Effect of Surface Roughness and Trips on the Drag of a Circular Cylinder at Subcritical Flow

Aerodynamic drag is the major source of resistive force on elite and amateur athletes in speed sports such as cycling. Over a circular cylinder, typically used as a low-order model of an athlete's leg or arm, passive flow control methods have shown to be effective towards reducing aerodynamic drag. In the range of Reynolds numbers experienced by cyclists, surface roughness and boundary layer trips can reduce the aerodynamic drag by transitioning the local boundary layer at the surfaces to turbulence resulting in a drag crisis at a lower range of Reynolds numbers. Experiments were conducted to study the impacts of surface roughness and trip on the drag and vortex shedding of a circular cylinder. The cylinder model was covered with textile sleeves made from seven different fabrics designed to provide a variety of surface roughness. Moreover, each type of fabric possessed one, two, three, and four spanwise seam configurations to trip the flow. The drag was measured in a closed-loop wind tunnel in the range of 40000 < Re < 120000 using two parallel mounted load cells and the Constant Temperature Anemometry technique was employed to measure the vortex shedding frequency. Results indicate that the drag reduction of the cylinder for the Reynolds number range of cyclists can be achieved when the surface roughness is combined with the seams. The number of seams and their location with respect to the freestream are shown to be influential parameters. For most of the fabrics, a drag reduction of up to 25% and a decrease in the vortex shedding frequency are observed compared to the smooth cylinder in the subcritical regime. However, for some cases with specific seam configurations, a significant decrease in the critical Reynolds number is detected, which leads to around 40% drag reduction and an increase in the vortex shedding frequency in the range of Reynolds numbers considered.