Uncertainty analysis of NSERC-NRC pod dynamometer system

The report presents a detailed uncertainty analysis for a podded propulsor open water test. A brief overview of the uncertainty analysis methodology has been provided, with a particular focus on the elements that are unique to the experiments with podded propulsors. The method used follows that of the American National Standards Institute (ANSI) and American Society of Mechanical Engineers (ASME) standard on Measurement uncertainty and the approach described by Coleman and steele in their 1999 book Experimental and Uncertainty Analysis for Engineers. The variables of interest in the podded propulsors uncertainty analysis were propeller thrust, unit thrust, propeller torque, forces and moments on the propulsor in the three orthogonal directions, propeller shaft rotational speed, carriage advance speed, azimuthing angle, water density (function of water temperature) and propeller diameter. The uncertainty analysis results of the experiments conducted using the NSERC-NRC dynamometer system were compared to that of a very high quality, well-established equipment (tests done in the IOT towing tank) used to measure the performance of some bare podded propellers and the results from the previous similar tests using the same equipment. Comparison of the results showed that the podded propulsor tests using the NSERC-NRC pod instrumentation in the current phase of tests provided the level of accuracy comparable with the established equipment. The uncertainty levels observed in the propeller thrust and unit thrust in the podded propulsor tests were found to be higher than the thrust uncertainty for the baseline tests, but less than the corresponding uncertainties found in the previous tests done on the same podded propulsors with same operating conditions with the same equipment. For majority of the cases, the primary element of the uncertainty of the performance coefficients was the bias error (90% or more on the total uncertainty). To reduce the overall uncertainty in the final results, the primary focuses should be to reduce the bias error in the equipment. The uncertainty analysis results provided strong evidence that the experimental data obtained using the NSERC-NRC dynamometer system presented the true performance characteristics of the model scale podded propulsors under consideration.

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