Coherent Vortical Structures and Their Relation to Hot/Cold Spots in a Thermal Turbulent Channel Flow

Abstract

Direct numerical simulations of a turbulent channel flow with a passive scalar at Re τ = 394 with blowing perturbations is carried out. The blowing is imposed through five spanwise jets located near the upstream end of the channel. Behind the blowing jets (about 1D, where D is the jet diameter), we observe regions of reversed flow responsible for the high temperature region at the wall: hot spots that contribute to further heating of the wall. In between the jets, low pressure regions accelerate the flow, creating long, thin, streaky structures. These structures contribute to the high temperature region near the wall. At the far downstream of the jet (about 3D), flow instabilities (high shear) created by the blowing generate coherent vortical structures. These structures move hot fluid near the wall to the outer region of the channel; thereby, these are responsible for cooling of the wall. Thus, for engineering applications where cooling of the wall is necessary, it is critical to promote the generation of coherent structures near the wall.

Role of vortical structures for enstrophy and scalar transport in flows with and without stable stratification

“…In particular, Kadoch et al [29] showed that in homogeneous, isotropic turbulence vortical structures are mainly responsible for turbulent transport and mixing of passive scalars. The impact of vortical structures on heat transport was investigated by Dharmarathne et al [9] in a thermal turbulent channel flow. They observed that vortical structures near the wall contribute to the removal of hot fluid from the wall to outer region.…”

Flow and Heat or Mass Transfer in the Chemical Process Industry

“…It was found that molecular diffusion and turbulence were both important in mixing when the turbulent flow was anisotropic. Dharmarathne et al [13] also used direct numerical simulations of turbulent flow to investigate the formation of hot and cold spots when jets of fluid with lower temperature were injected into a channel flow. Details of the development of coherent structures and their role in the transfer of heat in anisotropic turbulence were documented.…”