Buoyancy-driven secondary flows and momentum redistribution in high-pressure transcritical duct turbulence

In turbulent supercritical fluid flows, strong thermodynamic property variations shape the flow response to buoyancy and substantially modify turbulence dynamics. In square ducts, where turbulence is intrinsically coupled with secondary motions, these effects can fundamentally reshape cross-stream flow organization, redistribute streamwise momentum, and alter the turbulent balance. In this work, direct numerical simulations of high-pressure transcritical turbulent flow in a horizontal square duct are performed to isolate the effect of buoyancy orientation. Four configurations are considered, spanning stable and unstable stratification, as well as transverse buoyancy forcing, at comparable bulk Reynolds and Prandtl numbers. In all cases, a pseudo-boiling layer forms near the hot wall, but buoyancy direction strongly modifies its deformation, spatial organization, and mixing pattern. The resulting thermodynamic asymmetry reorganizes the topology and intensity of the secondary motions, which then act as the primary pathway through which buoyancy effects are transferred to the mean flow. Although bulk and wall-averaged quantities remain broadly similar across cases, secondary-flow intensity and topology vary strongly with buoyancy orientation, producing marked spatial redistribution of streamwise momentum. This redistribution displaces high-momentum regions, alters mean velocity gradients, and generates strongly non-uniform wall shear stress distributions. The pseudo-boiling region, therefore, behaves as an active wall-attached layer that modulates local viscous scaling and near-wall transport, while the net redistribution of streamwise momentum is governed primarily by the buoyancy-modified secondary-flow circulation. Consequently, the turbulence field responds through a spatially non-uniform restructuring of intensities, Reynolds-stress anisotropy, and turbulent kinetic-energy transport.