Abstract Qualitative and quantitative visualizations of transonic turbomachinery flows provide essential information on compressibility effects on boundary layer development, shock-boundary layer interactions and trailing edge flows. Background Oriented Schlieren (BOS) is a relatively new optical technique that allows capturing density gradient fields through turbomachinery cascades and thus, quantitative experimental data to validate transonic and supersonic blade designs. However, only very few experimental works in the open literature have successfully applied BOS to transonic turbine or compressor flows. The current study presents the application of BOS to a transonic low pressure turbine cascade, the VKI SPLEEN C1 cascade for a range of Reynolds numbers (70,000 to 140,000) and transonic Mach numbers (0.90 to 1.00). The linear turbine cascade is tested at the von Karman Institute in the S-1/C high-speed wind tunnel. The test section is instrumented with different BOS optical setups to visualize time-resolved density gradients through the turbine passage and at the trailing edge plane with dedicated field of views. The BOS images are processed using the cross-correlation algorithm, and the optical flow approach, recently introduced in BOS applications to gain spatial resolution and increased sensitivity. Steady-state density gradients of the cascade flow characterize the airfoil boundary layers, wakes and shock waves. The results from the two data reduction methods are assessed and compared against available RANS predictions. Time-resolved measurements reveal the low-frequency motion of weak shock waves generated in the blade passage using POD and spectra analysis.
Read full abstract7-days of FREE Audio papers, translation & more with Prime
7-days of FREE Prime access