Flow over a heated hollow sphere at elevated pressures: Near-wake thermal effect on flow structures

This experimental study investigates the effects of buoyancy on wake dynamics in buoyancy opposed-flow mixed convection by analyzing time-resolved particle image velocimetry data collected at an elevated pressure (3 MPa), which spans a range of Reynolds numbers (14 000 ≤ Re ≤ 28 500) and Richardson numbers (0.47 ≤ Ri ≤ 1.84). Changes in recirculation characteristics, turbulent kinetic energy (TKE) distribution, separation angle, and large-scale coherent structures are reported. Spectral proper orthogonal decomposition (SPOD) is applied to identify dominant flow structures and their spectral energy distribution across varying Ri. Results indicate that increasing buoyancy effects expands the recirculation zone, suggesting enhanced lateral mixing due to buoyancy-driven instabilities. The separation point shifts upstream with increased heating. Spanwise TKE distribution broadens with increasing Ri, highlighting a transition from shear-driven Kelvin–Helmholtz instability to buoyancy-driven Rayleigh–Taylor instability, fundamentally altering turbulence production. SPOD analysis reveals that in the unheated case, mode 1 energy is concentrated in a narrow frequency range, whereas heating redistributes spectral energy, broadening the bandwidth and stabilizing shear layer structures over a wider frequency range. Mode 2 captures lateral velocity fluctuations at the recirculation zone edges, with different frequencies corresponding to distinct shedding behaviors. The persistence of shear-layer mode shapes at higher Ri suggests an increase in coherent structure stability. These findings provide insight into the competition between opposing buoyancy and inertial forces in mixed convection flows, with implications for engineering applications such as thermal management in buoyancy-opposed configurations. Understanding the stabilization and redistribution of turbulent structures can aid in optimizing heat transfer and flow control strategies in high-temperature environments.