Buoyancy-induced effects on large-scale motions in differentially heated vertical channel flows studied in direct numerical simulations

Abstract

Direct numerical simulations of turbulent convection in a differentially heated vertical channel flow with Prandtl number Pr = 0.71 are conducted with a fourth-order accurate finite volume method for a bulk Reynolds number Re_b=4328 and three Grashof numbers Gr = {0, 6.4x10^5, 9.5x10^5}. The analysis of instantaneous flow field snapshots, first- and second-order moments, budget equations, power density spectra and quadrant analyses shows that the turbulent velocity fluctuations are attenuated in the aiding flow and enhanced in the opposing flow. In contrast, temperature fluctuations are attenuated in the opposing flow and enhanced in the aiding flow. The analyses further reveal that the low-momentum flow structures in the aiding flow are warmer than the high-momentum flow structures and vice versa in the opposing flow. Due to their different temperatures, buoyancy accelerates and decelerates the flow structures differently, which leads to reduced and increased pressure and shear fluctuations in the aiding and opposing flow. Thus, the redistribution of turbulent velocity fluctuations is lower in the aiding flow and higher in the opposing flow. The Reynolds shear stresses are consequently decreased in the former and increased in the latter, influencing the production of streamwise velocity fluctuations accordingly. In summary, the discussed physical mechanisms underline the indirect effect of buoyancy on the turbulent velocity fluctuations.