The Wake Flow Behind Azimuthing Thrusters: Measurements in Open Water, Under a Plate and Under a Barge

The wake flow behind a ducted azimuthing thruster was investigated. The thruster wake is an important factor in thruster interaction effects. Model tests were carried out for 3 different configurations; a thruster in open water conditions, a thruster under a flat plate and a thruster built into a barge. Two different thrusters were considered, a ‘normal’ thruster with a horizontal propeller axis and a ‘tilted’ thruster with a propeller axis and nozzle oriented 7 deg down-wards. In the tests the propeller thrust and torque were recorded, as well as the nozzle thrust and unit thrust. The velocities in the wake of the thruster were measured using a PIV (particle image velocimetry) system, for down-stream locations up to x/D = 19. The influence of the thruster tilt, the plate above the thruster and bilge radius on the thruster wake flow were investigated. Detailed PIV measurements were carried out on the wake flow behind the thruster in open water conditions. The PIV system used can measure 3D velocities in large set of points in a 2D plane, which is illuminated by a laser light beam. The flow velocities were measured in a large number of cross sections at different distances from the thruster. The PIV measurements provide a detailed image of the flow velocities in the thruster wake, showing the axial velocities, as well as the rotation and divergence of the wake. Subsequently, PIV measurements were carried out for the thruster under a flat plate and the thruster under a barge. The measurement results show a thruster wake that is deformed by the presence of the plate and the barge. The plate and the bottom of the barge form a flat plane above the thruster, clearly flattening the cross section of the thruster wake. Furthermore, the wake flow at the side of the barge, near the bilge radius, results in a low pressure region, causing the wake flow to diverge up as it flows from under the barge into the open water. This phenomenon is known as the Coanda effect and is strongly dependent on the bilge radius and the distance between the thruster and the side of the barge. The effect of both these parameters was confirmed in the model test results presented. The typical flow patterns observed as a result of the Coanda effect are illustrated in Figure 1 below. The results of the present model test research are used to further improve the understanding of the physics of thruster interaction effects. Furthermore, the results will serve as validation material for CFD calculations.