ABSTRACT RESUMEN A partir del programa original de Rogallo (1981) y del derivado po) Lee y Reynolds (1985)) se ha elaborado un programa de simulación directa de la turbulencia homogénea bidimensional. Este programa se ha usado para medir la velocidad de retorno a la isotropía en un flujo turbulento homogéneo inicialmente no isotrópico y para deducir e l efecto de los esfuerzos normales sobre la turbulencia bidimensional isotrópica. Los resultados obtenidos han permitido observar que (1) sí que existe retorno a la isotropía cuando se relaja u4 campo inicialmente no isotrópico y se mantiene la bidimensionalidad y (2) la deformación de un campo bidimensional isotrópico no introduce ninguna anisotropía. SUMMARY The effects of anisotropy on the decay of two-dimensional turbulence have been studied using a computer code for the direct numerical simulation of homogqneous two-dimensional turbulence, developed from the original code of Rogallo (1981) and the subsequent code of Lee and Reynolds (1985). Also, the effects of normal strain on isotropic two-dimensional turbulence have been established. Results show that (1) it exists return to isotropy during the relaxation process of an initially non isotropic, two-dimensional, homogeneous flow and (2) the deformation of an isotropic two-dimensionalfield by means of a plane strain does not droduce any anisotropic effect.

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    ABSTRACT: We compare results of high-resolution direct numerical simulation with equivalent two-point moment closure (the test-field model) for both randomly forced and spin-down problems. Our results indicate that moment closure is an adequate representation of observed spectra only if the random forcing is sufficiently strong to disrupt the dynamical tendency to form intermittent isolated vortices. For strong white-noise forcing near a lower-wavenumber cut-off, theory and simulation are in good agreement except in the dissipation range, with an enstrophy range less steep than the wavenumber to the minus fourth power. If the forcing is weak in amplitude, red noise, and at large wavenumbers, significant errors are made by the closure, particularly in the inverse-cascade range. For spin-down problems at large Reynolds numbers, the closure considerably overestimates enstrophy transfer to small scales, as well as energy transfer to large scales. We finally discuss the possibility that the closure errors are related to intermittency of various types. Intermittency can occur in either the inverse-cascade range (forced equilibrium) or the intermediate scales (spin-down), with isolated concentrations of vorticity forming the associated coherent structures, or it can occur in the dissipation range owing to the nonlinear amplification of variations in the cascade rate (Kraichnan 1967).
    Journal of Fluid Mechanics 03/1985; 153(-1):229 - 242. DOI:10.1017/S0022112085001239 · 2.38 Impact Factor
  • Studies in Applied Mathematics 01/1971; 50. · 1.25 Impact Factor
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    ABSTRACT: The spectral method of Orszag and Patterson (1972a, b) is used here to study pressure and velocity fluctuations in axisymmetric, homogeneous, incompressible, decaying turbulence at Reynolds numbers Re<sub>λ</sub> [less, similar] 40. In real space 32<sup>3</sup> points are treated. The return to isotropy is simulated for several different sets of anisotropic Gaussian initial conditions. All contributions to the spectral energy balance for the different velocity components are shown as a function of time and wavenumber. The return to isotropy is effected by the pressure-strain correlation. The rate of return is larger at high than at low wavenumbers. The inertial energy transfer tends to create anisotropy at high wavenumbers. This explains the overrelaxation found by Herring (1974). The pressure and the inertial energy transfer are zero initially as the triple correlations are zero for the Gaussian initial values. The two transfer terms are independent of each other but vary with the same characteristic time scale. The pressure-strain correlation becomes small for extremely large anisotropies. This can be explained kinematically. Rotta's (1951) model is approximately valid if the anisotropy is small and if the time scale of the mean flow is much larger than 0·2 L<sub>f</sub>/v, which is the time scale of the triple correlations (L<sub>f</sub> = integral length scale, v = root-mean-square velocity). The value of Rotta's constant is less dependent upon the Reynolds number if the pressure-strain correlation is scaled by v<sup>3</sup>/L<sub>f</sub> rather than by the dissipation. Lumley and Khajeh-Nouri's (1974) model can be used to account for the influence of large anisotropies. The effect of strain is studied by splitting the total flow field into large- and fine-scale motion. The empirical model of Naot, Shavit and Wolfshtein (1970) has been confirmed in this respect.
    Journal of Fluid Mechanics 10/1978; 88(1978):711-735. DOI:10.1017/S0022112078002359 · 2.38 Impact Factor
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