Introducing a single bubble event detection technique for air-water interfacial velocity measurements in unsteady turbulent bore

A breaking bore is an unsteady two-phase flow, characterised with intense air entrainment, large-scale turbulence and a highly fluctuating roller free surface. The combination of the presence of air bubbles and the unsteadiness of the flow constitutes a major challenge for experimental measurements of the bore roller velocity field in the bore roller. A novel single bubble event detection (SBED) technique was developed to provide pseudo-instantaneous interfacial velocities in the breaking roller, using an array of dual-tip phase-detection probes. The concept of the SBED technique is based upon cross-correlation analyses for each single bubble event travelling through both leading and trailing tips of the phase-detection probe. Several filtering criteria were combined with the SBED technique to improve its performance. This paper presents a detailed illustration of the SBED technique and its application to a breaking bore.