Study of flow at side weir in narrow flume using visualization techniques

The purpose of the research was to quantify characteristics of a subcritical flow at a rectangular sharp-crested side weir in a rectangular main channel using non-invasive measuring techniques based on the visualization of the flow. Experiments were carried out in physical models, including nine different dimensions of the side weir and nine combinations of the inflow and tailwater level for each weir, amounting to 81 test runs. Velocity vector fields were measured in various horizontal planes along the side weir using a high speed digital camera and electrolysis-induced hydrogen bubbles as flow tracers. Recorded films were converted into sequences of images which were used for numerical calculation of local velocities. Components of velocity vectors were determined with great spatial and time resolution. Longitudinal profiles of water surface elevation at each side weir were determined using photos of laser-induced vertical section planes. Measured discharges and flow depths were used to formulate new equation for the side weir discharge coefficient using dimensional analysis. The principal results indicated that velocity distribution along the side weir was distinctly non-uniform, with various velocity ratios increasing along the crest. However, the calculated energy grade line was parallel to the main channel bed, indicating that only friction losses were present. The proposed equations for the side weir discharge coefficient gave results that were in good accordance with two other studies.

► Visualization method allows non-invasive quantification of vector velocity fields.
► Laser-induced vertical planes allow non-contact survey of water surface profiles.
► Velocities at side weir are non-uniform, ratios us/ū and vs/us increase with x/L.
► Specific energy line is parallel to main channel bed, there are only friction losses.
► New equation for discharge coefficient is based on dimensional analysis, power-law.