Dual-mesh hybrid RANS-LES computations of turbulent natural convection flows

Abstract

Natural convection flows (buoyant flows) are important from the engineering point of view as they can be found in a number of engineering applications such as building ventilation, cooling of electronic equipment and passive safety nuclear plant designs. Buoyant flows are complicated from the RANS modelling point of view due to the twoway coupling that they feature between the flow and the thermal fields. On the other hand, the modelling of buoyant flows is easier with LES as a wide range of turbulence scales is resolved directly. Hence, an inaccurate modelling of the unresolved scales has a smaller impact on the LES results compared to RANS where all the turbulence scales are modelled. However, resolving the fine turbulence structures in the near-wall region with LES becomes very expensive at high Rayleigh numbers. This high cost of the LES can be overcome by using a hybrid RANS/LES approach such as the dual-mesh approach used here. In the dual-mesh framework, an auxiliary RANS simulation is used to resolve the near-wall region and this simulation is used to correct the near-wall statistics of an LES simulation which is done using a grid that is under-resolved near the walls. However, away from the wall the RANS simulation is corrected towards the LES. In this thesis, the dual-mesh approach was used for the first time to predict buoyant flows. The buoyant square cavity flow and the buoyant flow inside a cylindrical annuli were chosen here as they contain interesting features such as mean flow unsteadiness and the coexistence of laminar and turbulent regions. A heated forced convection channel flow was also studied here for the purpose of methodology and code validation. It has been found that the criteria that were used in the literature to determine the locations were the RANS corrects the LES and vice versa are not suitable for the buoyant flows studied here. Consequently, a new resolution criterion based on the turbulence lengthscales was designed here to be suitable for similar types of natural convection flows. The dual-mesh results of the studied cases were assessed by comparing them against available DNS and Quasi-DNS data from studies in the literature. It has been found that the correction of the under-resolved LES towards the RANS near the walls which is done when using the dual-mesh approach can allow the LES to perform well in capturing the outer edges of the turbulent boundary layers. In the buoyant cases studied here, these locations feature intermittency and interactions between the turbulent boundary layers and the laminar regions and the LES is superior to the RANS in capturing these effects.

Date of Award	1 Aug 2020
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Alistair Revell (Supervisor), Dominique Laurence (Supervisor) & Imran Afgan (Supervisor)