Systematic variations of parameters affecting waterjet-hull interaction
Examensarbete för masterexamen
Naval architecture and ocean engineering (MPNAV), MSc
The purpose of this thesis is to follow a methodology in which computational models with two degrees of freedom (2DOF), based on the finite volume method, are validated and used to investigate how certain phenomena and parameters, especially the longitudinal position of the waterjet intake, affect resistance and thrust deduction (t) for the STREAMLINE hull. To arrive at a scientific answer to this question a methodology is carried out comprised of a grid dependence study followed by three studies of systematic variation. These include variation of Froude number, variation of initial trim (LCG) and variation of waterjet intake positions. Comparing the output from the Froude number variation and initial trim studies with towing tank test data, error trends are found and are used to obtain the reliability of the computational models. These error trends can then give an indication of the validity of the results from the variation of waterjet intake position study. The results from the grid dependence study show an effective mesh size for the bare hull model of around 2.6 million elements. By applying this effective mesh size to the BH model, the error trends from the different parameter studies are obtained. The bare hull trends obtained from the Froude number and trim variation studies converge to an error span between 6-8%. A similar validation process using error trends for self propulsion has not been possible due to a lack of model test data. However, with the four model test data points available, it seems that the self propulsion model is reliable within at least a Froude number span of 0.44-0.48. It is found that the effect of varying the Froude number for four degrees bow up initial trim and 2DOF is similar to the effect of varying the Froude number for a zero degree initial trim with a fixed hull. It is also found that the simulated resistance peak for BH appears at a lower Froude number than the measured resistance peak for BH. Also, the SP resistance peak appears before the BH resistance peak. In all tested cases, the resistance peaks are found to be correlated with the transom clearance phenomena. The trim variation study shows that the optimum initial trim for the STREAMLINE hull at design speed (Fn = 0.996) is the even keel trim for both BH and SP. The position variation study shows that the optimum longitudinal position for the center of the waterjet intake is 465 mm fore of the transom. This study also shows that the difference in resistance between the best and worst position for each Froude number differs with about 2-6%. These results are a good start in gaining knowledge about waterjet-hull interaction and how waterjets can be delivered and installed to improve performance and sustainability.
Farkostteknik , Vehicle Engineering