A new pH-based tracing method for flow mixing studies in closed-loop experimental flumes: evaluation in an open-channel confluence

Laboratory experiments based on the dispersion and mixing of passive scalars in open-channel flows are invaluable to investigate flow patterns and to provide empirical values of the mixing coefficient in well-controlled conditions. The study details a new flow tracing method measuring the concentration of a passive scalar in laboratory open-channel flows based on pH measurements. Its main advantage is that the scalar exiting from the open-channel is eliminated downstream of the measuring locations, allowing long-time experiments in closed loop. The method is first validated in a uniform flow in a straight, rectangular channel. It is then used in a complex 3D flow, at the confluence of two open-channel flows. The pros and cons of the new method, in terms of performance and implementation, are finally discussed in view of other widely used tracing methods such as conductivity probes, fluorometers and video-imaging optical methods. The study confirms that the present tracing method is efficient for long-time closed-loop experiments. The calibration procedures are detailed, as well the sources of errors, leading to a relative uncertainty equal to a few percent of the concentration.