Flow Field Characteristics of Turbulent Annular Channels with Surface Roughness

Horizontal annular channels are widely encountered in fluid mechanics. Extensive research has explored how wall roughness alters total pressure loss in circular pipe flows. Meanwhile, existing literature has thoroughly discussed artificially enhanced rough structures (e.g., ribs, helical strips, and axial corrugations) for turbulence control and modulation. By contrast, relevant research on the inner cylinder roughness of annular channels remains insufficient. In practical engineering like petroleum drilling, inner pipes are usually treated as hydraulically smooth in theoretical and numerical models. However, machining inevitably generates tiny surface asperities on inner walls. Though this manufacturing micro-roughness is minimal in size, its true impact on flow characteristics is still unclear and requires a systematic numerical study. This study numerically explores the effects of practical inner-wall roughness on the hydrodynamic performance of annular flow. The simulation results reveal that compared with a fully smooth inner wall, surface roughness significantly rearranges the spatial distribution of major turbulent parameters (e.g., flow velocity, turbulence intensity, turbulent kinetic energy and Reynolds stress), and changes both the magnitude and position of their peak values. When the inner wall roughness is 0.4 mm, the near-wall Reynolds stress increases by 153.5%, the turbulence intensity increases by 52.4%, and the turbulent kinetic energy increases by 133.0%; the velocity peak shifts to the outer ring side, and the critical roughness is 0.06 mm. The findings confirm that considering inner tube roughness is critical for improving the prediction accuracy of both microscale turbulent behaviors and macro flow parameters in practical annular flow engineering. This work distinguishes itself from existing studies by systematically quantifying the response of full first- and second-order turbulent statistics to pipeline manufacturing micro-roughness, revealing the symmetry-breaking turbulent transport mechanism of annular flow, and quantitatively determining the critical inner-wall roughness that shifts the velocity peak to the annular geometric midpoint, which provides a targeted quantitative database for the correction of annular flow numerical simulations that ignore native micro-roughness.