Multiscale analysis of the Reynolds stress, dissipation, and subgrid-scale tensor in turbulent bubbly channel flows: Characterization of anisotropy and modeling implications

Abstract

A direct numerical simulation database of bubbly channel flows at friction Reynolds number 180 and with three different global void fractions has been used to perform a multiscale analysis of the anisotropy of the Reynolds stress tensor, the dissipation tensor, and the subgrid-scale (SGS) tensor in order to characterize the turbulence for a wide range of scales down to the smallest structures occurring in the flow. Based on the hypothesis of Kolmogorov, the non-linear turbulent energy transfer is expected to result in a loss of directional information such that, for a sufficiently high Reynolds number, the small-scale turbulence is expected to be isotropic and universal. The present analysis reveals that the presence of the bubbles increases the anisotropy of the flow which persists down to the smallest scales of motion, even for the dissipation and SGS tensor. This has implications for the complete landscape of turbulence modeling approaches ranging from large eddy simulation, over hybrid approaches to Reynolds averaged Navier–Stokes based modeling.