Rotation-induced drag suppression and thermal response around a pair of porous-wrapped cylinders

The present study investigates the coupled hydrodynamic and thermal behavior of two side-by-side counter-rotating porous-wrapped cylinders confined in a channel, motivated by the growing demand for simultaneous drag reduction and heat transfer enhancement in compact thermal systems. A unified Darcy-Brinkman-Forchheimer formulation under local thermal equilibrium conditions was employed to analyze the effects of rotation rate (0.5 ≤ α ≤ 3.0), gap ratio (2 ≤ g/D ≤ 4), and Darcy number ( Da = 10⁻⁴ to 3 × 10⁻³) at Re = 100 with a fixed porous layer thickness. The results reveal a progressive transition from periodic vortex shedding to vortex-shedding suppression and eventually to the formation of a doublet-like elliptical body with increasing rotation. A novel secondary stability regime is identified at g/D = 2 for 2.1 ≤ α ≤2.3, characterized by self-sustained bistable oscillations between elliptical-body configurations and asymmetric wake states, fundamentally different from the secondary instability previously reported for bare rotating cylinders. Spectral analysis demonstrates a systematic decrease in the dominant Strouhal number from St = 0.162 to 0.053 with increasing rotation at g/D = 2, followed by complete spectral collapse of dominant frequencies in vortex-suppressed regimes across all gap ratios. Mean drag coefficient decreases monotonically with increasing rotation until the onset of the elliptical-body regime, achieving near-zero or negative values indicating net thrust generation. Further increases in rotation restore drag through direct momentum opposition. Thermal analysis reveals that increasing rotation decreases the average Nusselt number due to flow trapping within the elliptical envelope, whereas higher permeability enhances heat transfer through combined conductive and convective mechanisms within the porous matrix. Notably, at small gap ratios and moderate-to-high permeability, a favorable trade-off between drag reduction and heat transfer emerges: Da = 10⁻³ achieves 83.96% drag reduction with only 12.19% heat transfer compromise, while Da = 3 × 10⁻³ yields 49.22% drag reduction coupled with 6.05% heat transfer enhancement. These findings establish counter-rotating porous-wrapped cylinders as versatile passive control system for simultaneous drag reduction and thermal management.