Large eddy simulations of transitional round jets: Influence of the Reynolds number on flow development and energy dissipation

Transitional round jets at Mach number M=0.9, with identical initial conditions except for the diameter, yielding Reynolds numbers over the range 1.7x10(3)<= Re-D <= 4x10(5), are computed by large eddy simulation (LES) using explicit selective/high-order filtering. The effects of the Reynolds number on the jet flows are first presented. As the Reynolds number decreases, the jets develop more slowly upstream from the end of the potential core, but more rapidly downstream. At lower Reynolds numbers, the decay of the centerline velocity and the jet spreading are indeed faster, and the turbulence intensities are higher after the potential core, in agreement with data of the literature. The integral length scales are also significantly larger. The results suggest moreover that the jet self-similar region is reached at shorter axial distances at lower Reynolds numbers. The influence of the Reynolds number on the energy-dissipation mechanisms involved in the LES, namely molecular viscosity and explicit filtering, is secondly investigated. At high Reynolds number, energy dissipation is mainly ensured by the explicit filtering, through the smaller scales discretized. As the Reynolds number decreases, the contribution of molecular viscosity increases and becomes predominant. Molecular viscosity is also shown to affect a large range of turbulent scales with a dissipation peak observed around the Taylor length scale.