The Hydrodynamics of Waterjet/Hull Interaction

Doktorsavhandling, 2014

The objective of the present investigation is to explore the physics behind the waterjet/hull interaction, and in particular the negative thrust deduction often reported in the semi-planing speed range. Another objective is to propose a validated numerical technique for computing the hydrodynamics of waterjet-driven hulls. The parameters that play a role in the waterjet/hull interaction are split into global effects (i.e. sinkage and trim variations) and local effects (other effects caused by the intake suction) and each are addressed individually in this thesis. Investigation of these parameters is carried out in two different ways. First, assuming the flow to be potential flow, an algorithm is developed for modelling the water/hull interaction. Then, in the second part of the thesis, a technique employing a Reynolds-Averaged Navier-Stokes (RANS) solver is employed for modelling the flow and understanding the interaction effects. The algorithm used in the first part is called the Pressure Jump Method in this thesis. This method is based on the equilibrium condition that the resistance forces are balanced by the thrust force created by the head increase through the waterjet pump. The Pressure Jump Method is coupled with a potential flow solver capable of non-linear free-surface modelling. Validation and verification of the method are accomplished by comparing the computational results with experimental data available from a test case. The resistance increment of the hull is also estimated using the Pressure Jump Method and the dominant parameters, which contribute to the thrust deduction, are determined. General sinkage and trim changes between the bare hull and the self-propelled hull are also estimated by approximating the waterjet- propelled hull as a flat plate with a rectangular hole representing the intake opening. In the second part of the thesis, a technique using a RANS solver with a Volume of Fluid (VOF) free-surface representation combined with a body force representation of the pump is developed and validated against measurements. Using the results of this technique, the thrust deduction fraction is studied in detail from very low to high speeds. It is revealed that, in the lower speed range, the transom clearance plays an important role in the behaviour of the thrust deduction fraction. Therefore, the transom clearance phenomenon is studied in detail. The reasons for the waterjet-driven hull resistance increment are identified through studying the hydrostatic and the hydrodynamic pressure resistance as well as the frictional resistance variations over the entire speed range. The difference between the net thrust and the gross thrust of the waterjet system, which has been a controversial issue, is also studied in this thesis and it is seen that this difference may well be the reason for the negative thrust deduction. Other issues investigated in the thesis are the shape of the capture area and the streamtube within which all water going into the intake is contained, the importance of the velocity profile at the intake and exit, the pressure distribution at the exit and the diameter and position of the vena-contracta.