DNS of heat transfer in a transitional channel flow accompanied by a turbulent puff-like structure

TL;DR

This study uses direct numerical simulations to investigate heat transfer in transitional channel flows, focusing on puff-like structures and their impact on turbulence and heat transport at low Reynolds numbers.

Contribution

It reveals the existence and influence of puff-like structures on heat transfer and turbulence in transitional channel flows at low Reynolds numbers, with detailed analysis of their spatial structure.

Findings

Puff-like structures significantly enhance heat transfer.

Localized turbulence appears as oblique bands with secondary flows.

Flow remains turbulent at Reynolds number as low as 60.

Abstract

Direct numerical simulations of turbulent heat transfer in fully-developed channel flows have been performed in a range of friction Reynolds number between 60 and 180, based on the friction velocity and the channel half width $\delta$, with emphasis on a puff-like structure, large-scale spatial intermittency. For the Reynolds numbers lower than 80 with a large computational domain of 51.2 x 2 x 22.5, the turbulent puff was observed and its significant influences on the momentum and heat transports were found. The spatial structure of the equilibrium puff, or the localized turbulence, was examined with taking account of two different thermal boundary conditions: the uniform heat-flux heating and the constant temperature difference between the walls. It was revealed that there existed a localized strong turbulent region in the form of an oblique band, along which a spanwise secondary flow was induced. In consequence, at the present lowest Reynolds number as low as 60, the flow remained turbulent and the larger Nusselt numbers than those without puff was obtained by the presence of puff.