An Experimental Investigation of the Perturbation of a Turbulent Boundary Layer by Two Cylinders Arranged in Tandem

Research into the effect of roughness on turbulent boundary layers is motivated by the ubiquity of rough surfaces in engineering applications and a desire to understand the behavior of turbulent flows over these surfaces. Prediction of drag over rough surfaces requires an understanding of flow over roughness, and as a result, many studies have been conducted on the effect of single roughness elements, as well as entire roughness fields immersed in a turbulent boundary layer. It is clear from these investigations that a link between these two approaches is required. To address this need, the current study examines the flow field surrounding two cylindrical roughness elements arranged in tandem. Experimental measurements were made in a 0.2 × 0.2 m² cross-section water channel using the particle image velocimetry (PIV) system at Union College. The spacing between the elements and the height ratio of the upstream to downstream elements were systematically varied to study their effects on the flow field.  The time-averaged flow field and turbulence statistics indicate the dependency of the flow features on the spacing between the two elements and the elements height ratio. These flow features include a recirculation zone and a downwash region (downward velocity) past the downstream cylinder, both of which decrease in size when an upstream element is present. Additionally, turbulence quantities including the Reynolds shear stress and turbulent kinetic energy are significantly altered when an upstream element is present.­