Waterjet-Hull Interaction

AuthorsTerwisga, T.J.C. van
Conference/JournalPhD-thesis, Delft University of Technology
Date25 Apr 1996

The main objective of this work is to develop and validate tools for the analysis of interaction effects in the powering characteristics of jet propelled vessels. Despite our knowledge about the hull and the waterjet in isolated conditions, a lack in knowledge with regard to the interference between hull system and jet system seems to exist. Many discrepancies between computed and actually measured power-speed relation of the prototype vessel are ascribed to interaction. Little knowledge is available on the mechanisms and the magnitude of these effects however. Misunderstandings in the field of jet propulsion are believed to often originate from a lack of clear definitions of concepts. It is demonstrated in Chapter 1 that a great deal of confusion can be found in the existing literature on definition and description of jet-hull interaction. Hence, this work starts with a theoretical model describing the complete waterjet-hull interaction. The effect of interaction on the hull is expressed in a hull resistance increment. The effect of interaction on the jet performance is expressed in a thrust deduction and so-called momentum and energy interaction efficiencies. The latter efficiencies account for the change in ingested momentum and energy flux due to the presence of the hull. Although a rough procedure for model propulsion tests was provided by the ITTC in 1987, this procedure was found to easily lead to large systematic errors, rendering the results of the tests doubtful. In addition, the data reduction procedure was based on an incomplete theoretical model. An improved experimental procedure based on thrust calibration through bollard pull tests is developed, together with a data reduction procedure that allows for quantification of the interaction parameters. Detailed computations and LDV measurements were made on the flow in the intake and aftbody region. They give insight into the validity of assumptions made in the experimental data reduction procedure. They show that a rectangular cross section of the imaginary streamtube upstream of the intake with an effective width of 1.3 times the geometric width, provides an adequate representation of the ingested flow. They also indicate that the jet system's thrust deduction fraction is not negligible in the speed range where the transom clears. Computations were conducted with a potential flow code and a Savitsky method, aimed at a direct computation of interaction. These computations did not show a satisfactory agreement with the experimental results. An empirical prediction model based on test results is recommended for preliminary power-speed computations. The present work provides a consistent set of definitions for a complete description of both the powering characteristics of the isolated hull system and jet system, and their interaction. An experimental procedure with a lower uncertainty level than hitherto published in the open literature is proposed for their quantification. The results of this work are hoped to contribute to a wider acceptation of the waterjet system and to smoother contractual negotiations, as the final performance of the hull-jet system is better predictable.


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Tom van Terwisga

Team leader Resistance and Propulsion

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