Abstract
Using various closure approximation theories, the evolution of scalar quantity spectrum in a statistically stationary isotropic turbulent velocity field is investigated to provide a unified comparison at Peclet number of 50. The phenomenological Heisenberg (eddy diffusivity) transfer function and the analytical approaches including the truncation, the quasi-normal, and the recent direct-interaction closure approximations are examined. As already known, the truncation and quasi-normal closures result in a negative-spectrum within short decay times. A modified quasi-normal closure formula is suggested as a means of approximating the simultaneous covariances, and the results thereof are comparable with the direct-interaction counterparts.

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