Boundary conditions

Experimental stator only results of the exit flow are different to the stage results due to not completely matching conditions of the experiment in the two test runs. In the stator only tests the average exit Mach number at 128% cax, stator was 1.00. In the stage experiments, the corresponding Mach num­ber was 1.05. These small differences can be significant for the aerodynamic rotor excitation in the studied transonic flow. In all computations, beside one (TRACE) the stage results have been used for setting up the computations, be­cause it is assumed that the boundary conditions from the stage experiments will lead to better agreement to the unsteady flow in the stage than the stator only computations. A Fourier analysis of the stator exit harmonics at midspan indicates that the low engine order harmonic content is only slightly influenced by the stator exit flow difference of the single passages. Closer to the end walls (at 20% and 9% of blade height), the differences between stator only and stage experiments are however remarkable. This can be seen in Figs. 4 and 5 when comparing the relative differences between stator only and stage result of the vane passing frequency (and multiples) to the ones in the low engine order frequency.

The mixed out flow parameters behind each passage of the stator are differ­ent, which is most probably caused by the geometric variation of the stator. To define a boundary condition for the computation, an arithmetic average of the mixed out flow parameters of each passage was specified.