Evolution of thermally stratified turbulent open channel flow after removal of the heat source
Access status:
Open Access
Type
ArticleAbstract
Evolution of thermally stratified open channel flow after removal of a volumetric heat source is investigated using direct numerical simulation. The heat source models radiative heating from above and varies with height due to progressive absorption. After removal of the heat source ...
See moreEvolution of thermally stratified open channel flow after removal of a volumetric heat source is investigated using direct numerical simulation. The heat source models radiative heating from above and varies with height due to progressive absorption. After removal of the heat source the initial stable stratification breaks down and the channel approaches a fully mixed isothermal state. The initial state consists of three distinct regions: a near-wall region where stratification plays only a minor role, a central region where stratification has a significant effect on flow dynamics and a near-surface region where buoyancy effects dominate. We find that a state of local energetic equilibrium observed in the central region of the channel in the initial state persists until the late stages of the destratification process. In this region local turbulence parameters such as eddy diffusivity kh and flux Richardson number Rf are found to be functions only of the Prandtl number Pr and a mixed parameter Q , which is equal to the ratio of the local buoyancy Reynolds number Reb and the friction Reynolds number Reτ . Close to the top and bottom boundaries turbulence is also affected by Reτ and vertical position z . In the initial heated equilibrium state the laminar surface layer is stabilised by the heat source, which acts as a potential energy sink. Removal of the heat source allows Kelvin–Helmholtz-like shear instabilities to form that lead to a rapid transition to turbulence and significantly enhance the mixing process. The destratifying flow is found to be governed by bulk parameters Reτ , Pr and the friction Richardson number Riτ . The overall destratification rate D is found to be a function of Riτ and Pr .
See less
See moreEvolution of thermally stratified open channel flow after removal of a volumetric heat source is investigated using direct numerical simulation. The heat source models radiative heating from above and varies with height due to progressive absorption. After removal of the heat source the initial stable stratification breaks down and the channel approaches a fully mixed isothermal state. The initial state consists of three distinct regions: a near-wall region where stratification plays only a minor role, a central region where stratification has a significant effect on flow dynamics and a near-surface region where buoyancy effects dominate. We find that a state of local energetic equilibrium observed in the central region of the channel in the initial state persists until the late stages of the destratification process. In this region local turbulence parameters such as eddy diffusivity kh and flux Richardson number Rf are found to be functions only of the Prandtl number Pr and a mixed parameter Q , which is equal to the ratio of the local buoyancy Reynolds number Reb and the friction Reynolds number Reτ . Close to the top and bottom boundaries turbulence is also affected by Reτ and vertical position z . In the initial heated equilibrium state the laminar surface layer is stabilised by the heat source, which acts as a potential energy sink. Removal of the heat source allows Kelvin–Helmholtz-like shear instabilities to form that lead to a rapid transition to turbulence and significantly enhance the mixing process. The destratifying flow is found to be governed by bulk parameters Reτ , Pr and the friction Richardson number Riτ . The overall destratification rate D is found to be a function of Riτ and Pr .
See less
Date
2019Source title
Journal of Fluid MechanicsVolume
876Publisher
Cambridge University PressFunding information
ARC DP150100912Licence
Creative Commons Attribution-NonCommercial-NoDerivatives 4.0Rights statement
This article has been published in a revised form in Journal of Fluid Mechanics http://doi.org/10.1017/jfm.2019.543. This version is free to view and download for private research and study only. Not for re-distribution or re-use. © copyright holder.Faculty/School
Faculty of Engineering, School of Aerospace Mechanical and Mechatronic EngineeringShare