LS-PIV procedure applied to a plunging water jet issuing from an overflow nappe

The aim of this work is to understand the behaviour of a sheet of water falling freely under gravity, subject to instability and breaking up into droplets. Such behaviour significantly affects the pressure force at the toe of large dams equipped with ogee-type weirs. In this laboratory experiment, the LS-PIV (Large Scale - PIV) procedure is applied to determine the surface velocity field of a water jet issuing from a sharpcrested weir. The fall is 5 meters high and the weir is 40 cm wide with a maximum discharge flow of 0.050m³/s. In order to obtain high resolution measurements, the water jet was analysed using a high speed video camera (1000 fps). The initial results have been post-processed with the FUDAA-LSPIV software, co-developed by EDF and IRSTEA. The patterns of deformation of the water-air interface were recorded to determine the velocity vectors. The deformation patterns are relatively small thanks to the high frequency acquisition of the video camera. No special tracers were required as the high frequency acquisition ensures a good correlation factor is consistently obtained. This method allows for continuous monitoring of a large velocity field at a high time frequency. The discussion covers details of the corrections that were made in order to integrate the nappe trajectory and presents the finding of a characteristic distance following which energy dissipation becomes significant.