Propeller Wake Measurement and Uncertainty Assessment by a Towing-Tank LDV

Abstract A towing-tank underwater Laser-Doppler Velocimeter (LDV) system equipped with an optical fp50 shift probe and a high-precision 3D moving coordinate frame was developed to measure propeller wake. The Nd: YAG-laser head was available in a power range from 50mW up to 500mW, different wavelengths and mounted directly in the probe. The towing carriage speed of initial point and predetermined moved points were repeatedly measured by the 2D probe. The test duration was changed to obtained appropriate sample sizes, which was applied into the later test. Uncertainty analysis of LDV calibration follows ITTC-recommended procedures and guidelines. The uncertainty assessment of test data follows ASME standard procedures to quantify the comparisons and reach conclusions regarding the quality. The uncertainty analysis results of the repeated uniform flow measurements on initial point presented high measurement accuracy of the probe. The uncertainty analysis results and the comparison between the initial point and predetermined moved points verified the high moving accuracy of the coordinate frame in uniform flow measurements. All completed repeated uniform flow measurements were considered as an important contribution to estimate systematic uncertainty of the whole LDV system. The propeller wake field of a modified E1619 was measured in open water test. According to the test condition, an in-house viscous CFD code was used to carried out simulation. The test results agreed well with the CFD results. The uncertainty assessment was carried out following the same standard procedures. This LDV measurement system is reliable enough, and its advantage of non-intrusive measurement can be perfectly presented in propeller wake measurement.