The wake characteristics of a wall-mounted finite foam-covered cylinder are explored by comparing the flow properties with the wake of a solid cylinder. The velocity data are acquired experimentally using planar particle image velocimetry (PIV) and numerically by large eddy simulation at Reynolds number of 35100 based on the flow depth and 13100 based on the cylinder diameter. The wake characteristics may vary significantly if the approach flow properties change and also depend on the relative height of the boundary layer thickness with respect to the cylinder height. Therefore, it is a necessity to achieve a fully developed approach flow to maintain the consistency and universality of the dataset. The fully developed flow is achieved numerically by recycling of flow variables from outlet to inlet, but it is not straightforward to achieve the fully developed flow experimentally. Therefore, experimental studies on the effects of the tripping intensity and the free surface perturbations are carried out for proper conditioning of the approach flow and to achieve a fully developed state in the open channel flume.
Two flow depths at a similar Reynolds number are used for the study of tripping and the intensity of tripping is gradually increased until the flow reaches the fully developed state. The boundary layer thickness is determined based on the wall-normal distribution of Reynolds stresses and higher-order moments and this is found to be more consistent than the classical definition which suggests a wall-normal position of 99% of maximum velocity. The fully developed flow is ensured by the self-similarity and comparing the experimental data with the literature. The flow properties of the fully developed state are characterized by using uniform momentum zone analysis. Compared to the momentum zones of developing flow, the fully developed flow shows a vertical variability in the quadrant events and higher shear contribution from the sweep events in the outer boundary layer which is caused by the existence of the free surface.
In the study of free surface perturbation, the effects of three different floaters are observed at a far downstream measuring station and compared with the fully developed flow. A dip in the mean velocity is noticed adjacent to the free surface and it gets larger with an increase in perturbation. It is also seen that the Reynolds shear stress becomes negative near the free surface and the dominant quadrant events shift from ejections and sweeps to inward and outward interactions due to the inverted shear layer developed from the floaters. The extent of turbulence penetration towards the bed is deeper with the increment in the level of perturbation. These floater boards are commonly used in open channel flow experiments to minimize the free surface fluctuations. However, they are found to have an unintended influence on the flow characteristics and therefore are not used in the study of the wake.
The wakes of the solid and foam-covered cylinders are developed computationally and experimentally under a fully developed approach flow. The PIV data are used to validate the computational model which is further used to reveal three-dimensionality in the flow characteristics. The iso-surface of λ2 is used to depict the instantaneous vortical structures such as horseshoe vortex, arch vortex, tip vortex etc. These vortical structures are prominent for the solid cylinder but broken. In case of foam-covered cylinders, the formation of the flow structure is highly influenced by the inner solid cylinder, top plate and the foam covering. Especially, the foam structure interrupts the formation of any large flow structures and suppresses the oscillating behaviour of the wake. Consequently, Reynolds stress generation in the near-wake region of the foam-covered cylinder is found to be much less and no dominant frequency is identified in the FFT analysis. Finally, spectral proper orthogonal decomposition (SPOD) modes are used to visualize the coherent structures around the body. In case of a solid cylinder, the tip vortices move downward and reattaches with the bottom wall within a short distance. However, this downstream reattachment length for the foam-covered cylinder is significantly higher since the flow through the porous foam structures lifts up the tip vortices. A difference in the reattachment pattern can also be seen on the top face of the cylinder, which occurs in a straight line for the solid cylinder but in a curved line for the foam-covered cylinder. The SPOD modes on the mid-horizontal plane depicts an asymmetric generation of side vortices due to the asymmetry in the foam structure
