Comment on Effect of viscous-convective subrange on passive scalar statistics at high Reynolds number

Recently, Shete et al. [Phys. Rev. Fluids 7, 024601 (2022)] explored the characteristics of passive scalars in the presence of a uniform mean gradient, mixed by stationary isotropic turbulence. They concluded that at high Reynolds and Schmidt numbers, the presence of both inertial-convective and viscous-convective ranges renders the statistics of the scalar and velocity fluctuations to behave similarly. However, their data included Schmidt numbers of 0.1, 0.7, 1.0, and 7.0, only the last of which can (at best) be regarded as moderately high. Additionally, they do not consider already available data in the literature at substantially higher Schmidt number of up to 512. By including these data, we demonstrate here that the differences between velocity and scalar statistics show no vanishing trends with increasing Reynolds and Schmidt numbers, and essential differences remain intact at all Reynolds and Schmidt numbers.

Automated summary

The study investigates the characteristics of passive scalars under a uniform mean gradient. The results indicate that at high Reynolds and Schmidt numbers, the statistics of scalar and velocity fluctuations behave similarly. However, previous data did not consider higher Schmidt numbers. By including these data, it is demonstrated that the differences between velocity and scalar statistics persist at all Reynolds and Schmidt numbers.