Abstract
This article introduces the SVG (saltโvelocity gauge), a novel automated technique for measuring flow velocity by means of salt tracing. SVG allows a high measuring rate (up to one every 2 seconds), short control section length (down to 10 cm), high accuracy (+[sol ]โ1ยท5 cm s^โ1^), and unbiased calculation of the mean velocity in experimental conditions with turbulent, supercritical flow.
A few cubic centimetres of saturated salt solution (NaCl) are injected into the flow at regular time intervals using a programmable solenoid valve. The tracer successively passes two conductivity probes placed a short distance downstream. The transformation of the signal between the two probes is modelled as a oneโdimensional diffusion wave equation. Model calibration gives an estimation of the mean velocity and the diffusion for each salt plume.
Two implementations of the SVG technique are described. The first was an outdoors simulated rainfall experiment in Senegal (conductivity probes at 40 cm apart, 8 Hz measurement rate, salt injections at 10 second intervals). Mean velocity was estimated to range between 0ยท1 and 0ยท3 m s^โ1^. The second was a laboratoryโbased flume experiment (conductivity probes at 10 cm apart, 32 Hz, salt injections at 2 second intervals). Another SVG with probes at 34 cm apart was used for comparison. An acoustic Doppler velocimeter (ADV) was also used to give an independent assessment of velocity. Using the 10 cm salt gauge, estimated mean velocity ranged from 0ยท6 to 0ยท9 m s^โ1^ with a standard deviation of 1ยท5 cm s^โ1^. Comparisons between ADV, 10 cm SVG and 34 cm SVG were consistent and demonstrated that the saltโtracing results were unbiased and independent of distance between probes. Most peaks were modelled with r^2^ > 90 per cent.
The SVG technology offers an alternative to the dyeโtracing technique, which has been severely criticized in the literature because of the wide interval of recommended values for the correction factor ฮฑ to be applied to the timings. This article demonstrates that a fixed value of ฮฑ is inappropriate, since the correction factor varies with velocity, diffusion and the length of the control section. Copyright ยฉ 2005 John Wiley & Sons, Ltd.