Measuring the turbulent characteristics in an open channel using the PIV method

Investigation of open channel flows is very important for a wide range of applications, including restoration and enhancement of river aquatic systems. As a result, the scientific community has focused on providing further insights on the flow characteristics in vegetated channels. Vegetation may be submerged or emerged, rigid or flexible with high or low density. For rigid vegetation, the hydraulic behaviour of the channel is similar to the behaviour of a channel with macro-roughness which could be caused by the presence of geometrical elements (e.g. cylinders, cubes). For flexible vegetation, both the flexibility of the vegetation and the hydrodynamic of the flow contribute to the generation of several formations such as erect, gently swaying, and prone.
In this study, the characteristics of turbulent flow in an open channel were studied experimentally using Particle Image Velocimetry (PIV). This method assumes that the particles of a fluid faithfully follow the flow dynamics, hence the motion of these seeding particles are used to calculate velocity information of the flow. The experiments were conducted for both impermeable and permeable beds in a channel of 6.5m length, 7.5 cm width and 25 cm height. Two grass-like vegetation types of different height (2 and 6 cm) were used to represent permeable beds. These conditions are typical of flows encountered in sediment transport problems. Hydraulic characteristics such as distributions of velocities, turbulent intensities and Reynolds stress are investigated at a fine resolution using the PIV. Velocity is measured above the vegetation at different heights. Results show that velocity over the vegetation region is a function of the vegetation height and the total flow depth; velocity decreases as the vegetation height increases. In addition, we show that velocities above the vegetation region are much lower than velocities above an impermeable bed. This is due to the turbulent shear stresses and the existence of turbulence in the vegetation region, which reduce the mean velocity above the vegetation region. In addition, results show a region of zero velocity; between 3 and 6 cm and 1 and 2 cm for a 6 cm and 2 cm vegetation. This result shows that 50% of the vegetation behaves like an impermeable bed.