High-Speed Schlieren and PIV in a Transonic Compressor Cascade with Shock Control Bumps

In the design of transonic turbomachinery blades, the reduction of shock instabilities and the associated flow separations is of importance to minimize viscous losses and ensure proper flow guiding over a wide operating range. Approaches using passive flow control with shock control bumps (SCB) can contribute to this. In the framework of the TEAMAero research project the effectiveness of SCBs was investigated in a non-proprietary transonic compressor cascade Team Aero (TCTA). The TCTA was operated in the transonic cascade wind tunnel of the DLR Institute of Propulsion Technology at M_1=1.22. To evaluate the effectiveness of flow control, high-speed schlieren visualizations (HSS) were conducted in three adjacent passages as well as HS-PIV both at frame rates near 20~kHz. In particular, HSS that covers adjacent blade passages allows for the identification of interactions between blade passages involving resonant low-frequency shock motions, which could be potentially decoupled by the SCBs. For the TCTA without SCBs, the resonant propagation of instabilities towards downstream passages was already observed. The mitigation of such resonances and interferences of the transonic flow through neighboring passages by flow control is of importance since it may reduce also precursors of rotating instabilities in real machines. Within this contribution it is demonstrated that the spectral modal decomposition and low order reconstruction of the HSS recordings allows deeper insights into these resonances for the TCTA with and without SCBs. In addition, the present contribution will provide deeper insight into the unsteady velocity distribution measured by High-Speed PIV using Frame-Straddling. Capabilities of the new 40W Double-Pulse Lasersystem is demonstrated.