Investigations of the Unsteady Shock-Boundary Layer Interaction in a Transonic Compressor Cascade

Abstract

The performance of compressor blades in transonic flow conditions is heavily hampered by unsteady flow effects caused by the Shock-Boundary Layer Interaction (SBLI). Even though these effects have been subject to research for a long period of time, very little is still known about the physical mechanisms driving the unsteadiness. In order to help elucidate the nature of these interactions in turbomachines, the recently designed Transonic Cascade TEAMAero was tested at the DLR's Transonic Cascade Wind Tunnel facility in Cologne. The cascade flow was measured with a unique high-speed Schlieren configuration capturing three adjacent passages simultaneously, along with unsteady total pressure measurements at the outlet, and unsteady acceleration measurements on the tunnel sidewalls. The results indicate that the main oscillation of the shocks is broadband at relatively low frequencies around 550 Hz for the aerodynamic design point, and 180 Hz for the off-design point. A further high-frequency tone is observed around 1140 Hz throughout. These frequencies were also observed with the different measurement devices employed. Cross-correlations between the signals indicate that there are different mechanisms at play between the passages that cause both upstream and downstream transmission of information. Because of this, the shock oscillations were not always in-phase between adjacent passages. The results help characterize the SBLI behavior of the cascade in the wind tunnel, aiding future numerical and experimental efforts aiming to decipher the inherent mechanisms causing it.