On the flow behaviour of confined finite-length wavy cylinders

Confined aspect-ratio of 6 wavy cylinders with a mean blockage-ratio of 0.5 were studied using time-resolved particle-image velocimetry at a sub-critical Reynolds number of 2700. Wavelengths and wave amplitudes of 2–4 and 0.1–0.3 mean diameters respectively were investigated. Results show that vortices are generally shed from the wavy cylinder and channel walls regularly, reminiscent of the unsteady symmetric flow configuration in confined non-wavy cylinders. Furthermore, vortex formation lengths for confined wavy cylinders are generally shorter than their unconfined counterparts, though their variations with respect to geometrical changes remain consistent with unconfined flow conditions. Gross cross-stream flow behaviour does not differ significantly between confined and unconfined wavy cylinders, indicating that finite-length effects are independent of the present confinement. Confined wavy cylinder wake regions are more sensitive towards geometrical changes and a combination of small wavelength and large wave amplitude leads to significant suppression of coherent cylinder and wall vortex-shedding. This is supported by phase-averaged flow reconstructions derived from Proper Orthogonal Decomposition analysis. Lastly, larger wave amplitudes lead to redistributions of dominant flow energy further downstream and to higher mode numbers, which suggests a causal link to the formation of stronger and more coherent streamwise vortices.