Numerical study of the coherent structures in a transitional vertical channel natural convection flow

Numerical simulations of a spatially developing transitional flow in a vertical channel with one side uniformly heated and subjected to random velocity fluctuations at the inlet have been performed. Two characteristic frequency bands are observed in the flow, near the heated wall. The ability of the Proper Orthogonal Decomposition and the time-domain Spectral Proper Orthogonal Decomposition (SPOD) to decompose the flow is assessed, and SPOD is shown to be a powerful tool, as it is capable of separating the most energetic modes into two great families whose frequency content matches the frequency bands previously identified. The spatial structure of the modes is described, and their contribution to the turbulent heat transfer and velocity-temperature correlation is evaluated. Finally, the modes are linked to coherent structures that are observed in instantaneous visualizations of the flow, and a scenario of the development of the coherent structures in the laminar-turbulent transitional process is proposed.