On the impact of the aspect ratio on the formation of large-Scale structures in turbulent mixed convection in a cuboidal sample

摘要

Temperature measurements combined with Particle Image Velocimetry (PIV) were performed in a cuboidal convection sample with a square cross section (width / height). The objective of the study is to analyze the influence of the aspect ratio Gamma(xz) = L/H and the Archimedes number Ar on the large-scale coherent structures in mixed convection. Air at ambient pressure was used as working fluid (Pr = 0.7). The aspect ratio was varied between 1 = Gamma(xz) = 4 (length / height) and the variation of the Archimedes number was achieved by prescribing the inflow velocity of an air jet, which was superimposed on the thermally induced flow via an inlet and outlet channel. The Rayleigh number was kept constant at Ra = 2.0 x 10(8). A model, adopted from pure thermal convection, was used to extract the number of convection rolls from the circumferential temperature distribution measured along the sidewalls of the convection sample. For a fixed Archimedes number (Ar approximate to 4), the number of thermally induced large-scale circulations (LSCTC) increased with the aspect ratio of the sample. Specifically, starting with Gamma = 2, a linear dependence with unity slope was found. A decrease of the Archimedes number led to a transition from the single ISCTC structures to FC-dominated LSCs (LSCFC) for Gamma = 1 and Gamma = 2, whereas the multi-roll structures found for Gamma = 3 and Gamma = 4 were more stable even for higher inflow velocities.