Experimental setup

1.1 Compressor cascade

The measurements were performed on a large-scale compressor cascade called V103-220 consisting of three NACA 65 blades, which represent the mid-span of the hub section of a stator blade in a highly loaded axial compres­sor. Only the center blade is used for the measurements. To achieve a higher resolution of the boundary layer effects, a blade chord length of l = 220 mm was chosen. The design conditions with an inlet Mach number of Mai = 0.67, a Reynolds number based on the blade chord length of Re1 = 450000 and an inlet ft>w angle of в 1 = 132° lead to a fully subsonic cascade ft>w. The geo­metrical data of the cascade as well as the definitions of distances are displayed in Fig 1.

1.2 The wake generator

The periodically unsteady flow caused by the relative motion of rotor and stator rows and its influence on the compressor cascade is simulated by a mov­ing bar type wake generator with a bar diameter of dbar = 2 mm. This so-called

EIZ (Erzeuger Instationaerer Zustroemung, see Fig. 2) and its constructional principles are explained by Acton and Fottner (1996) in more detail. The cylin­drical steel bars create a far wake very similar to the one produced by an actual airfoil (Pfeil and Eiffer 1976). Preliminary tests showed that the wakes shed by bars of 2 mm diameter are representative for the wakes of the V103 pro­file geometry regarding the wake width. The distance ratio between the bars and the cascade inlet plane is about x/l = 0.35 (see Fig. 1). Two different bar pitches of 40 mm and 120 mm were used. The belt mechanism drives the bars with speeds of up to 40 m/s. However, the maximum bar speed for the present investigation is 20 m/s, thus generating Strouhal numbers between 0.22 and 0.66 for the investigated test cases.

It should be noted that the maximum bar speed together with the axial ve­locities is still too slow to produce a Strouhal number and inlet velocity tri­angle representative for modern compressors. The wakes enters the cascade passage almost parallel to the blades. Therefore the data acquired with this setup cannot be transferred directly to real turbomachines. The measurements should be considered as basic investigations of the unsteady multimode transi­tion process. As the main purpose of the present experimental investigations is to obtain a deeper unterstanding of the ft>w phenomena and to provide a sound database for the validation of unsteady numerical flow solvers and particular transition models, the angle of the incoming wake is of minor importance.