Effect of the free-stream orientation on mixed convective flow past a porous cylinder

Numerical simulations are conducted for steady flow striking a porous circular cylinder from various orientations in the mixed convection regime over wide ranges of the Reynolds number (5 ≤ Re ≤ 40), Darcy number (10 ≤ Da ≤ 10), and Richardson number (0 ≤ Ri < 7). The free-stream orientation is defined as an angle (α) between the directions of the free stream and the gravitational acceleration, varying from 90° to 180°. For 90° ≤ α < 180°, the recirculating wakes present an asymmetric structure under thermal buoyancy, which is more obvious for relatively permeable cases. The impact of α on this asymmetry is inextricably dependent on Re, Ri, and Da. Diverse wake structures are identified owing to the interaction between the free-stream orientation and the strength of thermal buoyancy. Two or three regimes with various wake behaviours are distinguished, including the two-recirculating-wake regime, the single-recirculating-wake regime, and the no-recirculating-wake regime. Thermal buoyancy suppresses the development of the recirculating wakes. Less thermal energy is required for the complete suppression of the recirculating wakes at smaller Re and larger Da. The heat transfer rate in terms of the average Nusselt number (Nu) is enhanced when α increases, and the stimulative effect of thermal buoyancy on heat transfer at constant Re and Da is more pronounced at a higher α.