Twenty-Fourth Symposium on Naval Hydrodynamics (2003) / Chapter Skim
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Propeller Inflow at Full Scale During a Manoeuver
Propeller Inflow at Full Scale During a Manoeuver
Pages 552-567
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From page 552... ...
It was found that the transverse velocities in the propeller plane were almost fully responsible for changes in power absorption in a turn. The mean axial velocity was about equal for the inner and the outer propeller in a turn.
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From page 553... ...
These trials included wake field measurements and cavitation observations on the Canadian Forces Auxiliary Vessel CFA V QUEST and on the Australian patrol boat HMAS TOVVNSVILLE. The wake measurements on CFA V QUEST did not cover the whole propeller disk reliably, but the measurements on HMAS TOVVNSVILLE produced a complete dataset over a significant portion of the propeller disk.
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From page 554... ...
The drift angles involved in the present investigations were limited to approximately 5 degrees, in order to minimize the effect of the presence of the center propeller on the propeller inflow of either main propeller. MEASURING EQUIPMENT A special LDV head was developed for this project to be able to measure flow velocities up to a distance of about 4 meters from the head.
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From page 555... ...
In a turn the acceptance ratio was much lower than on a straight course, indicating that increased turbulence in the flow can complicate the LDV measurements. Observations of cavitation on the port propeller were made using the Osprey video camera illustrated in Figure 6.
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From page 556... ...
Since the horizontal velocity components could be negative, a Bragg cell was initially used in the LDV head. The Bragg cell gives a phase shift to the laser beam, causing the interference grid in the measuring volume to move at a certain speed.
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From page 557... ...
The turning diameter varied between 670 and 915 meters, the estimated ship speed during the turns varied between 12.4 and 12.9 knots and the drift angles varied between -1 and +5 degrees. These variations in conditions are incorporated in the measured wake field, as the time to measure the full wake field was many hours and the trial spanned over several days There is no reliable method to correct each inflow reading for the effects of wind and wave induced ship motions.
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From page 558... ...
EB to CL Figure 13: Horizontal velocity component in a turn to port (inner propeller, ship speed 6.39 m/see) Taking this into account the two axial wake peaks of the outer propeller are 0.89 (0.97 from Figure 14 minus 0.08)
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From page 559... ...
Again there is a stronger horizontal velocity component in the lower half of the propeller disk, as was the case for the inner propeller, but now especially in the outer sector. To analyze these measured propeller inflow data further the powering data of the propellers at full scale were measured.
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From page 560... ...
The transverse velocity distribution is given in Figure 17. (For the model scale data the propeller disk was projected on the measuring plane along the baseline, resulting is a slightly lower position of the indicated disk than at full scale.
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From page 561... ...
The relation between the wake distributions and the propeller performance will be further analyzed below using lifting surface calculations. The axial velocity distribution in front of the inner propeller with the model at 5 degrees drift is shown in Figure 18.
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From page 562... ...
Assuming that the measured wake distributions of the model at 5 degrees drift angle are representative of the wake in a turn at full scale, the vertical velocity component at model scale can be used to complete the full scale data. The validity of this assumption can be assessed by comparing the horizontal velocity components measured on the model and the ship.
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From page 563... ...
Adding in the vertical component from the model wake, the completed transverse velocity fields at full scale are given in Figures 23 and 24. These velocity fields, in combination with the measured axial velocity distributions, can now be used for calculations of cavitation inception on the propeller and for relating changes in the wake field to the power data.
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From page 564... ...
The velocity distribution of the measured total wake, made nondimensional with the ship or model speed, was used as input for the wake distribution. The mean axial velocity was adjusted to obtain the measured torque coefficient.
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From page 565... ...
So the effect of a turn on the cavity extent was small. 14 Figure 27: Cavitation on the ship's port propeller at 520 rpm on a straight course Figure 28: Cavitation on the ship's port propeller at 520 rpm in a turn to starboard (outer propeller)
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From page 566... ...
However, as the turning rate increases the transverse velocity becomes increasingly important, and above 0.5 times the turning rate of the full scale test, the inception speed continuously decreases. The minimum pressure under these conditions does not occur when the blade is in the top position, but when it is in the lower half of the propeller disk, where the larger transverse velocities have increased the blade loading and decreased the pressure.
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From page 567... ...
The combination of a small transverse velocity and a wake peak reduction caused the inception speed to increase initially at small rates of turn. For fuller ships the transverse velocity in the upper part of the propeller disk may not be negligible.
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