Assessment of the impact of bend type on flow characteristics in 180° compound bends

Abstract

River bends are rarely simple, i.e., the bend radius is rarely constant throughout the bend length. They are rather irregular, i.e. their shape in the plan can be approximated with different bend radii. Since the bulk of previous research is concerned with simple bends, this paper aims at studying the effect of variable bend curvature on flow in an irregular bend with two bend radii. The effect is studied numerically, using a 3D finite volume based model SSIIM1, which solves Reynolds-Averaged Navier-Stokes (RANS) equations with two-equation turbulence model closure. After successful calibration against experimental data from the simple 180° bend, the model is used to simulate flow in 180° variable curvature bends with the following combinations of relative curvature: mild-mild, mild-sharp and sharp-mild. Results have shown that: (1) The maximum spanwise water surface slope always develops at the entrance to the sharp bend, (2) the greatest slope develops in the sharp-mild layout, (3) the location where the maximum velocity path cuts the bend centerline does not change with the bend layout (simple or variable curvature), (4) the bend curvature sequence affects lateral velocity distributions of the streamwise velocity only in the middle of the bend, (5) the relative curvature sequence has an adverse effect on the size of two regions with maximum bed shear stress – one around the curvature inflection point and the other close to the bend exit.