Kinematics of Ship Wake Flow. The Seventh David W. Taylor Lectures.

For the usual merchant ship at constant speed in calm water one can calculate the wave drag approximately and even the three-dimensional boundary layer up to say 90 of the ship length. But we do not know where and how the boundary layer separates and mixes with the general secondary flow behind the ship to form the near-wake flow which the propeller has to face and which the propeller itself influences. Isotachs of the nominal or even of the effective wake can tell only part of the truth. Along the ship, vorticity is produced in the boundary layer, and the vector lies in the vertical plane, parallel to the frame and perpendicular to the main flow. So, a vorticity line, i.e., a line parallel to the vorticity vector at any point, would just go round the hull girthwise at the parallel midship. Yet, further on in the wake, one usually finds what looks like a strong longitudinal vortex pair, even when there are no bilge vortices as with our model. Hence, our starting question was how are these vorticity lines bent and bundled Usual boundary-layer tests normal to the hull cannot answer this question. What we wanted was a general survey of the details of the mean flow near the stern.