Partial sheltering effects on spanwise wake interference of unequal-height tandem finite wall-mounted cylinders

• Wake interference by the downstream cylinder (DC) enhanced near-wall pumping motion behind the upstream cylinder (UC). • Wake interference reduces the growth rates of the spanwise shear layers toward the free end of UC compared to SC and DC. • The attachment of spanwise shear layers from the UC to the DC accelerated vortex formation in the sheltered portion of DC. • Unlike the SC, where tip vortices dominated near the free end, the DC free end also exhibited Von Kármán vortex shedding. The effects of partial sheltering on the unsteady flow dynamics of unequal-height tandem finite wall-mounted cylinders fully submerged in a turbulent boundary layer (TBL) were investigated using time-resolved particle image velocimetry. The height ratio of the cylinders was kept constant at h / H = 0.75 , where h and H are the heights of the upstream cylinder (UC) and downstream cylinder (DC), respectively. The center-to-center spacing between the cylinders was 4 d , the Reynolds number based on the cylinder diameter ( d ) was 5540 and the submergence ratio was δ / H = 1.2 , where δ is the TBL thickness. Measurements were also performed for a single cylinder (SC) with a similar aspect ratio ( H / d = 7 ) as the DC. Spatio-temporal characteristics in three streamwise-spanwise planes along the height of the tandem cylinders (TC) and the SC were examined to understand the impact of partial sheltering on the mean wake structure, pumping motion of the reverse flow region, spanwise shear layer growth, and cellular vortex shedding behavior. The results showed that wake interference by the DC enhanced the near-wall pumping motion and integral time scales, while reducing the vortex shedding frequency behind the UC compared to the SC. For the DC, the attachment of the spanwise shear layers from the UC significantly accelerated vortex formation of smaller structures and delayed vortex formation at the unsheltered free end. As a result, the reverse flow region was reduced in the sheltered portion but enhanced near the free end of the DC.