verts the mechanical power (input) into hydraulic power (output). The ducting system leads the flow through the exterior to the pump and through the nozzle back to the environment. Standard locations for the flow, as defined by the International Towing Tank Conference (ITTC), are indicated in Figure 2.1, where the inflow capture area is marked as station 1a which is located one inlet diameter ahead of the inlet tangency. References to momentum flux stations throughout this thesis are, when not otherwise stated, referring to the ITTC momentum flux station definition as shown in Figure 2.1.
Figure 2.1: Control volume representing the hydrodynamic model of the waterjet as defined by ITTC (2002) showing momentum flux stations 0 to 7.
Processes throughout the system are presented in relation to the seven stations shown in Figure 2.1 and discussed by the 23rd ITTC Specialist Committee on Validation of Waterjet Test Procedures in ITTC (2002) as follows:
• Station 0: Far ahead of the ship in undisturbed flow. Water flows free with a speed matching the speed of the boat through water.
• Station 1: Stream tube inlet at one inlet width upstream of inlet tangency point. Part of the flow below the hull which enters the waterjet system. At this location the average velocity is smaller than at station 0 due to the boundary layer built up on the keel. The cross sectional area is typically modelled as a rectangle with 1.3 times the width of the duct pipe diameter and a height determined by the mass flow rate through the waterjet, as described in ITTC (1996) and van Terwisga (1996).
• Station 2: Duct inlet at the aft lip of the waterjet inlet. At this location the water enters the propulsion system.
• Station 3: Pump inlet just ahead of the pump blade tips. The wake field entering the pump is defined at this location, and depending on the operation of the vessel, the pressure can be lower or higher than the pressure at station 1. The pressure at
this point is dependent on the vessel speed, the mass flow rate, and the pressure loss experienced in the duct inlet (station 2).
• Station 4: Pump centre between pump rotor and stator. At this location, right after the rotor, the static pressure is at its maximum, the flow is rotational, but with the same average axial velocity as at station 3.
• Station 5: Pump outlet just aft of the pump stator. Running from station 4 to 6 guide vanes are installed, transforming static pressure into a dynamic pressure as the water is accelerating through the nozzle. Head rise is reduced by the guide vanes to produce a specific thrust and recovering some of the energy bound in the rotational flow. • Station 6: Nozzle outlet plane. Here the accelerated jet flow leaves the propulsion
system.
• Station 7: Jet maximum vena contracta where static pressure is near ambient in the jet. The stream continues to contract after leaving the nozzle (at station 6) which is due to the vena contracta phenomena discussed in ITTC (2005b). At this location the diameter of the waterjet is at its minimum and this location is considered the end point of the propulsion system. The maximum average velocity is reached at this location and the thrust of the waterjet propulsion system is determined.
The cross sectional area of a waterjet propulsion unit and the control volume usually applied for a waterjet system analysis is shown in Figure 2.2, where the numbering of the surface areas are the same as introduced by van Terwisga (1996).
Figure 2.2: Waterjet control volume as a cut through the ducting system. The numbered surface areas can be broken down as follows:
• Surface 1: Capture area. ITTC (2005a) locates this surface as ”far enough in front of the intake ramp tangency point, before inlet losses occur” and as a practical solution recommends one inlet diameter ahead of the inlet tangency point. The main reason for
selecting this area for the capture area is to avoid major flow distortions caused by the intake geometry.
• Surface 2: Intake throat where the sectional area of the channel is the smallest • Surface 3: Surface covering the waterjet inlet opening
• Surface 4: Outer lip surface
• Surface 5: Dividing stream tube. Imaginary Surface which separates the flow drawn into the ducting system from the rest of the flow field and is defined by the stagnation stream line from the lip surface.
• Surface 6: Nozzle discharge area • Surface 7: Vena contracta
• Surface 8: Boundary area of the pump control volume • Surface 9: Waterjet system internal material boundary
A basic overview of the main components found in a waterjet propulsion unit is given in Figure 2.3 which also shows the most important velocities which are the inlet velocity (VI),
pump velocity (VP ump), and jet velocity (VJ).
Figure 2.3: Main components of a waterjet propulsion unit with indicated inlet, pump, outlet, and ship velocities as well as stator, impeller and inlet duct.
Thrust describes the force a waterjet system produces on a vessel and thrust is defined as forward acting force produced by propelling water backwards. Newtons third law is followed by the system by exceeding a force on the water in the waterjet which then exerts a force on the waterjet system which is then transferred to the vessel, propelling it forward. A basic thrust equation is defined in Allison (1993) and shown in Equation 2.1, whereV1 andV7 are
the average axial velocities at stations 1 and 7 (see Figure 2.1), ˙m is the mass flow rate,ρ
is the fluid density, QJ is the volumetric flow rate, and T is the produced thrust. If the jet
is due to the dependency of the stream tube velocity (V1) on the vessel velocity through the
water. Thrust produced at zero speed is defined as Bollard pull and represents the maximum pulling force a waterjet propulsion system can produce at a specific rotational speed and torque.
T = ˙m(V7−V1) =ρQJ(V7−V1) (2.1)
The waterjet pump is the main component of the propulsion system. In the pump the energy is transferred from the engine to the water through the propeller blades. Many waterjet pumps consist of two stages; the nozzle stage, in which the guide vanes are integrated (see Figure 2.4), and the impeller stage.
Figure 2.4: 3D representation of a typical stock waterjet showing guide vanes (stator), impeller, waterjet nozzle, shaft/hub, and pump inlet and nozzle (outlet).
The impeller rotation adds energy to the fluid, creating a pressure rise and sustaining the flow rate of the waterjet system by counteracting the pressure loss throughout the system. Due to a high degree of swirl introduced by the rotating impeller, the static pressure is high after the impeller. Water accelerated though the nozzle transforms high static pressure to dynamic pressure (kinetic energy). The transformation from static to dynamic pressure is achieved by narrowing the nozzle and streamlining the flow in the axial direction using the guide vanes. Extra friction is introduced by the guide vanes, but the streamlining of the flow reduces the required head rise (by the conversion of swirl to axial flow) so that the overall pump efficiency is increased. Bounds in the swirl after the nozzle results in lost energy as it does not contribute to the propulsion of the vessel.