Twin propeller time-dependent scouring processes. Physical experiments

We present a new set of experiments in local scour due to confined twin propeller jets aiming to include propeller driving characteristics to the analysis of the scouring action during in-port manoeuvres. The maximum scour depth is studied as a function of the bed clearance, the wall clearance and the efflux velocity of the jet. Aiming to reproduce more realistic manoeuvres, both forward and backward rotation of the propellers are analysed showing a different behaviour. The obtained results are used to propose two different empirical models, one for each regime of rotation. A new non-dimensional parameter obtained from the Buckingham π analysis, proposed as wall Froude number, is used to find a clear threshold triggering the scouring mechanism. The experimental results show that the distance to the vertical wall is the main contributor to the higher scour depths rather than the propeller speed of rotation or the bed clearance.