Wake characteristics of single and tandem emergent cylinders in shallow open channel flow

The present thesis deals with the vertical variability of the wake characteristics behind emergent single and tandem circular cylinders in shallow open channel flow at two Reynolds numbers (ReD = 925, 3000) in order to conceptually investigate the flow past vegetation. The tandem configuration includes two and three slender cylinders arranged in-line with different gap lengths (G) while the cylinders are rigid with a small diameter (D = 6.4 mm). The experiments were conducted in a wide water flume, where a small blockage ratio (~ 0.5%) was achievable. Velocity measurement was performed using particle image velocimetry (PIV) in vertical and horizontal planes in the wake region. The focus of the study is to explore the wake flow characteristics in terms of mean velocity profiles, turbulent parameters and coherent structures along the water depth for all test cases. Proper orthogonal decomposition (POD) was applied to educe information about the coherent structures in the flow in the different horizontal planes.;The results show that the bed and the free surface influence the wake development at both Reynolds numbers for all test cases. The bed suppression effect was found stronger at the lower Reynolds number, as the transverse growth of the wake was more restricted compared to the high Reynolds number flow. The free surface shows more deformation in the wake region at high Reynolds number and results in lower turbulence compared to the low Reynolds number flow. The size, shape and development of the re-circulation region behind the single cylinder also indicate vertical variability at both Reynolds numbers.;The POD results indicate that in the mid-depth and near-free surface planes, the von Karman vortex street and the separated shear layer (SSL) instability are the dominant mechanisms at both Reynolds numbers. In the near-bed plane, the most energetic coherent structures are determined to be the base vortex, horseshoe vortex structures and SSL instability at low Reynolds number, while at high Reynolds number the dominant structures are von Karman vortex structures and the SSL instability.;In the case of tandem cylinders, the gap length (G) between the cylinders has a key role in the wake flow characteristics. At G = 1.9D, the downstream cylinder is sheltered by the lead cylinder for both Reynolds numbers, particularly in the mid-depth region. While at G = 3.9D the second cylinder has local effect on the wake flow for both Reynolds numbers, particularly in the mid-depth. A higher turbulence is noticed in the wake region for the larger gap. The bed suppression effect is noticed for all tandem cases with the stronger effect at lower Reynolds number. Wider turbulence profiles are noticed for the larger gap near the free surface at low Reynolds number, while such expansion is not observed at the high Reynolds number flow.