Effect of roughness and trips on the drag of a circular cylinder at subcritical flow

The aerodynamic impact of surface roughness and trips on a circular cylinder was investigated. Surface roughness was caused by textile sleeves of seven distinct fabrics varying in roughness. These sleeves were configured with one or two seams strategically positioned to influence the flow dynamics over a Reynolds number range of $4\times 10^4⩽\text{Re}⩽1.3\times 10^5$. Measurements of the aerodynamic drag, lift, unsteady flow, and mean flow field were made. It is shown that seam(s) can reduce cylinder drag across the Reynolds number range. The parameters influencing this reduction include the number, position, configuration of seams, and fabric roughness. For a single seam, drag reduction of up to 45% and an increase in vortex shedding frequency were observed, relative to a smooth cylinder. In the case of fabrics with two symmetrical seams about the cylinder stagnation point, a significant reduction in the critical Reynolds number occurred, accompanied by approximately 35% drag reduction across a broad range of Reynolds numbers. Furthermore, an increase in freestream turbulence intensity caused a considerable reduction in both the critical Reynolds number and drag. These variations in the forces acting on the cylinder were related to the behavior of shear layers and wake.