Turbulent puffs in a horizontal square duct under stable temperature stratification

Abstract

Direct numerical simulations of streamwise-localized turbulent flow (turbulent puff) in a horizontal square duct heated from above are performed at Richardson numbers 0 ≤ Ri ≤ 0.58 to characterize differences between isothermal and non-isothermal puffs. It is found that the structure of the non-isothermal puff exhibits significantly different properties from that of the isothermal puff. In particular, differences are observed in wall shear stresses. In the upstream part of the non-isothermal puff, higher shear appears on the horizontal walls than the vertical ones. In the downstream part, meanwhile, higher shear appears on the vertical walls than the horizontal ones. The higher shear on the horizontal walls in the upstream part is interpreted in terms of coherent streamwise vortices induced by buoyancy in the corner regions. The higher shear on the vertical walls in the downstream part is considered to be a consequence of higher wall-normal (horizontal) turbulence intensity on the vertical walls under the weaker constraint of the stable stratification.