Study of one-dimensional spectral dynamic equations of the Reynolds stresses in homogeneous anisotropic turbulence: Application to split-spectrum modeling

The CTR numerical data base generated by direct simulation of homogeneous anisotropic turbulence was used to calculate all of the terms in the spectral balance equations for the turbulent Reynolds stresses. The aim in not only to test the main closure assumptions used in the split-spectrum models, but also to try to devise improved hypotheses deduced from the statistical information. Numerical simulations of turbulent flows provide a large amount of data, a thought provoking wealth of information. The main advantage of this type of comparison is that a great variety of flows can be considered, and this is necessary to test closure hypotheses. Moreover various initial conditions can be introduced in the calculation, even if they are not experimentally feasible. All the terms in the spectral equations can be calculated. The limited Reynolds numbers of the simulations and the statistical noise caused by a small sample, particularly at the large scales, causes some difficulty in the interpretation of the results, but the method of approach proved to be a powerful tool for testing and improving spectral closures

Second-order modelling of variable density turbulent jets : evaluation in the near field region

International audienceThis paper is concerned with a complete second-order model of variable density turbulent Jets. Emphasis is given here on the near-fleld region of the flow where it is found that the influence of the density variations is quite important resulting in a complex behaviour of both the mean and turbulent velocity fields. Particular attention has been paid to the mesh grid and the initial conditions so that quantitative comparison with the experimental data obtained in the study developed in parallel to that one at I.M.S.T. can be made. Results relative to the velocity field only will be reported herein since quite few studies have been focusing so far on the near-field region where the model shows shoncomings which may not be visible when looking at results obtained in the far-field where pseudo-similarity is attained

Inertial range scaling of the scalar flux spectrum in two-dimensional turbulence

Two-dimensional statistically stationary isotropic turbulence with an imposed uniform scalar gradient is investigated. Dimensional arguments are presented to predict the inertial range scaling of the turbulent scalar flux spectrum in both the inverse cascade range and the enstrophy cascade range for small and unity Schmidt numbers. The scaling predictions are checked by direct numerical simulations and good agreement is observed

Inertial range scaling of scalar flux spectra in uniformly sheared turbulence

A model based on two-point closure theory of turbulence is proposed and applied to study the Reynolds number dependency of the scalar flux spectra in homogeneous shear flow with a cross-stream uniform scalar gradient. For the cross-stream scalar flux, in the inertial range the spectral behavior agrees with classical predictions and measurements. The streamwise scalar flux is found to be in good agreement with the results of atmospheric measurements. However, both the model results and the atmospheric measurements disagree with classical predictions. A detailed analysis of the different terms in the evolution equation for the streamwise scalar flux spectrum shows that nonlinear contributions are governing the inertial subrange of this spectrum and that these contributions are relatively more important than for the cross-stream flux. A new expression for the scalar flux spectra is proposed. It allows us to unify the description of the components in one single expression, leading to a classical K^-7/3 inertial range for the cross-stream component and to a new K^-23/9 scaling for the streamwise component that agrees better with atmospheric measurements than the K^-3 prediction of J. C. Wyngaard and O. R. Cot\'e [Quart. J. R. Met. Soc. 98, 590 (1972)]

Investigation of characteristic length scales in variable density turbulent jets

National audienceIt is now well established that density variations have a significant effect on the structure of turbulent jets

Investigation of characteristic scales in variable density turbulent jets using a second-order model

International audienceThis paper is concerned with the investigation of characteristic turbulent scales in circular and plane turbulent free jets with large density variations. The numerical study has been performed with a second-order turbulence model to confirm analytical relations about density and Reynolds number effects on dynamic and scalar turbulence scales derived from requirements of pseudo-self-preservation. The present results explain and quantify trends previously reported from visualizations of variable density turbulent jets performed by Pitts (NBSIR Report No. 86-3340, U.S. Department of Commerce, 1986). It is clearly shown that the parameter relevant for the evolution of these scales is the momentum flux Mj . In addition, the influence of this quantity on small scales is found to be larger than that on large scales