Turbulent Flow Characteristics Around Bank-Attached Vanes

Regarding the growing awareness of the environmental issues over the past decades, the main aspect of river engineering has been the environmental improvement of the rivers. River restoration refers to the environmental and ecological aspects of river engineering. New eco-friendly river restoration techniques such as vanes, deflectors, W-weir, U-weir, cross vane and J-hook vanes are generally designed and implemented in accordance with different hydraulic conditions to control the river bed, stabilize the banks, improve the river ecosystem conditions and to overcome the negative effects of the conventional structures on river natural functioning. These structures performance has been evaluated in many studies. Bank-attached vanes are one of the eco-friendly structures. In the present paper, the effect of the different geometry and permeability rates of these structures on turbulent flow pattern has been investigated.

Turbulent flows are simulated by solving the Navier-Stokes equations. FLUENT software and large eddy simulation (LES) mathematical model were used to solve the Navier-Stokes equations and simulate the turbulent flow field around bank-attached vanes in a straight channel. Smagorinsky-Lilly subgrid-scale model was used to model unclosed residual stress tensor (τ_ij) in the governing equations. It must be noted that subgrid-scale turbulence models in FLUENT employ the Boussinesq hypothesis to calculate subgrid-scale stresses (τ_kk). To simulate the flow field affected by different bank-attached vanes, boundary conditions, flow depth and approach flow velocity were determined. Streamwise measured and simulated free surface profiles around 30% permeable rectangular vane at different Y/L positions were compared to evaluate the numerical model accuracy. Numerical and experimental results agreed well since relative average error values were 4-5% in all cases. Furthermore, a mesh composed of approximately 150000 elements was considered as an optimum mesh for all created models to resolve flow characteristics. However, due to the different permeability rates, generating the optimum mesh for all cases it is not possible.

Velocity magnitude results around bank-attached vanes revealed the presence of two flow zones which are the main flow field, upstream and downstream separation region formed due to the local effects of the bank-attached vanes and channel constriction. These zones are separated by a fully turbulent and dynamic flow, named the detached shear layer. In general high velocity zones near the vanes tip region and channel bed effect the local scour hole characteristics, thus tip velocity and maximum velocity variations are investigated at the near bed horizontal plane. Moreover, parameters such as tip velocity, maximum velocity and flow separation angle are affected by the interaction of different eddies, varying geometry and permeability conditions. Results showed that tip velocity and maximum velocity ratios have declining trend with increasing permeability rate. Generally by increasing the permeability rate, bank-attached vanes do not have a significant effect on flow pattern. Bed shear stress is considered as one of the primary parameters that affect the main flow field and downstream separation zone, results showed that due to the local effects of the bank-attached vanes, channel constriction and interaction of the horseshoe vortices downstream of the simulated vanes, maximum bed shear stress values mainly occurred at X⁄L= 2.4 section. Furthermore, by increasing the permeability rate, maximum bed shear stress values shifted towards the channel centerline. In general, due to the triangular vanes cross-sectional opening (geometry), these structures effect on flow characteristics is smaller in comparison to the rectangular vanes.