Direct numerical simulations of high velocity ratio coaxial jets: mixing properties and influence of upstream conditions

Balarac, Guillaume; Si‐Ameur, Mohamed; Lesieur, Marcel; Métais, Olivier

doi:10.1080/14685240600833094

Cited by 21 publications

(24 citation statements)

References 21 publications

(44 reference statements)

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“…As in plane mixing layers 11 and single jets, 12 the coaxial jets develop counter-rotating pairs of streamwise vortices stretched between consecutive vortex rings: these play a major role in the mixing process. Indeed, in a recent numerical study, 13 we have shown that the ejections of the species seeded in the outer jet associated with these streamwise vortices strongly enhanced the jet mixing properties. Balarac et al 13 also demonstrated that the turbulent mixing process is strongly affected by the jet upstream conditions, which modify the transitional state.…”

Section: Introductionmentioning

confidence: 81%

Mixing enhancement in coaxial jets through inflow forcing: A numerical study

2007

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Direct numerical simulations are performed to analyze the flow dynamics and the mixing properties of natural ͑unforced͒ and excited coaxial jets at moderate Reynolds number. First, the study of the natural coaxial jet, with species injected in the outer jet alone, allows us to understand the role of the coherent vortices on the mixing process during the transition stage. It is observed that the global flow behavior is controlled by the dynamics of the outer shear layer during the transition. The streamwise vortices are shown to play a significant role in the mixing process since they initiate intense ejections from the seeding regions. Spots of pure ͑unmixed͒ species from the outer jet are seen to persist far downstream. Two different types of inflow forcing are then considered based on the information provided by the natural coaxial jet: first, a purely axisymmetric excitation and second, combined axisymmetric and azimuthal excitations all of moderate amplitude. These excitations are applied to the outer shear layer with a frequency corresponding to the periodic passage of the outer vortical structures. The goal of these excitations is to trigger the vortices formation and to control their dynamics to improve the mixing properties of the jet. With the purely axisymmetric excitation, the outer and inner Kelvin-Helmholtz vortices appear earlier than for the natural jet and the transition process is faster. This early three-dimensionality growth is due to a rapid appearance of streamwise vortices, stretched between consecutive vortex rings, which lead to enhanced mixing. For the combined axisymmetric and azimuthal excitations, the outer Kelvin-Helmholtz rings appear moreover with an azimuthal deformation from the beginning of the jet. This allows for the early generation of streamwise vortices. The vortex stretching phenomenon takes place near the jet inlet with a growth of the axial component of the vorticity fluctuations. The ejections of species from the outer jet thus appear sooner and with a larger intensity. Finally, the mixing efficiency is studied through the global mixedness and the intensity of segregation.

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Section: Introductionmentioning

confidence: 81%

Mixing enhancement in coaxial jets through inflow forcing: A numerical study

2007

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“…Stanley et al [13], Reichert and Biringen [14] did DNS of a compressible plane jet exhausting into a parallel stream. Balarac et al [15,16] performed DNS of coaxial jet at moderate Reynolds with high velocity ratio.…”

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confidence: 99%

Direct Numerical Simulation of Subsonic Round Turbulent Jet

Wang

et al. 2010

Flow Turbulence Combust

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Direct numerical simulation(DNS) of spatially developing round turbulent jet flow with Reynolds number 4,700 was carried out. Over 20 million grid points were used in this simulation. Fully compressible three-dimensional Navier-Stokes equations were solved. High order explicit spatial difference schemes and RungeKutta time integration scheme were used to calculate derivatives and time marching, respectively. Non-reflecting boundary conditions and exit zone techniques were adopted. Some refined computational grids were used in order to capture the smallest turbulent structures near the centerline of the jet. Low level disturbance were imposed on the jet inflow velocity to trigger the developing of turbulence. Turbulent statistics such as mean velocity, Reynolds stresses, third order velocity moments were obtained and compared with experimental data. One-dimensional velocity autospectra was also calculated. The inertial region where the spectra decays according to the k −5/3 was observed. The quantitative profiles of mean velocity and all of the third order velocity moments which were difficult to measure via experimental techniques were presented here in detail. The jet flow was proven to be close to fully self-similar around 19 jet diameters downstream of jet exit. The statistic data and revealed flow feature obtained in this paper can provide valuable reference for round turbulent jet research.

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“…The introduction of white noise perturbation avoids difficulties in specifying random perturbation with a given spectrum; this region is not representative for confrontation to experimental investigations as the causes of disturbances is linked to several sources like vibrations, pressure waves… However, it has been shown [3,4,21] that the consideration of hyperbolic profile let to develop easily the main instabilities and the flow features.…”

Section: Resultsmentioning

confidence: 99%