Investigations of Waterjet/Hull Interaction Effects

Licentiate thesis, 2012

A waterjet propulsor operates in a different way than a conventional propeller. This makes it hard to use the same concepts for studying the thrust and powering of these systems. The net thrust of the propeller can be obtained by measuring the force transmitted through its shaft, but since there is not just a single contact point between the waterjet unit and the hull, the net thrust measurement cannot be easily accomplished for the waterjet unit. Instead another thrust force, which is simpler to measure, is defined to express the magnitude of the waterjet unit thrust. The new thrust definition is called the gross thrust and is obtained by the measurement of the momentum flux change through the waterjet control volume. In this thesis, it has been tried to find out the links between these two thrust forces. The original work fulfilled in this thesis can be divided in to two main parts. The first part is an introduction to an iterative algorithm for modelling the effect of the waterjet on the hull. The algorithm is called the Pressure Jump Method. This method is based on the fact that the resistance forces are balanced with the thrust force created by the head increase through the waterjet pump. In this thesis, the Pressure Jump Method is coupled with a potential flow solver capable of non-linear free-surface modelling but there is not any limitation for the method to be used in combination with, e.g., RANS solvers. Validation and verification of the Pressure Jump Method is accomplished by comparing the computational results with experimental data available from a test case. The second part of the thesis is dedicated to investigate the individual contribution of different parameters that may influence the thrust deduction of a waterjet-propelled craft. In this part the results obtained from the Pressure Jump Method along with some extra calculations are employed to find out the dominant parameters, which contribute to the thrust deduction.