The flow structure around a surface piercing strut

Abstract

The flow near a surface piercing, symmetric body with a long draft is examined. The experiments are performed in a towing tank at 0.05⩽FrL⩽0.51, primarily focusing on FrL=0.25, and include velocity measurements using PIV as well as video and film photography above and below the free surface. The bow wave is mild (no bubble entrainment) for Froude numbers below 0.35; however, bow wave breaking and vorticity entrainment at the toe of the wave occur. Energy dissipation in the bow wave is significant and affects the flow behind it. At FrL⩾0.15, impingement of the flow on the model near x/L=0.41 generates a turbulent, bubbly wake. On the mid-body just behind this impingement is the origin of a second wave, containing several regions of counter-rotating vorticity which entrain bubbles from the free surface. The wave crest becomes milder and eventually irrotational with increasing distance from the model. At x/L=0.64, boundary layer separation begins at the intersection of the model and the free surface. The separated region grows, but never extends far from the free surface. The separation process originates from secondary flows associated with impingement and breaking at the root of the mid-body wave. At FrL=0.25, there is no reverse flow within the separated region, but at FrL>0.30, flow reversal does occur.