  We discuss the static force characteristic of annular gaps resulting from an
axial flow component. So far there is a severe lack of understanding of the
flow inside the annulus. First, the state-of-the-art modelling approaches to
describe the flow inside the annulus are recapped and discussed. The discussion
focuses in particular on the modelling of inertia effects. Second, a new
calculation method, the Clearance-Averaged Pressure Model (CAPM) is presented.
The CAPM uses an integro-differential approach in combination with power law
ansatz functions for the velocity profiles and a Hirs' model to calculate the
resulting pressure field. Third, for experimental validation, a setup is
presented using magnetic bearings to inherently measure the position as well as
the force on the rotor induced by the flow field inside the gap. The
experiments focus on the characteristic load behaviour, attitude angle and
pressure difference across the annulus. Fourth, the experimental results are
compared to the calculation results.
