Ecole Polytechnique Federale de Lausanne | Clanet C.,Ecole Polytechnique - Palaiseau
Journal of Fluid Mechanics | Year: 2010

Abdominal aortic aneurysms are a dilatation of the aorta, localized preferentially above the bifurcation of the iliac arteries, which increases in time. Understanding their localization and growth rate remain two open questions that can have either a biological or a physical origin. In order to identify the respective role of biological and physical processes, we address in this article these questions of the localization and growth using a simplified physical experiment in which water (blood) is pumped periodically (amplitude a, pulsation ) in an elastic membrane (aorta) (length L, cross-section A0 and elastic wave speed c0) and study the deformation of this membrane while decharging in a rigid tube (iliac artery; hydraulic loss K). We first show that this pulsed flow either leads to a homogenous deformation or inhomogenous deformation depending on the value of the non-dimensional parameter c0 2/(aL2K). These different regimes can be related to the aneurysm locations. In the second part, we study the growth of aneurysms and show that they only develop above a critical flow rate which scales as A0c0/K. © 2010 Cambridge University Press.

The basis of the partially integrated transport modeling (PITM) method was introduced by Schiestel and Dejoan ["Towards a new partially integrated transport model for coarse grid and unsteady turbulent flow simulations," Theor. Comput. Fluid Dyn.18, 443 (2005)]10.1007/s00162-004-0155-z and Chaouat and Schiestel ["A new partially integrated transport model for subgrid-scale stresses and dissipation rate for turbulent developing flows," Phys. Fluids17, 065106 (2005)]10.1063/1.1928607. This method provides a continuous approach for hybrid RANS-LES (Reynolds averaged Navier-Stokes equations-large eddy simulations) simulations with seamless coupling between RANS and LES regions. The main ingredient of the method is the new dissipation-rate equation that can be applied as a subfilter scale turbulence model. Then, it becomes easy to convert almost any usual RANS transport model into a subfilter scale model. In particular, the method can be applied to two equation models and to stress transport models as well. In the derivation of the method, the partial integration technique allows to keep a link between the spectral space and the physical space of the resulting model. The physical turbulent processes involving the production, dissipation, and flux transfer of the turbulent energy are introduced in the equations. The present work, after recalling the main building steps of the PITM method, brings further insight into the physical interpretation of the method, its underlying hypotheses and its internal acting mechanisms. In particular, the finiteness of the coefficients used in the dissipation-rate equation is discussed in detail from a theoretical point of view. Then, we consider the analytical example of self-similar turbulent flow for analyzing the dissipation-rate equation. From an analytical solution obtained by Taylor series expansions taking into account the Kovasznay hypothesis for evaluating the transfer term, we compute the functional coefficients and used in RANS and LES methodologies, respectively, and we demonstrate that both coefficients take on finite values when the Reynolds number goes to infinity. Finally, after briefly mentioning some flow illustrations to get a real appraisal of the PITM method in its capabilities to simulate unsteady flows on relatively coarse grids with a sufficient accuracy for engineering computations, we study the coefficient through one chosen example. © 2012 American Institute of Physics.

Chaouat B.,ONERA | Schiestel R.,IRPHE
Physics of Fluids | Year: 2013

The basis of the partially integrated transport modeling method was introduced in papers of Schiestel and Dejoan ["Towards a new partially integrated transport model for coarse grid and unsteady turbulent flow simulations," Theor. Comput. Fluid Dyn.18, 443 (2005)] and Chaouat and Schiestel ["A new partially integrated transport model for subgrid-scale stresses and dissipation rate for turbulent developing flows," Phys. Fluids17, 065106 (2005)]. This method provides a continuous approach for hybrid Reynolds averaged Navier-Stokes (RANS)-large eddy simulation (LES) simulations with seamless coupling between RANS and LES regions. The method, like in usual LES techniques, makes use of space filtering in the turbulent field. In the foundation papers cited above and in the main applications considered so far, the filter width has been supposed constant or at least slowly varying. In the present paper, we examine the effect of variable filter width in the model equations and how to account for this effect in practical numerical simulations. With the aim to illustrate the theoretical development of the effect of varying filter width in time and space on the governing equations of mass, momentum, and turbulence model, and to show the usefulness of the proposed approach, we perform then numerical simulations of isotropic decaying turbulence. © 2013 AIP Publishing LLC.

Mercier F.,IRSTEA | Bonelli S.,IRSTEA | Golay F.,Avenue Of Luniversite | Anselmet F.,IRPHE | And 2 more authors.
Acta Geotechnica | Year: 2015

This study focuses on the numerical modelling of the concentrated leak erosion of a cohesive soil by turbulent flow in axisymmetrical geometry, using the Hole Erosion Test (HET). The numerical model is based on the adaptive remeshing of the water/soil interface to ensure the accurate description of the mechanical phenomena occurring near the soil/water interface. The erosion law governing the interface motion is based on two erosion parameters: critical shear stress and the erosion coefficient. The model is first validated in the case of 2D piping erosion caused by laminar flow. Then, the numerical results are compared with the interpretation model of the HET. Three HETs performed on different soils are modelled with rather good accuracy. Lastly, a parametric analysis of the influence of the erosion parameters on erosion kinetics and the evolution of the channel diameter is performed. Finally, after this validation by comparison with both the experimental results and the interpretation of Bonelli et al. [2], our model is now able to accurately reproduce the erosion of a cohesive soil by a concentrated leak. It also provides a detailed description of all the averaged hydrodynamic flow quantities. This detailed description is essential in order to achieve better understanding of erosion processes. © 2014, Springer-Verlag Berlin Heidelberg.

Pierro B.D.,IRPHE | Abid M.,IRPHE
International Journal for Numerical Methods in Fluids | Year: 2013

This paper is devoted to the development of a parallel, spectral and second-order time-accurate method for solving the incompressible and variable density Navier-Stokes equations. The method is well suited for finite thickness density layers and is very efficient, especially for three-dimensional computations. It is based on an exact projection technique. To enforce incompressibility, for a non-homogeneous fluid, the pressure is computed using an iterative algorithm. A complete study of the convergence properties of this algorithm is done for different density variations. Numerical simulations showing, qualitatively, the capabilities of the developed Navier-Stokes solver for many realistic problems are presented. The numerical procedure is also validated quantitatively by reproducing growth rates from the linear instability theory in a three-dimensional direct numerical simulation of an unstable, non-homogeneous, flow configuration. It is also shown that, even in a turbulent flow, the spectral accuracy is recovered. © 2012 John Wiley & Sons, Ltd.

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