Mean Flow and Turbulence Characteristics around Multiple-Arm Instream Structures and Comparison with Single-Arm Structures

The characteristics of turbulent flows around two weir-like obstacles made of rocks submerged in an open channel, known as a cross vane and W-weir, were analyzed. These structures consisted of multiple structural arms that are angled upstream and constructed in such a way that they span the entire width of a channel. These weir-like (or multiple-arm) structures have been widely used for stream restoration purposes. Nevertheless, little is known about the flow structures and turbulent flow mechanisms associated with these structures. In this study, by carrying out numerical simulations, the turbulent flow fields around a cross vane and W-weir with complex and realistic rock geometries were investigated. For the numerical simulations of the turbulent flow fields, the large-eddy simulation (LES) model that solves the three-dimensional (3D) Navier-Stokes equations together with the curvilinear immersed boundary (CURVIB) method was employed. For the validation, the computed results were first compared to experimental data obtained in a laboratory flume using acoustic Doppler velocimetry (ADV). The time-averaged velocity fields obtained from the LES model were subsequently analyzed to investigate the 3D flow structures, secondary flow patterns, and turbulent flow mechanisms around the cross vane and W-weir. In addition, the LES results were compared with those of five other single-arm structure cases to obtain comprehensive understanding of the flow mechanisms of various instream structures. The LES results showed that high streamwise velocity cores and secondary flow cells form downstream of the multiple-arm structures, and the numbers of them depend on the the number of the arms. Compared to single-arm structures, the multiple-arm structures generally showed larger drag coefficients and energy dissipation ratios, and larger energy and momentum correction factors in the vicinity of the structures.