Figure 7: Predicted flow velocities through jet at 30knots.
the predicted wave pattern for 30 knots. Te CFD model for the propelled case is
also shown in Figure 6. Te internal details of the waterjet were not modelled. Instead the action of the waterjet was represented by placing sink and source planes inside the waterjet ahead and astern of where the stator and rotor would be located. Te strength of the sink and source where set to give the correct inflow and outflow velocities at these two planes. A cross section of the waterjet model in operation at 30knots giving a diagram of the flow velocities through the jet is shown in Figure 7.
Te method used to analyse the CFD
results is based on the recommendations of the ITTC (References 3 and 4). Tis method involves the analysis of the flow through a number of control planes. Tese are shown in Figure 8 and described in Table 1. Te determination of the portion of the
flow field to be analysed for control Plane 1 is less clear cut. For normal model test work where the boundary layer profile is measured at on location only or if not measured then is calculated using a simple boundary layer model, the ITTC recommends that it is assumed that the flow ingested by the waterjet comes from an area a certain amount wider than the intake duct and is of either rectangular or elliptical shape. Te height of the intake zone is adjusted so that the correct volume flow rate is captured. In cases where CFD calculations are available, a third option of determining the actual intake zone by consideration of the ingested streamlines exists. Figure 9 shows diagrams of these three options. In work summarised in Reference 3 it is reported that there is little difference
The Naval Architect July/August 2010
Figure 8: Definition of Control Planes.
Figure 9: Possible Definition of Intake zone on Plane 7, Rectangular (left), Elliptical (Middle), From CFD (Right).
Table 1: Definition of Control Planes.
Table 2: Effect of Different Intake Zone Assumptions.
between using these three methods. In view of the fact that in this case the hull has considerably more shape in the region of the intake than is the case for most waterjet type hulls it is interesting to compare the
results from these three methods. Table 2 gives the calculated results for
wake, inlet loss and estimated propulsion efficiency for a speed of 30knots for the three different assumed intake zone
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