Trans RINA, Vol 153, Part A1, Intl J Maritime Eng, Jan-Mar 2011 tests were performed with model advance speeds
corresponding to Froude numbers in the range 0.0434≤ Fn ≤ 0.217 and its associated propeller advance coefficient range 0.2 ≤ J ≤ 1. The propeller’s rotational velocity was kept at 6 rps for the duration of the experiment in order to avoid propeller ventilation, and ensure similar conditions as those for tests performed without the hull [2].
typically exist for Reynolds numbers below 0.2 x 106. The range of the Reynolds number for the strut part of
The range of the propeller blade’s Reynolds number was 0.32 x 106 -
0.35 x 106 (Rn). Viscous scale effects the body was from 45 2.58 10 1.29 10 (
R VL v
n ) . In a
previous study of the thruster in open water [2], tests were performed at both 6 and 12 rps at low azimuth angles, and the differences in KT and KQ between the two propeller speeds were
The
corresponding to the model speeds tested was from 6
1.496 10 - 7.48 10 . For hydrodynamic
nominal Reynolds 6
number range bodies, a
Reynolds number of 5 x 106 is usually considered adequate for model results to be representative of the full scale [8]. The attachment of boundary layer turbulence stimulators to the hull and pods would be expected to create effective Reynolds numbers that were higher. The hull model was equipped with a 1 mm diameter cotton tread at station 19½ as a turbulence stimulator, while there was no turbulence stimulation at the thruster body. However, since most of the thrusters’ body is inside the propeller slipstream, the flow is also expected to be turbulent without extra turbulence stimulation. Thus, the results are expected to be representative of full-scale performance.
3.2 DISCUSSION OF RESULTS IN CALM WATER
The time average of all the forces and moments on the propeller behind the ship hull are compared with open water tests in the pulling condition at the same heading angles and advance velocities, so that the ship hull wake effect on the propeller performance and shaft loads can be investigated.
Figure 3 and Figure 4 compare the propeller torque and thrust between open water conditions and behind the hull. At a higher oblique inflow angle, the effective advance velocity (axial component of oblique inflow) is lower (the effective advance coefficient
Je=J*cos,
where is the heading angle), leading to higher torque and thrust than with lower oblique inflow angles. In other words, as the propeller is subjected to a larger heading angle, the propeller blades experience higher angle of attack leading to larger loads.
-40 -30 -20 Heading angle -10 0 0.6 0.5 0.4 0.3 0.2 0.1 0 10 20
J=0.2-Hull J=0.6-Hull J=1-Hull J=0.2 J=0.6 J=1
30 40
Figure 3: Comparison of the thrust coefficient between open water and behind the hull conditions
within 6%, something that
confirms that the relatively low Reynolds numbers in these tests still can be expected to provide reliable results.
0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09
0 -40 -30 -20 -10 0 10 Heading angle
Figure 4: Comparison of the propeller torque coefficient between open water condition and behind the hull
The torque and thrust are asymmetric with respect to positive and negative heading angles when the propeller works behind the ship model, whereas this is not the case in open water conditions. For the port thruster studied here, negative heading angle means that the thruster points outwards. In the stern area of the ship, the flow is generally directed inwards (and upwards), so that when the thruster points outwards it is oriented (partly) in the direction of the incoming flow. Thus, the effective heading angle is less than the nominal azimuth angle.
In positive heading angles the propeller gives slightly higher thrust and torque behind the ship than open water conditions, since in this case the in-plane velocity from the hull wake and from the azimuth angle are adding, not cancelling as for negative heading angles. The in-plane velocity from the ship hull wake seems to be weaker in positive heading angles (inward toward the ship hull centre) than in negative heading angles. This is not unexpected, since the propeller position changes with heading angle, so a change in hull wake field at the propeller can be expected. In positive heading angles, when the propeller is moved closer to the centreline, the axial wake component is expected to increase, i.e. the
A-12 ©2011: The Royal Institution of Naval Architects
J=0.2-Hull J=0.6-Hull J=1-Hull J=0.2 J=0.6 J=1
20 30 40
Torque coefficient KQ
Thrust coefficient KT
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