Decay of the perturbations induced by a horizontal cylinder across an open-channel flow

A laboratory study is reported on the flow perturbations induced by a cylinder across an open-channel flow at a subcritical Reynolds number of Re_D = 10⁴ (based on the cylinder diameter). Inspired by field measurements on the Plizska River, Poland (Blanckaert et al., 2014) the investigated configuration is representative of large wood trunks that traverse a river.

The flow perturbation induced by cylinders has been abundantly investigated in infinite unbounded configurations. Cylinders placed in a bottom boundary layer, such as found in rivers, have hardly been investigated. Previous investigations have demonstrated the importance of different parameters such as the cylinder-based Reynolds number Re_D , the gap ratio G_b, defined as the ratio between the distance from the flume bed to the lower edge of the cylinder and the cylinder diameter D, the blockage ratio Br, defined as the ratio between D and the boundary layer thickness, and the boundary layer turbulence. However, these studies mainly focused on the frequency of the vortex shedding, characterized by the Strouhal number St and not on the characteristics of the wake.

Thus, a first aim of the present study is to analyze the flow characteristics in the wake of the cylinder, such as the half-width of the wake L0 , the velocity deficit Us , the decay of the perturbations in the streamwise velocity ū, turbulent kinetic energy k and Reynolds stresses.

A second aim is to analyze the effect of the bed boundary layer on the perturbations induced by the cylinder, and the effect of the latter on the bed boundary layer. This is important as these mutual interactions can have implications on the bed morphology, fluxes of matter including wake retention and hyporheic exchange and stream habitats in general.

The study reveals that important differences exist between the flow perturbations in unbounded and bounded configurations. In a bounded environment, the wake half-width L0 increases at a slower rate along the flow direction and is limited by the boundaries (the free-surface and the bed). Moreover, a faster recovery of the streamwise velocity ū and a faster decay of the velocity deficit Us were observed. These differences with the unbounded case are due to the confinement of the wake and the blockage created by the cylinder. Furthermore, the turbulence generated by the interaction of the wake and the boundary layer, as well as the ambient turbulence also have an impact on these differences.

 

Blanckaert, K., Han, R., Pilotto, F., and Pusch, M. (2014). Effects of Large Wood on Morphology, Flow and Turbulence in a Lowland River. In International Conference on Fluvial Hydraulics, River Flow 2014, pages 2493–2501