Twenty-Fourth Symposium on Naval Hydrodynamics (2003) / Chapter Skim
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Propeller Wake Analysis Behind a Ship by Stereo PIV
Propeller Wake Analysis Behind a Ship by Stereo PIV
Pages 789-805
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From page 789... ...
The blade viscous wake, which develops from the blade surface boundary layers, the trailing vortex sheets that are due to the radial gradient of the bound circulation, and the velocity fluctuation distributions are identified and discussed. The complex interaction between the hull wake and propeller is described through the evolution of the mean velocity components and the vorticity fields.
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From page 790... ...
. However, it is apparent that this information is not always sufficient to characterize the flow field especially for propeller flows where the presence of strong three-dimensional flow structures with strong velocity gradients requires the knowledge of all the velocity components.
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From page 791... ...
This result has been assessed on a test bench by measuring a known displacement of a reference object placed on a translation stage. IMAGE ANALYSIS AND STEREO RECONSTRUCTION As the first step toward the determination of the three velocity components at the measurement plane, the images are processed to obtain the vector fields viewed by the left camera and the right camera.
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From page 792... ...
The camera views were calibrated using a special target providing a mesh of 20X20 dots in two planes. This calibration was required to determine the transformation function needed to reconstruct the 3 velocity components from the two separate planar Figure 4: Longitudinal mean velocity component obtained by stereo reconstruction without (a)
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From page 793... ...
This led to the erroneous evaluation of the 3 components of the velocity field in the region extending along an horizontal radius from the hub. To overcome this problem the images have been pre-processed: a mean image (figure 3b)
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From page 794... ...
This error is essentially present in the measurement of the instantaneous flow field, in particular in the evaluation of the cross flow components which are directly measured by the underwater camera as explained before. The error in the measurement of the longitudinal component is related mainly to the stereo reconstruction.
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From page 795... ...
Although in the present case measurements of the propeller inflow were not exhaustive due to limited optical access with the Stereo-PIV set-up, this information can give an idea of the nominal wake and of the propeller inflow, even if obtained slightly downstream the propeller plane. The mean velocity field is characterized by a sharp and strong axial velocity defect due to the diminishing cross section of the hull at the stern.
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From page 796... ...
- the link of the tip vortex with the blade trailing vorticity is more apparent with respect to the mean velocity field where this information is almost confused or lost; - the vorticity field provides information of the radial distribution of the blade loading and points out to the differences between the five blades due to the different respective inflow conditions; - the wake at 6 and 12 o'clock position is strongly distorted and fragmented as a consequence of the strong inflow variations in these angular positions. An example of the important modifications of the shape and intensity of the wake released by the blade during the revolution, is shown in figure 13.
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From page 797... ...
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From page 798... ...
~ - ~= non Moo -also .0,40 -0.30 o.2o "o.1o O.OO O.lo 0.20 0.30 0.40 0.50 O.60 0.70 t.: jr Figure 11: Cross-flow V,W (m s) at different angles (0°,18°,36°,54°)
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From page 799... ...
Figure 12: Vorticity ( s-l) at different angles (0°,9°,18°,27°,36°,45°,54°,63°)
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From page 800... ...
that causes an axial acceleration as pointed out by the higher axial velocity achieved in the second measurement plane; the smoothing effect due to turbulent diffusion and strong dissipation between the first two planes and the last one which is farther downstream as also pointed out by the stronger radial gradients of the axial velocity component for the first two planes comparing with the last one; · the rapid decay of the tip vortex with respect to the hub vortex which is still characterizing the flow in the last measurement plane A better description of the vortices evolution is provided by the vorticity field shown in figure 16. The following features on the three different transversal planes can be noticed · a very strong distortion of the wake structure due to the different pitch of the wake shed from the trailing edge and the pitch of the tip vortices can be noticed from the first to the second plane; very strong diffusion and dissipation of the blade wake, which is spreading very quickly Dom the first to the second measurement plane.
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From page 801... ...
Figure 14: Total turbulence (m/s) at different angles (0°,9°,18°,27°,36°,45°,54°,63°)
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From page 802... ...
The rapid spreading of the propeller wake in the downstream flow where the wake is faded and smoothed by turbulent diffusion and viscous dissipation. From the experimental setup point of view and with regards to the fitture implementation in standard ship model testing procedures, the stereo-PIV has shown a number of advantages compared to the well assessed LDV technique.
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From page 803... ...
14411454. Stella, A., Guy, G., Di Felice, F., "Propeller flow field analysis by means of LDV phase sampling techniques", Experiments in Fluids, Vol.28, 2000, pp.
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From page 804... ...
225-252, 1990. Westerweel J., Van Oord, J., "Stereoscopic PIV measurements in a turbulent boundary layer", Particle Image Velocimetrv: progress toward industrial application Kluwer, Dordrecht, 1999.
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From page 805... ...
DISCUSSION B.-G Paik Pohang University of Science and Technology, Korea I think the ship wake is important for the understanding of propeller wake. Because the ship wake connects the propeller wake behind a ship with the propeller wake in P.O.W.
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