The characteristics of coherent structures in low Reynolds number mixed convection flows

Turbulent coherent structures generated in a channel flow at low Reynolds numbers during mixed convection have been experimentally studied using the particle image velocimetry (PIV) technique. The measurements are conducted in the channel cross-plane, the streamwise mid-vertical plane and two horizontal planes close to the bottom heated wall to capture the three-dimensional aspect. In the present study, Gr/Re² ranged between 9 and 206, implying that the natural convection was dominant over forced convection. An algorithm based on the velocity tensor second invariant (Q) is used to detect coherent structures. The location of each detected coherent structure is recorded, and vorticity and kinetic energy associated with each coherent structure are computed. The number and strength of the coherent structures are found to increase with an increase in the bottom wall temperature in all measurement planes. The strength and number of coherent structures show partial dependency on the flow rate. The number of coherent structures is found to be largest in the channel's lower half, where strong interactions between rising plumes, falling sheets and mean shear flow occur. However, on average, the most energetic coherent structures are present in the channel's upper region.