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A method of evaluating the identification uncertainty in identifying dynamic coefficients of hydrodynamic components, such as fluid-film bearings, seals, and squeezefilm dampers, using test rig data is extended to compare two types of tests: (1) experiments where the applied force is measured and (2) experiments where the hydrodynamic force produced by the test article is measured. Additionally, a comparison is made for each type of test on the sensitivity of the identification uncertainty to assuming the rotor in the test rig is rigid. The results show that validation of dynamic coefficients can be more accurate (i.e. – lower uncertainty) if the hydrodynamic force of the test article is measured. INTRODUCTION Industrial rotating machinery such as compressors and gas turbines are trending toward higher operating speed and higher unit load. The more demanding applications need more accurate prediction of system behaviour. Accurate rotordynamic analysis is critical in predicting operating characteristics of systems and ensuring safe, stable operation. Inadequate analysis can lead to millions of dollars in lost productivity or even catastrophic failure. (Gunter & Weaver, 2016; Rieger, McCloskey, & Dewey, 1990) Stability predictions for rotating machines require a rotor model and dynamic properties of system components such as bearings and seals. The dynamic properties of bearings and seals are typically expressed as stiffness and/or damping coefficients. Coefficients are predicted with codes using physics-based models such as a thermoelastohydrodynamic (TEHD) description. (He, 2003) An API-sponsored survey revealed that dynamic coefficient predictions could vary significantly. (Kocur, Nicholas, & Lee, 2007) The survey distributed the same bearing, seal, and rotor design to the various groups with rotordynamic modelling capabilities and requested dynamic coefficient predictions and stability predictions. The dynamic coefficient predictions of bearings and seals varied by as much as an order of magnitude for stiffness and damping. For stability predictions, a common eigensolver was used to isolate variation to modelling differences between groups. The stability predictions varied greatly due to the coefficient variations, often predicting a more stable system than the experimental data indicated. One of the conclusions of this survey is that “a gold standard of experimental data is needed for both tilting pad journal bearings and gas labyrinth seal dynamic coefficients.” Experimental Validation of Predictive Models Many test rigs have been developed for experimental model validation. (Dimond, Sheth, Allaire, & He, 2009; Tiwari, Lees, & Friswell, 2004) These test rigs have two typical forms: 1) the rotor is held rigidly with rolling element bearings and bearing housing motion is excited by a connected actuator 2) the housing is held via a rigid connection to the foundation and the rotor is excited. The first design is more common due to the implementation being simpler. Designs of the second type been implemented with unbalance force providing the excitation. Recently, active magnetic bearings (AMBs) have been used to apply the excitation force to the rotor. Their non-contact nature simplifies the application of force to the rotating shaft. In a typical dynamic coefficient identification experiment, the applied force on the excited component is measured and the resulting displacement of the excited component relative to the rigidly held component is measured. Using the force and displacement measurement, dynamic coefficients can be calculated. All measurements of physical quantities, whether directly from a sensor or constructed from the data from several sensors, have some uncertainty directly associated with their measurement principle and its embodiments. Schwartz et al. show that these uncertainties lead to unavoidable uncertainty in the identified dynamic coefficients. (Schwartz, Fittro, & Knospe, 2017) The identification uncertainty increases as frequency of excitation increases. The uncertainty is also a

Dynamic Coefficient Identification from Experiments: the Effects of Measuring Hydrodynamic Forces Vs. Applied Forces