  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).
