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the flow around the blades is fully attached to the blade surfaces. The flow between the propeller blades moves towards the stern as designed. (II) As the braking procedure continues, the propeller begins to turn opposite to its normal direction and the flow around the propeller blades starts to separate. Also, the flow begins to reverse upstream toward the bow. (III) As the braking procedure continues even further, the flow may reattach to the blade surfaces. At this time, the reversed propeller flow should be well established and extend to a location far ahead of the propeller toward bow. Flow visualization has shown that unsteady ring vortices are present during the second stage of the crashback process; that is, (II) above, while the flow inside the propeller disk was reversed.

Figure 2. Time history of propeller unsteady side force.

Attempts were made towards the modeling of the unsteady flow field. For example, a two dimensional quasi-steady approach was investigated by the authors. In that approach, a two-dimensional RANS code was exercised and the propeller was replaced by a specified time-dependent body force obtained from propeller lifting surface code. The locations of the computed ring vortex were examined and the side forces were estimated. Fig.4 presents a typical calculated result showing the formation of a vortical structure at J=–0.5. The advance ratio, J, is defined as Vo/nD, where Vo is the ship (or tunnel) speed, n is the propeller revolution per second, and D is the propeller diameter. Although, those results show the observed features of a moving vortex field, it cannot explain the origin of unsteadiness. Detailed quantitative measurements are needed for better understanding the phenomena and for advancing numerical modeling. The objective of the present research program was to measure the propeller induced flow field at the blade tip region. This paper presents the unsteady flow structures visualized by the use of laser sheet as well as the vortex velocity field measured by Particle Displacement Velocimetry.

Figure 3. Flow field during propeller crashback.

Figure 4. Numerically simulated velocity field at crashback, J=–0.5.

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