Use of vanes in sharp bends to improve open-channel flow characteristics : laboratory and numerical experiments

Sharp open-channel bends are commonly encountered in hydraulic engineering design. The flow around these bends is highly three-dimensional (3D) due to combined effects of secondary flow cells, large variation of free surface in the bend, and flow separation along the inner bend wall. Disturbances such as secondary flows and flow separation caused by the bend may persist for considerable distances in the downstream channel. A simple way of reducing these disturbances is through the insertion of continuous vertical vanes in the bend section. First, a Laser Doppler Anemometry (LDA) unit was used to measure the three-dimensional mean and turbulent velocity components of flow in an experimental rectangular open-channel bend. Flow characteristics of the bend with no vane were compared with those of bends having 1 or 3 vertical vanes. The key bend flow characteristics such as the size of the flow separation zone at the inner wall of the bend, the intensity of secondary flow, the level of turbulence generated, the extent of energy loss and the fluctuation of water surface were determined. Several 3D numerical simulations were tested using various grid generation methods, different turbulence models and water surface treatment methods to determine the complex flow field in the sharp bend and in the channel downstream of the bend. The simulations revealed that the Reynolds stress turbulence model (RSM) with body-fitted coordinates, combined with the volume of fluid (VOF) method for water surface representation, gave the best validation when compared to experimental data. Following this, the simulations were extended for the bend with vanes. This allowed a more detailed quantification of the mean and turbulent characteristics of flow between vanes, without experiencing the limitations encountered in experiments. Results from both experimental and numerical investigations confirm that placing vertical vanes in the bend is a very effective way of reducing the intensity of the secondary flow and the extent of the flow separation zone along the inner wall. Energy loss for bends with vanes is also slightly reduced compared to the no-vane case. The 3-vane configuration is particularly efficient at creating a very uniform flow downstream of the bend. KEYWORDS: 90° open-channel sharp bend; Vertical vanes; Flow separation; Laser Doppler Anemometry; Secondary flows; 3D simulation; Turbulence models; Water surface slope; Reynolds Stress Model (RSM