On Three-Dimensionality of Turbulent Buoyant Channel Flow

A three-dimensional buoyancy-driven turbulent boundary layer is generated in fully developed turbulent channel flow.Setting the buoyancy forces perpendicular to the mean flow direction and using high Grashof number result in strong crossflow which is expected to have a similar influence to that of the previously studied pressure- and shear-driven threedimensionalturbulence boundary layers (3DTBLs). The present buoyancy-driven configuration has the advantage of being inequilibrium and three-dimensional from inception. Few issues are presented and discussed in order to give betterunderstanding of the underlying physics associated with buoyancy-induced 3DTBL flows. The characteristics of the resulting3DTBL are examined in comparisons with the available data in the literature. Reduction of the structure parameter A1(defined as the ratio between the total Reynolds shear stress to twice the turbulent kinetic energy TKE) is noticed in the edgeof the inner layer which is attributed to the reduction of both Reynolds shear stress and turbulent kinetic energy. Explanationfor such reductions is given by examining the changes in coherent structures due to buoyancy effect. Comparisons of somerelevant quantities used in turbulence modeling with those of 2DTBL flow reveal the difficulty of establishing a universalmodel for a satisfactory 3DTBL flow prediction.