Investigation of Unsteady Flow Effects in an Axial Compressor Based on Whole Annulus Computations

Flows in the first stages of axial compressors are subject to a number of unsteady effects which are not generally taken into account in standard investigations, e.g. transition under the influence of impinging wakes as well as that of downstream potential effects, or the influence of unsteady blade row interactions. For numerical studies of these phenomena, usually, simplifications are made to the test case in question. These simplifications include domain scaling, quasi-unsteady modeling, or modeling assumptions regarding boundary layer development, like wall functions; and their impact on the final result can be quite high. This paper is concerned with the investigation of unsteady blade row interactions in a 4.5 stage research compressor without any of these simplifications. To this end, the whole annulus of the first two stages of the compressor was meshed and unsteady RANS simulations were carried out at two different operating points. Spatial and temporal resolutions were fine enough to allow the investigation of transitional phenomena on the blade surface using an integral multi-mode transition model. For each operating point, one revolution was recorded after computations reached a periodic state. Examination of the boundary layer parameters shows that transition plays a considerable role for the first two compressor stages which were investigated in more detail. It was evident that the development of the blade boundary layer depends not only on incoming wakes from upstream blade rows; the precise transition location is also highly dependent on the potential effect of downstream blade rows.