Procedure for Conducting a Towing Tank Test of a Waterjet Propelled Craft using Laser Doppler Velocimetery to Determine the Momentum and Energy Flux

Abstract

This paper describes the towing tank test procedure used for conducting propulsion tests of the Rolls-Royce Naval Marine (previously Bird-Johnson Co.) Advanced Waterjet 21 (AWJ-21™) propulsor. The tests were conducted on hull model 5565-1, an existing 22.5 scale model of a hull form representative of a future tumblehome naval destroyer. AII the test work described here took place at the Naval Surface Warfare Center, Carderock Divisions, David Taylor Model Basin. Design of the waterjet was carried out by Rolls-Royce Naval Marine in 1999 and the towing tank experiments were conducted in October 2000 and February 2001. The test procedure follow the guidelines for the "momentum flux" method as described in Appendix A of the 21st International Towing Tank Conference (ITTC) Waterjet Group Report (1996] and reproduced in the Quality Manual of the 22nd ITTC Special Committee on Waterjets Report [1999]. However, the ITTC procedure does not address the methods for obtaining the required data and does not provide guidance for scaling the model data to the full size ship. The paper deals with these issues and some of the measurement procedures that are specific to the AWJ-21™ test program. In particular, an accurate survey of the inflow and discharge velocity distribution is required to obtain the mass flow and head rise through the propulsor. In the present case, a Laser Doppler Velocimeter (LDV) is used to carry out these surveys. Because conducting LDV surveys is very time-consuming, it is not practical to determine the mass flow and head rise at every test speed by this method. In our case, the LDV surveys were conducted at only two speeds. · These data · are then used. to characterize the flow non-uniformity and to correlate with pitot-static probes located in the inlet and discharge of the jet system. With this correlation, the pitot-static probes are used to obtain the performance over the entire speed range. While a Laser Doppler Velocimeter provides a very accurate means of obtaining velocity, it does not provide the required static pressure in the flow. It was found that by locating the inlet and discharge survey planes properly, the variation in static pressure over the survey area is small compared to the dynamic pressure and can be neglected. This permits the single static tap on the pitot-static probe to provide the required mean pressure. This paper describes the procedure for using the LDV survey to obtain the waterjet performance.