Mean flow characteristics in horizontal and vertical channels with unstable stratification by buoyancy for liquid lead

In this work, a further study on turbulence states and structures in horizontal and vertical channels for liquid lead with unstable stratification by buoyancy is carried out based on the latest published high-fidelity direct numerical simulation (DNS) database. The mixed flow in horizontal and vertical channel conditions at $R{e}_{\tau }=150$, $Pr=0.025$, and $Ri=0$, 0.25, 0.5, 1 are considered. The turbulence states of liquid lead with different buoyant effects are studied with Lumley triangle. The results indicate that different buoyant effects have a remarkable impact on turbulence states, in which the collapse of invariant trajectories in outer region suggests similarity. Large-scale organized motions and large-scale thermal structures are identified through instantaneous fields and quantitatively analyzed using one-dimensional premultiplied wavenumber spectra. The typical spanwise wavelengths of large-scale organized motions and large-scale thermal structures induced by buoyancy are approximately $3\delta$ and $2\pi \delta$, respectively, where $\delta$ is channel half-width. Detailed analyses with intuitive instantaneous fields and quantitative premultiplied wavenumber spectra are performed to investigate physical mechanisms in mixed channel flows with different buoyant effects for liquid lead. It is expected that this study would give insights into mean flow characteristics in mixed horizontal and vertical channel flows with unstable stratification for liquid lead.