Identification of the dynamic force and moment characteristics of annular gaps using linear independent rotor whirling motions

TL;DR

This study experimentally determines the dynamic force and moment characteristics of annular gaps using a test rig with active magnetic bearings, providing new data on tilt and moment coefficients and their dependence on annulus length.

Contribution

It introduces a comprehensive experimental method to measure rotordynamic coefficients, including tilt and moment, and validates a new calculation approach for these coefficients.

Findings

Experimental data agree well with the new calculation method.

Annulus length significantly influences rotordynamic coefficients.

Additional coefficients are relevant even for short annuli, earlier than previously assumed.

Abstract

Nowadays, most studies on the dynamic properties of annular gaps focus only on the force characteristics due to translational motions, while the tilt and moment coefficients are less well studied. Therefore, there is hardly any reliable experimental data for the additional coefficients that can be used for validation purpose. To improve this, a test rig first presented by Kuhr (2022) is used to experimentally determine the dynamic force and moment characteristics of three annuli of different lengths. By using active magnetic bearings, the rotor is excited with user-defined frequencies and the rotor position and the forces and moments induced by the flow field in the annulus are measured. To obtain accurate and reliable experimental data, extensive preliminary studies are carried out to determine the known characteristics of the test rig rotor and the added mass and inertia imposed by the test rig. Subsequently, an elaborate uncertainty quantification is carried out to quantify the measurement uncertainties. The experimental results, i.e. the 48 rotordynamic coefficients, are compared to a new calculation method. It is shown that the presented experimental data agree well with the calculation method, especially for the additional rotordynamic tilt and moment coefficients. Furthermore, it is shown that the annulus length significantly influences the coefficients of the first sub-matrix. A dependence of the additional coefficients on the length is recognisable, but less pronounced. Even for the shortest investigated annulus, i.e. L = 1, the stiffness coefficients due to the forces from the angular motion of the rotor are of the same order of magnitude as the stiffness coefficients due to the forces from the translational motion. This supports recent results, indicating that the additional coefficients become relevant much earlier than assumed throughout the literature, cf. Childs (1993).