Large eddy simulations of wall heat transfer and coherent structures in mixed convection over a wavy wall

In this numerical study the mixed convective flow of water over a heated wavy surface over a range of Reynolds and Richardson numbers, including transitional and turbulent flow regimes (20 ≤ Re ≤ 2000 and 0.5 ≤ Ri ≤ 5000) is investigated. A dynamic Large Eddy Simulation (LES) approach is applied where the thermal buoyancy effects are represented by the Boussinesq approximation. The LES results show good agreement with available measurements including first and second order statistics of velocity and thermal fields. We focus our investigation on the thermal buoyancy effects on the wall heat transfer and the spatial reorganisation of the vortical flow structures. In order to characterise the reorganisation of the mean flow features, the vortical coherent structures are identified and extracted according to the swirling strength criteria. Interesting reorganisation of flow structures takes place between Re = 20 and Re = 200 where the initially spanwise oriented large coherent structures start to be streamwise oriented. With further increase of Re, these large structures disappear from the central part of the simulated domain and reappear in the proximity of the horizontal wavy wall for Re ≥ 1000. The imprints of this flow reorganisation are clearly visible in the distributions of the local heat transfer coefficient (Nusselt number) along the horizontal wavy wall. The integral heat transfer for the wavy wall configuration is significantly enhanced (≈2.5 times) for Re = 1000, 2000 in comparison with the standard flat horizontal wall configuration.