Investigations of the Boundary Layer on a Highly Loaded Compressor Cascade With Wake-Induced Transition

The flow field through a turbomachinery compressor cascades is significantly affected by the unsteady flow originating from the upstream blade rows. The interaction is caused by the wakes from the upstream blades, which affect the properties of the boundary layer of the downstream blades. In addition, pressure fluctuations exist between upstream and downstream blades. These phenomenona play a significant role in the loss generation process on turbomachinery blades because it influences the onset of transition in the boundary layer and has the potential to suppress a boundary layer separation in some cases. Extensive experimental investigations have been performed at the Institute of Jet Propulsion in Neubiberg, where these effects where studied in detail. The measurements were performed on a large scale compressor cascade called V103-220. The chord length of l = 220 mm chosen allowed the unsteady boundary layer development to be studied in great detail and provided high quality data for this complex flow, which can be used for the validation of CFD codes. Unsteady CFD calculations were performed using the RANS-code TRACE developed at DLR Cologne. A modern variant of the Wilcox k-ω turbulence model in combination with a newly implemented transition model was used, allowing a better determination of multimode transition. A multiblock grid with an O-type grid around the blade and a boundary layer resolution of y+<1 was used. Experimental and numerical results confirm that wake passing has a large influence on the unsteady boundary layer development also in this compressor flow case. The premature forced transition is followed by a stable calmed region, which partially suppresses laminar separation due to its higher shear stress level and delays the onset of transition in the path between wakes. In addition, it was found that the leakage from two slots, which are opened in the rig when the wake generator device is installed, changes the flow field considerably. This effect is not fully reproduced by the CFD calculations. To study this effect in more detail, three-dimensional steady and unsteady CFD calculations were undertaken and are being continued.