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Alain H. Cartellier - One of the best experts on this subject based on the ideXlab platform.

  • influence of gas turbulence on the instability of an air water Mixing Layer
    Physical Review Letters, 2015
    Co-Authors: Jean-philippe Matas, Sylvain Marty, Mohamed Seydou Dem, Alain H. Cartellier
    Abstract:

    We present the first evidence of the direct influence of gas turbulence on the shear instability of a planar air-water Mixing Layer. We show with two different experiments that increasing the level of velocity fluctuations in the gas phase continuously increases the frequency of the instability, up to a doubling of frequency for the largest turbulence intensity investigated. A modified spatiotemporal stability analysis taking turbulence into account via a simple Reynolds stress closure provides the right trend and magnitude for this effect.

  • Experimental and analytical study of the shear instability of a gas-liquid Mixing Layer
    Physics of Fluids, 2011
    Co-Authors: Jean-philippe Matas, Sylvain Marty, Alain H. Cartellier
    Abstract:

    We carry out an inviscid spatial linear stability analysis of a planar Mixing Layer, where a fast gas stream destabilizes a slower parallel liquid stream, and compare the predictions of this analysis with experimental results. We study how the value of the liquid velocity at the interface and the finite thickness of the gas jet affect the most unstable mode predicted by the inviscid analysis: in particular a zero interface velocity is considered to account for the presence in most experimental situations of a splitter splate separating the gas and the liquid. Results derived from this theory are compared with experimentally measured frequencies and growth rates: a good agreement is found between the experimental and predicted frequencies, while the experimental growth rates turn out to be much larger than expected.

Oleg Schilling - One of the best experts on this subject based on the ideXlab platform.

  • experimental characterization of initial conditions and spatio temporal evolution of a small atwood number rayleigh taylor Mixing Layer
    Journal of Fluid Mechanics, 2006
    Co-Authors: Nicholas J Mueschke, Malcolm J Andrews, Oleg Schilling
    Abstract:

    (Received 17 October 2005 and in revised form 11 April 2006) The initial multi-mode interfacial velocity and density perturbations present at the onset of a small-Atwood-number, incompressible, miscible Rayleigh–Taylor instabilitydriven Mixing Layer have been quantified using a combination of experimental techniques. The streamwise interfacial and spanwise interfacial perturbations were measured using high-resolution thermocouples and planar laser-induced fluorescence (PLIF), respectively. The initial multi-mode streamwise velocity perturbations at the twofluid density interface were measured using particle-image velocimetry (PIV). It was found that the measured initial conditions describe an initially anisotropic state, in which the perturbations in the streamwise and spanwise directions are independent of one another. The evolution of various fluctuating velocity and density statistics, together with velocity and density variance spectra, were measured using PIV and high-resolution thermocouple data. The evolution of the velocity and density statistics is used to investigate the early-time evolution and the onset of strongly nonlinear, transitional dynamics within the Mixing Layer. The early-time evolution of the density and vertical velocity variance spectra indicate that velocity fluctuations are the dominant mechanism driving the instability development. The implications of the present experimental measurements on the initialization of Reynolds-averaged turbulent transport and Mixing models and of direct and large-eddy simulations of Rayleigh–Taylor instability-induced turbulence are discussed.

  • experimental characterization of initial conditions and spatio temporal evolution of a small atwood number rayleigh taylor Mixing Layer
    Journal Name: Journal of Fluid Mechanics vol. 567 N A August 1 2006 pp. 27-63; Journal Volume: 567, 2005
    Co-Authors: Nicholas J Mueschke, Malcolm J Andrews, Oleg Schilling
    Abstract:

    The initial multi-mode interfacial velocity and density perturbations present at the onset of a small Atwood number, incompressible, miscible, Rayleigh-Taylor instability-driven Mixing Layer have been quantified using a combination of experimental techniques. The streamwise interfacial and spanwise interfacial perturbations were measured using high-resolution thermocouples and planar laser-induced fluorescence (PLIF), respectively. The initial multi-mode streamwise velocity perturbations at the two-fluid density interface were measured using particle-image velocimetry (PIV). It was found that the measured initial conditions describe an initially anisotropic state, in which the perturbations in the streamwise and spanwise directions are independent of one another. The evolution of various fluctuating velocity and density statistics, together with velocity and density variance spectra, were measured using PIV and high-resolution thermocouple data. The evolution of the velocity and density statistics is used to investigate the early-time evolution and the onset of strongly-nonlinear, transitional dynamics within the Mixing Layer. The early-time evolution of the density and vertical velocity variance spectra indicate that velocity fluctuations are the dominant mechanism driving the instability development. The implications of the present experimental measurements on the initialization of Reynolds-averaged turbulent transport and Mixing models and of direct and large-eddy simulations of Rayleigh-Taylor instability-induced turbulence are discussed.

Daniele Carati - One of the best experts on this subject based on the ideXlab platform.

  • direct numerical simulation and large eddy simulation of a shear free Mixing Layer
    Journal of Fluid Mechanics, 2004
    Co-Authors: Bernard Knaepen, Olivier Debliquy, Daniele Carati
    Abstract:

    High resolution direct numerical simulation (DNS) (512×1024×512) and large-eddy simulation (LES) of a shear-free Mixing Layer are presented. The geometry of the flow consists of two Layers with different turbulence intensities that are in contact and interact through a fairly thin Mixing Layer. This geometry is used to explore the influence of inhomogeneities in the characteristic length scales, times scales and energy scales on the turbulence properties. Comparison of DNS results is made with the Veeravalli & Warhaft ( J. Fluid Mech. 207 , 191–229, 1989) experiment. The LES is performed on a 32×64×32 grid using an eddy-viscosity model. The use of such a model appears to be justified by the very weak departures from isotropy that are observed in the shear-free Mixing Layer. The LES predictions are compared with the filtered DNS data and show that the eddy viscosity model performs very well in predicting the energy profile as well as the deviation from Gaussianity in the turbulent velocity field statistics.

  • high resolution dns of a shear free Mixing Layer and les
    2004
    Co-Authors: Olivier Debliquy, Bernard Knaepen, D Desmidts, Daniele Carati
    Abstract:

    Results from a high resolution (512 × 1024 × 512) DNS of a shear-free Mixing Layer are investigated. The Mixing Layer consists of the transition between two regions of homogeneous turbulence characterized by different turbulent intensities and energy spectra. The simulation has been initialised in order to reproduce the conditions of laboratory investigations of the same flow and an extensive comparison between the DNS and the experimental results is proposed. Also, the possibility to reproduce the main features of this flow using LES is explored.

  • dns and les of a shear free Mixing Layer
    Center for Turbulence Research Annual Research Briefs 2003, 2003
    Co-Authors: Bernard Knaepen, Olivier Debliquy, Daniele Carati
    Abstract:

    The purpose of this work is twofold. First, given the computational resources available today, it is possible to reach, using DNS, higher Reynolds numbers than in Briggs et al.. In the present study, the microscale Reynolds numbers reached in the low- and high-energy homogeneous regions are, respectively, 32 and 69. The results reported earlier can thus be complemented and their robustness in the presence of increased turbulence studied. The second aim of this work is to perform a detailed and documented LES of the shear-free Mixing Layer. In that respect, the creation of a DNS database at higher Reynolds number is necessary in order to make meaningful LES assessments. From the point of view of LES, the shear-free Mixing-Layer is interesting since it allows one to test how traditional LES models perform in the presence of an inhomogeneity without having to deal with difficult numerical issues. Indeed, as argued in Briggs et al., it is possible to use a spectral code to study the shear-free Mixing Layer and one can thus focus on the accuracy of the modelling while avoiding contamination of the results by commutation errors etc. This paper is organized as follows. First we detail the initialization procedure used in the simulation. Since the flow is not statistically stationary, this initialization procedure has a fairly strong influence on the evolution. Although we will focus here on the shear-free Mixing Layer, the method proposed in the present work can easily be used for other flows with one inhomogeneous direction. The next section of the article is devoted to the description of the DNS. All the relevant parameters are listed and comparison with the Veeravalli & Warhaft experiment is performed. The section on the LES of the shear-free Mixing Layer follows. A detailed comparison between the filtered DNS data and the LES predictions is presented. It is shown that simple eddy viscosity models perform very well for the present test case, most probably because the flow seems to be almost isotropic in the small-scale range that is not resolved by the LES.

  • large eddy simulation of a shear free magnetohydrodynamic Mixing Layer
    DNS LES - Progress and Challenges: Proceedings of the Third AFOSR International Conference on DNS LES, 2001
    Co-Authors: Olivier Debliquy, Bernard Knaepen, Daniele Carati, Leonidas Sakell, Chaoqun Liu, Thomas Beutner
    Abstract:

    Abstract : We present LES results of the evolution of a decaying magneto hydrodynamic (MHD) Mixing Layer using dynamic eddy-viscosity subgrid scale models. The LES results are obtained using a spectral code with a 32(exp 3) resolution and are compared to a direct numerical simulation (DNS) with 128(exp 3) Fourier modes. The evolution of the kinetic and magnetic energies is presented and their profiles along the inhomogeneous direction is also discussed.

Josette Bellan - One of the best experts on this subject based on the ideXlab platform.

  • direct numerical simulation of a confined three dimensional gas Mixing Layer with one evaporating hydrocarbon droplet laden stream
    Journal of Fluid Mechanics, 1999
    Co-Authors: Richard S Miller, Josette Bellan
    Abstract:

    Direct numerical simulations are performed of a confined three-dimensional, temporally developing, initially isothermal gas Mixing Layer with one stream laden with as many as 7.3×10 5 evaporating hydrocarbon droplets, at moderate gas temperature and subsonic Mach number. Complete two-way phase couplings of mass, momentum and energy are incorporated which are based on a thermodynamically self-consistent specification of the vapour enthalpy, internal energy and latent heat of vaporization. Effects of the initial liquid mass loading ratio ( ML ), initial Stokes number ( St 0 ), initial droplet temperature and flow three-dimensionality on the Mixing Layer growth and development are discussed. The dominant parameter governing flow modulation is found to be the liquid mass loading ratio. Variations in the initial Stokes number over the range 0.5[les ] St 0 [les ]2.0 do not cause significant modulations of either first- or second-order gas phase statistics. The Mixing Layer growth rate and kinetic energy are increasingly attenuated for increasing liquid loadings in the range 0[les ] ML [les ]0.35. The laden stream becomes saturated before evaporation is completed for all but the smallest liquid loadings owing to: (i) latent heat effects which reduce the gas temperature, and (ii) build up of the evaporated vapour mass fraction. However, droplets continue to be entrained into the Layer where they evaporate owing to contact with the relatively higher-temperature vapour-free gas stream. The droplets within the Layer are observed to be centrifuged out of high-vorticity regions and to migrate towards high-strain regions of the flow. This results in the formation of concentration streaks in spanwise braid regions which are wrapped around the periphery of secondary streamwise vortices. Persistent regions of positive and negative slip velocity and slip temperature are identified. The velocity component variances in both the streamwise and spanwise directions are found to be larger for the droplets than for the gas phase on the unladen stream side of the Layer; however, the cross-stream velocity and temperature variances are larger for the gas. Finally, both the mean streamwise gas velocity and droplet number density profiles are observed to coincide for all ML when the cross-stream coordinate is normalized by the instantaneous vorticity thickness; however, first-order thermodynamic profiles do not coincide.

Lawrence Ukeiley - One of the best experts on this subject based on the ideXlab platform.

  • examination of large scale structures in a turbulent plane Mixing Layer part 2 dynamical systems model
    Journal of Fluid Mechanics, 2001
    Co-Authors: Lawrence Ukeiley, Laurent Cordier, Remi Manceau, J Delville, Mark Glauser, Jeanpaul Bonnet
    Abstract:

    The temporal dynamics of large-scale structures in a plane turbulent Mixing Layer are studied through the development of a low-order dynamical system of ordinary differential equations (ODEs). This model is derived by projecting Navier–Stokes equations onto an empirical basis set from the proper orthogonal decomposition (POD) using a Galerkin method. To obtain this low-dimensional set of equations, a truncation is performed that only includes the first POD mode for selected streamwise/spanwise ( k 1 / k 3 ) modes. The initial truncations are for k 3 = 0; however, once these truncations are evaluated, non-zero spanwise wavenumbers are added. These truncated systems of equations are then examined in the pseudo-Fourier space in which they are solved and by reconstructing the velocity field. Two different methods for closing the mean streamwise velocity are evaluated that show the importance of introducing, into the low-order dynamical system, a term allowing feedback between the turbulent and mean flows. The results of the numerical simulations show a strongly periodic flow indicative of the spanwise vorticity. The simulated flow had the correct energy distributions in the cross-stream direction. These models also indicated that the events associated with the centre of the Mixing Layer lead the temporal dynamics. For truncations involving both spanwise and streamwise wavenumbers, the reconstructed velocity field exhibits the main spanwise and streamwise vortical structures known to exist in this flow. The streamwise aligned vorticity is shown to connect spanwise vortex tubes.

  • examination of large scale structures in a turbulent plane Mixing Layer part 1 proper orthogonal decomposition
    Journal of Fluid Mechanics, 1999
    Co-Authors: Joel Delville, Laurent Cordier, Lawrence Ukeiley, Jeanpaul Bonnet, Mark Glauser
    Abstract:

    Large-scale structures in a plane turbulent Mixing Layer are studied through the use of the proper orthogonal decomposition (POD). Extensive experimental measurements are obtained in a turbulent plane Mixing Layer by means of two cross-wire rakes aligned normal to the direction of the mean shear and perpendicular to the mean flow direction. The measurements are acquired well into the asymptotic region. From the measured velocities the two-point spectral tensor is calculated as a function of separation in the cross-stream direction and spanwise and streamwise wavenumbers. The continuity equation is then used for the calculation of the non-measured components of the tensor. The POD is applied using the cross-spectral tensor as its kernel. This decomposition yields an optimal basis set in the mean square sense. The energy contained in the POD modes converges rapidly with the first mode being dominant (49% of the turbulent kinetic energy). Examination of these modes shows that the first mode contains evidence of both known flow organizations in the Mixing Layer, i.e. quasi-two-dimensional spanwise structures and streamwise aligned vortices. Using the shot-noise theory the dominant mode of the POD is transformed back into physical space. This structure is also indicative of the known flow organizations.