Flow organization in non-Oberbeck-Boussinesq Rayleigh-Benard convection in water

TL;DR

This study numerically investigates how non-Oberbeck-Boussinesq effects influence flow organization in water-based Rayleigh-Benard convection, analyzing temperature, velocity, and heat transfer across various conditions up to high Rayleigh numbers.

Contribution

It provides a detailed numerical analysis of NOB effects on flow structures and heat transfer in water, extending understanding beyond previous experimental data.

Findings

Flow features depend on Rayleigh number and temperature difference.

Center convection roll remains characterized by a single velocity scale.

Results align with experimental NOB data for center temperature and Nusselt number ratio.

Abstract

Non-Oberbeck-Boussinesq (NOB) effects on the flow organization in two-dimensional Rayleigh-Benard turbulence are numerically analyzed. The working fluid is water. We focus on the temperature profiles, the center temperature, the Nusselt number, and on the analysis of the velocity field. Several velocity amplitudes (or Reynolds numbers) and several kinetic profiles are introduced and studied; these together describe the various features of the rather complex flow organization. The results are presented both as functions of the Rayleigh number Ra (with Ra up to 10^8) for fixed temperature difference (Delta) between top and bottom plates and as functions of Delta ("non-Oberbeck-Boussinesqness") for fixed Ra with Delta up to 60 K. All results are consistent with the available experimental NOB data for the center temperature Tc and the Nusselt number ratio Nu_{NOB}/Nu_{OB} (the label OB meaning that the Oberbeck-Boussinesq conditions are valid). Beyond Ra ~ 10^6 the flow consists of a large diagonal center convection roll and two smaller rolls in the upper and lower corners. In the NOB case the center convection roll is still characterized by only one velocity scale.